FXUS66 KSTO 192138
AFDSTO
Area Forecast Discussion
National Weather Service Sacramento CA
238 PM PDT Tue Aug 19 2025
SYNOPSIS...
A gradual warming trend through the week is expected to peak late
this week, with above normal temperatures accompanied by
widespread Moderate HeatRisk and areas of Major HeatRisk from
Thursday through early next week. Onshore flow expected to
persist, although generally lighter mid to late week, then
becoming breezier by the weekend. Isolated mountain thunderstorm
chances increase Friday-Monday, mainly along the Sierra from I-80
southward as monsoonal moisture aloft approaches the region.
.KEY MESSAGES...
* Today - Tuesday:
- Near to below normal temperatures and Minor HeatRisk.
- Afternoon highs the mid 80s to low 90 for much of the Valley
and foothills.
- Periodically breezy onshore flow is forecast at times,
strongest in the Delta vicinity and over the Sierra Crest.
* Wednesday - Weekend:
- Slightly above normal temperatures Wednesday, trend well above
normal Thursday into the weekend as high pressure builds back
in over the Western US.
- 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 Thu-
Sat: 60-95%
- Widespread Moderate HeatRisk and areas of Major HeatRisk
expected Thursday-Saturday, followed by a gradual cooling trend
late in the weekend.
- A Heat Advisory has been issued for the Valley, Delta,
foothills and Coast Range from 11 AM Friday through 11 PM
Saturday.
- Dry weather with low daytime humidity in the teens to 20s and
limited overnight recovery anticipated mid to late week. This
will bring periods of elevated fire weather conditions.
- Monsoonal moisture advection will bring a slight chance of
afternoon/evening thunderstorms to the Sierra crest south of
I-80 Friday into the weekend. Thunderstorm chances expand
northward Saturday and Sunday.
* Early Next Week:
- Slight cooling trend is expected on Monday, but widespread
Moderate HeatRisk still persists across the Valley and
foothills.
- Isolated mountain thunderstorm chances (10-25%) over the Sierra
through early next week.
.Changes from previous forecast...
- Heat Advisory issued for the Valley, foothills, Delta, and
Coast Range from 11 AM Friday through 11 PM Saturday.
- Widespread Moderate HeatRisk lingers into Monday.
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