842
FXUS66 KSTO 232031
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
131 PM PDT Sat Aug 23 2025

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

.DISCUSSION...

.SYNOPSIS...
A Heat Advisory remains in effect for the Delta, Valley, and
adjacent foothills until 11 pm this evening. Elevated fire weather
conditions continue to accompany the hot and dry conditions
today, with near daily mountain thunderstorm chances persisting
into early next week. A gradual cooling trend begins by Sunday
with near normal temperatures returning by late next week.

&&

.KEY MESSAGES...

* Today - Sunday:
 - Heat Advisory remains in effect for the Coastal Range, Delta,
   Valley, and adjacent foothills through 11 pm PDT this evening.
 - High temperatures in the upper 90s to triple digits expected
   through the Valley/foothills today, with a gradual cool down
   beginning on Sunday.
 - Widespread Moderate HeatRisk and areas of Major HeatRisk.
 - Probability of Valley/foothills high temperatures > 100F: 60-95%

 - Elevated fire weather conditions due to isolated mountain
   thunderstorm chances along the Sierra this weekend. Hot and dry
   weather with low daytime humidity in the low teens to 20s and
   limited overnight recovery will add to elevated fire weather
   concerns elsewhere.
 - Monsoonal moisture advection will bring isolated to scattered
   thunderstorm chances to the region this weekend.
   --Highest chances along Sierra from I-80 southward this
    afternoon/evening, expanding northward to include the entire
    Sierra/southern Cascades extent on Sunday.
   --Slight chance of nocturnal thunderstorms along Sierra south
    of I-80
 - Probability of mountain thunderstorms:
   --Today: 15-25%
   --Sunday AM: 10-15%
   --Sunday Afternoon/Evening: 15-30%

* Next Week:
 - Gradual cooling trend continues into the week ahead 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 to scattered 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% on Monday,
   decreasing to 10-20% by Tuesday.

.Changes from previous forecast...
- Increasing chances for isolated nocturnal thunderstorm
  development south of I-80 from 3 am Sunday - 8 am Sunday.

&&

.AVIATION...
VFR conditions expected at TAF sites across interior NorCal next
24 hours. MVFR/IFR conditions possible in isolated to scattered
mountain showers/thunderstorms along the Sierra from I-80
southward after 21z. Surface winds generally 12 kts or less,
although southwest gusts to 20 kts through Delta and vicinity
after 00z.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 11 PM PDT this evening 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.

&&

$$