diff --git a/config/runtime.exs b/config/runtime.exs
index 5da21990..da5d8e78 100644
--- a/config/runtime.exs
+++ b/config/runtime.exs
@@ -247,6 +247,13 @@ if config_env() == :prod do
        # so duplicate fires for an already-seeded run_time are a single
        # idempotent INSERT batch.
        {"5,20,35,50 * * * *", Microwaveprop.Workers.PropagationGridWorker},
+       # HRDPS Canadian propagation chain. ECCC publishes HRDPS f000
+       # ~3-4h after each 00/06/12/18Z cycle, so seeding at HH:35 of
+       # each cycle hour (5h after cycle, 30 min offset from HRRR's
+       # */15 schedule) gives a comfortable margin before the f000
+       # files land. The Rust prop-grid-rs HRDPS branch (run_chain_step_hrdps)
+       # drains the resulting source='hrdps' rows.
+       {"35 5,11,17,23 * * *", Microwaveprop.Workers.HrdpsGridWorker},
        # GEFS runs publish ~3-4h after the 00/06/12/18Z cycle; seed the
        # Day 2-7 outlook 5h after each run to stay safely past NOMADS'
        # publication lag. Seeder enqueues f024-f168 at 6-hour cadence
diff --git a/lib/microwaveprop/propagation.ex b/lib/microwaveprop/propagation.ex
index c1e2db29..2bf742d0 100644
--- a/lib/microwaveprop/propagation.ex
+++ b/lib/microwaveprop/propagation.ex
@@ -345,27 +345,38 @@ defmodule Microwaveprop.Propagation do
   end
 
   @doc """
-  Load the full CONUS score set for `{band_mhz, valid_time}` from the
-  on-disk binary file and broadcast it to every `ScoreCache` in the
-  cluster. Called from `PropagationGridWorker` after each forecast
-  hour so all pods have a warm cache by the time clients begin
-  requesting the new hour.
+  Load the full North America score set for `{band_mhz, valid_time}` from
+  the on-disk binary files (HRRR `.prop` + HRDPS `.hrdps.prop`, merged) and
+  broadcast it to every `ScoreCache` in the cluster. Called from
+  `PropagationGridWorker` after each forecast hour so all pods have a warm
+  cache by the time clients begin requesting the new hour.
 
-  Returns `{:error, reason}` when the score file is missing or
-  corrupt — callers distinguish those from the successful empty-grid
-  case to avoid poisoning the cache with `[]` on a bad read.
+  Returns `{:error, :enoent}` only when neither file exists. A single
+  missing file (HRDPS pre-cycle, HRRR briefly absent) is OK — the cache
+  warms with whichever side is available.
   """
   @spec warm_cache_and_broadcast(non_neg_integer(), DateTime.t()) ::
           :ok | {:error, :enoent | :invalid_format}
   def warm_cache_and_broadcast(band_mhz, valid_time) do
-    case ScoresFile.read(band_mhz, valid_time) do
-      {:ok, payload} ->
-        scores = ScoresFile.extract_points(payload, nil)
-        ScoreCache.broadcast_put(band_mhz, valid_time, scores)
-        :ok
+    hrrr =
+      case ScoresFile.read(band_mhz, valid_time) do
+        {:ok, payload} -> ScoresFile.extract_points(payload, nil)
+        _ -> nil
+      end
 
-      {:error, reason} ->
-        {:error, reason}
+    hrdps =
+      case ScoresFile.read_hrdps(band_mhz, valid_time) do
+        {:ok, payload} -> ScoresFile.extract_points(payload, nil)
+        _ -> nil
+      end
+
+    case {hrrr, hrdps} do
+      {nil, nil} ->
+        {:error, :enoent}
+
+      {h, c} ->
+        ScoreCache.broadcast_put(band_mhz, valid_time, (h || []) ++ (c || []))
+        :ok
     end
   end
 
diff --git a/lib/microwaveprop/propagation/scores_file.ex b/lib/microwaveprop/propagation/scores_file.ex
index 9d5062df..e511a3eb 100644
--- a/lib/microwaveprop/propagation/scores_file.ex
+++ b/lib/microwaveprop/propagation/scores_file.ex
@@ -50,13 +50,25 @@ defmodule Microwaveprop.Propagation.ScoresFile do
     Application.get_env(:microwaveprop, :propagation_scores_dir, "/data/scores")
   end
 
-  @doc "Returns the full path for a `(band_mhz, valid_time)` score file."
+  @doc "Returns the full path for a `(band_mhz, valid_time)` HRRR score file."
   @spec path_for(non_neg_integer(), DateTime.t()) :: String.t()
   def path_for(band_mhz, %DateTime{} = valid_time) when is_integer(band_mhz) do
     iso = valid_time |> DateTime.truncate(:second) |> DateTime.to_iso8601()
     Path.join([base_dir(), Integer.to_string(band_mhz), "#{iso}.prop"])
   end
 
+  @doc """
+  Returns the path for the HRDPS-source companion score file. Sibling of
+  `path_for/2` — both files coexist for the same `(band, valid_time)`.
+  Read-side functions transparently merge the two so callers get the
+  union of CONUS + Canadian scored cells.
+  """
+  @spec path_for_hrdps(non_neg_integer(), DateTime.t()) :: String.t()
+  def path_for_hrdps(band_mhz, %DateTime{} = valid_time) when is_integer(band_mhz) do
+    iso = valid_time |> DateTime.truncate(:second) |> DateTime.to_iso8601()
+    Path.join([base_dir(), Integer.to_string(band_mhz), "#{iso}.hrdps.prop"])
+  end
+
   @doc false
   @spec legacy_path_for(non_neg_integer(), DateTime.t()) :: String.t()
   def legacy_path_for(band_mhz, %DateTime{} = valid_time) when is_integer(band_mhz) do
@@ -241,9 +253,35 @@ defmodule Microwaveprop.Propagation.ScoresFile do
   @spec read_bounds(non_neg_integer(), DateTime.t(), map() | nil) ::
           [%{lat: float(), lon: float(), score: non_neg_integer()}]
   def read_bounds(band_mhz, %DateTime{} = valid_time, bounds \\ nil) do
-    case read(band_mhz, valid_time) do
-      {:ok, payload} -> extract_points(payload, bounds)
-      _ -> []
+    hrrr_cells =
+      case read(band_mhz, valid_time) do
+        {:ok, payload} -> extract_points(payload, bounds)
+        _ -> []
+      end
+
+    hrdps_cells =
+      case read_hrdps(band_mhz, valid_time) do
+        {:ok, payload} -> extract_points(payload, bounds)
+        _ -> []
+      end
+
+    # The two grids are disjoint by construction (Grid.hrdps_only_points
+    # excludes CONUS), so concat without dedup. Order isn't load-bearing —
+    # callers downstream of /scores/cells don't depend on ordering.
+    hrrr_cells ++ hrdps_cells
+  end
+
+  @doc """
+  Read the HRDPS-source score file for `(band_mhz, valid_time)`. Same
+  decode shape as `read/2`; uses the `.hrdps.prop` extension.
+  """
+  @spec read_hrdps(non_neg_integer(), DateTime.t()) ::
+          {:ok, map()} | {:error, :enoent | :invalid_format}
+  def read_hrdps(band_mhz, %DateTime{} = valid_time) when is_integer(band_mhz) do
+    case File.read(path_for_hrdps(band_mhz, valid_time)) do
+      {:ok, binary} -> decode(binary)
+      {:error, :enoent} -> {:error, :enoent}
+      {:error, other} -> {:error, other}
     end
   end
 
@@ -261,7 +299,11 @@ defmodule Microwaveprop.Propagation.ScoresFile do
   @spec read_point(non_neg_integer(), DateTime.t(), float(), float()) :: non_neg_integer() | nil
   def read_point(band_mhz, %DateTime{} = valid_time, lat, lon) do
     Enum.find_value(
-      [path_for(band_mhz, valid_time), legacy_path_for(band_mhz, valid_time)],
+      [
+        path_for(band_mhz, valid_time),
+        path_for_hrdps(band_mhz, valid_time),
+        legacy_path_for(band_mhz, valid_time)
+      ],
       &open_and_read_point(&1, lat, lon)
     )
   end
diff --git a/lib/microwaveprop/workers/hrdps_grid_worker.ex b/lib/microwaveprop/workers/hrdps_grid_worker.ex
index 9723fb29..65aeee02 100644
--- a/lib/microwaveprop/workers/hrdps_grid_worker.ex
+++ b/lib/microwaveprop/workers/hrdps_grid_worker.ex
@@ -13,11 +13,10 @@ defmodule Microwaveprop.Workers.HrdpsGridWorker do
 
   ## Activation
 
-  **Not yet wired into runtime.exs cron.** The Rust prop-grid-rs worker
-  needs an HRDPS branch (plan stage 4) before it can drain
-  `source: "hrdps"` rows correctly. Until then this module is dormant
-  scaffolding — building blocks ready for the cron wire-up once the
-  Rust counterpart ships.
+  Active. The Rust prop-grid-rs worker handles `source: "hrdps"` rows via
+  `pipeline::run_chain_step_hrdps`, fetching from MSC Datamart, writing
+  scores to `<base>/<band>/<iso>.hrdps.prop` (sibling of the HRRR
+  `.prop` file for the same cycle). Cron entry lives in `runtime.exs`.
   """
 
   use Oban.Worker,
diff --git a/lib/microwaveprop_web/live/map_live.ex b/lib/microwaveprop_web/live/map_live.ex
index 16cae79e..8700c0ad 100644
--- a/lib/microwaveprop_web/live/map_live.ex
+++ b/lib/microwaveprop_web/live/map_live.ex
@@ -15,13 +15,14 @@ defmodule MicrowavepropWeb.MapLive do
   @default_center %{lat: 32.897, lon: -97.038}
   @default_zoom 7
 
-  # Continental-US bounding box (excludes Alaska, Hawaii, Puerto Rico). Used
-  # to decide whether to show the "data is CONUS-only" banner to visitors
-  # whose Cloudflare geolocation falls outside the dataset's coverage.
+  # North American coverage bbox: HRRR (CONUS) + HRDPS (Canadian extent
+  # to 60°N — Arctic deferred until SRTM gap is filled). Used to decide
+  # whether to show the "out of coverage area" banner to visitors whose
+  # Cloudflare geolocation falls outside the combined dataset.
   @conus_lat_min 24.5
-  @conus_lat_max 49.5
-  @conus_lon_min -125.0
-  @conus_lon_max -66.5
+  @conus_lat_max 60.0
+  @conus_lon_min -141.0
+  @conus_lon_max -52.0
 
   # How often to re-query oban_jobs for pipeline state while the map is
   # open. Short enough that the "Updating…" chip shows up within a page-
diff --git a/lib/mix/tasks/hrdps_probe.ex b/lib/mix/tasks/hrdps_probe.ex
deleted file mode 100644
index 4bf96ac1..00000000
--- a/lib/mix/tasks/hrdps_probe.ex
+++ /dev/null
@@ -1,165 +0,0 @@
-defmodule Mix.Tasks.Hrdps.Probe do
-  @shortdoc "Throwaway probe: fetch one HRDPS variable, decode at sample cities"
-  @moduledoc """
-  One-shot probe for the Canadian HRDPS model. Fetches a single GRIB2
-  file from MSC Datamart and uses `wgrib2 -lon` to extract values at
-  five Canadian cities, printing a table for visual sanity-checking.
-
-  Designed as a wedge to retire risk before committing to a real
-  HrdpsClient: validates URL pattern, GRIB2 decode, and rotated-lat/lon
-  reprojection without touching the DB, Oban, or the score grid.
-
-  Throwaway code. Will be deleted once the real client lands.
-
-      mix hrdps.probe                 # latest cycle, f000, 2m temperature
-      mix hrdps.probe --hour 6        # f006 from latest cycle
-      mix hrdps.probe --cycle 20260429T12Z
-
-  URL pattern (current as of 2026-04-29; ECCC reorganized away from the
-  old `/model_hrdps/` root to a date-prefixed structure):
-
-      https://dd.weather.gc.ca/{YYYYMMDD}/WXO-DD/model_hrdps/continental/2.5km/
-        {HH}/{FFF}/{YYYYMMDD}T{HH}Z_MSC_HRDPS_{VAR}_{LEVEL}_RLatLon0.0225_PT{FFF}H.grib2
-  """
-  use Mix.Task
-
-  @datamart_base "https://dd.weather.gc.ca"
-
-  # Variable / level slug for 2m temperature in HRDPS naming. HRRR-side
-  # equivalent is `TMP:2 m above ground`; HRDPS uses MSC's per-variable
-  # filename convention.
-  @variable_slug "TMP_AGL-2m"
-
-  @cities [
-    {"YYZ Toronto", 43.68, -79.63},
-    {"YVR Vancouver", 49.19, -123.18},
-    {"YYC Calgary", 51.11, -114.02},
-    {"YOW Ottawa", 45.32, -75.67},
-    {"YHZ Halifax", 44.88, -63.51}
-  ]
-
-  @impl Mix.Task
-  def run(argv) do
-    {opts, _, _} =
-      OptionParser.parse(argv,
-        strict: [cycle: :string, hour: :integer],
-        aliases: [c: :cycle, h: :hour]
-      )
-
-    {:ok, _} = Application.ensure_all_started(:req)
-
-    forecast_hour = Keyword.get(opts, :hour, 0)
-    cycle = opts |> Keyword.get(:cycle) |> resolve_cycle()
-
-    url = build_url(cycle, forecast_hour)
-    IO.puts("Fetching: #{url}")
-
-    {:ok, path} = download(url)
-    IO.puts("Downloaded #{path} (#{File.stat!(path).size} bytes)")
-
-    print_inventory(path)
-    print_city_table(path, cycle, forecast_hour)
-  end
-
-  # ----- Cycle resolution -----
-
-  defp resolve_cycle(nil), do: latest_published_cycle()
-
-  defp resolve_cycle(<<year::binary-size(4), month::binary-size(2), day::binary-size(2), "T", hour::binary-size(2), "Z">>) do
-    %{date: "#{year}#{month}#{day}", hour: hour}
-  end
-
-  defp resolve_cycle(other) do
-    Mix.raise("Invalid --cycle #{inspect(other)} — expected YYYYMMDDTHHZ")
-  end
-
-  # HRDPS publishes ~3-4h after each cycle (00/06/12/18Z). Walk back from
-  # "now - 4h" until we find a cycle whose root directory exists.
-  defp latest_published_cycle do
-    now = DateTime.utc_now()
-    candidates = for h <- 0..3, do: DateTime.add(now, -((4 + h * 6) * 3600), :second)
-
-    candidates
-    |> Enum.map(&snap_to_cycle/1)
-    |> Enum.uniq()
-    |> Enum.find(&cycle_published?/1)
-    |> case do
-      nil -> Mix.raise("No published HRDPS cycle found in the last 24h")
-      cycle -> cycle
-    end
-  end
-
-  defp snap_to_cycle(%DateTime{} = dt) do
-    cycle_hour = div(dt.hour, 6) * 6
-    %{date: Calendar.strftime(dt, "%Y%m%d"), hour: String.pad_leading("#{cycle_hour}", 2, "0")}
-  end
-
-  defp cycle_published?(%{date: date, hour: hour}) do
-    url = "#{@datamart_base}/#{date}/WXO-DD/model_hrdps/continental/2.5km/#{hour}/000/"
-
-    case Req.head(url, retry: false, connect_options: [timeout: 5_000]) do
-      {:ok, %{status: 200}} -> true
-      _ -> false
-    end
-  end
-
-  # ----- URL + download -----
-
-  defp build_url(%{date: date, hour: hour}, forecast_hour) do
-    fff = String.pad_leading("#{forecast_hour}", 3, "0")
-    filename = "#{date}T#{hour}Z_MSC_HRDPS_#{@variable_slug}_RLatLon0.0225_PT#{fff}H.grib2"
-
-    "#{@datamart_base}/#{date}/WXO-DD/model_hrdps/continental/2.5km/#{hour}/#{fff}/#{filename}"
-  end
-
-  defp download(url) do
-    path = Path.join(System.tmp_dir!(), "hrdps_probe_#{System.unique_integer([:positive])}.grib2")
-
-    case Req.get(url, into: File.stream!(path), retry: false) do
-      {:ok, %{status: 200}} -> {:ok, path}
-      {:ok, %{status: status}} -> Mix.raise("Datamart returned HTTP #{status} for #{url}")
-      {:error, reason} -> Mix.raise("Fetch failed: #{inspect(reason)}")
-    end
-  end
-
-  # ----- wgrib2 inspection -----
-
-  defp print_inventory(path) do
-    {output, 0} = System.cmd("wgrib2", [path], stderr_to_stdout: true)
-    IO.puts("\nGRIB2 inventory:")
-    IO.puts("  " <> String.trim(output))
-  end
-
-  defp print_city_table(path, cycle, forecast_hour) do
-    args = [path] ++ Enum.flat_map(@cities, fn {_, lat, lon} -> ["-lon", "#{lon}", "#{lat}"] end)
-    {output, 0} = System.cmd("wgrib2", args, stderr_to_stdout: true)
-
-    values = parse_wgrib2_lon_output(output)
-
-    IO.puts("\nHRDPS 2m temperature, cycle #{cycle.date}T#{cycle.hour}Z f#{pad3(forecast_hour)}:\n")
-    IO.puts("  city                  lat,lon               temp")
-    IO.puts("  --------------------  --------------------  ------")
-
-    @cities
-    |> Enum.zip(values)
-    |> Enum.each(fn {{name, lat, lon}, kelvin} ->
-      celsius = kelvin - 273.15
-
-      IO.puts("  #{String.pad_trailing(name, 20)}  #{format_coord(lat, lon)}  #{format_temp(celsius)}")
-    end)
-  end
-
-  # wgrib2 -lon emits one line for the GRIB record with all `lon=…lat=…val=…`
-  # entries colon-separated. For five cities we get five `val=` matches.
-  defp parse_wgrib2_lon_output(output) do
-    ~r/val=([\-\d\.]+)/
-    |> Regex.scan(output)
-    |> Enum.map(fn [_, v] -> String.to_float(v) end)
-  end
-
-  defp format_coord(lat, lon), do: "#{:io_lib.format("~6.2f", [lat])},#{:io_lib.format("~7.2f", [lon])}"
-
-  defp format_temp(celsius), do: "#{:io_lib.format("~5.1f", [celsius])}°C"
-
-  defp pad3(n), do: String.pad_leading("#{n}", 3, "0")
-end
diff --git a/test/microwaveprop/propagation/scores_file_test.exs b/test/microwaveprop/propagation/scores_file_test.exs
index 309b4552..3b294e66 100644
--- a/test/microwaveprop/propagation/scores_file_test.exs
+++ b/test/microwaveprop/propagation/scores_file_test.exs
@@ -253,6 +253,38 @@ defmodule Microwaveprop.Propagation.ScoresFileTest do
     test "returns an empty list when the file doesn't exist" do
       assert ScoresFile.read_bounds(10_000, ~U[2026-04-14 18:00:00Z]) == []
     end
+
+    test "merges cells from a sibling .hrdps.prop file when present" do
+      # Write the HRRR file via the public writer, then copy it to the
+      # HRDPS path. Both files now contain the same cell — read_bounds
+      # should return both, doubling the cell list. (Production HRDPS
+      # files have a different bbox; this test only verifies the merge
+      # plumbing routes through both paths.)
+      vt = ~U[2026-04-14 18:00:00Z]
+      ScoresFile.write!(10_000, vt, [%{lat: 25.0, lon: -125.0, score: 42}])
+
+      hrrr_path = ScoresFile.path_for(10_000, vt)
+      hrdps_path = ScoresFile.path_for_hrdps(10_000, vt)
+      File.cp!(hrrr_path, hrdps_path)
+
+      scores = ScoresFile.read_bounds(10_000, vt)
+      assert length(scores) == 2
+      assert Enum.all?(scores, &(&1.score == 42))
+    end
+
+    test "returns HRDPS cells alone when only the .hrdps.prop file exists" do
+      vt = ~U[2026-04-14 18:00:00Z]
+      # Use write! to land a file at the HRRR path, then move it to
+      # the HRDPS path so only the HRDPS file remains.
+      ScoresFile.write!(10_000, vt, [%{lat: 25.0, lon: -125.0, score: 80}])
+
+      File.rename!(
+        ScoresFile.path_for(10_000, vt),
+        ScoresFile.path_for_hrdps(10_000, vt)
+      )
+
+      assert [%{score: 80}] = ScoresFile.read_bounds(10_000, vt)
+    end
   end
 
   describe "read_point/4" do