Backfill NWS's missing pressure & UV from Open-Meteo (with source tooltip)

NWS doesn't publish pressure (its pressure series is empty) or a UV index.
When the selected source is NWS, merge current.pressure_msl, current.uv_index
and daily.uv_index_max from Open-Meteo and tag them via data._fallback so the
Current + Details views show a tooltip noting the value came from the
non-selected source (Open-Meteo) instead of showing 0 hPa / --.

src/components/CurrentWeather.svelte

@@ -36,6 +36,8 @@
 
   const unit = $derived(app.settings.units === 'imperial' ? '°F' : '°C')
   const windUnit = $derived(app.settings.units === 'imperial' ? 'mph' : 'km/h')
+  const fbPressure = $derived(!!data?._fallback?.pressure)
+  const fbUv = $derived(!!data?._fallback?.uv)
 </script>
 
 {#if current}
@@ -69,12 +71,12 @@
         <span class="metric-value">{Math.round(current.wind_speed_10m)} {windUnit}</span>
         <span class="metric-sub">{windCompass(current.wind_direction_10m)}</span>
       </div>
-      <div class="metric-card">
+      <div class="metric-card" title={fbPressure ? 'Pressure from Open-Meteo — NWS does not provide it' : undefined}>
         <span class="metric-icon">🌡️</span>
         <span class="metric-label">Pressure</span>
         <span class="metric-value">{Math.round(current.pressure_msl || 0)} hPa</span>
       </div>
-      <div class="metric-card">
+      <div class="metric-card" title={fbUv ? 'UV index from Open-Meteo — NWS does not provide it' : undefined}>
         <span class="metric-icon">☀️</span>
         <span class="metric-label">UV Index</span>
         <span class="metric-value">{current.uv_index != null ? Math.round(current.uv_index) : '--'}</span>

src/components/WeatherDetail.svelte

@@ -9,6 +9,8 @@
 
   const unit = $derived(app.settings.units === 'imperial' ? '°F' : '°C')
   const windUnit = $derived(app.settings.units === 'imperial' ? 'mph' : 'km/h')
+  const fbPressure = $derived(!!data?._fallback?.pressure)
+  const fbUv = $derived(!!data?._fallback?.uv)
 
   function windCompass(deg) {
     if (deg == null) return '--'
@@ -95,7 +97,7 @@
           </div>
           <div class="detail-row">
             <span>Pressure</span>
-            <span class="detail-value">{Math.round(current.pressure_msl || 0)} hPa</span>
+            <span class="detail-value" title={fbPressure ? 'Pressure from Open-Meteo — NWS does not provide it' : undefined}>{Math.round(current.pressure_msl || 0)} hPa</span>
           </div>
           <div class="detail-row">
             <span>Feels Like</span>
@@ -117,7 +119,7 @@
         <div class="detail-body">
           <div class="detail-row">
             <span>UV Index</span>
-            <span class="detail-value">
+            <span class="detail-value" title={fbUv ? 'UV index from Open-Meteo — NWS does not provide it' : undefined}>
               {current.uv_index != null ? Math.round(current.uv_index) : '--'}
               {#if current.uv_index != null}
                 <span class="uv-badge uv-{getUVLevel(current.uv_index).toLowerCase().replace(' ', '-')}">{getUVLevel(current.uv_index)}</span>
@@ -134,7 +136,7 @@
           </div>
           <div class="detail-row">
             <span>Max UV Today</span>
-            <span class="detail-value">{daily?.uv_index_max?.[0] != null ? Math.round(daily.uv_index_max[0]) : '--'}</span>
+            <span class="detail-value" title={fbUv ? 'UV from Open-Meteo — NWS does not provide it' : undefined}>{daily?.uv_index_max?.[0] != null ? Math.round(daily.uv_index_max[0]) : '--'}</span>
           </div>
         </div>
       </div>

src/lib/api/nws.js

@@ -252,3 +252,29 @@ export async function fetchForecastNWS(lat, lon, units = 'imperial') {
 
   return normalizeNWS({ points, forecast, hourly, grid, timeZone: tz, units })
 }
+
+/**
+ * Backfill fields that NWS does not provide (pressure, UV index) from an
+ * Open-Meteo response, tagging them so the UI can show a tooltip that the
+ * value came from the non-selected source.
+ */
+export function mergeFallbackData(nwsData, omData) {
+  const fallback = {}
+  const cur = nwsData.current || {}
+  const omCur = omData?.current || {}
+  if (omCur.pressure_msl != null) {
+    cur.pressure_msl = omCur.pressure_msl
+    fallback.pressure = true
+  }
+  if (omCur.uv_index != null) {
+    cur.uv_index = omCur.uv_index
+    fallback.uv = true
+  }
+  const omDaily = omData?.daily || {}
+  if (omDaily.uv_index_max?.length && nwsData.daily) {
+    nwsData.daily.uv_index_max = omDaily.uv_index_max
+    fallback.uv = true
+  }
+  if (Object.keys(fallback).length) nwsData._fallback = fallback
+  return nwsData
+}

src/lib/stores/app.svelte.js

@@ -4,7 +4,7 @@
 
 import { getLocations, loadSettings } from '../storage/db.js'
 import { fetchForecast } from '../api/weather.js'
-import { fetchForecastNWS } from '../api/nws.js'
+import { fetchForecastNWS, mergeFallbackData } from '../api/nws.js'
 
 export class AppStore {
   // Location state
@@ -116,6 +116,14 @@ export class AppStore {
       if (source === 'nws') {
         try {
           data = await fetchForecastNWS(location.lat, location.lon, units)
+          // NWS omits pressure + UV index — backfill those from Open-Meteo and
+          // flag them so the UI can tooltip that they came from the other source.
+          try {
+            const om = await fetchForecast(location.lat, location.lon, units)
+            data = mergeFallbackData(data, om)
+          } catch (omErr) {
+            console.warn('Open-Meteo fields unavailable:', omErr.message)
+          }
         } catch (nwsErr) {
           // NWS is US-only and can be unavailable — fall back to Open-Meteo
           // so the app keeps working rather than breaking.

tests/nws.test.js

@@ -8,6 +8,7 @@ import {
   dateKey,
   localTimeString,
   normalizeNWS,
+  mergeFallbackData,
 } from '../src/lib/api/nws.js'
 
 const FIX = (name) => JSON.parse(readFileSync(path.join('tests/fixtures/nws', name), 'utf8'))
@@ -103,3 +104,26 @@ describe('normalizeNWS (recorded live fixture for Rogers County, OK)', () => {
     expect(m.current.wind_speed_10m).toBeCloseTo(10 * 1.609344, 1)
   })
 })
+
+describe('mergeFallbackData', () => {
+  it('backfills pressure + UV from Open-Meteo and tags them (non-selected source)', () => {
+    const nws = {
+      _source: 'nws',
+      current: { temperature_2m: 103, pressure_msl: null, uv_index: null },
+      daily: { time: ['2026-08-16'], uv_index_max: [] },
+    }
+    const om = { current: { pressure_msl: 1011.2, uv_index: 7.4 }, daily: { uv_index_max: [7.2, 6, 5] } }
+    const r = mergeFallbackData(nws, om)
+    expect(r.current.pressure_msl).toBe(1011.2)
+    expect(r.current.uv_index).toBe(7.4)
+    expect(r.daily.uv_index_max).toEqual([7.2, 6, 5])
+    expect(r._fallback).toMatchObject({ pressure: true, uv: true })
+  })
+
+  it('does not tag when Open-Meteo lacks the fields', () => {
+    const nws = { current: { pressure_msl: null, uv_index: null }, daily: {} }
+    const om = { current: { pressure_msl: null, uv_index: null }, daily: {} }
+    const r = mergeFallbackData(nws, om)
+    expect(r._fallback).toBeUndefined()
+  })
+})