diff --git a/lib/microwaveprop_web/live/map_live.ex b/lib/microwaveprop_web/live/map_live.ex
index a3bbe388..40f645b7 100644
--- a/lib/microwaveprop_web/live/map_live.ex
+++ b/lib/microwaveprop_web/live/map_live.ex
@@ -15,6 +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.
+  @conus_lat_min 24.5
+  @conus_lat_max 49.5
+  @conus_lon_min -125.0
+  @conus_lon_max -66.5
+
   # 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-
   # visit for an in-progress hourly run, long enough to be cheap.
@@ -53,6 +61,7 @@ defmodule MicrowavepropWeb.MapLive do
     valid_times = Propagation.available_valid_times(selected_band)
     selected_time = recovered_time || closest_to_now(valid_times)
     center = initial_center(session)
+    visitor = visitor_location(session)
     bounds = bounds_around(center)
     initial_scores = Propagation.scores_at(selected_band, selected_time, bounds)
 
@@ -73,6 +82,7 @@ defmodule MicrowavepropWeb.MapLive do
        bounds: bounds,
        initial_center: center,
        initial_zoom: @default_zoom,
+       outside_conus: outside_conus?(visitor),
        grid_visible: false,
        radar_visible: false,
        antenna_height_ft: 33
@@ -86,6 +96,25 @@ defmodule MicrowavepropWeb.MapLive do
 
   defp initial_center(_session), do: @default_center
 
+  # Return the visitor's Cloudflare geolocation as a %{lat:, lon:} map, or
+  # nil when the CF headers weren't set (local dev, direct connect, tests
+  # without a stubbed session).
+  defp visitor_location(%{"cf_lat" => lat, "cf_lon" => lon}) when is_float(lat) and is_float(lon) do
+    %{lat: lat, lon: lon}
+  end
+
+  defp visitor_location(_session), do: nil
+
+  # True when the visitor's location is known and falls outside the CONUS
+  # bounding box. Missing geolocation returns false so we don't flash a
+  # banner at anyone we can't actually place.
+  defp outside_conus?(nil), do: false
+
+  defp outside_conus?(%{lat: lat, lon: lon}) do
+    lat < @conus_lat_min or lat > @conus_lat_max or
+      lon < @conus_lon_min or lon > @conus_lon_max
+  end
+
   # Build a bounding box roughly matching the hardcoded default (~3.4° × 9°)
   # around a given center so the initial HRRR score query still returns a
   # useful tile set for the visible area.
@@ -486,6 +515,22 @@ defmodule MicrowavepropWeb.MapLive do
     <div class="flex w-screen h-screen overflow-hidden">
       <%!-- Map container --%>
       <div class="relative flex-1 min-w-0">
+        <%!-- CONUS-only data banner (shown only when visitor geolocation is
+             outside the continental US) --%>
+        <div
+          :if={@outside_conus}
+          id="conus-banner"
+          role="status"
+          class="alert alert-info absolute top-2 left-1/2 -translate-x-1/2 z-[1002] max-w-[calc(100vw-1rem)] md:max-w-xl shadow-lg text-sm"
+        >
+          <.icon name="hero-information-circle" class="size-5 shrink-0" />
+          <span>
+            This data set currently covers the continental United States only.
+            You're viewing the map centered on your location, but propagation
+            data won't be available there yet.
+          </span>
+        </div>
+
         <%!-- Full-page map --%>
         <div
           id="propagation-map"
diff --git a/test/microwaveprop_web/live/map_live_test.exs b/test/microwaveprop_web/live/map_live_test.exs
index b8c3717f..b69b1feb 100644
--- a/test/microwaveprop_web/live/map_live_test.exs
+++ b/test/microwaveprop_web/live/map_live_test.exs
@@ -51,6 +51,30 @@ defmodule MicrowavepropWeb.MapLiveTest do
     end
   end
 
+  describe "CONUS-only banner" do
+    @banner_copy "This data set currently covers the continental United States only."
+
+    test "is hidden when no CF lat/lon in session", %{conn: conn} do
+      {:ok, _lv, html} = live(conn, ~p"/map")
+      refute html =~ ~s(id="conus-banner")
+      refute html =~ @banner_copy
+    end
+
+    test "is hidden when CF lat/lon is inside CONUS (DFW)", %{conn: conn} do
+      conn = Phoenix.ConnTest.init_test_session(conn, %{cf_lat: 32.9, cf_lon: -97.0})
+      {:ok, _lv, html} = live(conn, ~p"/map")
+      refute html =~ ~s(id="conus-banner")
+      refute html =~ @banner_copy
+    end
+
+    test "is visible when CF lat/lon is outside CONUS (London)", %{conn: conn} do
+      conn = Phoenix.ConnTest.init_test_session(conn, %{cf_lat: 51.5, cf_lon: -0.1})
+      {:ok, _lv, html} = live(conn, ~p"/map")
+      assert html =~ ~s(id="conus-banner")
+      assert html =~ @banner_copy
+    end
+  end
+
   describe "select_band" do
     test "updates selected band on event", %{conn: conn} do
       {:ok, lv, _html} = live(conn, ~p"/map")