diff --git a/web/src/layers/index.ts b/web/src/layers/index.ts
index 69f9033..52037f1 100644
--- a/web/src/layers/index.ts
+++ b/web/src/layers/index.ts
@@ -2,7 +2,7 @@ import { alerts } from './alerts';
 import { cape } from './cape';
 import { precip } from './precip';
 import { temp } from './temp';
-import type { WeatherLayer } from './types';
+import { defineLayer, type WeatherLayer } from './types';
 import { wind } from './wind';
 
 export type { LayerCtx, WeatherLayer } from './types';
@@ -10,6 +10,14 @@ export type { LayerCtx, WeatherLayer } from './types';
 /**
  * The layer registry. Array order is paint order (the filled rasters at the
  * bottom, vector/point layers on top). Adding a layer is a one-line change here
- * plus its module — `App` never has to learn the layer exists.
+ * plus its module — `App` never has to learn the layer exists. Each layer is
+ * wrapped in `defineLayer`, which type-checks it as its own `WeatherLayer<D>`
+ * and erases the data type to `unknown` for the shared array (no `any`).
  */
-export const LAYERS: WeatherLayer<any>[] = [precip, cape, alerts, wind, temp];
+export const LAYERS: WeatherLayer<unknown>[] = [
+  defineLayer(precip),
+  defineLayer(cape),
+  defineLayer(alerts),
+  defineLayer(wind),
+  defineLayer(temp),
+];
diff --git a/web/src/layers/temp.tsx b/web/src/layers/temp.tsx
index 4411594..ccddd8e 100644
--- a/web/src/layers/temp.tsx
+++ b/web/src/layers/temp.tsx
@@ -6,8 +6,8 @@ import {
 import { TileLayer } from '@deck.gl/geo-layers';
 import { TextLayer } from '@deck.gl/layers';
 import { GRID_ZOOM_SPLIT, WEATHER_BASE } from '../config';
-import type { CityForecast, CityTileIndex } from '../generated/weather';
-import { age, type LatticeFc } from '../weather';
+import type { CityTileIndex } from '../generated/weather';
+import { age, type CityTileFc, type LatticeFc } from '../weather';
 import { Swatch } from './swatch';
 import type { WeatherLayer } from './types';
 
@@ -47,20 +47,12 @@ interface TempData {
   lattice: LatticeFc | null;
 }
 
-interface CityTile {
-  hours: number[];
-  features: {
-    geometry: { coordinates: [number, number] };
-    properties: CityForecast;
-  }[];
-}
-
 /** Flatten a city tile into one record per (city, forecast hour). */
-function explodeCities(tile: CityTile | null): TempRecord[] {
+function explodeCities(tile: CityTileFc | null): TempRecord[] {
   if (!tile) return [];
   const out: TempRecord[] = [];
   for (const f of tile.features) {
-    const position = f.geometry.coordinates;
+    const position = f.geometry.coordinates as [number, number];
     const { name, t } = f.properties;
     for (let k = 0; k < t.length; k++) {
       out.push({ position, hour: tile.hours[k], temp: t[k], name });
diff --git a/web/src/layers/types.ts b/web/src/layers/types.ts
index 13efe1b..85d345d 100644
--- a/web/src/layers/types.ts
+++ b/web/src/layers/types.ts
@@ -44,3 +44,18 @@ export interface WeatherLayer<D = unknown> {
   /** Optional tooltip for a picked object belonging to this layer. */
   tooltip?: (object: any) => string | null;
 }
+
+/**
+ * Register a layer into the registry, erasing its data type to `unknown`.
+ *
+ * `WeatherLayer<D>` is invariant in `D` (it both produces `D` via `select` and
+ * consumes it via `build`), so a `WeatherLayer<SpecificData>` is not directly
+ * assignable to a shared `WeatherLayer<unknown>[]`. This helper type-checks each
+ * layer as its own `WeatherLayer<D>` (so `select`→`build` stays sound per layer)
+ * and erases only the registry's view — far tighter than typing the array
+ * `WeatherLayer<any>[]`, which would accept a malformed layer. `App` only ever
+ * pairs a layer's own `select` with its own `build`, so the erasure is safe.
+ */
+export function defineLayer<D>(layer: WeatherLayer<D>): WeatherLayer<unknown> {
+  return layer as WeatherLayer<unknown>;
+}
diff --git a/web/src/weather.ts b/web/src/weather.ts
index a31e874..1ae722d 100644
--- a/web/src/weather.ts
+++ b/web/src/weather.ts
@@ -10,6 +10,7 @@ import type { FeatureCollection, Geometry, Point } from './generated/geojson';
 import type {
   AlertProps,
   CapeTexIndex,
+  CityForecast,
   CityTileIndex,
   LatticeForecast,
   RefcTexIndex,
@@ -37,6 +38,17 @@ export type LatticeFc = FeatureCollection<Point, LatticeForecast> & {
   hours: number[];
 };
 
+/** A single point-forecast city tile (the `citytile/{snapshotMs}/{z}/{x}/{y}`
+ * pyramid): a FeatureCollection of GFS-sampled cities, each carrying a
+ * temperature series, plus the snapshot + hour axis. Same envelope as the
+ * lattice, but per-tile rather than global — and built from the generated
+ * `CityForecast`/`Point` so it tracks the Rust contract instead of being
+ * hand-declared at the use site. */
+export type CityTileFc = FeatureCollection<Point, CityForecast> & {
+  snapshotMs: number;
+  hours: number[];
+};
+
 /** Human "N min ago" for a feed's `generated_ms` (or model snapshot) time. */
 export function age(ms?: number): string {
   if (!ms) return '—';