Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 230421
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
921 PM PDT Wed Mar 22 2017

.Synopsis...
Unsettled weather will continue into next week as a series of
weather systems move through the region.

&&

.Discussion...
Upper low pressure system is still over the area with scattered
showers over the foothills and mountains with some lingering
isolated showers in the valley. The center of upper low will move
to near Las Vegas overnight and as it does the showers will
rapidly diminish between 10 pm and 1 am tonight.

Snow level is near 4500 feet and with the showers expected to
rapidly diminish the timing looks good to let the winter weather
advisory expire at 10 pm. Only minor accumulations are expected
before the showers end.

Thursday will be dry during the day. The GFS is slightly faster
than the NAM in bringing in precipitation Thursday night. Snow
levels will start out lower with this system (near 6000 ft) so
expected some travel problems going over the crest starting
Friday morning.
.Previous Discussion...
Upper trough is positioned over California this afternoon with
disturbance dropping south through it across NorCal. Morning`s
precip band has shifted east and weakened over the Sierra...but
not before producing a 4 to 6 hour burst of heavy snow. Afternoon
and evenings concerns shift to thunderstorm development with hail
being the main concern due to cold air aloft/steep lapse rates.
Shear is less than yesterday...but stronger than 12z model
forecast as southeast low winds of 10-20 mph continue Red Bluff
to Sacramento. Activity will shift over Sierra late this
afternoon/evening with another burst of heavy snow possible. Snow
levels will drop to 4000 feet at times in heavier activity.

Brief break between storms during the day Thursday before the next
system moves in during the night into Saturday. Valley rainfall
amounts will range from 1-2 inches Redding area to 0.25-0.50
Stockton/Modesto. Mountain precip amounts will be 1-2 inches
liquid. Snowfall amounts 6-12 inches could fall with rather low
snow levels down to 4500 feet.

&&
.EXTENDED DISCUSSION (Sunday THROUGH Wednesday)

Next in a series of Pacific frontal systems pushes into Norcal
on Sunday. All areas of the CWA should see at least a chance
of precipitation by afternoon. Precipitation amounts with this
week system are expected to be fairly light so even with moderate
snow levels, impacts are expected to be minimal. Models diverge
as to how fast this system will exit the region. ECMWF and GEM
models quickly shift the upper trough axis eastward into the
Great Basin by Monday while the slower GFS keeps cyclonic flow
and shower threat over the forecast area Monday. Models diverge
even more Tuesday and Wednesday. Have kept slight threat of
precipitation over the northern mountains Tuesday with ECMWF
and GEM showing some sort of trough over Pacific Northwest but
GFS puts a fairly strong ridge over Norcal Tuesday so confidence
in the forecast for this time period is low. Models seem to be
settling in on idea of west coast ridging around the middle of
next week for at least a brief drying period but considering poor
model consistency, confidence remains low.

&&

.AVIATION...

VFR ceilings central valley and MVFR/IFR over mountains as upper
level trough moves through Norcal today. Snow level near 4500
feet. Improving conditions after 09Z.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 10 PM PDT this evening for West
Slope Northern Sierra Nevada-Western Plumas County/Lassen Park.

&&

$$



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