fix(weather): default selected time to current hour, not clock-closest

Past minute 30 of the wall clock the next forecast hour is closer to
utc_now() than the current hour, so the picker would land on (e.g.)
19:00Z when the user is at 18:43Z. HRRR has 19:00Z, but the HRDPS
chain only seeds the *current* hour (18:00Z), so the dual-source map
showed an empty Canadian half whenever we crossed the half-hour mark.

Switch to "most-recent valid_time <= now", with a fallback to the
closest future when nothing on disk is yet at-or-before now (cold
start). Hoist the picker into a public function so it can be unit-
tested deterministically — anchoring the integration test on
DateTime.utc_now() truncated to the hour would have only verified the
new behavior past minute 30.

/weather-ca delegates to the same picker so the two maps agree on
"current."

lib/microwaveprop_web/live/weather_ca_map_live.ex

@@ -97,11 +97,11 @@ defmodule MicrowavepropWeb.WeatherCaMapLive do
 
   defp parse_float_param(_value, _min, _max, default), do: default
 
-  defp pick_initial_valid_time([]), do: nil
-
+  # Delegate to WeatherMapLive's picker so the two maps agree on what
+  # "current" means: most-recent <=now, with a fall-through to the
+  # closest future when nothing is yet at-or-before now.
   defp pick_initial_valid_time(valid_times) do
-    now = DateTime.utc_now()
-    Enum.min_by(valid_times, fn vt -> abs(DateTime.diff(vt, now, :second)) end)
+    MicrowavepropWeb.WeatherMapLive.pick_initial_valid_time(valid_times)
   end
 
   # Don't filter by recency. HRDPS publishes 4×/day with a 3-4h

lib/microwaveprop_web/live/weather_map_live.ex

@@ -113,11 +113,25 @@ defmodule MicrowavepropWeb.WeatherMapLive do
 
   defp parse_float_param(_value, _min, _max, default), do: default
 
-  defp pick_initial_valid_time([]), do: nil
+  @doc false
+  # Public for direct unit testing: anchoring an integration test on a
+  # truncated `utc_now()` would only assert the new behavior past minute
+  # 30 of the wall clock.
+  def pick_initial_valid_time(valid_times, now \\ DateTime.utc_now())
+  def pick_initial_valid_time([], _now), do: nil
 
-  defp pick_initial_valid_time(valid_times) do
-    now = DateTime.utc_now()
-    Enum.min_by(valid_times, fn vt -> abs(DateTime.diff(vt, now, :second)) end)
+  def pick_initial_valid_time(valid_times, now) do
+    # Prefer the most recent valid_time that is <= now (the "current
+    # hour" — what HRRR analysis just delivered and what the HRDPS chain
+    # seeds when fired this hour). Fall back to the closest absolute
+    # match only when nothing on disk is yet at-or-before now.
+    past_or_equal =
+      Enum.filter(valid_times, fn vt -> DateTime.compare(vt, now) != :gt end)
+
+    case past_or_equal do
+      [] -> Enum.min_by(valid_times, fn vt -> abs(DateTime.diff(vt, now, :second)) end)
+      list -> Enum.max_by(list, &DateTime.to_unix/1)
+    end
   end
 
   # Drop valid_times older than one hour ago. The HRRR scalar cache can

test/microwaveprop_web/live/weather_map_live_test.exs

@@ -131,6 +131,33 @@ defmodule MicrowavepropWeb.WeatherMapLiveTest do
       refute html =~ ~s(data-selected-time="#{future_iso}")
     end
 
+    test "pick_initial_valid_time prefers the most-recent <=now over a clock-closer future hour" do
+      # Deterministic unit test against the picker (the integration
+      # version would only fail past minute 30 of the wall clock).
+      now = ~U[2026-04-30 18:43:00Z]
+      current_hour = ~U[2026-04-30 18:00:00Z]
+      next_hour = ~U[2026-04-30 19:00:00Z]
+
+      # next_hour is closer to `now` (17 min vs 43 min), but the user
+      # wants the *current hour* so HRRR-analysis-now and HRDPS-seeded-
+      # for-now align on the dual-source map.
+      assert MicrowavepropWeb.WeatherMapLive.pick_initial_valid_time(
+               [current_hour, next_hour],
+               now
+             ) == current_hour
+    end
+
+    test "pick_initial_valid_time falls back to the closest future when nothing is <=now" do
+      # Cold-start case: no analysis-or-past data on disk yet, only
+      # forecast hours. The picker should still return something so the
+      # map is non-blank.
+      now = ~U[2026-04-30 18:00:00Z]
+      f1 = ~U[2026-04-30 19:00:00Z]
+      f6 = ~U[2026-05-01 00:00:00Z]
+
+      assert MicrowavepropWeb.WeatherMapLive.pick_initial_valid_time([f1, f6], now) == f1
+    end
+
     test "select_time pushes an overlay refresh for the selected hour", %{conn: conn} do
       base =
         DateTime.utc_now()