Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 162330
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
330 PM PST Thu Feb 16 2017


.Synopsis...
Wet weather returns with several storm system passages expected
through next week. Sierra mountain travel issues at times. Gusty
winds today and Monday. Increased flood threat Monday.

&&

.Discussion (Today through Sunday)...

Current model analysis and water vapor imagery shows the
shortwave trough that brought morning rain moving across the
Sierra in the early afternoon. The trough base was still working
across the Sierra, which was causing some heavier showers and some
isolated thunderstorms in the northern Sacramento Valley. Surface
pressure gradient in the northern Sacramento Valley has continued
to cause some gusty south winds in the northern Valley up to 40
mph at times, with isolated wind gusts up to 30 mph for the
central valley (in particular the Oroville and Chico areas).

Latest model guidance has suggested that wind gusts in the
northern Valley will continue through mid to late afternoon before
diminishing this evening as the shortwave trough aloft passes
through the region. Model guidance is still in good agreement to
having an upper low deepening off the coast and moving onshore
Friday evening, which will bring more rounds for scattered
precipitation for the day and increasing chances for Friday
evening. With a stronger low level jet associated with the
incoming upper trough and tightening pressure gradient, the
southern Sacramento valley and northern San Joaquin valley will
likely see increased wind gusts Friday through Friday night. Snow
amounts will increase for the Sierra at elevations higher than
5500 feet and could see heavier snow accumulations Friday
afternoon through Saturday morning.

The upper level trough continues to linger through the weekend,
which will keep chances for rain for the valley and snow for the
higher elevations. However, most of the storm is expected to
affect southern California during the weekend.

&&

.EXTENDED DISCUSSION (Monday THROUGH Thursday)

Extended models continue to show the strongest system in the
forecast period moving in by early Monday and continuing into
Tuesday. While there have been some variations in the exact track
of the storm, confidence is high for moderate to heavy
precipitation for the northern Sierra/southern Cascades. What is
trickier is which area will be the focus of heaviest
precipitation, whether it will be over the Feather River basin or
over the American River basin. It looks like the most favorable
area will be over the Feather River Basin, with 4 to 8 inches of
rain/water equivalent for Monday and Tuesday. The American River
Basin (I80 and Highway 50 corridor) should see water
equivalent/rain of about 4 to 6 inches, with 1 to 2 feet of snow
around and above pass levels. The Valley is forecast to receive
about 1-3 inches of rain. This could bring some flooding issues.
Additional runoff in the mountains could bring additional
mudslides.

Snow levels are another challenging aspect of the forecast, due to
uncertainty in how far north the storm will track. During the day
Monday, snow levels are forecast to climb as warm, moist air
moves up from the southwest, in the 6000-7000 foot range over the
southern Cascades and mountains of western Plumas County, the
7500-8000 foot range for the northern Sierra. Snow levels will
drop some Monday night, but exactly how quickly and how far
remains uncertain, due to large model run timing differences. The
latest GFS drops snow levels to 3500-4000 feet by daybreak
Tuesday, while the ECMWF keeps levels around 6000-6500 feet. These
levels will affect how much precipitation ends up as snow, and
how much is runoff. For now, am going with a blend of both,
somewhat favoring the ECMWF for now, which would limit heavier
snow to higher elevations.

In addition to precipitation, strong, gusty winds are expected
Monday. Gusts of 40 to 50 mph are possible in the Valley, gusting
over 55 mph in the Sierra. This coupled with saturated wet ground
could bring down trees, causing power outages.

The main upper trough is forecast to move inland later Tuesday
into Wednesday resulting in showers across the area, with lowering
snow levels and lighter precipitation amounts. Snowfall could
cause additional travel problems across the mountains, with snow
levels dropping to 3500-4000 feet.

A period of drier weather is expected for late in the week, with
mainly just some lingering snow showers in the mountains. EK

&&

.AVIATION...

Scattered showers and isolated thunderstorms are possible behind
the front through this evening, mainly north of KSMF.
Southeasterly surface wind gusts 25-35 kts in the northern
Sacramento Valley through 00z. Improving conditions and
decreasing wind expected this evening. storng southerly winds
return Friday morning from SMF southward, gusting to 45kt.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 7 AM Friday to 10 AM PST Saturday
for West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.

Wind Advisory from 7 AM to 10 PM PST Friday for Carquinez Strait
and Delta-Northern Sacramento Valley-Northern San Joaquin Valley-
Southern Sacramento Valley.

Flash Flood Watch through Friday morning for Carquinez Strait
and Delta-Central Sacramento Valley.

&&

$$



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