Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 301045
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
345 AM PDT Sat May 30 2020

.Synopsis...
Low pressure moves through today bringing cooler weather with
showers and thunderstorms with possible locally heavy
precipitation. Threat of mountain showers and afternoon
thunderstorms continues early next week.

&&

.DISCUSSION...
561 DM closed upper low approaching the Bay Area this morning with
center progged to move inland invof of KSFO around 18z, then
track through interior NorCal this afternoon. Forecast PWS over
the CWA running around 1 to 1.5 inches to suggest potential for
heavy precipitation. CAPE values trend up from the south this
morning, spreading over much of the CWA this afternoon with values
from 200-500 J/Kg. Elevated instability progs indicating
widespread thunderstorm potential today with large areas of
modified TT`s above 30 over CWA. As upper low progresses inland
later today, increasing SEly flow and upper level divergence
combine over western portions of Shasta/Tehama counties, extending
into the Coastal Range. HREF precip probabilities showing area of
0.5 inch/hr exceedance above 50 percent in watch area between 3
and 6 pm. The Carr-Hirz-Delta burn scar in Western Shasta will
need to be watched. Flash flood potential will be dependent on
cell speed and if training occurs over same region. Small hail
will also be possible with stronger cells. Gusty winds can be
expected, especially in the vicinity of thunderstorms. Clouds and
synoptic cooling will result in high temperatures today upwards
of 20 to 30 degrees lower than Friday.

Models progress upper low through Central Oregon tonight resulting
in storm potential lifting northward through the CWA this evening
and diminishing overnight. Secondary weaker trough moves through
Sunday with possible late day showers over the higher elevations
of Lassen Park and the Northern Sierra Nevada. High temps rebound
some Sunday but remain below normal.

Short wave energy continues to feed into long wave trough
early next week developing another upper low that remains quasi-
stationary off the SoCal coast into midweek. Southerly diffluent
flow will continue a threat of showers and afternoon thunderstorms
over the mountains Mon/Tue. AMS warms Monday back to near normal
with highs Tuesday forecast 5 to 10 degrees above normal.

PCH

&&

.EXTENDED DISCUSSION (Wednesday THROUGH Saturday)...
Ensembles and deterministic models continue to show a cutoff low
stalling just off the southern California coast for the middle of
next week. Depending on how far north this low sets up, embedded
shortwaves along the northern portion of the low could produce
mountain shower and thunderstorm chances each afternoon next week,
mainly over the northern Sierra. Other than that, dry and warm
conditions are expected. Temperatures will warm back up to around
5 to 10 degrees above normal Wednesday through Thursday, in the
low to mid 90s for the Valley and mountain highs in the 60s to
70s.

Ensembles still indicate a trough approaching from the Gulf of
Alaska by Friday. GEFS and EPS continue to show cooling trend,
with high temperatures near normal levels. There is also a chance
of precipitation by later Friday for the northern mountains, with
best potential later in the weekend. EK

&&

.AVIATION...
VFR conditions at TAF sites will give way to scattered MVFR and
isolated IFR in showers and thunderstorms today. Isolated elevated
thunderstorms are developing in the Valley early this morning
over the central and southern Sacramento Valley. A more extensive
band of showers and will set up over the northern Sacramento
Valley 15-03z. Scattered thunderstorms develop to the south after
18z. Delta Breeze 22-29 kts with gusts around 40 kts through the
Carquinez Strait and into the Sac Delta. Widespread IFR/LIFR
conditions over mtns today obscuring higher terrain.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through this evening for Mountains
Southwestern Shasta County to Western Colusa County-Northern
Sacramento Valley-Shasta Lake Area / Northern Shasta County.

&&

$$



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