FXUS66 KSTO 231051

Area Forecast Discussion
National Weather Service Sacramento CA
351 AM PDT Thu May 23 2019

Scattered light showers and Isolated thunderstorms over the Sierra
today and all interior Norcal mountains Friday as one low pressure
system moves east and another drops in to take its place.
Scattered showers and thunderstorms most areas over the weekend
with below normal daytime temperatures. Afternoon mountain showers
continuing through mid week.


Mainly dry this morning as Norcal sits under northerly upper flow
between high pressure over the eastern Pacific and a low pressure
system now centered near southern Nevada. Some cloud cover
associated with the low is spilling over mainly the eastern half
of the forecast area this morning and some scattered northern
Sierra showers/thunderstorms are not out of the question later
today. Elsewhere, a little more sun should bring up daytime highs
today to close to normal for this time of year. An upper low, now
over western Canada, is forecast to drop into the Pacific
Northwest tonight and early Friday. Brief upper ridging over the
state between this low and the Great Basin low should bring a
short period of clearing most areas. This period of clearing will
br brief, however, with afternoon heating, divergent flow aloft
and orographics bringing afternoon showers and thunderstorms to
the mountains Friday afternoon with some threat of showers to the
northern Sacramento valley as well. The low over the Pacific
Northwest now looks to be an inside slider type dropping into the
Great Basin on Saturday. Therefore, the greatest risk of any
shower activity will be over the eastern CWA with enough
instability for thunderstorms over the Sierra Cascade range.
Another cold upper low is now forecast to drop out of the Pacific
Northwest on Sunday but this system should take a more westerly
track. As a result, scattered showers will be a possibility over
the entire CWA and north state with a threat of thunderstorms over
the Sierra Cascades. Daytime highs will be taking a drop Saturday
and Sunday as these two cold systems drop through the north state
especially on Sunday when daytime highs are expected to come in
from 10 to 15 degrees below normal. For the most part, snow levels
should be above pass levels so mountain travel impacts should be
minimal this weekend although snow levels may drop to pass levels
at times causing periods of slick roads. Sunday's cold upper low
is forecast to move east of the state Sunday night but will likely
continue to have impacts on the CWA weather going in to the early
part of next week.


Models have come into better agreement with progression of upper
low into the Desert SW Monday, but then diverge significantly
beyond this time. Although ensemble spread increases during the
latter part of the extended, general consensus is that long wave
troughing remains along 120W. Under broad scale weak synoptic
lift, best chances for showers will be terrain induced forcing
over the mountains in the afternoons. Below normal temperatures
progged into Monday with high temps returning to near normal
Tuesday, then above normal Wednesday and Thursday.



Lcl MVFR/IFR conds poss mnly ovr fthls and mtns this aftn into eve
for shwrs and isold tstms, otrw VFR conds ovr Intr NorCal nxt 24
hrs. Nly flow alf with areas of Nly sfc wnd gsts to 25 kts in
Cntrl Vly, mnly N of KOVE, and adjcnt fthls/mtns til abt 02z Thu.



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