Area Forecast Discussion
Issued by NWS Sacramento, CA

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254
FXUS66 KSTO 192138
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
238 PM PDT Tue Aug 19 2025

For additional details on weather and expected impacts over the
next 7 days, please visit weather.gov/sto/briefing.

.DISCUSSION...

.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.

&&

.AVIATION...
VFR conditions next 24 hours across interior NorCal. Sustained
surface winds generally at or below 12 kts, except in the Delta
with west-southwest surface winds 12-15 kts with gusts 15 to 20
kts through 18Z Wednesday.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from 11 AM Friday to 11 PM PDT Saturday for
Carquinez Strait and Delta-Central Sacramento Valley-Mountains
Southwestern Shasta County to Western Colusa County-Northern
Sacramento Valley-Northern San Joaquin Valley-Shasta Lake Area /
Northern Shasta County-Southern Sacramento Valley.

&&

$$