FXUS66 KSTO 222036
AFDSTO
Area Forecast Discussion
National Weather Service Sacramento CA
136 PM PDT Fri Aug 22 2025
SYNOPSIS...
Triple digit high temperatures and limited overnight relief
culminate in widespread Moderate HeatRisk and areas of Major
HeatRisk today and Saturday. As a result, a Heat Advisory remains
in effect for the Coastal Range, Delta, Valley, and adjacent
foothills until 11 pm PDT Saturday night. Elevated fire weather
conditions accompany the hot and dry conditions today and
Saturday, with near daily mountain thunderstorm chances
anticipated today into early next week. A gradual cooling trend
begins by Sunday as near normal temperatures return by late next
week.
.KEY MESSAGES...
* Today - Saturday:
- Heat Advisory remains in effect for the Coastal Range, Delta,
Valley, and adjacent foothills through 11 pm PDT Saturday
night.
- High temperatures in the upper 90s to triple digits expected
through the Valley/foothills, with limited overnight relief.
- Probability of Valley/foothills high temperatures > 100F:
65-100%
- Widespread Moderate HeatRisk and areas of Major HeatRisk.
- Elevated fire weather conditions due to isolated mountain
thunderstorm chances along the Sierra, as well as hot and dry
weather elsewhere, with low daytime humidity in the low teens
to 20s and limited overnight recovery.
- Monsoonal moisture advection will bring chances of isolated
afternoon/evening thunderstorms to the Sierra crest from I-80
southward.
- Probability of mountain thunderstorm development: 10-20%
* Sunday - Next Week:
- Gradual cooling trend from Sunday onward as Four Corners
ridging breaks down and weak offshore troughing begins to
develop.
- Widespread Moderate HeatRisk through Tuesday, becoming
primarily Minor HeatRisk from midweek onward.
- Isolated mountain thunderstorm chances spread northward to
include the Sierra/southern Cascades and Shasta County
mountains as monsoonal moisture advection continues.
- Highest mountain thunderstorm chances around 15-30% expected
Sunday and Monday, decreasing to 10-20% through midweek.
.Changes from previous forecast...
- Highest mountain thunderstorm potential expected Sunday and
Monday, although impacts from dry lightning outside of
precipitation cores possible today and Saturday as well.
- A more gradual cooling trend is expected into next week with
areas of Moderate HeatRisk lingering through Tuesday.
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