135
FXUS66 KSTO 172103
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
203 PM PDT Sat May 17 2025

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

.Discussion...

.Synopsis...
Breezy and mostly dry conditions continue today for most of the
area. An isolated thunderstorm is possible over the mountains and
foothills of the Sierra mainly north and along I-80 through late
this evening. A few observations of lightning from early morning
and afternoon storms have been observed along the foothills of
Plumas County. The Fire Weather Watch has been upgraded to a Red
Flag Warning in effect from 8 AM PDT Sunday - 8 PM PDT Monday, as
a weather system will follow a classic inside slider pattern and
bring gusty north winds and low daytime relative humidity values
to the Valley. Fine fuels are dry, and in combination with the
dry north winds and low RH values, are the biggest threats for
fire starts and rapid growth.

Dry and hot conditions will follow on Tuesday with areas of
Moderate HeatRisk in the Valley and elevated fire weather
conditions due to continued drying and low RH values. Upper level
ridging looks to build in the Pacific Ocean until at least next
Friday, when heights will flatten slightly and lower high
temperatures slightly into the low 90s with Minor HeatRisk.

&&

.Key Messages...

* Red Flag Warning in effect from 8 am PDT Sunday through 8 pm
  PDT Monday for Valley and Delta locations below 1000 feet.

* Today:
 -Weather system begins to drop southward from Pacific NW.
 -10-25% probability of isolated thunderstorms across northeast
  foothills and Sierra/southern Cascades from Interstate 80
  northward. Best chances for t-storm development through this
  afternoon, chances diminish into the late evening hours.
 -Little to no rainfall anticipated outside of any thunderstorms
  that develop, as most energy with this system is in the Great
  Basin region. Little to no snow expected either, some light
  flurries above 7500 feet south of US-50.
 -Temperatures cooling to near normal, with breezy winds
  throughout the day.

* Sunday - Monday:
 -Current ensemble trends indicate the expected weather system
  following an inside slider trajectory through the region.
 -Gusty north to east winds expected to develop Sunday and Monday
  as this system progresses, with lingering breezy winds into
  Tuesday.
 -Probability of gusts 35 mph or higher:
 --Sunday: 40-70%, highest along and west of I-5 in the
  Sacramento Valley.
 --Monday: 20-40% across the central/southern Sacramento Valley
  and Delta.
 -Critical fire weather conditions expected Sunday and Monday, due
  to the combination of gusty northerly winds and low relative
  humidity in the central/southern Sacramento Valley, Delta, and
  northern San Joaquin Valley. Highest threat area(s): Central and
  Southern Sacramento Valley along and west of the I-5 corridor,
  and the Northern San Joaquin Valley. Winds have trended slightly
  lower and RH values slightly higher in the northern Sacramento
  Valley, however critical fire weather conditions are still
  expected.

* Tuesday - Thursday:
 -Warming trend expected early to mid next week, with areas of
  Moderate HeatRisk across the Valley.
 -North to east winds expected to remain diurnally breezy while
  very dry conditions prevail.
 -Elevated fire weather conditions anticipated.

.Changes from previous forecast...
 -Fire Weather Watch was upgraded to a Red Flag Warning.


&&
.AVIATION...

VFR conditions expected to prevail over Valley TAF sites next 24
hours. Northwest to west wind gusts 15-25 kts across portions of
the Valley, Delta, and mountains until 06z Sunday. Northerly wind
gusts 15-30 kts across the Valley and Delta 15z Sunday through
00z Tuesday. Isolated showers and thunderstorms possible over the
Sierra and southern Cascades until 00z Sunday.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from 8 AM Sunday to 8 PM PDT Monday for
Carquinez Strait and Delta-Central Sacramento Valley in Glenn,
Colusa, Yuba, Northern Sutter, and Butte County Below 1000 Ft-
Northern Sacramento Valley to Southern Tehama County Line Below
1000 Ft-Northern San Joaquin Valley in San Joaquin and
Stanislaus Counties Below 1000 ft-Southern Sacramento Valley in
Yolo-Sacramento Far Western Placer, southern Sutter and Solano
County Below 1000 Ft.

&&

$$