827
FXUS66 KSTO 241959
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
1259 PM PDT Sun Aug 24 2025

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

.DISCUSSION...

.SYNOPSIS...
Continued summer-like high temperatures today, with widespread
Moderate HeatRisk in the Valley and thermal belts of the
foothills. Afternoon/evening thunderstorm chances in the
mountains through Wednesday from monsoonal moisture surge;
elevated fire weather conditions in the mountains due to t-storm
potential. Gradual cooling temperatures by mid-week with upper
80s to mid 90s across the Valley.

&&

.KEY MESSAGES...

* Today:
 - High temperatures in the upper 90s to triple digits expected
   through the Valley/foothills today. Stronger Delta Breeze
   influence this morning and tonight has/will allow for better
   overnight recoveries, with mostly Moderate HeatRisk across the
   Valley today.
 - Probability of high temperatures > 100F: 40-90%, best chances
   in Redding/Shasta County, 10-20% elsewhere
 - Monsoonal moisture advection will bring isolated to scattered
   thunderstorm chances to the region. Storms are expected to have
   rain reach the ground, however due to recent heat and drying
   fuels, lightning strikes outside precipitation cores may lead
   to new fire starts, prompting elevated fire weather conditions.
 - Probability of mountain thunderstorms:
   --Today: 20-40% across Sierra and Southern Cascades, best
    chances along the crest and east.

* Monday - Wednesday:
 - Gradual cooling trend continues into the week ahead as Four
   Corners ridging breaks down and weak offshore troughing begins
   to develop.
 - Widespread Moderate HeatRisk Monday, becoming primarily Minor
   HeatRisk from Tuesday 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 20-40% on Monday,
   decreasing to 15-35% by Tuesday; with roughly 15-25% chances in
   the northern Sacramento Valley.
 - Continued elevated fire weather concerns from lightning strikes
   away from precipitation cores from thunderstorms

* Rest of the week:
 - Lowering thunderstorm chances and seasonable temperatures,
   with generally Minor HeatRisk
 - High temperatures mainly in the 90s across the Valley and Delta
 - Off-shore troughing looks to remain dominate synoptic feature,
   which will help keep temperatures in check as we move into the
   last weekend of August.

.Changes from previous forecast...
- HeatRisk decreased to mostly Minor on Tuesday. Overnight
  recoveries from enhanced onshore flow will help lower the
  HeatRisk.

&&

.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 through 03z.
Surface winds generally 12 kts or less, although southwest gusts
to 20 kts through Delta and vicinity after 00z.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$