Weathercast

Weathercast is a rain-only nowcasting app for iOS, Android, and web. It answers one question for a selected location: when will rain reach me during the next 120 minutes?

The client is intentionally honest about data quality. It shows timing, intensity, duration, confidence, freshness, and a regional coverage tier. It does not claim street-level precision when only numerical guidance is available.

Product surface

• Native Expo Router navigation with Now, Radar, Places, and Accuracy tabs
• Foreground location or manual place search; no account or background tracking
• 15-minute rain timeline, onset window, event duration, intensity, and confidence
• Offline display of the most recent successful nowcast
• Local rain alerts in development and production builds
• Licensed radar tile integration point with an explicit unavailable state
• Accessible light/dark UI adapted to iOS, Android, tablet, and web
• Provider-independent /v1/nowcast contract for a calibrated production backend
• Owned Bun /v1/nowcast service with rate limits, request validation, single-flight deduplication, and an immutable forecast archive
• Prospective radar-study runner with frozen cohorts, scheduled batch issuance, and reproducible source provenance
• Leakage-safe isotonic calibration with immutable train/validation/evaluation partitions and paired holdout scoring

Run locally

Requirements: Bun, Xcode for iOS, and Android Studio for Android.

bun install
bun start

Expo Go can exercise forecast and navigation flows. Android notifications require a development build:

bunx expo run:android
bunx expo run:ios

Configuration

Copy .env.example to .env.local when using owned services.

Variable	Purpose
EXPO_PUBLIC_NOWCAST_API_URL	Base URL for the production nowcast API
EXPO_PUBLIC_RADAR_MANIFEST_URL	Licensed radar-frame manifest URL
EXPO_PUBLIC_GOOGLE_MAPS_API_KEY	Android-restricted Google Maps SDK key
EXPO_PUBLIC_OPEN_METEO_HOST	Evaluation fallback host override

If no nowcast API is configured, the app uses Open-Meteo's 15-minute numerical forecast as a Standard coverage evaluation fallback. That path is not radar nowcasting and its free/open-access terms are not a substitute for a commercial data agreement. Production distribution must configure an owned backend and licensed providers. Android safely shows a no-map state when no Maps key is configured instead of initializing the native Google Maps SDK.

The production EAS profile fails closed when the nowcast API, radar manifest, or restricted Android Maps key is missing. Preview and development profiles retain honest fallback states for engineering work.

Run the owned API locally with bun run api. Its default Open-Meteo adapter is evaluation-only, always returns Standard/uncalibrated coverage, and is rejected when NODE_ENV=production. Production requires the licensed normalized-upstream adapter described in server/README.md.

Quality gates

bun run check
bunx expo-doctor
bun run export

Tests cover rain-event extraction, confidence behavior, alert decisions, persisted preferences, API validation/rate limiting, exact 120-minute contracts, immutable archive-before-serve behavior, conditional requests, and 50-way request deduplication. See release checklist for store and operational gates.

Architecture

The mobile app never contains provider credentials. A production deployment uses:

radar / satellite / stations / NWP
                  ↓
        ingestion + immutable archive
                  ↓
       QC + ensemble nowcast + calibration
                  ↓
          Weathercast point API / tile CDN
                  ↓
             Expo mobile client

Read architecture, API contract, and data sourcing.

Accuracy policy

“Most accurate” is a measured result, not UI copy. Every issued forecast must be archived and verified against independent observations. Public comparisons require a defined region, period, horizon, sample size, and metric. The client therefore shows no fabricated benchmark or uncalibrated numeric confidence score.

The backend preregisters held-out radar studies and creates immutable per-horizon reliability reports. Each schema-v3 study freezes its input-frame count and ensemble size; issuance loads those values from the archive, and runtime drift is rejected. A report cannot become publication-eligible before the fixed study window ends, below 95% complete cohort issuance, below any registered sample gate, or without preregistered runtime parameters. Calibration uses disjoint prospective training, validation, and untouched evaluation studies. A versioned isotonic artifact must improve validation Brier, bind before evaluation begins, retain raw issuance-time probabilities, and improve paired Brier on the final holdout before its model-evidence promotion gate opens. This evidence scope evaluates Weathercast itself; a claim that Weathercast beats another provider still requires a separately lawful, preregistered paired comparison.

License

MIT. Weather data and map layers retain their own licenses and attribution requirements.