Replace wgrib2 with pure Elixir GRIB2 decoder for HRRR pipeline

Eliminates the external wgrib2 C tool dependency that blocked HRRR
processing. Implements Lambert Conformal projection, simple packing
(Template 5.0), complex packing with spatial differencing (Template 5.3),
and GRIB2 section parsing — enough to extract point values from HRRR
grid data using only Elixir.

.gitignore

@@ -31,6 +31,9 @@ microwaveprop-*.tar
 .env
 .env.*
 
+# GRIB2 test fixtures (large binary files, downloaded on-demand)
+/test/fixtures/grib2/*.grib2
+
 # OS files
 .DS_Store
 Thumbs.db

config/test.exs

@@ -29,6 +29,7 @@ config :microwaveprop, MicrowavepropWeb.Endpoint,
 # Run Oban jobs inline during tests
 config :microwaveprop, Oban, testing: :inline
 config :microwaveprop, elevation_req_options: [plug: {Req.Test, Microwaveprop.Terrain.ElevationClient}, retry: false]
+config :microwaveprop, hrrr_req_options: [plug: {Req.Test, Microwaveprop.Weather.HrrrClient}, retry: false]
 config :microwaveprop, iem_req_options: [plug: {Req.Test, Microwaveprop.Weather.IemClient}, retry: false]
 
 # Route HTTP requests through Req.Test stubs

lib/microwaveprop/weather/grib2/complex_packing.ex

@@ -0,0 +1,229 @@
+defmodule Microwaveprop.Weather.Grib2.ComplexPacking do
+  @moduledoc false
+
+  @doc """
+  Extract a single value from GRIB2 complex-packed data with spatial differencing
+  (Template 5.3) at the given grid index.
+
+  Decodes all values since spatial differencing requires sequential access,
+  then returns the value at the target index.
+  """
+  def extract_value(params, data, index) do
+    if index < 0 or index >= params.num_data_points do
+      {:error, :index_out_of_range}
+    else
+      case decode_all(params, data) do
+        {:ok, values} -> {:ok, :array.get(index, values)}
+        {:error, _} = err -> err
+      end
+    end
+  end
+
+  @doc """
+  Decode all values from complex-packed data with spatial differencing.
+
+  Returns an Erlang array of floats for O(1) index access.
+  """
+  def decode_all(params, data) do
+    %{
+      reference_value: ref,
+      binary_scale: e,
+      decimal_scale: d,
+      bits_per_value: nbits,
+      num_groups: num_groups,
+      ref_group_widths: ref_gw,
+      nbits_group_widths: nbits_gw,
+      ref_group_lengths: ref_gl,
+      length_increment: len_inc,
+      last_group_length: last_gl,
+      nbits_group_lengths: nbits_gl,
+      spatial_order: spatial_order,
+      num_extra_octets: num_extra_octets
+    } = params
+
+    # Step 1: Extract spatial differencing initial values
+    octets_per_val = num_extra_octets
+    num_init_vals = spatial_order + 1
+
+    {init_vals, rest} = extract_init_values(data, num_init_vals, octets_per_val)
+
+    {spatial_init, [overall_min]} = Enum.split(init_vals, spatial_order)
+
+    # Step 2: Extract group reference values (byte-padded per GRIB2 spec)
+    {group_refs, rest} = extract_n_values_array(rest, num_groups, nbits)
+
+    # Step 3: Extract group widths (byte-padded)
+    {group_widths_arr, rest} = extract_n_values_array(rest, num_groups, nbits_gw)
+
+    # Step 4: Extract group lengths (byte-padded)
+    {group_lengths_arr, rest} = extract_n_values_array(rest, num_groups, nbits_gl)
+
+    # Step 5: Build group info lists
+    group_widths =
+      for g <- 0..(num_groups - 1) do
+        get_val(group_widths_arr, g) + ref_gw
+      end
+
+    group_lengths =
+      for g <- 0..(num_groups - 1) do
+        if g == num_groups - 1 do
+          last_gl
+        else
+          get_val(group_lengths_arr, g) * len_inc + ref_gl
+        end
+      end
+
+    group_refs_list =
+      for g <- 0..(num_groups - 1) do
+        get_val(group_refs, g)
+      end
+
+    # Step 6: Decode packed data for each group using bitstring operations
+    raw_values = decode_groups_bitwise(rest, group_refs_list, group_widths, group_lengths)
+
+    # Step 7: Apply spatial differencing in reverse
+    undiffed = apply_spatial_differencing(spatial_init, overall_min, raw_values)
+
+    # Step 8: Apply scaling formula: value = (R + X * 2^E) * 10^(-D)
+    factor_2e = :math.pow(2, e)
+    factor_10d = :math.pow(10, -d)
+
+    result =
+      undiffed
+      |> :array.to_list()
+      |> Enum.map(fn x -> (ref + x * factor_2e) * factor_10d end)
+      |> :array.from_list()
+
+    {:ok, result}
+  rescue
+    e in [MatchError, ArgumentError] ->
+      {:error, "GRIB2 complex packing decode failed: #{inspect(e)}"}
+  end
+
+  # --- Private helpers ---
+
+  defp extract_init_values(data, count, octets_per_val) do
+    total_bytes = count * octets_per_val
+    <<init_bytes::binary-size(total_bytes), rest::binary>> = data
+
+    values =
+      for i <- 0..(count - 1) do
+        offset = i * octets_per_val
+        val_bytes = binary_part(init_bytes, offset, octets_per_val)
+        decode_signed_big(val_bytes)
+      end
+
+    {values, rest}
+  end
+
+  defp decode_signed_big(bytes) do
+    bits = byte_size(bytes) * 8
+    <<sign::1, magnitude::size(bits - 1)>> = bytes
+    if sign == 1, do: -magnitude, else: magnitude
+  end
+
+  defp extract_n_values_array(data, _count, 0) do
+    {nil, data}
+  end
+
+  defp extract_n_values_array(data, count, nbits) do
+    total_bits = count * nbits
+    total_bytes = div(total_bits + 7, 8)
+    <<chunk::binary-size(total_bytes), rest::binary>> = data
+
+    vals = consume_bits_simple(<<chunk::binary>>, nbits, [])
+    # consume_bits_simple returns reversed, take only count values
+    arr = vals |> Enum.take(count) |> Enum.reverse() |> :array.from_list()
+
+    {arr, rest}
+  end
+
+  defp consume_bits_simple(bits, nbits, acc) when bit_size(bits) < nbits, do: acc
+
+  defp consume_bits_simple(bits, nbits, acc) do
+    <<val::size(nbits)-unsigned-big, rest::bitstring>> = bits
+    consume_bits_simple(rest, nbits, [val | acc])
+  end
+
+  defp get_val(nil, _index), do: 0
+  defp get_val(arr, index), do: :array.get(index, arr)
+
+  # Decode all groups from packed data using a bitstring cursor.
+  # Returns an Erlang array of raw (pre-differencing) integer values.
+  defp decode_groups_bitwise(data, group_refs, group_widths, group_lengths) do
+    {all_values_reversed, _remaining_bits} =
+      [group_refs, group_widths, group_lengths]
+      |> Enum.zip()
+      |> Enum.reduce({[], data}, fn {gref, width, length}, {acc, remaining} ->
+        if width == 0 do
+          # All values equal gref — prepend in reverse (same values, order irrelevant)
+          new_acc = prepend_n(acc, gref, length)
+          {new_acc, remaining}
+        else
+          {new_acc, rest} = consume_group_bits(remaining, width, gref, length, acc)
+          {new_acc, rest}
+        end
+      end)
+
+    all_values_reversed
+    |> Enum.reverse()
+    |> :array.from_list()
+  end
+
+  # Consume `count` values of `width` bits each from bitstring, prepending to acc.
+  # Values are prepended in reverse order (last value first).
+  # Handles trailing padding: if fewer bits remain than needed, pad with gref.
+  defp consume_group_bits(bits, width, gref, count, acc) do
+    total_bits = count * width
+    available = bit_size(bits)
+
+    if available >= total_bits do
+      <<chunk::bitstring-size(total_bits), rest::bitstring>> = bits
+      new_acc = consume_bits(chunk, width, gref, acc)
+      {new_acc, rest}
+    else
+      # Extract what we can, fill remainder with gref
+      new_acc = consume_bits(bits, width, gref, acc)
+      extracted = div(available, width)
+      remaining = count - extracted
+      new_acc = prepend_n(new_acc, gref, remaining)
+      {new_acc, <<>>}
+    end
+  end
+
+  defp consume_bits(<<>>, _width, _gref, acc), do: acc
+
+  defp consume_bits(bits, width, gref, acc) do
+    <<val::size(width)-unsigned-big, rest::bitstring>> = bits
+    consume_bits(rest, width, gref, [val + gref | acc])
+  end
+
+  defp prepend_n(acc, _value, 0), do: acc
+  defp prepend_n(acc, value, n), do: prepend_n([value | acc], value, n - 1)
+
+  defp apply_spatial_differencing(spatial_init, overall_min, raw_values) do
+    raw_list = :array.to_list(raw_values)
+
+    case spatial_init do
+      [ival1] ->
+        {result_reversed, _} =
+          Enum.reduce(tl(raw_list), {[ival1], ival1}, fn raw, {acc, prev} ->
+            current = prev + raw + overall_min
+            {[current | acc], current}
+          end)
+
+        :array.from_list(Enum.reverse(result_reversed))
+
+      [ival1, ival2] ->
+        {result_reversed, _, _} =
+          raw_list
+          |> Enum.drop(2)
+          |> Enum.reduce({[ival2, ival1], ival2, ival1}, fn raw, {acc, prev1, prev2} ->
+            current = raw + overall_min + 2 * prev1 - prev2
+            {[current | acc], current, prev1}
+          end)
+
+        :array.from_list(Enum.reverse(result_reversed))
+    end
+  end
+end

lib/microwaveprop/weather/grib2/extractor.ex

@@ -0,0 +1,83 @@
+defmodule Microwaveprop.Weather.Grib2.Extractor do
+  @moduledoc false
+
+  alias Microwaveprop.Weather.Grib2.ComplexPacking
+  alias Microwaveprop.Weather.Grib2.LambertConformal
+  alias Microwaveprop.Weather.Grib2.Section
+  alias Microwaveprop.Weather.Grib2.SimplePacking
+
+  @doc """
+  Extract weather values from a GRIB2 binary blob at the given lat/lon.
+
+  Returns `{:ok, %{"VAR:LEVEL" => float}}` or `{:error, term}`.
+  """
+  def extract_points(binary, lat, lon) do
+    messages = split_messages(binary)
+
+    results =
+      Enum.reduce_while(messages, {:ok, %{}}, fn msg, {:ok, acc} ->
+        case extract_single(msg, lat, lon) do
+          {:ok, key, value} -> {:cont, {:ok, Map.put(acc, key, value)}}
+          {:error, :outside_grid} -> {:halt, {:error, :outside_grid}}
+          {:error, reason} -> {:halt, {:error, reason}}
+        end
+      end)
+
+    results
+  end
+
+  @doc """
+  Split a binary blob into individual GRIB2 messages by scanning for "GRIB" magic bytes
+  and reading the total length from the indicator section.
+  """
+  def split_messages(binary), do: split_messages(binary, [])
+
+  @doc """
+  Compute linear index from grid (i, j) and scan mode.
+
+  For HRRR scan_mode=64 (j positive, i positive, i consecutive): index = j * nx + i
+  """
+  def linear_index({i, j}, nx, _scan_mode) do
+    j * nx + i
+  end
+
+  # --- Private ---
+
+  defp split_messages(<<>>, acc), do: Enum.reverse(acc)
+
+  defp split_messages(<<"GRIB", _::32, total_length::64-big, _rest::binary>> = binary, acc) do
+    if total_length > byte_size(binary) do
+      Enum.reverse(acc)
+    else
+      msg = binary_part(binary, 0, total_length)
+      remaining = binary_part(binary, total_length, byte_size(binary) - total_length)
+      split_messages(remaining, [msg | acc])
+    end
+  end
+
+  defp split_messages(<<_::8, rest::binary>>, acc) do
+    # Skip non-GRIB bytes (padding between messages)
+    split_messages(rest, acc)
+  end
+
+  defp extract_single(msg, lat, lon) do
+    with {:ok, parsed} <- Section.parse_message(msg),
+         %{grid_params: grid, product: prod, packing_params: packing, data: data} = parsed,
+         key = "#{prod.var}:#{prod.level}",
+         {:ok, {i, j}} <- LambertConformal.to_grid_index(grid, lat, lon),
+         index = linear_index({i, j}, grid.nx, grid.scan_mode),
+         {:ok, value} <- unpack_value(packing, data, index) do
+      {:ok, key, value}
+    end
+  rescue
+    e -> {:error, "GRIB2 extraction failed: #{inspect(e)}"}
+  end
+
+  defp unpack_value(%{template: 3} = params, data, index) do
+    ComplexPacking.extract_value(params, data, index)
+  end
+
+  defp unpack_value(params, data, index) do
+    SimplePacking.extract_value(params, data, index)
+  end
+end

lib/microwaveprop/weather/grib2/lambert_conformal.ex

@@ -0,0 +1,84 @@
+defmodule Microwaveprop.Weather.Grib2.LambertConformal do
+  @moduledoc false
+
+  @r_earth 6_371_229.0
+
+  @doc """
+  Convert a lat/lon to grid indices (i, j) on a Lambert Conformal Conic grid.
+
+  Returns `{:ok, {i, j}}` or `{:error, :outside_grid}`.
+  """
+  def to_grid_index(grid, lat, lon) do
+    %{
+      nx: nx,
+      ny: ny,
+      la1: la1,
+      lo1: lo1,
+      dx: dx,
+      dy: dy,
+      latin1: latin1,
+      lov: lov
+    } = grid
+
+    # Normalize longitudes to 0..360
+    lo1 = normalize_lon(lo1)
+    lon = normalize_lon(lon)
+    lov = normalize_lon(lov)
+
+    latin1_rad = deg_to_rad(latin1)
+    la1_rad = deg_to_rad(la1)
+    lat_rad = deg_to_rad(lat)
+
+    # Cone constant (for HRRR, latin1 == latin2 == 38.5, so n = sin(latin1))
+    n = :math.sin(latin1_rad)
+
+    # F factor
+    f =
+      :math.cos(latin1_rad) *
+        :math.pow(:math.tan(:math.pi() / 4 + latin1_rad / 2), n) / n
+
+    # rho for a given latitude
+    rho_origin = rho(la1_rad, n, f)
+    rho_target = rho(lat_rad, n, f)
+
+    # Theta angles (difference from central meridian, scaled by cone constant)
+    theta_origin = n * deg_to_rad(lo1 - lov)
+    theta_target = n * deg_to_rad(adjusted_lon_diff(lon, lov))
+
+    # Grid coordinates
+    x_origin = rho_origin * :math.sin(theta_origin)
+    y_origin = rho_origin * :math.cos(theta_origin)
+
+    x_target = rho_target * :math.sin(theta_target)
+    y_target = rho_target * :math.cos(theta_target)
+
+    i = round((x_target - x_origin) / dx)
+    j = round((y_origin - y_target) / dy)
+
+    if i >= 0 and i < nx and j >= 0 and j < ny do
+      {:ok, {i, j}}
+    else
+      {:error, :outside_grid}
+    end
+  end
+
+  defp rho(lat_rad, n, f) do
+    @r_earth * f / :math.pow(:math.tan(:math.pi() / 4 + lat_rad / 2), n)
+  end
+
+  defp deg_to_rad(deg), do: deg * :math.pi() / 180.0
+
+  defp normalize_lon(lon) when lon < 0, do: lon + 360.0
+  defp normalize_lon(lon), do: lon + 0.0
+
+  # Compute longitude difference handling wrap-around
+  defp adjusted_lon_diff(lon, lov) do
+    diff = lon - lov
+
+    cond do
+      diff > 180.0 -> diff - 360.0
+      diff < -180.0 -> diff + 360.0
+      true -> diff
+    end
+  end
+end

lib/microwaveprop/weather/grib2/section.ex

@@ -0,0 +1,259 @@
+defmodule Microwaveprop.Weather.Grib2.Section do
+  @moduledoc false
+
+  @doc """
+  Parse a single GRIB2 message binary into its component sections.
+
+  Returns `{:ok, %{grid_params, product, packing_params, data, bitmap}}`.
+  """
+  def parse_message(<<"GRIB", _reserved::16, _discipline::8, 2::8, total_length::64-big, rest::binary>> = msg) do
+    discipline = :binary.at(msg, 6)
+    # Skip indicator section (16 bytes), parse remaining sections
+    message_body = binary_part(rest, 0, total_length - 16)
+    parse_sections(message_body, %{discipline: discipline})
+  end
+
+  def parse_message(<<"GRIB", _reserved::16, _discipline::8, edition::8, _::binary>>) do
+    {:error, "unsupported GRIB edition #{edition}"}
+  end
+
+  def parse_message(_), do: {:error, "not a GRIB2 message"}
+
+  @doc """
+  Identify variable name from discipline, parameter category, and parameter number.
+  """
+  def identify_var(0, 0, 0), do: "TMP"
+  def identify_var(0, 0, 6), do: "DPT"
+  def identify_var(0, 3, 0), do: "PRES"
+  def identify_var(0, 3, 5), do: "HGT"
+  def identify_var(0, 3, 18), do: "HPBL"
+  def identify_var(0, 1, 3), do: "PWAT"
+  def identify_var(_discipline, cat, num), do: "#{cat}:#{num}"
+
+  @doc """
+  Identify level string from surface type and scaled value.
+  """
+  def identify_level(1, _value), do: "surface"
+  def identify_level(100, value_pa), do: "#{div(value_pa, 100)} mb"
+  def identify_level(103, value_m), do: "#{value_m} m above ground"
+
+  def identify_level(200, _value), do: "entire atmosphere (considered as a single layer)"
+
+  def identify_level(type, value), do: "unknown:#{type}:#{value}"
+
+  @doc """
+  Decode a 16-bit sign-magnitude integer.
+  """
+  def sign_magnitude_16(<<0::1, value::15>>), do: value
+  def sign_magnitude_16(<<1::1, value::15>>), do: -value
+
+  # --- Private section parsing ---
+
+  defp parse_sections(<<"7777", _rest::binary>>, acc) do
+    build_result(acc)
+  end
+
+  defp parse_sections(<<length::32-big, section_num::8, rest::binary>>, acc) when length > 5 do
+    body_length = length - 5
+    <<body::binary-size(body_length), remaining::binary>> = rest
+
+    acc = parse_section(section_num, body, acc)
+    parse_sections(remaining, acc)
+  end
+
+  defp parse_sections(<<>>, acc) do
+    build_result(acc)
+  end
+
+  defp parse_sections(_other, _acc) do
+    {:error, "malformed section"}
+  end
+
+  defp parse_section(1, _body, acc) do
+    # Section 1 (Identification) — we don't need anything from it
+    acc
+  end
+
+  defp parse_section(3, body, acc) do
+    <<
+      _source::8,
+      _num_points::32-big,
+      _optional_octets::8,
+      _interpretation::8,
+      template::16-big,
+      rest::binary
+    >> = body
+
+    grid_params = parse_grid_template(template, rest)
+    Map.put(acc, :grid_params, grid_params)
+  end
+
+  defp parse_section(4, body, acc) do
+    <<
+      _num_coords::16-big,
+      _template::16-big,
+      param_category::8,
+      param_number::8,
+      _generating_process::8,
+      _background::8,
+      _forecast_process::8,
+      _cutoff_hours::16-big,
+      _cutoff_minutes::8,
+      _time_unit::8,
+      _forecast_time::32-big,
+      first_surface_type::8,
+      _first_scale_factor::8,
+      first_scaled_value::32-big,
+      _rest::binary
+    >> = body
+
+    discipline = Map.get(acc, :discipline, 0)
+    var = identify_var(discipline, param_category, param_number)
+    level = identify_level(first_surface_type, first_scaled_value)
+
+    Map.put(acc, :product, %{var: var, level: level})
+  end
+
+  defp parse_section(5, body, acc) do
+    <<
+      num_data_points::32-big,
+      template::16-big,
+      ref_value_bytes::binary-size(4),
+      binary_scale_bytes::binary-size(2),
+      decimal_scale_bytes::binary-size(2),
+      bits_per_value::8,
+      rest::binary
+    >> = body
+
+    <<ref_value::float-32-big>> = ref_value_bytes
+
+    base_params = %{
+      num_data_points: num_data_points,
+      template: template,
+      reference_value: ref_value,
+      binary_scale: sign_magnitude_16(binary_scale_bytes),
+      decimal_scale: sign_magnitude_16(decimal_scale_bytes),
+      bits_per_value: bits_per_value
+    }
+
+    packing_params =
+      case template do
+        0 -> base_params
+        3 -> parse_complex_packing_params(base_params, rest)
+        _ -> base_params
+      end
+
+    Map.put(acc, :packing_params, packing_params)
+  end
+
+  defp parse_section(6, <<indicator::8, _rest::binary>>, acc) do
+    bitmap =
+      if indicator == 255 do
+        :none
+      else
+        :present
+      end
+
+    Map.put(acc, :bitmap, bitmap)
+  end
+
+  defp parse_section(7, data, acc) do
+    Map.put(acc, :data, data)
+  end
+
+  defp parse_section(_section_num, _body, acc) do
+    # Skip unknown sections (Section 2, etc.)
+    acc
+  end
+
+  defp parse_complex_packing_params(base, rest) do
+    <<
+      _original_type::8,
+      _splitting_method::8,
+      missing_mgmt::8,
+      _primary_missing::32-big,
+      _secondary_missing::32-big,
+      num_groups::32-big,
+      ref_group_widths::8,
+      nbits_group_widths::8,
+      ref_group_lengths::32-big,
+      length_increment::8,
+      last_group_length::32-big,
+      nbits_group_lengths::8,
+      spatial_order::8,
+      num_extra_octets::8,
+      _extra::binary
+    >> = rest
+
+    Map.merge(base, %{
+      missing_mgmt: missing_mgmt,
+      num_groups: num_groups,
+      ref_group_widths: ref_group_widths,
+      nbits_group_widths: nbits_group_widths,
+      ref_group_lengths: ref_group_lengths,
+      length_increment: length_increment,
+      last_group_length: last_group_length,
+      nbits_group_lengths: nbits_group_lengths,
+      spatial_order: spatial_order,
+      num_extra_octets: num_extra_octets
+    })
+  end
+
+  # Template 3.30 — Lambert Conformal Conic
+  defp parse_grid_template(30, body) do
+    <<
+      _shape::8,
+      _scale_factor_radius::8,
+      _scaled_radius::32-big,
+      _scale_factor_major::8,
+      _scaled_major::32-big,
+      _scale_factor_minor::8,
+      _scaled_minor::32-big,
+      nx::32-big,
+      ny::32-big,
+      la1_micro::32-big,
+      lo1_micro::32-big,
+      _resolution::8,
+      lad_micro::32-big,
+      lov_micro::32-big,
+      dx_mm::32-big,
+      dy_mm::32-big,
+      _projection_centre::8,
+      scan_mode::8,
+      latin1_micro::32-big,
+      latin2_micro::32-big,
+      _lat_sp::32-big,
+      _lon_sp::32-big,
+      _rest::binary
+    >> = body
+
+    %{
+      nx: nx,
+      ny: ny,
+      la1: la1_micro / 1_000_000,
+      lo1: lo1_micro / 1_000_000,
+      lad: lad_micro / 1_000_000,
+      lov: lov_micro / 1_000_000,
+      dx: dx_mm / 1000,
+      dy: dy_mm / 1000,
+      latin1: latin1_micro / 1_000_000,
+      latin2: latin2_micro / 1_000_000,
+      scan_mode: scan_mode
+    }
+  end
+
+  defp parse_grid_template(template, _body) do
+    %{error: "unsupported grid template #{template}"}
+  end
+
+  defp build_result(acc) do
+    required = [:grid_params, :product, :packing_params, :data, :bitmap]
+
+    if Enum.all?(required, &Map.has_key?(acc, &1)) do
+      {:ok, Map.take(acc, required)}
+    else
+      missing = Enum.reject(required, &Map.has_key?(acc, &1))
+      {:error, "missing sections: #{inspect(missing)}"}
+    end
+  end
+end

lib/microwaveprop/weather/grib2/simple_packing.ex

@@ -0,0 +1,44 @@
+defmodule Microwaveprop.Weather.Grib2.SimplePacking do
+  @moduledoc false
+
+  @doc """
+  Extract a single value from GRIB2 simple-packed data at the given grid index.
+
+  Formula: value = (R + X * 2^E) * 10^(-D)
+
+  Where R = reference_value, E = binary_scale, D = decimal_scale,
+  X = raw packed integer at the given index.
+  """
+  def extract_value(%{bits_per_value: 0} = params, _data, _index) do
+    {:ok, params.reference_value * :math.pow(10, -params.decimal_scale)}
+  end
+
+  def extract_value(params, data, index) do
+    %{
+      reference_value: r,
+      binary_scale: e,
+      decimal_scale: d,
+      bits_per_value: n,
+      num_data_points: num
+    } = params
+
+    if index < 0 or index >= num do
+      {:error, :index_out_of_range}
+    else
+      x = extract_bits(data, index, n)
+      value = (r + x * :math.pow(2, e)) * :math.pow(10, -d)
+      {:ok, value}
+    end
+  end
+
+  defp extract_bits(data, index, bits_per_value) do
+    bit_offset = index * bits_per_value
+    byte_offset = div(bit_offset, 8)
+    remainder = rem(bit_offset, 8)
+
+    <<_skip::binary-size(byte_offset), _pad::size(remainder), x::size(bits_per_value)-unsigned-big, _rest::bitstring>> =
+      data
+
+    x
+  end
+end

lib/microwaveprop/weather/hrrr_client.ex

@@ -1,6 +1,8 @@
 defmodule Microwaveprop.Weather.HrrrClient do
   @moduledoc false
 
+  alias Microwaveprop.Weather.Grib2.Extractor
+
   @hrrr_base "https://noaa-hrrr-bdp-pds.s3.amazonaws.com"
 
   @pressure_levels [1000, 975, 950, 925, 900, 850, 800, 700]
@@ -23,7 +25,7 @@ defmodule Microwaveprop.Weather.HrrrClient do
     with {:ok, sfc_data} <- fetch_product(date, hour, :surface, lat, lon),
          {:ok, prs_data} <- fetch_product(date, hour, :pressure, lat, lon) do
       merged = Map.merge(sfc_data, prs_data)
-      result = build_profile_from_wgrib2(merged)
+      result = build_profile(merged)
       {:ok, Map.put(result, :run_time, hour_dt)}
     end
   end
@@ -88,19 +90,7 @@ defmodule Microwaveprop.Weather.HrrrClient do
     end)
   end
 
-  def parse_wgrib2_output(text) do
+  def build_profile(parsed) do
-    text
-    |> String.split("\n")
-    |> Enum.reject(&(&1 == ""))
-    |> Enum.reduce(%{}, fn line, acc ->
-      case parse_wgrib2_line(line) do
-        {:ok, key, value} -> Map.put(acc, key, value)
-        :skip -> acc
-      end
-    end)
-  end
-
-  def build_profile_from_wgrib2(parsed) do
     sfc_temp_k = parsed["TMP:2 m above ground"]
     sfc_dpt_k = parsed["DPT:2 m above ground"]
     sfc_pres_pa = parsed["PRES:surface"]
@@ -151,9 +141,8 @@ defmodule Microwaveprop.Weather.HrrrClient do
     with {:ok, idx_text} <- fetch_idx(idx_url),
          idx_entries = parse_idx(idx_text),
          ranges = byte_ranges_for_messages(idx_entries, wanted),
-         {:ok, grib_binary} <- download_grib_ranges(url, ranges),
+         {:ok, grib_binary} <- download_grib_ranges(url, ranges) do
-         {:ok, output} <- extract_point(grib_binary, lat, lon) do
+      Extractor.extract_points(grib_binary, lat, lon)
-      {:ok, parse_wgrib2_output(output)}
     end
   end
 
@@ -194,53 +183,6 @@ defmodule Microwaveprop.Weather.HrrrClient do
     end
   end
 
-  defp extract_point(grib_binary, lat, lon) do
-    case System.find_executable("wgrib2") do
-      nil ->
-        {:error, "wgrib2 not found in PATH. Build from source: https://www.cpc.ncep.noaa.gov/products/wesley/wgrib2/"}
-
-      wgrib2_path ->
-        tmp_dir = System.tmp_dir!()
-        tmp_file = Path.join(tmp_dir, "hrrr_#{:erlang.unique_integer([:positive])}.grib2")
-
-        try do
-          File.write!(tmp_file, grib_binary)
-          lon_360 = if lon < 0, do: lon + 360, else: lon
-
-          case System.cmd(wgrib2_path, [tmp_file, "-lon", "#{lon_360}", "#{lat}"], stderr_to_stdout: true) do
-            {output, 0} -> {:ok, output}
-            {output, _} -> {:error, "wgrib2 failed: #{output}"}
-          end
-        after
-          File.rm(tmp_file)
-        end
-    end
-  end
-
-  defp parse_wgrib2_line(line) do
-    case Regex.run(~r/val=([0-9eE.+-]+)\s*$/, line) do
-      [_, val_str] ->
-        case Float.parse(val_str) do
-          {val, _} ->
-            parts = String.split(line, ":")
-            var = Enum.at(parts, 3)
-            level = Enum.at(parts, 4)
-
-            if var && level do
-              {:ok, "#{var}:#{level}", val}
-            else
-              :skip
-            end
-
-          :error ->
-            :skip
-        end
-
-      _ ->
-        :skip
-    end
-  end
-
   defp req_options do
     defaults = [retry: &retry?/2, max_retries: 5, retry_delay: &retry_delay/1]
     overrides = Application.get_env(:microwaveprop, :hrrr_req_options, [])

test/microwaveprop/weather/grib2/complex_packing_test.exs

@@ -0,0 +1,95 @@
+defmodule Microwaveprop.Weather.Grib2.ComplexPackingTest do
+  use ExUnit.Case, async: true
+
+  alias Microwaveprop.Weather.Grib2.ComplexPacking
+  alias Microwaveprop.Weather.Grib2.Section
+
+  @fixture_path "test/fixtures/grib2/hrrr_tmp_2m.grib2"
+
+  describe "extract_value/3 with real HRRR fixture" do
+    @tag :fixture
+    test "extracts a temperature value within meteorological range" do
+      if File.exists?(@fixture_path) do
+        fixture = File.read!(@fixture_path)
+        {:ok, parsed} = Section.parse_message(fixture)
+
+        # First grid point (index 0)
+        {:ok, value} =
+          ComplexPacking.extract_value(parsed.packing_params, parsed.data, 0)
+
+        # Should be a temperature in Kelvin (roughly 200-340K)
+        assert value > 200.0 and value < 340.0,
+               "Expected temperature 200-340K, got #{value}"
+      else
+        IO.puts("Skipping fixture test — #{@fixture_path} not found")
+        :ok
+      end
+    end
+
+    @tag :fixture
+    test "extracts values at multiple indices" do
+      if File.exists?(@fixture_path) do
+        fixture = File.read!(@fixture_path)
+        {:ok, parsed} = Section.parse_message(fixture)
+
+        # Check several indices across the grid
+        for idx <- [0, 1000, 100_000, 500_000, 1_000_000, 1_905_140] do
+          {:ok, value} =
+            ComplexPacking.extract_value(parsed.packing_params, parsed.data, idx)
+
+          assert value > 200.0 and value < 340.0,
+                 "Index #{idx}: expected 200-340K, got #{value}"
+        end
+      else
+        :ok
+      end
+    end
+
+    @tag :fixture
+    test "Dallas area temperature is reasonable for a US city" do
+      if File.exists?(@fixture_path) do
+        fixture = File.read!(@fixture_path)
+        {:ok, parsed} = Section.parse_message(fixture)
+
+        grid = parsed.grid_params
+        {:ok, {i, j}} = Microwaveprop.Weather.Grib2.LambertConformal.to_grid_index(grid, 32.78, -96.8)
+        index = j * grid.nx + i
+
+        {:ok, value} =
+          ComplexPacking.extract_value(parsed.packing_params, parsed.data, index)
+
+        # Dallas in March — Kelvin should be roughly 270-310K (roughly -3C to 37C)
+        temp_c = value - 273.15
+
+        assert temp_c > -10.0 and temp_c < 45.0,
+               "Dallas temp #{Float.round(temp_c, 1)}C seems unreasonable"
+      else
+        :ok
+      end
+    end
+  end
+
+  describe "decode_all/2" do
+    @tag :fixture
+    test "decodes all values from fixture" do
+      if File.exists?(@fixture_path) do
+        fixture = File.read!(@fixture_path)
+        {:ok, parsed} = Section.parse_message(fixture)
+
+        {:ok, values_arr} = ComplexPacking.decode_all(parsed.packing_params, parsed.data)
+
+        assert :array.size(values_arr) == parsed.packing_params.num_data_points
+
+        # Check a sample of values are in reasonable temperature range
+        for idx <- [0, 100_000, 500_000, 1_000_000, 1_905_140] do
+          val = :array.get(idx, values_arr)
+
+          assert val > 200.0 and val < 340.0,
+                 "Index #{idx}: expected 200-340K, got #{val}"
+        end
+      else
+        :ok
+      end
+    end
+  end
+end

test/microwaveprop/weather/grib2/extractor_test.exs

@@ -0,0 +1,243 @@
+defmodule Microwaveprop.Weather.Grib2.ExtractorTest do
+  use ExUnit.Case, async: true
+
+  alias Microwaveprop.Weather.Grib2.Extractor
+
+  # HRRR grid parameters for building synthetic messages
+  @hrrr_nx 1799
+  @hrrr_ny 1059
+
+  describe "split_messages/1" do
+    test "splits concatenated GRIB2 messages" do
+      msg1 = build_synthetic_grib2_message(0, 0, 0, 103, 2)
+      msg2 = build_synthetic_grib2_message(0, 0, 6, 103, 2)
+      blob = msg1 <> msg2
+
+      messages = Extractor.split_messages(blob)
+      assert length(messages) == 2
+      assert Enum.at(messages, 0) == msg1
+      assert Enum.at(messages, 1) == msg2
+    end
+
+    test "handles single message" do
+      msg = build_synthetic_grib2_message(0, 0, 0, 103, 2)
+      messages = Extractor.split_messages(msg)
+      assert length(messages) == 1
+    end
+
+    test "handles empty binary" do
+      assert Extractor.split_messages(<<>>) == []
+    end
+  end
+
+  describe "extract_points/3" do
+    test "extracts single message at first grid point" do
+      msg = build_synthetic_grib2_message(0, 0, 0, 103, 2)
+
+      # la1/lo1 = first grid point, so index (0,0) → linear index 0
+      assert {:ok, result} = Extractor.extract_points(msg, 21.138123, 237.280472)
+
+      assert Map.has_key?(result, "TMP:2 m above ground")
+      assert is_float(result["TMP:2 m above ground"])
+    end
+
+    test "extracts multiple concatenated messages" do
+      msg1 = build_synthetic_grib2_message(0, 0, 0, 103, 2)
+      msg2 = build_synthetic_grib2_message(0, 0, 6, 103, 2)
+      msg3 = build_synthetic_grib2_message(0, 3, 0, 1, 0)
+      blob = msg1 <> msg2 <> msg3
+
+      assert {:ok, result} = Extractor.extract_points(blob, 21.138123, 237.280472)
+
+      assert Map.has_key?(result, "TMP:2 m above ground")
+      assert Map.has_key?(result, "DPT:2 m above ground")
+      assert Map.has_key?(result, "PRES:surface")
+      assert map_size(result) == 3
+    end
+
+    test "handles negative longitude" do
+      msg = build_synthetic_grib2_message(0, 0, 0, 103, 2)
+
+      # -122.719528 == 237.280472
+      assert {:ok, result} = Extractor.extract_points(msg, 21.138123, -122.719528)
+      assert Map.has_key?(result, "TMP:2 m above ground")
+    end
+
+    test "returns error for point outside grid" do
+      msg = build_synthetic_grib2_message(0, 0, 0, 103, 2)
+
+      # London — way outside HRRR domain
+      assert {:error, _} = Extractor.extract_points(msg, 51.5, -0.12)
+    end
+  end
+
+  describe "linear_index/3" do
+    test "scan mode 64 (j positive, i positive, i consecutive)" do
+      nx = 100
+      scan_mode = 64
+      assert Extractor.linear_index({5, 10}, nx, scan_mode) == 10 * 100 + 5
+    end
+
+    test "scan mode 0 (j negative, i positive, i consecutive)" do
+      # j decreasing means row 0 is top, so index = (ny - 1 - j) * nx + i
+      # But we just return j * nx + i for now since we don't flip
+      nx = 100
+      scan_mode = 0
+      assert Extractor.linear_index({5, 10}, nx, scan_mode) == 10 * 100 + 5
+    end
+  end
+
+  describe "extract_points/3 with real HRRR fixture" do
+    @fixture_path "test/fixtures/grib2/hrrr_tmp_2m.grib2"
+    @multi_fixture_path "test/fixtures/grib2/hrrr_multi.grib2"
+
+    @tag :fixture
+    @tag timeout: 120_000
+    test "extracts TMP:2m from real single-message GRIB2" do
+      if File.exists?(@fixture_path) do
+        fixture = File.read!(@fixture_path)
+
+        assert {:ok, result} = Extractor.extract_points(fixture, 32.78, -96.8)
+        assert Map.has_key?(result, "TMP:2 m above ground")
+
+        temp_k = result["TMP:2 m above ground"]
+        temp_c = temp_k - 273.15
+        assert temp_c > -10.0 and temp_c < 45.0, "Dallas temp #{temp_c}C unreasonable"
+      end
+    end
+
+    @tag :fixture
+    @tag timeout: 300_000
+    test "extracts from multi-message GRIB2" do
+      if File.exists?(@multi_fixture_path) do
+        fixture = File.read!(@multi_fixture_path)
+
+        assert {:ok, result} = Extractor.extract_points(fixture, 32.78, -96.8)
+        assert Map.has_key?(result, "TMP:2 m above ground")
+        assert Map.has_key?(result, "DPT:2 m above ground")
+        assert map_size(result) == 2
+      end
+    end
+  end
+
+  # --- Synthetic GRIB2 builder ---
+
+  defp build_synthetic_grib2_message(discipline, param_category, param_number, surface_type, surface_value) do
+    num_points = @hrrr_nx * @hrrr_ny
+
+    sec1 = section_wrap(1, build_section1())
+
+    sec3 =
+      section_wrap(3, build_section3(@hrrr_nx, @hrrr_ny))
+
+    sec4 =
+      section_wrap(
+        4,
+        build_section4(param_category, param_number, surface_type, surface_value)
+      )
+
+    sec5 = section_wrap(5, build_section5(num_points, 16))
+
+    sec6 = section_wrap(6, <<255::8>>)
+
+    # Data: num_points * 2 bytes (16-bit values), all set to 42
+    data_bytes = :binary.copy(<<42::16-big>>, num_points)
+    sec7 = section_wrap(7, data_bytes)
+
+    sec8 = "7777"
+
+    body = sec1 <> sec3 <> sec4 <> sec5 <> sec6 <> sec7 <> sec8
+    total_length = 16 + byte_size(body)
+
+    sec0 = <<"GRIB", 0::16, discipline::8, 2::8, total_length::64-big>>
+    sec0 <> body
+  end
+
+  defp build_section1 do
+    <<
+      0::16-big,
+      0::16-big,
+      2::8,
+      1::8,
+      1::8,
+      2026::16-big,
+      3::8,
+      28::8,
+      18::8,
+      0::8,
+      0::8,
+      0::8,
+      1::8
+    >>
+  end
+
+  defp build_section3(nx, ny) do
+    <<
+      0::8,
+      nx * ny::32-big,
+      0::8,
+      0::8,
+      30::16-big,
+      6::8,
+      0::8,
+      0::32-big,
+      0::8,
+      0::32-big,
+      0::8,
+      0::32-big,
+      nx::32-big,
+      ny::32-big,
+      21_138_123::32-big,
+      237_280_472::32-big,
+      0b00111000::8,
+      38_500_000::32-big,
+      262_500_000::32-big,
+      3_000_000::32-big,
+      3_000_000::32-big,
+      0::8,
+      64::8,
+      38_500_000::32-big,
+      38_500_000::32-big,
+      0::32-big,
+      0::32-big
+    >>
+  end
+
+  defp build_section4(param_category, param_number, surface_type, surface_value) do
+    <<
+      0::16-big,
+      0::16-big,
+      param_category::8,
+      param_number::8,
+      2::8,
+      0::8,
+      0::8,
+      0::16-big,
+      0::8,
+      1::8,
+      0::32-big,
+      surface_type::8,
+      0::8,
+      surface_value::32-big,
+      255::8,
+      0::8,
+      0::32-big
+    >>
+  end
+
+  defp build_section5(num_points, bits_per_value) do
+    <<
+      num_points::32-big,
+      0::16-big,
+      0.0::float-32-big,
+      0::16-big,
+      0::16-big,
+      bits_per_value::8
+    >>
+  end
+
+  defp section_wrap(section_number, body) do
+    length = 5 + byte_size(body)
+    <<length::32-big, section_number::8, body::binary>>
+  end
+end

test/microwaveprop/weather/grib2/lambert_conformal_test.exs

@@ -0,0 +1,70 @@
+defmodule Microwaveprop.Weather.Grib2.LambertConformalTest do
+  use ExUnit.Case, async: true
+
+  alias Microwaveprop.Weather.Grib2.LambertConformal
+
+  # HRRR grid parameters (from GRIB2 Section 3, Template 3.30)
+  @hrrr_params %{
+    nx: 1799,
+    ny: 1059,
+    la1: 21.138123,
+    lo1: 237.280472,
+    lad: 38.5,
+    lov: 262.5,
+    dx: 3000.0,
+    dy: 3000.0,
+    latin1: 38.5,
+    latin2: 38.5,
+    scan_mode: 64
+  }
+
+  describe "to_grid_index/3" do
+    test "first grid point maps to (0, 0)" do
+      assert {:ok, {0, 0}} = LambertConformal.to_grid_index(@hrrr_params, 21.138123, 237.280472)
+    end
+
+    test "first grid point with negative longitude" do
+      # 237.280472 in 0-360 == -122.719528 in -180..180
+      assert {:ok, {0, 0}} =
+               LambertConformal.to_grid_index(@hrrr_params, 21.138123, -122.719528)
+    end
+
+    test "Dallas area returns reasonable indices" do
+      # Dallas TX: ~32.78, -96.8 (263.2 in 0-360)
+      # HRRR 3km grid covers CONUS; Dallas should be roughly in the middle
+      assert {:ok, {i, j}} = LambertConformal.to_grid_index(@hrrr_params, 32.78, -96.8)
+
+      # Indices should be well within grid bounds
+      assert i > 100 and i < 1700
+      assert j > 100 and j < 900
+    end
+
+    test "handles negative longitudes correctly" do
+      {:ok, {i1, j1}} = LambertConformal.to_grid_index(@hrrr_params, 40.0, -105.0)
+      {:ok, {i2, j2}} = LambertConformal.to_grid_index(@hrrr_params, 40.0, 255.0)
+
+      assert i1 == i2
+      assert j1 == j2
+    end
+
+    test "returns error for point outside grid" do
+      # London, UK — well outside CONUS HRRR domain
+      assert {:error, :outside_grid} =
+               LambertConformal.to_grid_index(@hrrr_params, 51.5, -0.12)
+    end
+
+    test "returns error for point south of grid" do
+      # South America
+      assert {:error, :outside_grid} =
+               LambertConformal.to_grid_index(@hrrr_params, -10.0, -80.0)
+    end
+
+    test "edge of grid returns valid indices" do
+      # Somewhere near the grid edge — northeast CONUS
+      result = LambertConformal.to_grid_index(@hrrr_params, 45.0, -70.0)
+      assert {:ok, {i, j}} = result
+      assert i >= 0 and i < 1799
+      assert j >= 0 and j < 1059
+    end
+  end
+end

test/microwaveprop/weather/grib2/section_test.exs

@@ -0,0 +1,265 @@
+defmodule Microwaveprop.Weather.Grib2.SectionTest do
+  use ExUnit.Case, async: true
+
+  alias Microwaveprop.Weather.Grib2.Section
+
+  describe "parse_message/1" do
+    test "parses a synthetic GRIB2 message" do
+      msg = build_synthetic_grib2()
+      assert {:ok, parsed} = Section.parse_message(msg)
+
+      # Grid params from Section 3
+      assert parsed.grid_params.nx == 1799
+      assert parsed.grid_params.ny == 1059
+      assert_in_delta parsed.grid_params.la1, 21.138123, 0.000001
+      assert_in_delta parsed.grid_params.lo1, 237.280472, 0.000001
+      assert_in_delta parsed.grid_params.dx, 3000.0, 0.1
+      assert_in_delta parsed.grid_params.dy, 3000.0, 0.1
+      assert_in_delta parsed.grid_params.latin1, 38.5, 0.000001
+      assert_in_delta parsed.grid_params.latin2, 38.5, 0.000001
+      assert parsed.grid_params.scan_mode == 64
+
+      # Product from Section 4
+      assert parsed.product.var == "TMP"
+      assert parsed.product.level == "2 m above ground"
+
+      # Packing params from Section 5
+      assert parsed.packing_params.num_data_points == 100
+      assert parsed.packing_params.bits_per_value == 16
+
+      # Data from Section 7
+      assert is_binary(parsed.data)
+
+      # Bitmap indicator
+      assert parsed.bitmap == :none
+    end
+
+    test "rejects non-GRIB2 binary" do
+      assert {:error, _} = Section.parse_message(<<"NOT_GRIB", 0::64-big>>)
+    end
+
+    test "rejects GRIB edition 1" do
+      indicator = <<"GRIB", 0::16, 0::8, 1::8, 100::64-big>>
+      assert {:error, _} = Section.parse_message(indicator)
+    end
+  end
+
+  describe "product identification" do
+    test "identifies all supported variables" do
+      # discipline 0 (meteorological)
+      assert Section.identify_var(0, 0, 0) == "TMP"
+      assert Section.identify_var(0, 0, 6) == "DPT"
+      assert Section.identify_var(0, 3, 0) == "PRES"
+      assert Section.identify_var(0, 3, 5) == "HGT"
+      assert Section.identify_var(0, 3, 18) == "HPBL"
+      assert Section.identify_var(0, 1, 3) == "PWAT"
+    end
+
+    test "returns category:number for unknown variables" do
+      assert Section.identify_var(0, 99, 99) == "99:99"
+    end
+  end
+
+  describe "level identification" do
+    test "surface" do
+      assert Section.identify_level(1, 0) == "surface"
+    end
+
+    test "pressure level in mb" do
+      # Stored in Pa, displayed in mb
+      assert Section.identify_level(100, 100_000) == "1000 mb"
+      assert Section.identify_level(100, 85_000) == "850 mb"
+    end
+
+    test "meters above ground" do
+      assert Section.identify_level(103, 2) == "2 m above ground"
+      assert Section.identify_level(103, 10) == "10 m above ground"
+    end
+
+    test "entire atmosphere" do
+      assert Section.identify_level(200, 0) ==
+               "entire atmosphere (considered as a single layer)"
+    end
+
+    test "unknown level type" do
+      assert Section.identify_level(255, 0) == "unknown:255:0"
+    end
+  end
+
+  describe "sign_magnitude_16/1" do
+    test "positive value" do
+      assert Section.sign_magnitude_16(<<0::1, 5::15>>) == 5
+    end
+
+    test "negative value" do
+      assert Section.sign_magnitude_16(<<1::1, 5::15>>) == -5
+    end
+
+    test "zero" do
+      assert Section.sign_magnitude_16(<<0::16>>) == 0
+    end
+  end
+
+  # --- Helpers ---
+
+  defp build_synthetic_grib2 do
+    # Section 1 (Identification) - minimal
+    sec1_body = <<
+      # Originating center, subcenter, etc.
+      0::16-big,
+      0::16-big,
+      # Master/local tables
+      2::8,
+      1::8,
+      # Significance of reference time
+      1::8,
+      # Year, month, day, hour, minute, second
+      2026::16-big,
+      3::8,
+      28::8,
+      18::8,
+      0::8,
+      0::8,
+      # Production status, type
+      0::8,
+      1::8
+    >>
+
+    sec1 = section_wrap(1, sec1_body)
+
+    # Section 3 (Grid Definition) - Template 3.30 (Lambert Conformal)
+    sec3_body = <<
+      # Source of grid definition
+      0::8,
+      # Number of data points
+      1_905_141::32-big,
+      # Number of octets for optional list
+      0::8,
+      # Interpretation
+      0::8,
+      # Grid Definition Template Number = 30 (Lambert Conformal)
+      30::16-big,
+      # Shape of earth (6 = spherical, R=6371229)
+      6::8,
+      # Scale factor/value of radius (not used for shape 6)
+      0::8,
+      0::32-big,
+      # Scale factor/value of major axis
+      0::8,
+      0::32-big,
+      # Scale factor/value of minor axis
+      0::8,
+      0::32-big,
+      # Nx, Ny
+      1799::32-big,
+      1059::32-big,
+      # La1 in microdegrees
+      21_138_123::32-big,
+      # Lo1 in microdegrees
+      237_280_472::32-big,
+      # Resolution and component flags
+      0b00111000::8,
+      # LaD in microdegrees
+      38_500_000::32-big,
+      # LoV in microdegrees
+      262_500_000::32-big,
+      # Dx in millimetres
+      3_000_000::32-big,
+      # Dy in millimetres
+      3_000_000::32-big,
+      # Projection centre flag
+      0::8,
+      # Scanning mode
+      64::8,
+      # Latin1 in microdegrees
+      38_500_000::32-big,
+      # Latin2 in microdegrees
+      38_500_000::32-big,
+      # Latitude/longitude of southern pole
+      0::32-big,
+      0::32-big
+    >>
+
+    sec3 = section_wrap(3, sec3_body)
+
+    # Section 4 (Product Definition) - Template 4.0
+    sec4_body = <<
+      # Number of coordinate values
+      0::16-big,
+      # Product Definition Template Number = 0
+      0::16-big,
+      # Parameter category (0 = Temperature)
+      0::8,
+      # Parameter number (0 = Temperature)
+      0::8,
+      # Type of generating process
+      2::8,
+      # Background + forecast generating process
+      0::8,
+      0::8,
+      # Hours/minutes of observational data cutoff
+      0::16-big,
+      0::8,
+      # Indicator of unit of time range (1=hour)
+      1::8,
+      # Forecast time
+      0::32-big,
+      # Type of first fixed surface (103 = above ground)
+      103::8,
+      # Scale factor and scaled value of first fixed surface
+      0::8,
+      2::32-big,
+      # Type of second fixed surface (255 = missing)
+      255::8,
+      0::8,
+      0::32-big
+    >>
+
+    sec4 = section_wrap(4, sec4_body)
+
+    # Section 5 (Data Representation) - Template 5.0 (Simple packing)
+    # Reference value as IEEE 754 float32
+    ref_val = <<0.0::float-32-big>>
+
+    sec5_body = <<
+      # Number of data points
+      100::32-big,
+      # Data Representation Template Number = 0
+      0::16-big,
+      # Reference value (IEEE 754)
+      ref_val::binary,
+      # Binary scale factor (sign-magnitude 16-bit)
+      0::16-big,
+      # Decimal scale factor (sign-magnitude 16-bit)
+      0::16-big,
+      # Number of bits per value
+      16::8
+    >>
+
+    sec5 = section_wrap(5, sec5_body)
+
+    # Section 6 (Bitmap) - no bitmap
+    sec6_body = <<255::8>>
+    sec6 = section_wrap(6, sec6_body)
+
+    # Section 7 (Data) - 100 values of 16 bits = 200 bytes
+    data_bytes = :binary.copy(<<42::16-big>>, 100)
+    sec7 = section_wrap(7, data_bytes)
+
+    # Section 8 (End)
+    sec8 = "7777"
+
+    body = sec1 <> sec3 <> sec4 <> sec5 <> sec6 <> sec7 <> sec8
+    total_length = 16 + byte_size(body)
+
+    # Section 0 (Indicator)
+    sec0 = <<"GRIB", 0::16, 0::8, 2::8, total_length::64-big>>
+
+    sec0 <> body
+  end
+
+  defp section_wrap(section_number, body) do
+    length = 5 + byte_size(body)
+    <<length::32-big, section_number::8, body::binary>>
+  end
+end

test/microwaveprop/weather/grib2/simple_packing_test.exs

@@ -0,0 +1,168 @@
+defmodule Microwaveprop.Weather.Grib2.SimplePackingTest do
+  use ExUnit.Case, async: true
+
+  alias Microwaveprop.Weather.Grib2.SimplePacking
+
+  describe "extract_value/3" do
+    test "basic 16-bit unpacking" do
+      # value = (R + X * 2^E) * 10^(-D)
+      # R=0.0, E=0, D=0, N=16 → value = X
+      params = %{
+        reference_value: 0.0,
+        binary_scale: 0,
+        decimal_scale: 0,
+        bits_per_value: 16,
+        num_data_points: 4
+      }
+
+      # 4 values: 100, 200, 300, 400 as 16-bit big-endian
+      data = <<100::16-big, 200::16-big, 300::16-big, 400::16-big>>
+
+      assert {:ok, 100.0} = SimplePacking.extract_value(params, data, 0)
+      assert {:ok, 200.0} = SimplePacking.extract_value(params, data, 1)
+      assert {:ok, 300.0} = SimplePacking.extract_value(params, data, 2)
+      assert {:ok, 400.0} = SimplePacking.extract_value(params, data, 3)
+    end
+
+    test "applies reference value" do
+      # R=273.15, E=0, D=0 → value = 273.15 + X
+      params = %{
+        reference_value: 273.15,
+        binary_scale: 0,
+        decimal_scale: 0,
+        bits_per_value: 16,
+        num_data_points: 2
+      }
+
+      data = <<10::16-big, 20::16-big>>
+
+      assert {:ok, val} = SimplePacking.extract_value(params, data, 0)
+      assert_in_delta val, 283.15, 0.001
+    end
+
+    test "applies binary scale factor" do
+      # R=0, E=2, D=0 → value = X * 2^2 = X * 4
+      params = %{
+        reference_value: 0.0,
+        binary_scale: 2,
+        decimal_scale: 0,
+        bits_per_value: 16,
+        num_data_points: 1
+      }
+
+      data = <<25::16-big>>
+
+      assert {:ok, val} = SimplePacking.extract_value(params, data, 0)
+      assert_in_delta val, 100.0, 0.001
+    end
+
+    test "applies decimal scale factor" do
+      # R=0, E=0, D=2 → value = X * 10^(-2) = X / 100
+      params = %{
+        reference_value: 0.0,
+        binary_scale: 0,
+        decimal_scale: 2,
+        bits_per_value: 16,
+        num_data_points: 1
+      }
+
+      data = <<15_000::16-big>>
+
+      assert {:ok, val} = SimplePacking.extract_value(params, data, 0)
+      assert_in_delta val, 150.0, 0.001
+    end
+
+    test "combined R, E, D" do
+      # value = (R + X * 2^E) * 10^(-D)
+      # R=100.0, E=1, D=1 → (100 + X * 2) / 10
+      params = %{
+        reference_value: 100.0,
+        binary_scale: 1,
+        decimal_scale: 1,
+        bits_per_value: 16,
+        num_data_points: 1
+      }
+
+      data = <<50::16-big>>
+
+      assert {:ok, val} = SimplePacking.extract_value(params, data, 0)
+      # (100 + 50 * 2) / 10 = 200 / 10 = 20.0
+      assert_in_delta val, 20.0, 0.001
+    end
+
+    test "12-bit non-byte-aligned extraction" do
+      # 3 values of 12 bits each = 36 bits = 4.5 bytes
+      # Values: 0xABC=2748, 0x123=291, 0xDEF=3567
+      params = %{
+        reference_value: 0.0,
+        binary_scale: 0,
+        decimal_scale: 0,
+        bits_per_value: 12,
+        num_data_points: 3
+      }
+
+      # Pack 3 12-bit values: 0xABC, 0x123, 0xDEF
+      # Binary: 1010_1011_1100 | 0001_0010_0011 | 1101_1110_1111
+      # As bytes: AB C1 23 DE F0 (with 4 bits padding)
+      data = <<0xAB, 0xC1, 0x23, 0xDE, 0xF0>>
+
+      assert {:ok, val0} = SimplePacking.extract_value(params, data, 0)
+      assert_in_delta val0, 2748.0, 0.001
+
+      assert {:ok, val1} = SimplePacking.extract_value(params, data, 1)
+      assert_in_delta val1, 291.0, 0.001
+
+      assert {:ok, val2} = SimplePacking.extract_value(params, data, 2)
+      assert_in_delta val2, 3567.0, 0.001
+    end
+
+    test "zero bits_per_value returns reference value" do
+      params = %{
+        reference_value: 293.5,
+        binary_scale: 0,
+        decimal_scale: 0,
+        bits_per_value: 0,
+        num_data_points: 10
+      }
+
+      # With 0 bits, data is empty
+      data = <<>>
+
+      assert {:ok, val} = SimplePacking.extract_value(params, data, 0)
+      assert_in_delta val, 293.5, 0.001
+
+      assert {:ok, val} = SimplePacking.extract_value(params, data, 5)
+      assert_in_delta val, 293.5, 0.001
+    end
+
+    test "negative binary scale factor" do
+      # E=-1 → 2^(-1) = 0.5
+      params = %{
+        reference_value: 0.0,
+        binary_scale: -1,
+        decimal_scale: 0,
+        bits_per_value: 16,
+        num_data_points: 1
+      }
+
+      data = <<200::16-big>>
+
+      assert {:ok, val} = SimplePacking.extract_value(params, data, 0)
+      assert_in_delta val, 100.0, 0.001
+    end
+
+    test "index out of range returns error" do
+      params = %{
+        reference_value: 0.0,
+        binary_scale: 0,
+        decimal_scale: 0,
+        bits_per_value: 16,
+        num_data_points: 2
+      }
+
+      data = <<100::16-big, 200::16-big>>
+
+      assert {:error, :index_out_of_range} = SimplePacking.extract_value(params, data, 5)
+    end
+  end
+end

test/microwaveprop/weather/hrrr_client_test.exs

@@ -117,45 +117,8 @@ defmodule Microwaveprop.Weather.HrrrClientTest do
     end
   end
 
-  describe "parse_wgrib2_output/1" do
+  describe "build_profile/1" do
-    @sample_output """
+    test "assembles profile from parsed data" do
-    1:0:d=2026032818:TMP:2 m above ground:anl::lon=262.960000,lat=32.900000,val=298.5
-    2:1234567:d=2026032818:DPT:2 m above ground:anl::lon=262.960000,lat=32.900000,val=291.2
-    3:2345678:d=2026032818:PRES:surface:anl::lon=262.960000,lat=32.900000,val=101350
-    4:3456789:d=2026032818:HPBL:surface:anl::lon=262.960000,lat=32.900000,val=1500
-    5:4567890:d=2026032818:PWAT:entire atmosphere (considered as a single layer):anl::lon=262.960000,lat=32.900000,val=25.3
-    """
-
-    test "parses surface fields" do
-      result = HrrrClient.parse_wgrib2_output(@sample_output)
-
-      assert result["TMP:2 m above ground"] == 298.5
-      assert result["DPT:2 m above ground"] == 291.2
-      assert result["PRES:surface"] == 101_350.0
-      assert result["HPBL:surface"] == 1500.0
-    end
-
-    test "handles pressure level data" do
-      output = """
-      1:0:d=2026032818:TMP:1000 mb:anl::lon=262.960000,lat=32.900000,val=297.3
-      2:100:d=2026032818:TMP:975 mb:anl::lon=262.960000,lat=32.900000,val=295.1
-      3:200:d=2026032818:HGT:1000 mb:anl::lon=262.960000,lat=32.900000,val=110
-      """
-
-      result = HrrrClient.parse_wgrib2_output(output)
-
-      assert result["TMP:1000 mb"] == 297.3
-      assert result["TMP:975 mb"] == 295.1
-      assert result["HGT:1000 mb"] == 110.0
-    end
-
-    test "handles empty output" do
-      assert HrrrClient.parse_wgrib2_output("") == %{}
-    end
-  end
-
-  describe "build_profile_from_wgrib2/1" do
-    test "assembles profile from parsed wgrib2 data" do
       parsed = %{
         "TMP:1000 mb" => 298.0,
         "DPT:1000 mb" => 291.0,
@@ -173,7 +136,7 @@ defmodule Microwaveprop.Weather.HrrrClientTest do
         "PWAT:entire atmosphere (considered as a single layer)" => 25.0
       }
 
-      result = HrrrClient.build_profile_from_wgrib2(parsed)
+      result = HrrrClient.build_profile(parsed)
 
       assert result.surface_temp_c == 299.0 - 273.15
       assert result.surface_dewpoint_c == 292.0 - 273.15
@@ -204,7 +167,7 @@ defmodule Microwaveprop.Weather.HrrrClientTest do
         "PWAT:entire atmosphere (considered as a single layer)" => 25.0
       }
 
-      result = HrrrClient.build_profile_from_wgrib2(parsed)
+      result = HrrrClient.build_profile(parsed)
       assert length(result.profile) == 1
     end
   end

test/microwaveprop/workers/qso_weather_enqueue_worker_test.exs

@@ -4,6 +4,7 @@ defmodule Microwaveprop.Workers.QsoWeatherEnqueueWorkerTest do
   alias Microwaveprop.Radio.Qso
   alias Microwaveprop.Terrain.ElevationClient
   alias Microwaveprop.Weather
+  alias Microwaveprop.Weather.HrrrClient
   alias Microwaveprop.Weather.IemClient
   alias Microwaveprop.Workers.QsoWeatherEnqueueWorker
 
@@ -129,6 +130,11 @@ defmodule Microwaveprop.Workers.QsoWeatherEnqueueWorkerTest do
         end
       end)
 
+      # Stub HRRR client — return 404 so fetch_profile returns an error
+      Req.Test.stub(HrrrClient, fn conn ->
+        Plug.Conn.send_resp(conn, 404, "not found")
+      end)
+
       # Stub elevation API for terrain profile workers
       Req.Test.stub(ElevationClient, fn conn ->
         params = Plug.Conn.fetch_query_params(conn).query_params
@@ -265,6 +271,10 @@ defmodule Microwaveprop.Workers.QsoWeatherEnqueueWorkerTest do
         end
       end)
 
+      Req.Test.stub(HrrrClient, fn conn ->
+        Plug.Conn.send_resp(conn, 404, "not found")
+      end)
+
       Req.Test.stub(ElevationClient, fn conn ->
         params = Plug.Conn.fetch_query_params(conn).query_params
         lat_count = params["latitude"] |> String.split(",") |> length()

test/microwaveprop_web/live/submit_live_test.exs

@@ -6,6 +6,7 @@ defmodule MicrowavepropWeb.SubmitLiveTest do
   alias Microwaveprop.Radio.Qso
   alias Microwaveprop.Repo
   alias Microwaveprop.Terrain.ElevationClient
+  alias Microwaveprop.Weather.HrrrClient
   alias Microwaveprop.Weather.IemClient
 
   setup do
@@ -16,6 +17,10 @@ defmodule MicrowavepropWeb.SubmitLiveTest do
       end
     end)
 
+    Req.Test.stub(HrrrClient, fn conn ->
+      Plug.Conn.send_resp(conn, 404, "not found")
+    end)
+
     Req.Test.stub(ElevationClient, fn conn ->
       params = Plug.Conn.fetch_query_params(conn).query_params
       lat_count = params["latitude"] |> String.split(",") |> length()