012
FXUS66 KSTO 202017
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
117 PM PDT Fri Jun 20 2025

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

.DISCUSSION...

.SYNOPSIS...
Seasonably cool high temperatures today, along with enhanced
onshore flow leading to breezy winds across the region. Elevated
fire weather conditions continue today across the Sacramento
Valley. Area of low pressure in the PacNW expected to move inland
today, which will introduce northerly winds and low daytime
humidity values Saturday morning through Sunday morning. The Fire
Weather Watch has been upgraded to a Red Flag Warning beginning
Saturday 11 AM through 9 AM Sunday.

A shortwave trough will eject from the parent low Sunday
night/Monday morning, and move through the area on Monday. A
slight chance (5-10%) of an afternoon shower primarily in the
Sierra and Southern Cascades is possible Monday. Warming trend
begins on Monday continuing into midweek, with seasonable high
temperatures in the 90s across the Valley.

&&

.KEY MESSAGES...

* Today:

 -Breezy onshore flow and low humidity values will lead to periods
  of elevated fire weather conditions today, mainly across the
  northern Sacramento Valley and northern San Joaquin Valley.

 -Seasonably cool high temperatures expected, with current
  temperatures reading in the mid to upper 70s across the Valley
  floor.

 -Please continue to practice fire safety such as, properly
  extinguishing any campfires, not driving over dry grass,
  checking your vehicle for chains or hanging objects that may
  cause sparks.

* Saturday and Sunday:

 -Predominately dry weather system in the PacNW moves onshore in
  WA and OR. This system will be responsible for a wind pattern
  change from onshore, to north winds. Minimum daytime humidity
  values will fall in response to wind change across the Valley,
  with 15-25% RH values expected.

 -The Fire Weather Watch has been upgraded to a Red Flag Warning,
  which will begin 11 AM Saturday morning and continue through 9
  AM Sunday morning. Highest area of concern: along and west of
  the I-5 corridor in the northern and central Sacramento Valley,
  and the northern San Joaquin Valley. North winds of 10-20 mph,
  with gusts 30-40 mph; strongest gusts Saturday
  afternoon/evening. Daytime RH values around 15-25%.

 -Winds weaken and return to onshore/diurnal pattern late Sunday
  night into Monday morning.

 -Probability of wind gusts > 30 mph:
*70-100% Saturday across the Northern/Central Sacramento Valley
     and northern San Joaquin Valley.
*50-70% Sunday across the Sacramento Valley and Delta.

* Next Week:

 -A shortwave trough is projected to eject from the parent low on
  Monday, and likely move through the mountains throughout the
  day. There is a non-zero chance of an isolated afternoon shower
  in the Sierra and southern Cascades, however no impacts are
  expected. While t-storm chances remain low ( less than 10%),
  there could be enough forcing from the shortwave and help from
  topographic influences to get an isolated t-storm fired up
  Monday afternoon/evening. Best chances currently reside south of
  HWY-50 in the Sierra.

 -Dry conditions with a warming trend as we move into the middle
  of next week. Broad zonal flow will allow onshore winds to
  continue in the Delta and adjacent areas. Stagnate flow
  elsewhere will allow temperatures to begin to climb into the
  upper 90s by Tuesday in the northern Sacramento Valley. Minor
  HeatRisk is expected at this time, thanks to Delta Breeze
  helping improve overnight relief with low temps in the mid 50s
  to mid 60s in the Valley/Delta.

 -Clusters Analysis relatively confident in broad zonal flow
  continuing into next weekend.

.Changes from previous forecast...
 -Fire Weather Watch has been upgraded to a Red Flag Warning. Fire
  Zones 220 and 266 were removed from the Watch/Warning, however
  elevated fire weather conditions will persist through Sunday in
  those zones.

&&

.AVIATION...

VFR conditions continue at TAF sites over the next 24 hours.
Gusty west to southwest wind gusts around 15-30 kts across
interior NorCal until around 00z Friday - 06z Saturday. Gusty
north to northwest 10-20 kts with gusts 20-35 kts develop after
03z-06z Saturday. Strongest winds are expected through the Delta
and vicinity and over the mountains today, then across the Valley
and Coastal Range on Saturday.
&&

.STO WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from 11 AM Saturday to 9 AM PDT Sunday for
Carquinez Strait and Delta-Central Sacramento Valley in Glenn,
Colusa, Yuba, Northern Sutter, and Butte County Below 1000 Ft-
Eastern Mendocino NF-Northern Sacramento Valley to Southern
Tehama County Line Below 1000 Ft-Northern San Joaquin Valley in
San Joaquin and Stanislaus Counties Below 1000 ft-Southeast Edge
Shasta-Trinity NF and Western Portions of Tehama-Glenn Unit-
Southern Sacramento Valley in Yolo-Sacramento Far Western
Placer, southern Sutter and Solano County Below 1000 Ft.

&&

$$