378
FXUS66 KSTO 181928
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
1228 PM PDT Sat May 18 2024

.SYNOPSIS...
Seasonable temperatures will continue through the weekend and into
early next week. Mostly sunny and dry conditions with periods of
breezy winds expected through the next 7 days. Isolated showers
and thunderstorms over high Sierra south of Highway 50 today.

&&

.DISCUSSION...

Morning marine stratus deck still lingering around portions of the
Valley at the time of this writing. Analysis of GOES-18 Satellite
reveals the stratus deck is beginning to erode, albeit slowly, as
we approach the afternoon hours, where it should finally lift out
of the area. Sunny and mostly dry conditions should prevail for
the remainder of the day across interior NorCal, except for a few
isolated showers and possible t-storms over the high Sierra south
of Highway 50. Breezy onshore winds are expected to continue
throughout the day as well, which will help usher in more
seasonably high temperatures for the area.

The best chances (15-30%) of an isolated shower or thunderstorm to
develop this afternoon and into the evening hours exists in
southeastern Tuolumne County. Recent runs of CAMs (Convective
Allowing Models) reveals decent levels of instability, and limited
inhibition over the aforementioned area. Only forcing mechanism
will be the mountains themselves, so main impacts with any
thunderstorm development will be brief heavy rain, lightning,
locally gusty winds, and small hail. Any storms that do develop
should move eastward after forming. Recent National Blend of
Models (NBM) runs have introduced a slight (10-20%) chance of a
stray shower forming over northern Butte County and southeastern
Tehama County later in the late evening hours as well, so we
cannot rule out a stray shower or two in those areas this evening.

Getting into the overall upper level pattern for the area, our
current westerly flow will begin to transition into a
northwesterly component tonight and into tomorrow as a trough to
our north in British Columbia begins to slide southward and phase
with a closed low in SoCal on Monday. This will setup a northerly
component Monday and stronger pressure gradients over the area,
which will result in a breezy northerly wind day for the region.
The NBM is still advertising around a 50-75% probability of wind
gusts exceeding 40 mph Monday in the usual wind prone area in the
western Sacramento Valley, along the I-5 corridor. Minimum
relative humidities (RH) values will fall to below 15% Monday, so
locally elevated fire weather conditions may exist throughout the
morning and evening hours. Tuesday, the pressure gradient will
weaken and allow for northerly winds to weaken slightly, and allow
high temperatures to warm into the mid 80s to low 90s for the
Valley/Delta and upper 60s to low 80s in the foothills/mountains.
&&

.EXTENDED DISCUSSION (Wednesday THROUGH Saturday)...

Wednesday the area will be under northwesterly flow aloft, which
will help keep temperatures warm in the 80s for the Valley/Delta
and 60s to 80s for the foothills/mountains. On Thursday, an upper
level low is progged to move into the PacNW which will extend
troughing into NorCal. Global ensembles differ with the strength,
location, and timing of troughing moving into our area, so
precipitation chances remain relatively uncertain at this point.
Cluster analysis is in relatively good agreement of the upper
level pattern change, however there are discrepancies in location
and strength as well. The NBM begins to introduce precipitation
chances Friday afternoon/evening for the northern and eastern
foothills through Saturday evening. Right now, current NBM
probabilities of exceeding 0.50" inches are less than 15%,
highlighting the uncertainty with regards to the location/strength
of the troughing.

High temperatures should cool through the extended period, with
near normal temperatures returning Friday then below normal
temperatures on Saturday. Saturday temps may not break the 80
degree mark in the Valley, with cooler 50s to low 70s in the
foothills and mountains. Stay tuned for forecast updates as we
head into the Memorial Day weekend.

&&

.AVIATION...

VFR conditions prevail for interior NorCal through the forecast
period. Breezy south to west surface winds expected after 21z, with
some gusts 25 to 30 kts across the Delta and to 20 kts in the
Sacramento vicinity. Winds then become lighter overnight. Isolated
showers/storms possible over the Sierra crest, south of Highway 50
20z-03z.

&&

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

$$