576
FXUS66 KSTO 181746
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
1046 AM PDT Wed Sep 18 2024


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

&&

.Discussion Update...
There is a 30-40% chance of scattered showers and thunderstorms
for eastern portions of the Park Fire burn scar this afternoon and
evening. The main concern is for these storms moving from east to
west into that area, bringing the potential for heavy rainfall,
which could cause debris flow and flooding impacts there. Because
of this, a Flash Flood Watch has been issued for the Park Fire
burn scar from 1 pm PDT to 10 pm PDT today.

.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 probability is 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 bring further
south, this will result in more southeasterly steering flow and
more 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 return to the Delta, Valley, and foothills of the Sierra
Monday through Tuesday of next week. 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.

&&

.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, northeasterly wind gusts up to
20 knots will be possible this evening (04z-09z Thurs).

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until 10 PM PDT this evening for Central
Sacramento Valley-Northeast Foothills/Sacramento Valley-Northern
Sacramento Valley-Western Plumas County/Lassen Park.

&&

$$