536
FXUS66 KSTO 041941
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
1241 PM PDT Thu Sep 4 2025

.DISCUSSION...

.SYNOPSIS...
A progressive weather system brings thunderstorm chances today
mainly in the mountains. Temperatures continue a cooling trend,
with widespread Minor HeatRisk the rest of the week. Smoke from
new fire starts will impact local air quality. Pattern change
expected early next week with cooler temperatures and
precipitation chances through at least mid-week.

&&

.KEY MESSAGES...

* Today and Friday:
 - High temperatures continue on a gradual cooling trend with
   widespread Minor HeatRisk today and tomorrow
 - Thunderstorm chances today and tomorrow have trended lower with
   this afternoon`s forecast package. Chances are around 5-10%
   along the Sierra crest, best chances south of HWY-50. Friday
   t-storm chances south of HWY-50 around 5%; low confidence on
   any storm development.
 - Smoke from the recent fire starts will continue to impact air
   quality around the region. Visit airnow.gov to check the Air
   Quality Index for your area.

* Saturday and Sunday:
 - Mostly calm weather expected on Saturday, as upper level trough
   moves out of the area. High temperatures will continue to cool
   into the mid to upper 80s, with mostly mid 80s for the Valley
   by Sunday.
 - Upper level low develops in PacNW by Sunday, forecast to remain
   slightly offshore and help enhance onshore flow, with periods
   of breezy winds in the Delta and vicinity
 - Sunday evening shower chances in the Coastal Range and northern
   Sacramento Valley. Daytime humidity values surge into the mid
   30s to mid 40s across the Valley.

* Monday - Wednesday:
 - Precipitation chances continue on Monday - Wednesday, as upper
   level trough is progged to move inland over NorCal.
- Monday precipitation chances highest in the northern
          Sacramento Valley
        - Tuesday precipitation chances are mainly north of
          Sacramento County and in the higher elevations
- Forecast will likely change as ensembles come into better
          agreement
 - Uncertainty exists on precipitation forecast, as well as any
   convection developing
- Ensembles confident on overall upper level low development
        - Available moisture will be the main forecast challenge
 - Currently, mostly light rain showers with a few embedded
   rumbles of thunder seem most likely; the NBM has backed off on
   total QPF Monday - Wednesday, so a great deal of uncertainty
   exists. Areas in the northern Sacramento Valley, southern
   Cascades, Sierra, and Coast Range stand the best chances to see
   some measurable precipitation/thunder at this time.
- Early NBM probs for 0.25" of rain around 30-50% for the
          aforementioned areas, Monday - Wednesday

.Changes from previous forecast...
- Thunderstorm chances for today and Friday have decreased
- Precipitation amounts and areal coverage of next week`s weather
  system have decreased, much uncertainty remains on track,
  moisture, and potential impacts.

&&

.AVIATION...

VFR conditions are expected over the next 24 hours. General afternoon
winds will be 10-15 kts across the Sacramento Valley with gusts
up to 15-25 kts, strongest in the Delta until 03z Fri with
lingering gusts through the Delta through 12z Fri. Slight chance
for isolated showers over the mountains until 03z Fri.

&&

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

$$