Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 241805 AAA

Area Forecast Discussion
National Weather Service Sacramento CA
953 AM PST TUE NOV 24 2015

A cold Pacific storm will bring heavy snow in the mountains with
snow extending down into the upper foothills with some showers
continuing into Wednesday. Isolated Valley/foothill thunderstorms
this afternoon and overnight, mainly west of I5. Dry and cold
weather expected Thanksgiving with lingering flurries over the
Sierra crest.


.DISCUSSION... A cold system from the Gulf of Alaska with a
fastmoving cold front pushing through Northern California. Radar and
surface observations show precipitation spreading across the area.
The snow profiler at Shasta Dam shows snow levels dropping to below
5000 feet.
Observations at Blue Canyon (5300 ft) currently has wet snow at 34
degrees, while the webcam at Kingvale (6100 ft) has wet snow along
the road. Donner Summit (7200 ft)shows snow starting to accumulate
on the road. Trucks are being screened for chains at Applegate.
Further north in Tehama and Plumas counties, Chain controls are in
effect on Highways 36 and 44. Snow has been reported sticking on the
ground in Chester and is visible on a webcam along the sides of the

Increasingly heavy snow is expected across Sierra highways such
as I80 and Highway 50. Roads will get more slippery and visibility
will decrease this morning. The worst conditions are expected early
this afternoon, with conditions likely improving by early evening.
Travel in the mountains is strongly discouraged. Snow levels will
continue to lower overnight into early Wednesday, with snow showers
continuing over the mountains and foothills. While several inches of
additional accumulation is possible, overall snow amounts/intensity
from these showers is looking less impressive, so may need to end
the Winter Storm Warning early.

Precipitation has reached the I80 corridor in the Valley, with
rain from Fairfield to Sacramento. The cold front is moving
quickly to the southeast and will bring rain into the northern San
Joaquin Valley within the next hour.

Behind the main front, there could be some isolated thunderstorms
in the Sacramento Valley this afternoon, mainly east of I5. The
HRRR and NAM models are not showing a lot of instability, but
because the front is moving quickly, there could be enough
clearing to allow some sun and additional destabilization of the
atmosphere. Due to significant shear along and just behind the
front, any storms the develop could have some rotation. Model
soundings suggest the storms would be fairly low topped at around
10 kft, which would be a limiting factor for the strength of any
thunderstorms that form. Overnight, another vort lobe pushes
through along the coast. This could bring some late evening/early
morning showers and thunderstorms over the Coastal Range and into
the Delta area. EK


Sat/Sun: The Rex Block Ridge will remain in place Saturday with
the high pressure cell over Wrn Canada and low pressure cell over
CA/NV. This lingering low will continue a chance of light showers
over the area. The potential for freezing/near freezing
temperatures exists in the Valley and very cold temperatures in
the mountains and mountain valleys, dropping into the teens and
single digits for the weekend. A limiting factor for the cold
morning temperatures will be mixing winds and cloud cover.

Monday: The dynamic, parent upper trough extending from the Gulf
of Alaska Sunday/Monday will finally push the blocking pattern
eastward, which will bring subtle ridging or a col area to NorCal
with a break in the precip chances. Monday adds some uncertainty
with the EC and GFS showing dirty ridging (i.e. EC is sharper than
GFS). Worse case scenario would be lingering snow showers mainly
over the Sierra and colder temps.

Tue/Wed: EC and GFS both have rather large and expansive low
pressure areas over the Gulf of Alaska. Both bring in shortwaves
supporting cold fronts into NorCal sometime between Tuesday and
Wednesday. The EC brings in multiple shortwaves from Tue to Thu
with lower snow levels, while the GFS has more of a steady
southwest flow aloft with higher snow levels as seen in an
Atmospheric River scenario Tue and Wed. Either way, we bumped up
the chance of precip Tue into Wed and increased snow levels a bit
for the first days of December.         JClapp


In Central Valley and adjacent lower foothills, widespread
MVFR/IFR developing around 13z with areas of southerly sfc wind
gusts to 25 kts. Widespread MVFR/IFR with local LIFR in northern
mountains after 12z, spreading to Sierra by 16-18z Tue. Sfc wind
gusts up to 45 kts over higher mountain terrain today. Snow
levels 040 to 060 this morning, lowering 020 to 025 this
aftn/evening.       JClapp


.STO Watches/Warnings/Advisories...
Winter Storm Warning until noon PST Wednesday for Motherlode-
Northeast Foothills/Sacramento Valley-West Slope Northern Sierra
Nevada-Western Plumas County/Lassen Park.

Winter Weather Advisory until 6 PM PST this evening for Burney
Basin / Eastern Shasta County-Mountains Southwestern Shasta
County to Northern Lake County-Shasta Lake Area / Northern
Shasta County.


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