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FXUS66 KSTO 282222
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
322 PM PDT Thu May 28 2020

.Synopsis...
Hot weather will come to an end due to a pattern change that begins
Friday. Low pressure will move into Norcal with cooler temperatures
and a chance of showers and thunderstorms over the weekend.

&&

.DISCUSSION...
Marine stratus has returned to the coast and with it a "Delta
trickle" to help cool things a tad in the marine/Delta influenced
areas. At 20z temps were running generally 2-7 degrees cooler than
yesterday in this area, and 6-19 degrees cooler at VCB and through
the Strait at Vallejo, respectively. Farther inland, temps were
running on par or even warmer than yesterday, especially in the
Sierra.

Looks as if RDD and RBL will hit the century mark today. For RDD it
will be the initial triple digit reading for the current heat wave,
but that site has been surpassed a few times already by DTS, SAC,
SCK and MOD during this heat wave.

This heat wave will be "blown away" by increasing onshore gradients
and the Delta Breeze later tonight and Fri. By Fri morning, both the
MAV and MET guidance for SUU have sustained wind speeds from 22-29
kts as thermal differences from land-sea and the approaching upper low
drive onshore momentum well inland. Over the Nrn Sac Vly, the flow
will turn up-Valley on Fri cooling their temps probably into the
mid, maybe upper, 90s.

When one concern is alleviated (the heat), another one emerges
(thunderstorms) with the potential for locally heavy rains. The
closed upper level low vcnty 28N/128W will tend to phase a couple
of PWAT plumes as it moves towards the MTR/SFO Bay areas early Sat
morning, and then over Norcal on Sat. The NAEFS anomaly table shows
about a +3 PW anomaly over Norcal on Sat, and about a 5 year return
interval, highlighting the potential for some heavier rainfall.

Although the NAM 3km REF prog keeps our area quiet on Fri, it
explodes an area of SE to NW oriented convection roughly from
BLU-OVE-N of Alder Springs late Fri nite and in the wee hours of
Sat morning. This is right in the area of strong PVA and upper
level diffluence in the NE quadrant of the approaching closed low.
Forecast soundings look quite wet during this time with PWs
above 1.0" at BAB, CIC, RBL, RDD. Moist SEly flow around the
closed low should favor the east side of the Trinity Alps/Coastal
Range (our western Tehama/Shasta Counties) for some heavier
rainfall. Earlier indications suggest some areas in Shasta County
could receive .60" in the 6 hr period early Sat morning, and
about 1.50" for the Sat/Sat nite time frame.

The shower and thunderstorm threat will diminish by late
Saturday afternoon and evening, with a residual chance of showers
and storms into Sun over the Nrn mtns and Sierra with the
approach of a secondary Pac trof. This will continue onshore flow
and cloud cover over our CWA, and although temps will recover some
on Sun it will still be much cooler than normal. After this big
heat wave (Max temps some 15 to 20 degrees above normal) how
about this for a cool down...6 to 20 degrees below normal on Sat,
and 5 to 12 degrees below normal on Sun? JHM

&&

.EXTENDED DISCUSSION (Monday THROUGH Thursday)...

No major changed to the previous forecast. A highly amplified
pattern is in place to start the work week as an elongated
longwave trough sits off the California coast while a strong ridge
occupies the middle of the country. Embedded shortwaves advancing
into northern California will ignite scattered showers across the
foothills and mountains, especially on Monday and Tuesday.
Ensembles gradually cut off this trough in the vicinity of
southern California although its exact position is debatable given
the typical forecast spread at these ranges. There is a slight
chance of afternoon showers are possible over the mountains every
day. Upper ridging curling up and around the low center should
prompt a notable warm up after starting a few degrees below
climatology. High temperatures are likely to return to the upper
80s/low 90s by mid- week. ~BRO/HEC

&&

.AVIATION...

VFR SKC conditions over Interior NorCal through Fri. Delta Breeze
expected to increase to 22-29 kts with gusts around 40 kts late
tonight and Fri through the Carquinez Strait and into the Sac Delta.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Excessive Heat Warning until 7 PM PDT this evening for Carquinez
Strait and Delta-Central Sacramento Valley-Motherlode-Mountains
Southwestern Shasta County to Western Colusa County-Northeast
Foothills/Sacramento Valley-Northern Sacramento Valley-Southern
Sacramento Valley.

Excessive Heat Warning until 7 PM PDT this evening for Northern
San Joaquin Valley-Shasta Lake Area / Northern Shasta County.

&&

$$
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