Area Forecast Discussion
Issued by NWS Sacramento, CA

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000
FXUS66 KSTO 152023 CCA
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
1000 AM PDT Wed Apr 15 2015

.Synopsis...
High pressure builds over NorCal behind a low pressure system
moving eastward across the Great Basin. Breezy to windy conditions
continue today before slackening Thursday. Gradual warming trend
through the end of the work week.

&&

.Discussion...
Upper flow expected to become more meridional over Norcal today as
upper low settles in over the Great Basin today...and over the
4-corners on Thu. 925 mbs winds forecast to slacken to 15-25 kts
this afternoon when deeper mixing will occur which makes advisory
winds more problematic as pressure gradients (MFR-SAC) weaken from
10+ mbs at 12z Wed to 8 mbs by 00z Thu. KDAX VWP shows 2 kft winds
decreasing as well from 40 kts at 1530z to 30 kts at 1612z seemingly
confirming the 925 mbs wind forecast. We may have to cancel the
advsry during or for the afternoon...but then have to consider
reissuing it as Ely gradients increase tonite.

Pressure rises over NV tonite are forecast to increase the RNO-SAC
gradient to 7 mbs by 12z Thu...almost doubling it from this morning.
Instead of a Nly barrier jet developing along the W side of the Sac
Vly ala this morning...the downslope/katabatic wind is forecast to
develop from the NE Sac Vly foothills into Srn Lake/Napa and
portions of Solano Co centered around 12z Thu. Sites such as Jarbo
Gap...Knoxville Creek...High Glade LO will be susceptible to this
pressure pattern and gusty winds. Pressure gradients and upper
support weaken drastically Thu afternoon and winds are expected to
diminish.

Another chilly/cold nite is expected in the NE mtn valleys/basins
due to NEly flow from the cold NE plateaus. Otherwise...adiabatic
warming and mixing will bring a rapid warmup to the region as high
pressure strengthens over Norcal and the air mass modifies. As 850
mbs temps warm into the upper single digits and teens Wed-Fri...max
temps will warm into the high 70s and 80s...assisted by adiabatic
warming/drying effects from Nly winds at times. Warming aloft will
result in a strong subsidence inversion...conducive for mild
overnite temps and low daytime RH and poor to moderate RH recovery
in the foothills and thermal belt areas at night.

A weak closed low is forecast to develop over northwest California
Saturday. Enough moisture and instability may be present for a few
late day showers and thunderstorms over the mountains.    JHM

&&

.Extended Discussion (Sunday through Wednesday)...
A weak upper disturbance is forecast to remain off the Northern
California coast Sunday and possibly into Monday. Depending on the
track of this low, we could potentially see continued convection
across interior NorCal mountains into early next week (the GFS is
the most bullish on this solution). Regardless, temperatures are
likely to remain well above normal in this timeframe.

A deeper, colder trough is likely to approach the West Coast early
next week, possibly moving inland midweek. We`re still seeing
considerable run-to-run model inconsistencies, which isn`t
surprising given that this will likely form a closed low. The GFS,
ECMWF, and GEM all bring at least some precipitation across
portions of the forecast area, though timing is still quite
variable. This at least gives us confidence to boost our forecast
precipitation chances. This pattern change will also likely bring
a pretty drastic temperature drop to around or below normal. Dang

&&

.Aviation...
VFR conditions the next 24 hours. Gusty north winds are expected
to strengthen again today, with 15-25 kts sustained and local
gusts to 35 kt possible across the Valley. Winds will decrease
across the Valley this evening, but NE winds will strengthen
across the Sierra, with local gusts above 40 kt possible
overnight. Dang

&&

.STO Watches/Warnings/Advisories...
wind advisory until 6 pm pdt this evening carquinez strait and
delta...central sacramento valley...northern sacramento valley...
southern sacramento valley.

&&

$$







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