Preserve metric forecast units and overnight alerts

docs/internal/facts.md

@@ -37,6 +37,8 @@ Report identity controls the summary shape:
 `DaypartSummaries` is collected from the resulting daily summaries.
 The detailed grouping, daypart-window, and alert rules are owned by
 [forecast derivation](forecast-derivation.md).
+Daily alert overlaps remain scoped to the civil day, while overnight daypart
+summaries retain alerts that overlap their complete next-day window.
 
 ## Missing data and failures
 

docs/internal/forecast-derivation.md

@@ -33,9 +33,17 @@ sorts periods, records the maximum and first precipitation, groups contiguous
 periods at or above its package-owned probability threshold, and records
 thunder mentions.
 
+Daypart temperature and apparent-temperature ranges are Fahrenheit values, and
+timed wind maxima are mph values. When only metric source fields are present,
+they are converted to those units before they are stored or evaluated against
+indicator thresholds; populated US-customary fields take precedence.
+
 Alert overlap parsing supports the normalized alert payload's available timing
 fields. Unparseable alerts and invalid intervals are ignored; valid overlaps
 are clipped to the requested period and ordered by alert start time.
+`DailySummary.AlertOverlaps` is limited to the local civil day, while each
+daypart evaluates the full alert run against its complete window, including the
+next-day portion of an overnight window.
 
 ## Missing data and failures