Phase 7: Regionalized seasonal scoring

Add Propagation.Region module with 8 CONUS climate zones (gulf_coast,
southeast, southern_plains, corn_belt, northeast, desert_southwest,
pacific_northwest, mountain_west) and per-region monthly seasonal
adjustment multipliers.

The scorer's score_season now takes lat/lon and applies a regional
multiplier from Region.seasonal_adjustment on top of the band's
seasonal_base + seasonal_adj. Gulf coast August gets a 1.15x boost
(drier, better for ducting) while Corn Belt August gets a 0.80x
penalty (corn evapotranspiration = miserable dewpoints).

Adjustments are hand-tuned starting points from the meteorologist's
qualitative guidance. Phase 9 recalibration will refine them from
backtest data.

lib/microwaveprop/propagation/region.ex

@@ -0,0 +1,111 @@
+defmodule Microwaveprop.Propagation.Region do
+  @moduledoc """
+  Classifies CONUS grid points into climatological regions for
+  band-specific seasonal scoring adjustments.
+
+  The meteorologist's April 2026 review noted that the existing
+  uniform seasonal scoring is wrong: Gulf coast August is drier and
+  better for propagation than June/July, while Iowa August is peak
+  corn evapotranspiration with brutally high dewpoints. Same month,
+  opposite effect.
+
+  This module provides:
+  - `for_point/2` — classifies a lat/lon into one of ~8 regions
+  - `seasonal_adjustment/2` — returns a multiplier (0.7-1.3) that
+    the scorer applies on top of the band's `seasonal_base` score.
+
+  Region boundaries are deliberately simple bounding boxes rather
+  than precise climate zone shapefiles. The scoring impact of getting
+  a boundary pixel wrong is a few points out of 100; the impact of
+  having no regional adjustment at all is the entire seasonal factor
+  being wrong for half the country.
+  """
+
+  @regions [
+    {:gulf_coast, {25.0, 32.0}, {-100.0, -80.0}},
+    {:southeast, {30.0, 37.0}, {-90.0, -75.0}},
+    {:southern_plains, {32.0, 38.0}, {-105.0, -93.0}},
+    {:corn_belt, {38.0, 48.0}, {-100.0, -82.0}},
+    {:northeast, {38.0, 48.0}, {-82.0, -67.0}},
+    {:desert_southwest, {30.0, 38.0}, {-120.0, -105.0}},
+    {:pacific_northwest, {42.0, 50.0}, {-125.0, -115.0}},
+    {:mountain_west, {38.0, 48.0}, {-115.0, -100.0}}
+  ]
+
+  @doc """
+  Classify a lat/lon point into a climatological region.
+
+  Returns an atom like `:gulf_coast`, `:corn_belt`, etc.
+  Returns `:other` for points outside any defined region (including
+  non-CONUS).
+  """
+  @spec for_point(float, float) :: atom
+  def for_point(lat, lon) do
+    Enum.find_value(@regions, :other, fn {name, {lat_min, lat_max}, {lon_min, lon_max}} ->
+      if lat >= lat_min and lat < lat_max and lon >= lon_min and lon < lon_max do
+        name
+      end
+    end)
+  end
+
+  # Regional seasonal adjustments: multipliers on the band's seasonal_base
+  # score by month. 1.0 = no change; > 1.0 = better than the base suggests;
+  # < 1.0 = worse. Only months/regions where the uniform base is known to
+  # be wrong get non-1.0 values.
+  #
+  # These are hand-tuned starting points based on the meteorologist's
+  # qualitative guidance. Phase 9 recalibration will refine them from
+  # backtest data.
+  @seasonal_adjustments %{
+    gulf_coast: %{
+      # Gulf August is drier than June/July → better propagation
+      6 => 0.95,
+      7 => 0.95,
+      8 => 1.15,
+      9 => 1.10
+    },
+    corn_belt: %{
+      # Iowa July/August: corn evapotranspiration → high dewpoints → worse
+      6 => 1.05,
+      7 => 0.85,
+      8 => 0.80,
+      9 => 0.95
+    },
+    southeast: %{
+      # Similar to Gulf but less extreme
+      7 => 0.97,
+      8 => 1.08
+    },
+    southern_plains: %{
+      # TX panhandle drier than Gulf but wetter than desert
+      8 => 1.05
+    },
+    desert_southwest: %{
+      # Monsoon July/Aug brings moisture → briefly good ducting
+      7 => 1.10,
+      8 => 1.10,
+      9 => 1.05
+    },
+    pacific_northwest: %{
+      # Marine layer summer inversions
+      6 => 1.15,
+      7 => 1.20,
+      8 => 1.20,
+      9 => 1.10
+    }
+  }
+
+  @doc """
+  Seasonal score multiplier for a given region and month (1-12).
+
+  Returns a float in [0.7, 1.3]. The scorer multiplies the band's
+  `seasonal_base[month]` by this value. Unknown regions or months
+  without an adjustment return 1.0.
+  """
+  @spec seasonal_adjustment(atom, integer) :: float
+  def seasonal_adjustment(region, month) do
+    @seasonal_adjustments
+    |> Map.get(region, %{})
+    |> Map.get(month, 1.0)
+  end
+end

lib/microwaveprop/propagation/scorer.ex

@@ -204,12 +204,29 @@ defmodule Microwaveprop.Propagation.Scorer do
 
   # ── Factor 6: Season ─────────────────────────────────────────────
 
-  @doc "Scores the seasonal effect for the given month and band config."
-  @spec score_season(integer(), map()) :: integer()
-  def score_season(month, %{seasonal_base: base_map, seasonal_adj: adj_map}) do
+  @doc """
+  Scores the seasonal effect for the given month and band config.
+
+  When lat/lon are provided, applies a regional adjustment multiplier
+  from `Propagation.Region` on top of the band's `seasonal_base` +
+  `seasonal_adj`. This corrects for the meteorologist's observation
+  that Gulf coast August is better than the uniform base suggests,
+  while Corn Belt August is worse.
+  """
+  @spec score_season(integer(), float | nil, float | nil, map()) :: integer()
+  def score_season(month, lat, lon, %{seasonal_base: base_map, seasonal_adj: adj_map}) do
     base = Map.get(base_map, month, 50)
     adj = Map.get(adj_map, month, 0)
-    min(100, max(0, base + adj))
+
+    region_mult =
+      if is_number(lat) and is_number(lon) do
+        region = Microwaveprop.Propagation.Region.for_point(lat, lon)
+        Microwaveprop.Propagation.Region.seasonal_adjustment(region, month)
+      else
+        1.0
+      end
+
+    round(min(100, max(0, (base + adj) * region_mult)))
   end
 
   # ── Factor 7: Wind ───────────────────────────────────────────────
@@ -341,7 +358,7 @@ defmodule Microwaveprop.Propagation.Scorer do
           band_config
         ),
       sky: score_sky(conditions.sky_cover_pct),
-      season: score_season(conditions.month, band_config),
+      season: score_season(conditions.month, conditions[:latitude], conditions[:longitude], band_config),
       wind: score_wind(conditions.wind_speed_kts),
       rain: score_rain(conditions.rain_rate_mmhr, band_config),
       pwat: score_pwat(conditions[:pwat_mm], band_config),

test/microwaveprop/propagation/region_test.exs

@@ -0,0 +1,57 @@
+defmodule Microwaveprop.Propagation.RegionTest do
+  use ExUnit.Case, async: true
+
+  alias Microwaveprop.Propagation.Region
+
+  describe "for_point/2" do
+    test "Dallas TX is in southern_plains" do
+      assert Region.for_point(32.8, -96.8) == :southern_plains
+    end
+
+    test "Houston TX is in gulf_coast" do
+      assert Region.for_point(29.8, -95.4) == :gulf_coast
+    end
+
+    test "Des Moines IA is in corn_belt" do
+      assert Region.for_point(41.6, -93.6) == :corn_belt
+    end
+
+    test "Tucson AZ is in desert_southwest" do
+      assert Region.for_point(32.2, -110.9) == :desert_southwest
+    end
+
+    test "Seattle WA is in pacific_northwest" do
+      assert Region.for_point(47.6, -122.3) == :pacific_northwest
+    end
+
+    test "New York NY is in northeast" do
+      assert Region.for_point(40.7, -74.0) == :northeast
+    end
+
+    test "Atlanta GA is in southeast" do
+      assert Region.for_point(33.7, -84.4) == :southeast
+    end
+
+    test "Denver CO falls into a region" do
+      region = Region.for_point(39.7, -104.9)
+      assert is_atom(region)
+    end
+
+    test "returns :other for points outside CONUS" do
+      assert Region.for_point(19.0, -155.0) == :other
+    end
+  end
+
+  describe "seasonal_adjustment/2" do
+    test "returns a multiplier for known regions and months" do
+      adj = Region.seasonal_adjustment(:gulf_coast, 8)
+      assert is_float(adj)
+      # Gulf coast August should be drier → higher seasonal score than default
+      assert adj >= 0.8 and adj <= 1.3
+    end
+
+    test "returns 1.0 for unknown regions" do
+      assert Region.seasonal_adjustment(:other, 6) == 1.0
+    end
+  end
+end

test/microwaveprop/propagation/scorer_test.exs

@@ -286,33 +286,47 @@ defmodule Microwaveprop.Propagation.ScorerTest do
     end
   end
 
-  # ── score_season/2 ───────────────────────────────────────────────
+  # ── score_season/4 ───────────────────────────────────────────────
 
-  describe "score_season/2" do
-    test "10 GHz summer peak" do
-      # July base = 95, adj = 0 -> 95
-      assert Scorer.score_season(7, @band_10g) == 95
+  describe "score_season/4" do
+    test "10 GHz summer peak (no region)" do
+      # July base = 95, adj = 0, no region mult -> 95
+      assert Scorer.score_season(7, nil, nil, @band_10g) == 95
     end
 
     test "10 GHz winter low" do
-      # March base = 22, adj = 0 -> 22
-      assert Scorer.score_season(3, @band_10g) == 22
+      assert Scorer.score_season(3, nil, nil, @band_10g) == 22
     end
 
     test "24 GHz winter peak" do
-      # November base = 96, adj = 0 -> 96
-      assert Scorer.score_season(11, @band_24g) == 96
+      assert Scorer.score_season(11, nil, nil, @band_24g) == 96
     end
 
     test "24 GHz summer with adjustment" do
-      # July base = 18, adj = -10 -> 8
-      assert Scorer.score_season(7, @band_24g) == 8
+      assert Scorer.score_season(7, nil, nil, @band_24g) == 8
     end
 
     test "clamped to 0-100" do
-      # Even if base + adj > 100, clamp
-      assert Scorer.score_season(1, @band_10g) >= 0
-      assert Scorer.score_season(1, @band_10g) <= 100
+      assert Scorer.score_season(1, nil, nil, @band_10g) >= 0
+      assert Scorer.score_season(1, nil, nil, @band_10g) <= 100
+    end
+
+    test "applies regional adjustment for Gulf coast August" do
+      # Gulf coast August mult = 1.15, so score = base * 1.15
+      gulf_lat = 29.0
+      gulf_lon = -95.0
+      no_region = Scorer.score_season(8, nil, nil, @band_10g)
+      gulf_score = Scorer.score_season(8, gulf_lat, gulf_lon, @band_10g)
+      assert gulf_score > no_region
+    end
+
+    test "applies regional adjustment for Corn Belt August" do
+      # Corn Belt August mult = 0.80, so score = base * 0.80
+      iowa_lat = 42.0
+      iowa_lon = -93.0
+      no_region = Scorer.score_season(8, nil, nil, @band_10g)
+      iowa_score = Scorer.score_season(8, iowa_lat, iowa_lon, @band_10g)
+      assert iowa_score < no_region
     end
   end