Cache the weather forecast for offline fallback

Radar flyout now also shows a 3-day Open-Meteo forecast (free, no API
key) for the map's current center. Radar frames and the forecast fetch
independently — one being unreachable doesn't hide the other. The last
successful forecast is cached in localStorage and shown (with a
"cached from" note) when the live fetch fails, so there's still
something useful once actually offline.

Radar tile images themselves aren't cached — that needs a Service
Worker, left as a separate follow-up (see TODO.adoc).

public/css/style.css

@@ -193,6 +193,36 @@ html, body {
   white-space: nowrap;
 }
 
+.radar-control__forecast {
+  margin-top: 6px;
+  padding-top: 6px;
+  border-top: 1px solid #eee;
+}
+
+.radar-control__forecast-day {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 2px 0;
+  white-space: nowrap;
+}
+
+.radar-control__forecast-day span:first-child {
+  width: 2.2em;
+  color: #888;
+}
+
+.radar-control__forecast-day span:nth-child(3) {
+  margin-left: auto;
+  font-variant-numeric: tabular-nums;
+}
+
+.radar-control__forecast-stale {
+  margin: 4px 0 0;
+  color: #888;
+  font-size: 11px;
+}
+
 .basemap-panel {
   position: fixed;
   top: 0;

public/js/radar.js

@@ -2,6 +2,34 @@ import { RADAR_ICON } from "/js/icons.js";
 
 const RAINVIEWER_API = "https://api.rainviewer.com/public/weather-maps.json";
 
+// Open-Meteo: free, no API key. Only the forecast (small JSON) is cached
+// here, in localStorage — the radar tile images above aren't: actually
+// serving those without a live network needs a Service Worker (intercepting
+// the tile requests and answering from a cache), which is its own separate
+// piece of infrastructure (registration, cache strategy, invalidation) and
+// deliberately left for later rather than folded into this. A cached
+// forecast still gives *something* useful once genuinely offline, which the
+// radar animation above can't.
+const FORECAST_API = "https://api.open-meteo.com/v1/forecast";
+const FORECAST_DAYS = 3;
+const FORECAST_CACHE_KEY = "maptool:forecast";
+
+// A small subset of WMO weather codes (open-meteo's `weather_code`) mapped
+// to one glyph each — not the full table, just enough to tell clear/cloudy/
+// rain/snow/storm apart at a glance.
+const WEATHER_EMOJI = [
+  [[0], "☀️"],
+  [[1, 2, 3], "⛅"],
+  [[45, 48], "🌫️"],
+  [[51, 53, 55, 56, 57, 61, 63, 65, 66, 67, 80, 81, 82], "🌧️"],
+  [[71, 73, 75, 77, 85, 86], "❄️"],
+  [[95, 96, 99], "⛈️"],
+];
+
+function weatherEmoji(code) {
+  return WEATHER_EMOJI.find(([codes]) => codes.includes(code))?.[1] ?? "🌡️";
+}
+
 // Matches the default window used by github.com/jpettitt/weather-radar-card
 // (60 minutes of history at RainViewer's native 10-minute cadence = 7
 // frames, the last one being "now").
@@ -42,14 +70,17 @@ export class RadarControl {
           <input type="range" class="radar-control__scrub" min="0" max="0" step="1" value="0">
           <span class="radar-control__time"></span>
         </div>
+        <div class="radar-control__forecast"></div>
       </div>
     `;
 
     this._icon = this._container.querySelector(".sidebar-icon");
     this._flyout = this._container.querySelector(".sidebar-flyout");
+    this._playback = this._container.querySelector(".radar-control__playback");
     this._playButton = this._container.querySelector(".radar-control__play");
     this._scrub = this._container.querySelector(".radar-control__scrub");
     this._time = this._container.querySelector(".radar-control__time");
+    this._forecastEl = this._container.querySelector(".radar-control__forecast");
 
     this._icon.addEventListener("click", () => this._setEnabled(!this._enabled));
     this._playButton.addEventListener("click", () => this._togglePlayback());
@@ -69,30 +100,87 @@ export class RadarControl {
       this._removeLayers();
       this._frames = [];
       this._updateFrameUI();
+      this._renderForecast(null);
       return;
     }
 
     this._icon.disabled = true;
-    let frames;
-    try {
-      frames = await fetchFrames();
-    } catch (err) {
-      this._enabled = false;
-      this._icon.classList.remove("active");
-      this._icon.disabled = false;
-      return;
-    }
+    // Independent fetches: RainViewer being unreachable shouldn't hide a
+    // forecast that Open-Meteo still answered, or vice versa — especially
+    // since only the forecast has a cached fallback to fall back to.
+    const [framesResult, forecastResult] = await Promise.allSettled([fetchFrames(), this._loadForecast()]);
     this._icon.disabled = false;
 
-    // The icon may have been clicked again (disabling) while the fetch was
-    // in flight.
-    if (!this._enabled || !frames.length) return;
+    // The icon may have been clicked again (disabling) while the fetches
+    // were in flight.
+    if (!this._enabled) return;
+
+    const frames = framesResult.status === "fulfilled" ? framesResult.value : [];
+    const forecast = forecastResult.status === "fulfilled" ? forecastResult.value : null;
+
+    // Nothing at all came back (both offline, no cached forecast either) —
+    // same "turn back off" story the radar-only fetch used to have on its
+    // own failure.
+    if (frames.length === 0 && !forecast) {
+      this._enabled = false;
+      this._icon.classList.remove("active");
+    } else if (frames.length > 0) {
+      this._frames = frames;
+      this._frameIndex = frames.length - 1;
+      this._addLayers();
+      this._startAnimation();
+    }
 
-    this._frames = frames;
-    this._frameIndex = frames.length - 1;
-    this._addLayers();
     this._updateFrameUI();
-    this._startAnimation();
+    this._renderForecast(forecast);
+  }
+
+  // cachedAt is null for a fresh fetch, or the cache's own timestamp when
+  // falling back to it — _renderForecast uses that to show a "cached" note
+  // instead of claiming this is live data.
+  async _loadForecast() {
+    const { lat, lng } = this._map.getCenter();
+    try {
+      const data = await fetchForecast(lat, lng);
+      writeCachedForecast(data);
+      return { data, cachedAt: null };
+    } catch (err) {
+      const cached = readCachedForecast();
+      if (!cached) throw err;
+      return cached;
+    }
+  }
+
+  _renderForecast(result) {
+    if (!result) {
+      this._forecastEl.innerHTML = "";
+      return;
+    }
+
+    const { data, cachedAt } = result;
+    const days = data.daily.time.map((date, i) => ({
+      date,
+      code: data.daily.weather_code[i],
+      max: Math.round(data.daily.temperature_2m_max[i]),
+      min: Math.round(data.daily.temperature_2m_min[i]),
+      precip: data.daily.precipitation_probability_max[i],
+    }));
+
+    const rows = days
+      .map(
+        (day) => `
+          <div class="radar-control__forecast-day">
+            <span>${new Date(day.date).toLocaleDateString([], { weekday: "short" })}</span>
+            <span>${weatherEmoji(day.code)}</span>
+            <span>${day.max}° / ${day.min}°</span>
+            <span>${day.precip}%</span>
+          </div>`,
+      )
+      .join("");
+    const staleNote = cachedAt
+      ? `<p class="radar-control__forecast-stale">Cached from ${new Date(cachedAt).toLocaleString([], { dateStyle: "short", timeStyle: "short" })}</p>`
+      : "";
+    this._forecastEl.innerHTML = rows + staleNote;
   }
 
   _addLayers() {
@@ -164,7 +252,10 @@ export class RadarControl {
   }
 
   _updateFrameUI() {
-    this._flyout.hidden = this._frames.length === 0;
+    // Shown whenever there's radar frames, a forecast, or both — either
+    // one on its own is still worth showing the flyout for.
+    this._flyout.hidden = !this._enabled;
+    this._playback.hidden = this._frames.length === 0;
     this._scrub.max = String(Math.max(0, this._frames.length - 1));
     this._scrub.value = String(this._frameIndex);
     const frame = this._frames[this._frameIndex];
@@ -192,6 +283,39 @@ async function fetchFrames() {
   return past.slice(-FRAME_COUNT).map((frame) => ({ ...frame, host }));
 }
 
+async function fetchForecast(lat, lon) {
+  const url = new URL(FORECAST_API);
+  url.searchParams.set("latitude", lat);
+  url.searchParams.set("longitude", lon);
+  url.searchParams.set("daily", "weather_code,temperature_2m_max,temperature_2m_min,precipitation_probability_max");
+  url.searchParams.set("timezone", "auto");
+  url.searchParams.set("forecast_days", String(FORECAST_DAYS));
+  const response = await fetch(url);
+  if (!response.ok) throw new Error(`Open-Meteo request failed: ${response.status}`);
+  return response.json();
+}
+
+// localStorage, not IndexedDB: this is one small JSON object (a few days of
+// daily figures), not worth a heavier async storage API for. Wrapped since
+// private browsing / storage-blocked contexts can throw on access, not just
+// on being full — a forecast is a nice-to-have, never worth crashing over.
+function writeCachedForecast(data) {
+  try {
+    localStorage.setItem(FORECAST_CACHE_KEY, JSON.stringify({ data, cachedAt: Date.now() }));
+  } catch (err) {
+    // Ignore — see above.
+  }
+}
+
+function readCachedForecast() {
+  try {
+    const raw = localStorage.getItem(FORECAST_CACHE_KEY);
+    return raw ? JSON.parse(raw) : null;
+  } catch (err) {
+    return null;
+  }
+}
+
 function firstSymbolLayerId(map) {
   const layers = map.getStyle()?.layers || [];
   const symbolLayer = layers.find((layer) => layer.type === "symbol");