Weight NWS weather-code mapping by precipitation probability

NWS text like 'Slight Chance Showers And Thunderstorms' (~17-24%) was mapping
straight to WMO 95 / thunderstorm icon, over-stating a mere chance of storms.
shortForecastToWmo now takes the period's precip probability (plus chance-like
wording) and downgrades low-chance storms to light showers (80) and low-chance
rain to light rain (61), keeping real/severe storms at 95. This also stops the
false thunderstorm alert banner for low-chance days. Today/Monday for Rogers
County now render as showers instead of storms.

src/lib/api/nws.js

@@ -57,14 +57,33 @@ export function parseWind(s) {
   return Math.max(...nums.map(Number))
 }
 
-/** Map an NWS shortForecast string to a WMO code for icons/severity. */
+/**
-export function shortForecastToWmo(text) {
+ * Map an NWS shortForecast string to a WMO code for icons/severity.
+ * @param {string} text - NWS shortForecast text, e.g. "Slight Chance Showers And Thunderstorms"
+ * @param {number|null} prob - precipitation probability (0-100) for that period,
+ *   so low-chance wording does not render as a full weather condition.
+ */
+export function shortForecastToWmo(text, prob = null) {
   const t = (text || '').toLowerCase()
-  if (/thunderstorm|tstm|severe thunder/.test(t)) return /\bsevere\b/.test(t) ? 96 : 95
+  const chanceLike = () =>
-  if (/freez|x-ice|ice pellets/.test(t) && /rain|drizzle|shower/.test(t)) return 66
+    (prob != null && prob < 30) ||
-  if (/snow|blizzard|flurr|sleet|snow shower/.test(t)) return /heavy|blizzard/.test(t) ? 75 : 71
+    /\bslight chance\b|chance of|scattered|isolated|fewer chance|\bchance\b/.test(t)
-  if (/freez|ice/.test(t)) return 66
+
-  if (/rain|shower|drizzle|sprinkl/.test(t)) return /heavy|strong|torrential/.test(t) ? 65 : /slight|isolated|scattered/.test(t) ? 61 : 63
+  if (/thunderstorm|tstm/.test(t)) {
+    // An actual severe/thunderstorm is a real condition; a mere *chance* of
+    // one is just light showers until something organises.
+    const severe = /\bsevere\b|\bstrong\b|warning|watch/.test(t)
+    if (severe) return 95
+    if (chanceLike()) return 80 // light rain showers, not a full thunderstorm
+    return 95
+  }
+  if (/freez|x-ice|ice pellets|freezing/.test(t) && /rain|drizzle|shower/.test(t)) return 66
+  if (/snow|blizzard|flurr|snow shower/.test(t)) return /heavy|blizzard/.test(t) ? 75 : 71
+  if (/rain|shower|drizzle|sprinkl/.test(t)) {
+    if (/heavy|strong|torrential/.test(t)) return 65
+    if (chanceLike()) return 61
+    return 63
+  }
   if (/fog|haze|smoke|brume/.test(t)) return 45
   if (/partly (cloudy|sunny)/.test(t)) return 2
   if (/overcast|covering|cloudy/.test(t)) return 3
@@ -123,7 +142,7 @@ function buildDaily(periods, tz, units) {
       else d.lows.push(c)
     }
     d.precip.push(prob(pd))
-    d.codes.push(shortForecastToWmo(pd.shortForecast))
+    d.codes.push(shortForecastToWmo(pd.shortForecast, prob(pd)))
     const ws = parseWind(pd.windSpeed)
     if (ws != null) d.winds.push(ws)
   }
@@ -166,7 +185,7 @@ export function normalizeNWS(src) {
       temperature_2m: temp(cur.temperature, units),
       apparent_temperature: apparent,
       relative_humidity_2m: cur.relativeHumidity?.value ?? null,
-      weather_code: shortForecastToWmo(cur.shortForecast),
+      weather_code: shortForecastToWmo(cur.shortForecast, prob(cur)),
       wind_speed_10m: ws != null ? speed(ws, units) : null,
       wind_direction_10m: compassToDeg(cur.windDirection),
       wind_gusts_10m: null,
@@ -182,7 +201,7 @@ export function normalizeNWS(src) {
     time.push(localTimeString(pd.startTime, tz))
     temperature_2m.push(temp(pd.temperature, units))
     precipitation_probability.push(prob(pd))
-    weather_code.push(shortForecastToWmo(pd.shortForecast))
+    weather_code.push(shortForecastToWmo(pd.shortForecast, prob(pd)))
     const ws = parseWind(pd.windSpeed)
     wind_speed_10m.push(ws != null ? speed(ws, units) : null)
   }

tests/nws.test.js

@@ -18,12 +18,26 @@ describe('nws helpers', () => {
     expect(shortForecastToWmo('Mostly Sunny')).toBe(2)
     expect(shortForecastToWmo('Partly Cloudy')).toBe(2)
     expect(shortForecastToWmo('Overcast')).toBe(3)
-    expect(shortForecastToWmo('Slight Chance Showers And Thunderstorms')).toBe(95)
     expect(shortForecastToWmo('Heavy Rain')).toBe(65)
     expect(shortForecastToWmo('Light Snow')).toBe(71)
     expect(shortForecastToWmo('Areas of Fog')).toBe(45)
   })
 
+  it('downgrades low-chance thunderstorms so they are not shown as storms', () => {
+    // The word matters less than the raw probability.
+    // 17% "slight chance" of showers/thunderstorms -> light rain showers, NOT a storm.
+    expect(shortForecastToWmo('Slight Chance Showers And Thunderstorms')).toBe(80)
+    expect(shortForecastToWmo('Slight Chance Showers And Thunderstorms', 17)).toBe(80)
+    expect(shortForecastToWmo('Thunderstorms', 20)).toBe(80)
+    expect(shortForecastToWmo('Isolated Thunderstorms', 20)).toBe(80)
+    // A real storm (wordy + high probability) stays a storm.
+    expect(shortForecastToWmo('Thunderstorms', 70)).toBe(95)
+    expect(shortForecastToWmo('Severe Thunderstorm Warning', 80)).toBe(95)
+    // Low-chance plain rain downgrades to light rain too.
+    expect(shortForecastToWmo('Chance of Rain', 15)).toBe(61)
+    expect(shortForecastToWmo('Showers And Thunderstorms', 50)).toBe(95)
+  })
+
   it('compassToDeg maps compass points to degrees', () => {
     expect(compassToDeg('N')).toBe(0)
     expect(compassToDeg('NE')).toBe(45)
@@ -69,6 +83,10 @@ describe('normalizeNWS (recorded live fixture for Rogers County, OK)', () => {
     expect(Math.max(...data.daily.temperature_2m_max)).toBeLessThan(115)
     expect(data.daily.temperature_2m_min[0]).toBe(75)
     expect(data.daily.precipitation_probability_max[0]).toBe(24)
+    // Today/Monday are only ~17-24% "slight chance" of storms -> light showers.
+    expect(data.daily.weather_code[0]).toBe(80)
+    expect(data.daily.weather_code[1]).toBe(80)
+    expect(data.current.weather_code).toBe(2) // "Mostly Sunny" hour
   })
 
   it('builds a realistic current from the first hourly period', () => {