diff --git a/lib/microwaveprop/weather.ex b/lib/microwaveprop/weather.ex
index 55a059c1..f6d1775f 100644
--- a/lib/microwaveprop/weather.ex
+++ b/lib/microwaveprop/weather.ex
@@ -933,10 +933,20 @@ defmodule Microwaveprop.Weather do
   because the cache mixes HRRR + HRDPS rows by design — caching this
   separately would double the per-pod memory budget for marginal benefit
   (Canadian viewport reads are infrequent compared to CONUS).
+
+  The /weather (dual-source) timeline picks valid_times from HRRR's
+  hourly cadence, but HRDPS publishes 4×/day with multi-hour latency,
+  so the requested time will frequently miss any on-disk HRDPS dir.
+  Snapping to the nearest available HRDPS time within a 6 h window
+  keeps the Canadian overlay rendering slightly stale instead of
+  disappearing entirely. /weather-ca picks times from HRDPS-only listings
+  so the exact time always matches and the snap is a no-op.
   """
+  @hrdps_nearest_window_seconds 6 * 3600
+
   @spec weather_grid_hrdps_at(DateTime.t(), %{optional(String.t()) => float()} | nil) :: [map()]
   def weather_grid_hrdps_at(%DateTime{} = valid_time, bounds) do
-    ScalarFile.read_bounds_hrdps(valid_time, bounds)
+    ScalarFile.read_bounds_hrdps_nearest(valid_time, bounds, @hrdps_nearest_window_seconds)
   end
 
   @doc """
diff --git a/lib/microwaveprop/weather/scalar_file.ex b/lib/microwaveprop/weather/scalar_file.ex
index 50ac4f67..fbbadfdd 100644
--- a/lib/microwaveprop/weather/scalar_file.ex
+++ b/lib/microwaveprop/weather/scalar_file.ex
@@ -191,6 +191,39 @@ defmodule Microwaveprop.Weather.ScalarFile do
     read_chunks(list_chunk_files_hrdps(valid_time), bounds)
   end
 
+  @doc """
+  Read HRDPS rows for the closest available valid_time within
+  `window_seconds` of `target`. Returns rows from the snapped time, or
+  `[]` if no HRDPS dir falls inside the window.
+
+  HRDPS publishes 4×/day with ~3-4 h latency, so the /weather timeline
+  (driven by HRRR's hourly cadence) will frequently request a time that
+  has no HRDPS dir on disk. Snapping to the nearest available time lets
+  the Canadian overlay render slightly stale rather than disappear
+  entirely whenever the user's selected forecast hour misses HRDPS.
+  """
+  @spec read_bounds_hrdps_nearest(DateTime.t(), bounds() | nil, non_neg_integer()) :: [row()]
+  def read_bounds_hrdps_nearest(%DateTime{} = target, bounds, window_seconds)
+      when is_integer(window_seconds) and window_seconds >= 0 do
+    case nearest_hrdps_valid_time(target, window_seconds) do
+      nil -> []
+      vt -> read_chunks(list_chunk_files_hrdps(vt), bounds)
+    end
+  end
+
+  defp nearest_hrdps_valid_time(target, window_seconds) do
+    target_unix = DateTime.to_unix(target)
+
+    list_valid_times_hrdps()
+    |> Enum.map(fn vt -> {vt, abs(DateTime.to_unix(vt) - target_unix)} end)
+    |> Enum.filter(fn {_vt, delta} -> delta <= window_seconds end)
+    |> Enum.min_by(fn {_vt, delta} -> delta end, fn -> nil end)
+    |> case do
+      nil -> nil
+      {vt, _delta} -> vt
+    end
+  end
+
   defp read_chunks([], _bounds), do: []
 
   defp read_chunks(files, bounds) do
diff --git a/test/microwaveprop/weather/scalar_file_test.exs b/test/microwaveprop/weather/scalar_file_test.exs
index ecf62724..bffccb10 100644
--- a/test/microwaveprop/weather/scalar_file_test.exs
+++ b/test/microwaveprop/weather/scalar_file_test.exs
@@ -246,6 +246,58 @@ defmodule Microwaveprop.Weather.ScalarFileTest do
     end
   end
 
+  # The /weather (dual-source) timeline picks valid_times from HRRR's
+  # hourly cadence. HRDPS publishes 4×/day with multi-hour latency, so
+  # the requested time will frequently miss any HRDPS dir. Snapping to
+  # the closest HRDPS dir within a window lets the Canadian overlay
+  # render slightly stale rather than disappear.
+  describe "read_bounds_hrdps_nearest/3" do
+    test "returns rows from the exact time when its HRDPS dir exists" do
+      vt = ~U[2026-04-29 12:00:00Z]
+      hand_write_chunk(ScalarFile.dir_for_hrdps(vt), 53.0, -60.0, 8.0)
+
+      assert [%{lat: 53.0, lon: -60.0, temperature: 8.0}] =
+               ScalarFile.read_bounds_hrdps_nearest(vt, nil, 6 * 3600)
+    end
+
+    test "snaps to the closest HRDPS dir within the window when exact time is absent" do
+      requested = ~U[2026-04-29 14:00:00Z]
+      closer = ~U[2026-04-29 13:00:00Z]
+      farther = ~U[2026-04-29 10:00:00Z]
+
+      hand_write_chunk(ScalarFile.dir_for_hrdps(closer), 53.0, -60.0, 7.0)
+      hand_write_chunk(ScalarFile.dir_for_hrdps(farther), 53.0, -60.0, 99.0)
+
+      assert [%{temperature: 7.0}] =
+               ScalarFile.read_bounds_hrdps_nearest(requested, nil, 6 * 3600)
+    end
+
+    test "snaps forward as well as backward (HRDPS forecast can lead requested time)" do
+      requested = ~U[2026-04-29 12:00:00Z]
+      future = ~U[2026-04-29 13:00:00Z]
+
+      hand_write_chunk(ScalarFile.dir_for_hrdps(future), 53.0, -60.0, 11.0)
+
+      assert [%{temperature: 11.0}] =
+               ScalarFile.read_bounds_hrdps_nearest(requested, nil, 6 * 3600)
+    end
+
+    test "returns [] when no HRDPS dir exists inside the window" do
+      requested = ~U[2026-04-29 14:00:00Z]
+      stale = ~U[2026-04-29 02:00:00Z]
+
+      hand_write_chunk(ScalarFile.dir_for_hrdps(stale), 53.0, -60.0, 8.0)
+
+      # 12h gap, 6h window → not eligible.
+      assert ScalarFile.read_bounds_hrdps_nearest(requested, nil, 6 * 3600) == []
+    end
+
+    test "returns [] when no HRDPS dirs exist at all" do
+      requested = ~U[2026-04-29 14:00:00Z]
+      assert ScalarFile.read_bounds_hrdps_nearest(requested, nil, 6 * 3600) == []
+    end
+  end
+
   defp hand_write_chunk(dir, lat, lon, temp) do
     File.mkdir_p!(dir)
     lat_band = (lat / 5) |> Float.floor() |> trunc()