876
FXUS66 KSTO 192222
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
322 PM PDT Thu Sep 19 2024


.Synopsis...
A cool pattern is over interior NorCal today, brining gusty winds
to the Delta, along with rain showers and a chance for isolated
thunderstorms over the Sierra (primarily south of I-80). Drier
weather starts tomorrow with an overall warming trend beginning.
Increased high temperatures with Moderate HeatRisk and elevated
fire weather concerns return to the Valley and foothills early
next week.

&&

.Discussion...
An upper lverl low off the coast is bringing in moisture from
east to west over the Sierra and foothills, bringing one last day
of showers and thunderstorms to that area this afternoon and
evening. While a few showers and thunderstorms have developed over
western Plumas County this afternoon, most thunderstorm
development has been south of I-80. A few light showers may reach
the Park Fire burn scar, but no debris flow impacts are expected
today. Remnants of Sierra storms have the potential to bring light
showers/virga to the far eastern portions of the Valley,
particularly the eastern portions of the northern San Joaquin
Valley. Dry conditions are expected elsewhere. The center of the
low pressure system will gradually move in Southern California
Friday, with dry weather returning to interior Northern
California.

High pressure begins to build in Friday as eastern Pacific ridging
builds eastward. Temperatures begin to warm to near normal by
Saturday, with highs above normal on Sunday. The NBM shows a 60 to
90% chance of high temperatures greater than 90 deg F on Friday
over the central and northern Sacramento Valley. Probabilities
extend into the Delta, southern Sacramento Valley, and northern
San Joaquin by Saturday and Sunday. This warming trend continues
into early next week, accompanied by a drying trend.

As the previous system propagates eastward, a steepening pressure
gradient develops as ridging begins to build over Northern
California. As a result, periodically breezy north winds will
permeate throughout the Valley and easterly/downslope winds over
the Sierra over the weekend and into early next week. Gusts will
generally be 15-20 mph with daytime relative humidities will be
in the mid teens to mid 20s, initially over the northern
Sacramento Valley and adjacent foothills Saturday-Sunday
resulting in localized elevated fire weather concerns.

&&


.EXTENDED DISCUSSION (Monday THROUGH Thursday)...

Early next week, we`re expecting drier and warmer conditions as
ridging builds in over the west coast; we`ll also see a return of
Moderate HeatRisk to much of the Valley and foothills. NBM is
currently showing a 60 to 85% probability of temperatures over 95
in the Valley on Monday and Tuesday, and 20 to 40% of
temperatures above 100 (though the risk of this is limited to the
northern San Joaquin Valley and Sacramento Metropolitan area). In
addition to the warm temperatures on Monday, with the positioning
of the ridge just over an upper low to the south, the resulting
pressure gradient will bring breezy north winds to parts of the
Sacramento Valley (primarily along the I-5 corridor and in the
northern Sacramento Valley). Gusts are currently expected to fall
around 15 to 25 mph. Tuesday, as the upper level pattern continues
to progress, breezy winds will shift to the Sierra and foothills,
with easterly downslope winds gusting around 20 to 30 mph. These
winds combined with lower relative humidities in the teens will
bring elevated fire weather conditions early in the week.

Midweek is expected to cool a couple degrees as weak troughing
begins to develop offshore, though remaining above normal for the
time of year. NBM currently has a decent amount of spread in
possible temperatures on Wednesday and Thursday, due in large
part to uncertainty within ensembles and clusters about the timing
of the trough moving into the area. That said, the NBM is
projecting a 25-50% probability of temperatures below 90 degrees
on Wednesday and Thursday in the Valley, which would put Valley
locations back near climatological normal.
&&

.AVIATION...
Exiting weather system will keep MVFR/IFR conditions
along the foothills and Sierra/Nevada Mountains near showers and
thunderstorms for the next 24 hours. Gusty and erratic winds near
storms should be expected. Elsewhere VFR conditions with variable
winds 12kts or less is expected.

&&

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

$$