Area Forecast Discussion
Issued by NWS Reno, NV

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000
FXUS65 KREV 110024 CCA
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
340 PM PST Sat Dec 10 2016

.SYNOPSIS...

Heavy rain, flooding and high elevation snow will continue into
this evening, then diminishing overnight as the storm system
pushes out of the area. Sunday and Monday will see a break in
significant precipitation before another round of storms moves
into the region Tuesday through Thursday and brings the threat
for more significant precipitation.

&&

.SHORT TERM...

The warm atmospheric river (AR) storm continues to affect the
region this afternoon with heavy rainfall and high elevation
snowfall. Deep subtropical moisture tap and strong 700mb flow has
contributed to heavy precipitation rates in the region, with
flooding issues along the Western Nevada Sierra Front.

Rainfall totals in the Sierra and Lake Tahoe Basin have generally
been around 2 to 4 inches, with around a couple inches of
rainfall in the foothills west of Reno and Carson City. A few
locations have seen localized heavier amounts of 3.5 to 4.5
inches of rainfall in the Carson Range west of Washoe Valley, near
the Little Valley and Davis Creek areas. East of US-395, lesser
rainfall totals of 0.25-0.50" have been observed, including Sparks
and Virginia City. Numerous reports of minor flooding have
occurred today, especially around Boomtown-Verdi-Mogul area,
Southwest Reno, as well as Washoe Valley, Bowers Mansion, Davis
Creek and down into the Carson City/ Eagle Valley areas as well.
Flooding reports have been mostly minor, but conditions are
expected to continue over the next few hours before the
precipitation ends. There is still a flood warning in effect for
the Truckee River near Truckee, CA. See our hydrology section
below for more details on flood potential through tonight.

Snow levels have fallen to around 7000-8000 feet this afternoon,
with some heavier precipitation pulling down snow levels to near
Lake Tahoe level earlier today. Highest elevations around Lake
Tahoe have gotten 2-3 feet of heavy wet Sierra cement, especially
above 8000 feet where this storm has remained snow for the entire
duration.

There will be one more push of heavy precipitation between now
and 5pm this evening, then the moisture surge will push south into
Mono County with precipitation diminishing for the Reno- Carson-
Tahoe area after 8pm. Snow levels will come down this evening,
with some several inches of snow accumulations expected around pass
level and up to 6 inches above 8000 feet. Anyone with travel plans
over the Sierra should check road conditions before heading out and
expect slick driving conditions tonight over passes, including Echo
and Donner as well.

Gusty winds up to 40 mph will continue through the rest of the
evening, especially east of Highway 395. We will cancel the wind
advisories for Mono-Mineral-Lyon-Churchill-Pershing counties, but
we will leave the lake wind advisory for Tahoe up through tonight.
Ridge winds will be near 100 mph through this evening, then drop
off to around 60-80 mph as the storm moves away after midnight.
We get a break in the weather Sunday and Monday, with breezy and
mild conditions before the next wet storm system moves into the
region Tuesday and Wednesday. Hoon

.LONG TERM...

The most substantial changes to the long term forecast were to lower
snow levels Tuesday, especially north of I-80 in the Basin, and
increase QPF substantially Tuesday through Thursday.

It is starting to look like a period of light to briefly moderate
snow is on tap for the lower valleys of northeast CA and western NV
from about highway 50 north Tuesday. On Sunday, much drier air will
work into the region behind today`s atmospheric river while the next
PWAT plume/atmospheric river reloads in the eastern Pacific. Late
Monday night and Tuesday, an upper low off the Pacific Northwest
coast will pick up the next moisture plume and push in into northern
CA and NV. The plume will then ride up and over the colder and drier
air in the low levels (warm air advection - WAA) over northeast
CA/western NV, providing lift for precipitation.

As the precipitation will fall into the drier atmosphere over the
region and provide wet bulb cooling of the airmass to allow for
mostly snow, at least through Tuesday morning during the strongest
period of WAA. This snow could cause widespread impacts to the
morning commute Tuesday for the northern Sierra, northeast CA, and
western NV (especially north and west of Fallon). Simulations are
in good agreement so confidence is high in impacts Tuesday morning
for northeast CA, and has risen to medium for far western NV. By
Tuesday afternoon and evening, WAA moves off to the north and
mixing should allow snow levels to jump significantly as warmer
air aloft mixes out any low level cold air.

Moving on to Tuesday night through early Thursday, flooding (at
least for small streams and poor drainage areas) concerns are rising
as simulations are coming in line for significant precipitation as
an atmospheric river gets stuck pointed at northern CA and northwest
NV ahead of low pressure in the eastern Pacific. At least several
inches of liquid precipitation equivalent are likely in the
northern Sierra, with a widespread half-inch to an inch of
precipitation projected out into western NV north of highway 50.
Snow levels are expected to rise again to above 7000-8000 feet.
With all the rainfall that has already occurred today, soil conditions
are primed for significant runoff and at least localized flooding.

Thursday afternoon and Thursday night, a strong cold front is
well agreed upon by models. This will serve to squelch the threat
for more flooding as snow levels crash to valley floors. Friday
and Saturday look very chilly indeed as cold continental air from
Canada invades CA and NV. Highs are not likely to get above the
mid or upper 30s for lower valleys Friday, with getting above
freezing difficult by next Saturday. Snyder

&&

.AVIATION...

Widespread MVFR CIGS with terrain obscuration in the northern Sierra
should improve dramatically this evening as a moisture plume moves
off to the south, with CIGS/VIS improving in Mono County west of
highway 395 after midnight. Rain could end as as brief period of
light snow for KTVL/KTRK this evening but accumulations should
remain very light at those terminals. Elsewhere into western NV and
eastern Mono County, occasional MVFR VIS/CIGS in -RA/RA will improve
this evening.

VFR conditions are expected on Sunday and Monday (except areas of
fog and/or low stratus possible Sunday morning for some
Sierra/northeast CA valleys) before the next rain and snow event
moves into the region on Tuesday. Snyder

&&

.HYDROLOGY...

Flood warnings are in effect for the creeks and streams flowing out
of the Carson Range and into areas from South Reno to Washoe Valley
to Carson City. Rainfall amounts in the Carson Range north of US-50
have exceeded 3.5 inches, with some locations as high as 4.5 to 5
inches.

Heavy rainfall potential around the Tahoe Basin and far western
Nevada will start to diminish after 4pm. Latest radar and satellite
trends show the back edge of precipitation is currently passing
south of Highway 36 in Lassen county. The back edge of the
precipitation band should reach I-80 by 5-7pm, and by 6-9 pm for
Highway 50.

Flows in small creeks around the Tahoe Basin should decrease quickly
this evening as the heavy rainfall ends. However high flows may
continue well into this evening for the western Nevada creeks as
water from the higher elevations flows into the valleys. Brong

&&

.REV Watches/Warnings/Advisories...
NV...Lake Wind Advisory until 4 AM PST Sunday for Lake Tahoe in
     NVZ002.

     Flood Watch until 10 PM PST this evening NVZ002-003.

CA...Lake Wind Advisory until 4 AM PST Sunday for Lake Tahoe in
     CAZ072.

     Flood Watch until 10 PM PST this evening CAZ072.

&&

$$

For more information from the National Weather Service visit...
http://weather.gov/reno



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