459
FXUS66 KSTO 180958
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
258 AM PDT Wed Sep 18 2024

.Synopsis...
Cooler, unsettled weather through this evening. Lingering showers
over the Sierra Nevada Thursday, otherwise drier and warmer
conditions into early next week.

&&

.Discussion...
Our current weather system continues to trek along the California
coast as it drops down from the Pacific NW today through Thursday
PM. Compared to Sunday`s system, this system is tracking further
south with a warmer airmass associated with it. This translates to
a weaker thunderstorm signal for our area and lesser chances for
snow along the peak of the Sierra. Current probabilities show a
15-40% of thunderstorm development over the Sierra and adjacent
foothills with highest chances (30-40%) along the ridge of the
Sierra this afternoon and evening (Wed 21z - Thu 03z). Forecast
show around 0.10-0.50 over the Sierra and foothills, up to just
under an inch along the Sierra crest. Probabilities of exceeding
0.25" for the aforementioned area are 55-85% over the Sierra with
30-50% over the foothills. The northern Sacramento Valley and
lower portions of the Valley south of I-80 (below 1000 feet) have
10-30% chance, which is more so indicating the slight chance of
showers moving through the Valley than any organized stratiform
precipitation.

This weather pattern will bring another chance of debris flow
issues over the Park Fire burn scar, albeit less synoptic support
compared to last weekend. A few hi-res model simulations show
isolated thunderstorms tracking over the northeastern portion of
the burn scar early Wednesday afternoon from east to west until
Wednesday evening around 8pm local.

As this system moves further into Southern California on
Thursday, we`ll see another round of thunderstorm chances confined
to the Sierra crest during the afternoon and evening hours. NBM
probabilities have 15-30% along the Sierra crest, with the highest
chances south of US 50. Given the center of the low being further
south, this will result in more southeasterly steering flow and downslope
storm motion.

By Friday, heights are expected to rise behind the weather
system, returning temperatures to near normal by weekend`s end.
Friday will see afternoon highs warm to the mid 80s to low 90s in
the Valley with upper 60s to mid 80s across the mountains to
foothills areas. With the warming temperatures and reoccurring light
to breezy north winds in the Valley through the weekend, daytime
humidities lower back to 20-30% over the Northern Sacramento
Valley through the weekend and into early next week.

&&


.EXTENDED DISCUSSION (Sunday THROUGH Wednesday)...
Dry weather is expected through the extended forecast period as
cluster analysis and ensemble guidance indicate high pressure
extending inland over Northern California. Temperatures will warm
up through the weekend and into early next week with highs in the
mid to upper 90s across the Valley and Delta. Areas of Moderate
HeatRisk also return to the Delta, Valley, and foothills of the
Sierra Monday-Tuesday as temperatures rise 5-10 degrees above normal.
By the late weekend, the previous weather system will be located
just south of the Great Basin resulting in periods of light to
breezy northerly flow across the more windprone areas of the
Sacramento Valley (i.e. the northern Sac Valley and along the I-5
corridor in the Sacramento Valley) Sunday through Tuesday. Daytime
humidities will decrease gradually through this period into the
upper teens to upper 20s, which may lead to localized fire weather
conditions in the northern Sacramento Valley and adjacent
foothills. An onshore flow pattern looks to return to interior
NorCal by Wednesday, alongside slightly cooler temperatures as
weak upper level troughing begins to set up offshore.

&&

.AVIATION...
Mainly VFR conditions and surface winds below 12 knots over
interior Northern California during the next 24 hours. Isolated
MVFR/IFR conditions may occur due to scattered rain showers and
isolated thunderstorms over the foothills and Sierra (between 22z
Wed-08z Thurs). Over the Sierra, easterly wind gusts up to 20
knots will be possible this evening (04z-09z Thurs).

&&

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

$$