Implement derived forecast modules

internal/forecast/derive.go

@@ -65,6 +65,38 @@ type AlertOverlap struct {
 	Description string          `json:"description,omitempty"`
 }
 
+type PrecipTiming struct {
+	MaxPrecipitationProbability *TimedValue `json:"maxPrecipitationProbability,omitempty"`
+	FirstPrecipitation          *TimedValue `json:"firstPrecipitation,omitempty"`
+	LastPrecipitation           *TimedValue `json:"lastPrecipitation,omitempty"`
+	ThunderMentioned            bool        `json:"thunderMentioned,omitempty"`
+}
+
+func BuildPrecipTiming(periods []weatherdata.ForecastPeriod) PrecipTiming {
+	var timing PrecipTiming
+	for _, forecastPeriod := range periods {
+		setMaxTimedValue(&timing.MaxPrecipitationProbability, forecastPeriod.ProbabilityOfPrecipitationPercent, forecastPeriod.StartTime)
+		if forecastPeriod.ProbabilityOfPrecipitationPercent != nil && *forecastPeriod.ProbabilityOfPrecipitationPercent > 0 {
+			value := TimedValue{
+				Value: *forecastPeriod.ProbabilityOfPrecipitationPercent,
+				Time:  forecastPeriod.StartTime,
+			}
+			if timing.FirstPrecipitation == nil || value.Time.Before(timing.FirstPrecipitation.Time) {
+				copied := value
+				timing.FirstPrecipitation = &copied
+			}
+			if timing.LastPrecipitation == nil || value.Time.After(timing.LastPrecipitation.Time) {
+				copied := value
+				timing.LastPrecipitation = &copied
+			}
+		}
+		if mentionsThunder(forecastPeriod.TextDescription) {
+			timing.ThunderMentioned = true
+		}
+	}
+	return timing
+}
+
 func BuildDailySummary(bundle *weatherdata.Bundle, date time.Time, location *time.Location, dayparts []DaypartDefinition) (*DailySummary, error) {
 	if bundle == nil {
 		return nil, fmt.Errorf("forecast bundle is required")
@@ -298,6 +330,10 @@ func indicatorsForText(text string) Indicators {
 	}
 }
 
+func mentionsThunder(text string) bool {
+	return strings.Contains(strings.ToLower(text), "thunder")
+}
+
 func numericIndicators(period weatherdata.ForecastPeriod) Indicators {
 	windGust := firstValue(period.WindGustMph, period.WindGustKmh)
 	windSpeed := firstValue(period.WindSpeedMph, period.WindSpeedKmh)

internal/forecast/derive_test.go

@@ -210,6 +210,47 @@ func TestAlertOverlap(t *testing.T) {
 	}
 }
 
+func TestBuildPrecipTimingTracksRainAndThunder(t *testing.T) {
+	location := time.FixedZone("Test", -5*60*60)
+	periods := []weatherdata.ForecastPeriod{
+		hour(location, "2026-05-29T08:00:00-05:00", "2026-05-29T09:00:00-05:00", "Cloudy", 70, nil, ptr(0), nil, nil),
+		hour(location, "2026-05-29T09:00:00-05:00", "2026-05-29T10:00:00-05:00", "Showers", 70, nil, ptr(30), nil, nil),
+		hour(location, "2026-05-29T12:00:00-05:00", "2026-05-29T13:00:00-05:00", "Thunderstorms", 70, nil, ptr(80), nil, nil),
+		hour(location, "2026-05-29T18:00:00-05:00", "2026-05-29T19:00:00-05:00", "Dry", 70, nil, ptr(0), nil, nil),
+	}
+
+	timing := BuildPrecipTiming(periods)
+
+	if timing.FirstPrecipitation == nil || timing.FirstPrecipitation.Time.Format(time.RFC3339) != "2026-05-29T09:00:00-05:00" {
+		t.Fatalf("FirstPrecipitation = %#v, want 9 AM shower", timing.FirstPrecipitation)
+	}
+	if timing.LastPrecipitation == nil || timing.LastPrecipitation.Time.Format(time.RFC3339) != "2026-05-29T12:00:00-05:00" {
+		t.Fatalf("LastPrecipitation = %#v, want noon thunderstorm", timing.LastPrecipitation)
+	}
+	if timing.MaxPrecipitationProbability == nil || timing.MaxPrecipitationProbability.Value != 80 {
+		t.Fatalf("MaxPrecipitationProbability = %#v, want 80", timing.MaxPrecipitationProbability)
+	}
+	if !timing.ThunderMentioned {
+		t.Fatal("ThunderMentioned = false, want true")
+	}
+}
+
+func TestBuildPrecipTimingHandlesDryForecast(t *testing.T) {
+	location := time.FixedZone("Test", -5*60*60)
+	periods := []weatherdata.ForecastPeriod{
+		hour(location, "2026-05-29T08:00:00-05:00", "2026-05-29T09:00:00-05:00", "Sunny", 70, nil, ptr(0), nil, nil),
+	}
+
+	timing := BuildPrecipTiming(periods)
+
+	if timing.FirstPrecipitation != nil || timing.LastPrecipitation != nil || timing.ThunderMentioned {
+		t.Fatalf("dry timing = %#v, want no precip timing and no thunder", timing)
+	}
+	if timing.MaxPrecipitationProbability == nil || timing.MaxPrecipitationProbability.Value != 0 {
+		t.Fatalf("dry max precip = %#v, want checked zero chance", timing.MaxPrecipitationProbability)
+	}
+}
+
 func TestThresholdHelpers(t *testing.T) {
 	if !DifferenceAtLeast(50, 56, 5) {
 		t.Fatal("DifferenceAtLeast = false, want true")