diff --git a/rust/prop_grid_rs/src/grid.rs b/rust/prop_grid_rs/src/grid.rs
index eb002d1b..de3c9709 100644
--- a/rust/prop_grid_rs/src/grid.rs
+++ b/rust/prop_grid_rs/src/grid.rs
@@ -1,10 +1,13 @@
 //! Grid definitions at 0.125° resolution. 1:1 port of
-//! `lib/microwaveprop/propagation/grid.ex`. Two regions, disjoint by
-//! construction:
+//! `lib/microwaveprop/propagation/grid.ex`. Two regions, disjoint on
+//! HRRR's *interior*:
 //!
 //!   * `conus_points()` — HRRR coverage (lat 25-50, lon -125 to -66).
-//!   * `hrdps_only_points()` — Canadian extent covered by HRDPS but not
-//!     HRRR (lat 49-60 minus the HRRR overlap, lon -141 to -52).
+//!   * `hrdps_only_points()` — HRDPS bbox (lat 49-60, lon -141 to -52)
+//!     minus HRRR's interior (lat < 50 inside HRRR's lon range). The
+//!     lat=50 boundary row is kept so HRDPS's halfStep painting closes
+//!     the seam between HRRR and HRDPS on /weather; the merge layer
+//!     dedupes by exact (lat, lon) for the merged read path.
 //!
 //! Coverage stops at 60°N for v1 (SRTM elevation cuts off there; Arctic
 //! CDEM coverage is deferred to a follow-up plan).
@@ -110,7 +113,14 @@ pub fn hrdps_only_points() -> Vec<(f64, f64)> {
 
 #[inline]
 fn in_conus_bbox(lat: f64, lon: f64) -> bool {
-    (LAT_MIN..=LAT_MAX).contains(&lat) && (LON_MIN..=LON_MAX).contains(&lon)
+    // Exclusive on the north edge: HRDPS keeps the lat=LAT_MAX (50.0)
+    // boundary row so its halfStep painting (±0.25°) overlaps HRRR's top
+    // edge (±0.0625°) on /weather, closing a ~0.19° blank strip the
+    // user would otherwise see between HRRR and HRDPS coverage. The
+    // merge_prefer_hrrr layer dedupes by exact (lat, lon) on the merged
+    // read path, and the canvas overlay paints HRRR above HRDPS, so the
+    // overlap is only visible north of HRRR's coverage.
+    (LAT_MIN..LAT_MAX).contains(&lat) && (LON_MIN..=LON_MAX).contains(&lon)
 }
 
 /// Matches Elixir's `Float.round(x, 3)` — banker's rounding isn't used,
@@ -166,16 +176,21 @@ mod tests {
     }
 
     #[test]
-    fn hrdps_only_points_are_disjoint_from_conus() {
+    fn hrdps_only_points_disjoint_from_conus_interior() {
+        // Disjointness now holds for CONUS *interior* (lat < LAT_MAX); the
+        // lat=LAT_MAX boundary row is intentionally shared so HRDPS fills
+        // the seam between HRRR and Canada on /weather. See
+        // `hrdps_keeps_lat_50_boundary_inside_conus_lons`.
         let conus: std::collections::HashSet<(u64, u64)> = conus_points()
             .into_iter()
+            .filter(|(la, _)| *la < LAT_MAX)
             .map(|(la, lo)| (la.to_bits(), lo.to_bits()))
             .collect();
 
         for (la, lo) in hrdps_only_points() {
             assert!(
                 !conus.contains(&(la.to_bits(), lo.to_bits())),
-                "hrdps point {la},{lo} overlaps conus"
+                "hrdps point {la},{lo} overlaps conus interior"
             );
         }
     }
@@ -194,4 +209,24 @@ mod tests {
         let any_above_50 = hrdps_only_points().iter().any(|(la, _)| *la > 50.0);
         assert!(any_above_50);
     }
+
+    #[test]
+    fn hrdps_keeps_lat_50_boundary_inside_conus_lons() {
+        // HRRR tops out at 50.0°N (halfStep=0.0625 → paints to 50.0625°N).
+        // HRDPS at 0.5° step would otherwise jump to 50.5°N (halfStep=0.25
+        // → paints down to 50.25°N), leaving a ~21 km blank strip on the
+        // map. Keep the lat=50 row in HRDPS so it paints from 49.75°N to
+        // 50.25°N and overlaps HRRR's top edge — the merge layer dedupes
+        // by exact (lat, lon) match for the merged read path, and the
+        // /weather canvas paints HRRR on top of HRDPS so the overlap is
+        // invisible everywhere HRRR exists.
+        let points = hrdps_only_points();
+        let has_50_in_conus = points
+            .iter()
+            .any(|(la, lo)| (*la - 50.0).abs() < 1e-9 && (-125.0..=-66.0).contains(lo));
+        assert!(
+            has_50_in_conus,
+            "expected hrdps_only_points to include the lat=50 row inside CONUS lon range"
+        );
+    }
 }