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Area forecast discussion
National Weather Service Sacramento California
255 PM PST Sat Dec 7 2019

Synopsis...
a strong winter storm will bring periods of heavy Rain, Mountain
snow, scattered thunderstorms, strong winds, and hazardous travel
this weekend. Dry conditions return next week with near average
temperatures. More widespread precipitation could return late in
the week.

&&

Discussion...
early afternoon satellite imagery shows the primary upper low just
off the far northern California coast. The associated surface wave
indicated by the distinct curl in the visible/water vapor imagery
is moving into The Crescent city area. The attendant cold front
currently exiting the coastal range eventually moves through the
Sacramento Valley during the early/mid-afternoon. While rather
mild conditions have prevailed over sections of the valley given
limited cloudiness, showers and scattered thunderstorms will be on
the increase during the afternoon and evening hours. Such
activity has begun to evolve along and west of I-5 in the northern
Sacramento Valley.

The 12z sounding from koak showed quite a bit of low-level shear,
particularly between 0-1 km with storm-relative helicities of
nearly 200 m2/s2 accompanied by sufficient deep-layer shear (0-6
km of 63 knots). In these situations, some rotation may be
observed within some of the healthier storms. As such, the Storm
Prediction Center currently has a marginal risk out for a couple
weak tornadoes across the Sacramento Valley. The threat should
wind down by later in the evening given a loss of daytime heating.
Otherwise, heavy downpours will be the biggest issue across the
valley and foothills leading to ponding on roads and minor street
flooding. If any such stronger radar echoes impact a burn scar, a
potential for debris flows would be possible. Conditions should
begin to improve beyond Sunday morning as the upper trough
departs. Winds will be on the decrease along with a gradual
increase in sunshine.

A high-impact winter storm continues to affect the Sierra Nevada
chain with a Winter Storm Warning remaining in effect until 1 PM
Sunday. Periods of heavy snow will remain likely above 6,500 feet
while snow levels are expected to drop down to around 5,500 to
6,000 feet late tonight into Sunday morning. Travel will remain
hazardous and is not recommended with the potential for whiteout
conditions at times as well as major travel delays. Storm total
snowfall will easily reach the 1 to 3 foot range in many
locations, locally higher in heavy snow bands and over mountain
summits. Conditions to turn more showery in nature during the
overnight hours into the first half of Sunday. A gradual
improvement in the weather is forecast by late Sunday afternoon.

Mid-level heights will continue to build on Monday and Tuesday as
a transient ridge swings through. Mostly sunny skies and near
average temperatures are expected to begin the week. A rather weak
shortwave enters the picture late Tuesday into Wednesday morning
bringing a slight chance for showers up toward Shasta County and
the far northern sections of the coastal range. Otherwise, expect
clouds to be plentiful given the trough passage. ~Bro

&&

Extended discussion (wednesday through saturday)...
ensemble guidance continues to indicate zonal flow across the
eastern Pacific later next week with ridging across norcal. The
pattern will bring the potential for light warm-advection
precipitation across the northern mountains. Late next week,
models are showing a shortwave trough moving into the pacnw. Both
the European model (ecmwf) ensemble and the gefs have trended deeper with this
system. In addition, the U.S. West Coast Arkansas landfall tool is
showing a weak tpw plume (ar). Thus, precipitation chances could
extend further south including the I-80 corridor and the Sierra.
Gefs plumes indicating around 0.30 inches of precipitation for blu
and about 0.35 inches for rdd, so overall amounts could remain on
the light side. Temperatures will be slightly above average.

&&
&&

Aviation...



Widespread MVFR, with local IFR, through 18z-20z Sunday as
Pacific storm system continues to bring showers to the region.
Southerly surface wind gusts to 25-30 kts, strongest over the
northern Sacramento Valley, gradually tapering after 00z Sunday.
Isolated thunderstorms possible through 02z Sunday accompanied by
small hail and surface wind gusts to 45 kts. Snow levels
6500-7500 ft.
&&

Sto watches/warnings/advisories...
Flash Flood Watch until 6 PM PST this evening for central
Sacramento Valley-Clear Lake/Southern Lake County-mountains
southwestern Shasta County to Northern Lake County-northeast
foothills/Sacramento Valley-northern Sacramento Valley-Shasta
Lake area / northern Shasta County-western Plumas County/Lassen
park.

Winter Storm Warning until 1 PM PST Sunday for West Slope
northern Sierra Nevada-western Plumas County/Lassen park.

Wind Advisory until 4 PM PST this afternoon for central
Sacramento Valley-mountains southwestern Shasta County to
Northern Lake County-northeast foothills/Sacramento Valley-

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