diff --git a/web/src/layers/alerts.tsx b/web/src/layers/alerts.tsx
index 0da6fc3..c10d633 100644
--- a/web/src/layers/alerts.tsx
+++ b/web/src/layers/alerts.tsx
@@ -1,13 +1,18 @@
 import type { Color } from '@deck.gl/core';
 import { GeoJsonLayer } from '@deck.gl/layers';
 import type { Geometry } from '../generated/geojson';
-import type { AlertProps } from '../generated/weather';
+import type { AlertProps, CityTileIndex } from '../generated/weather';
 import { age, type WeatherFc } from '../weather';
 import { Swatch } from './swatch';
 import type { WeatherLayer } from './types';
 
 type AlertFc = WeatherFc<Geometry, AlertProps>;
 
+/** Alerts plus the timeline axis (the citytile snapshot) so the layer can show
+ * only the alerts in effect at the map-wide forecast time, like every other
+ * time-aware layer. */
+type AlertData = { fc: AlertFc; axis: CityTileIndex | null };
+
 const FILL: Record<string, Color> = {
   Extreme: [168, 0, 90, 80],
   Severe: [220, 60, 30, 70],
@@ -24,29 +29,67 @@ const LINE: Record<string, Color> = {
   Unknown: [110, 110, 110, 220],
 };
 
-export const alerts: WeatherLayer<AlertFc> = {
+/** Wall-clock ms the timeline currently points at, or null before the axis
+ * loads (in which case we don't filter — show all active alerts). */
+function validMs(axis: CityTileIndex | null, time: number): number | null {
+  return axis ? axis.snapshotMs + time * 3_600_000 : null;
+}
+
+/** Alerts in effect at `at` (ms). A null onset/expires is treated as unbounded
+ * on that side; a null `at` means "don't filter". NWS `/alerts/active` only
+ * returns currently-active alerts, so scrubbing forward expires them and any
+ * future-onset alert in the feed appears at its onset. */
+function activeAt(
+  features: AlertFc['features'],
+  at: number | null,
+): AlertFc['features'] {
+  if (at == null) return features;
+  return features.filter((f) => {
+    const onset = f.properties.onset ? Date.parse(f.properties.onset) : NaN;
+    const expires = f.properties.expires
+      ? Date.parse(f.properties.expires)
+      : NaN;
+    if (!Number.isNaN(onset) && at < onset) return false;
+    if (!Number.isNaN(expires) && at > expires) return false;
+    return true;
+  });
+}
+
+export const alerts: WeatherLayer<AlertData> = {
   id: 'alerts',
-  label: (fc) => `NWS alerts${fc ? ` (${fc.features.length})` : ''}`,
+  label: (d) => `NWS alerts${d ? ` (${d.fc.features.length})` : ''}`,
   legend: <Swatch className="bg-[#dc3c1e]" />,
   defaultVisible: true,
-  select: (w) => w.alerts,
-  build: (fc) => [
-    new GeoJsonLayer({
-      id: 'alerts',
-      data: fc as any,
-      pickable: true,
-      autoHighlight: true,
-      highlightColor: [255, 255, 255, 40],
-      stroked: true,
-      filled: true,
-      lineWidthMinPixels: 1.5,
-      getFillColor: (f: any) => FILL[f.properties.severity] ?? FILL.Unknown,
-      getLineColor: (f: any) => LINE[f.properties.severity] ?? LINE.Unknown,
-    }),
-  ],
-  controls: (_ctx, fc) => (
-    <div className="text-slate-400 text-xs">{age(fc?.generated_ms)}</div>
-  ),
+  select: (w) => (w.alerts ? { fc: w.alerts, axis: w.cityTiles } : null),
+  build: (d, ctx) => {
+    const features = activeAt(d.fc.features, validMs(d.axis, ctx.time));
+    const fc = { ...d.fc, features };
+    return [
+      new GeoJsonLayer({
+        id: 'alerts',
+        data: fc as any,
+        pickable: true,
+        autoHighlight: true,
+        highlightColor: [255, 255, 255, 40],
+        stroked: true,
+        filled: true,
+        lineWidthMinPixels: 1.5,
+        getFillColor: (f: any) => FILL[f.properties.severity] ?? FILL.Unknown,
+        getLineColor: (f: any) => LINE[f.properties.severity] ?? LINE.Unknown,
+      }),
+    ];
+  },
+  controls: (ctx, d) => {
+    if (!d) return null;
+    const total = d.fc.features.length;
+    const shown = activeAt(d.fc.features, validMs(d.axis, ctx.time)).length;
+    return (
+      <div className="text-slate-400 text-xs">
+        {shown < total ? `${shown} of ${total} in effect · ` : ''}
+        {age(d.fc.generated_ms)}
+      </div>
+    );
+  },
   tooltip: (o) => {
     const p = o?.properties;
     if (!p?.event) return null;