Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 092332 CCA
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
326 PM PST Fri Dec 9 2016

.SYNOPSIS...

Periods of heavy rain, high elevation snow, and strong gusty winds
will impact the Sierra tonight through Saturday with periods of
light to moderate precipitation in western Nevada. Sunday and
Monday will see a break in significant precipitation before
another round of storms moves into the region the midweek.

&&

.SHORT TERM...

Latest weather situation is rapidly developing and has changed
significantly since early this morning. A moderate atmospheric
river (AR) storm is still expected to move through the region
tonight through Saturday, but models have signifcantly increased
their precipitation for this storm and will bring multiple
impacts to the region including small creek and stream rises,
rises on the mainstem Truckee River, strong gusty winds, and
heavy high elevation snow.

Winds: Winds are already increasing this afternoon, with gusts
around 40-50 mph. Wind Advisories have been issued for most areas
from Reno-Tahoe northward to the Oregon border through tonight
with areas east of US-395 having wind impacts on Saturday. 700mb
winds will continue to increase through this evening and into
Saturday morning with up to 60-70kts tomorrow morning. Moisture
will be increasing ahead of this AR system, so that will help to
dampen the winds on Saturday as the precipitation begins to
spillover into the valleys of western NV. We expect to see
widespread wind gusts of 50-60 mph in the valleys through tonight,
with wind prone areas gusting to 70mph+. Highest wind areas will
include the US-395 corridor tonight, with ridge winds in the Sierra
gusting over 100 mph tonight and Saturday. By Saturday, as the
heavier precip moves in, the area of high winds will shift east of
US-395, to areas including Virginia City Highlands, Fernley, Silver
Springs, Dayton, Fallon, Yerington, Haawthorne. Travel restrictions
for high profile vehicles, aviation shear/turbulence, and dangerous
boating conditions on Lake Tahoe can be expected today.

Precipitation: This AR storm is slowly making its way onshore into
Northern CA with high snow levels and abundant moisture.
Precipitable water amounts offshore are over 1.5", a very impressive
amount of moisture. Snow levels will start out around 8500-9000 feet
tonight as the first precipitation pushes in, then snow levels will
raise up to around 9000-9500 feet early Saturday morning. Models
have trended higher QPF amounts, so we have also raised our rainfall
amounts as well. A period of heaviest rainfall will come in between
3am and 10am Saturday morning, with rainfall amounts of up to 0.25-
0.50" per hour in the Sierra. This will lead to rises on the Truckee
River and other creeks and streams in the Sierra and into western
NV. See our hydrology discussion below for more details on flooding
potential and heavy rainfall. A good amount of spillover is expected
into the valley of western NV on Saturday as well, with rainfall up
to 0.50-1.0" in the Reno-Sparks-Carson-Minden areas.

Heavy Snow: With the high snow levels and heavy precipitation,
elevations above 9000 feet in the Sierra will likely see up to 1 to
3 feet of snow through Saturday evening. The main areas of concern
will be in Mono County along the Sierra Crest, where this heavy
snowfall looks to have the highest chance of accumulating. Most
mountain passes will be spared from the snow, with the exception of
Mount Rose Highway and Carson Pass which could see significant
snowfall accumulation and travel issues through Saturday night. Snow
levels do look to come down Saturday evening at the end of the
event, which will bring a few inches of snow down to pass level,
including Donner/Echo. Dry zonal flow will move in Sunday and
Monday, with breezy, mild conditions. Hoon

.LONG TERM...Monday night through Friday...

The stormy period will continue next week. Precipitation/snow
impacts are more uncertain that usual with large discrepancies
between simulations regarding the orientation and focus of the
systems. We shall do our best to break it down below...

Monday night through Tuesday night...the last couple runs of the GFS
are at odds with the ECMWF simulations with the orientation of flow
over CA and NV. The GFS flow is flatter/more zonal and allows deeper
cold air to press down into the northern Sierra and western NV while
the ECMWF builds a weak ridge over the region with low level cold
air bottled up in extreme northern NV and northeast CA into Tuesday.
The discrepancies seem to be related to the existence of an upstream
kicker which the GFS depicts. If there is a kicker right on the
heels of the upper low off the Pacific Coast, then the flow would
likely stay flatter with less time for the low to dig southward.
These discrepancies will will become an issue for snow levels and
precipitation type late Monday night and Tuesday as a period of warm
air advection (WAA) brings the chance for precipitation to the
region.

With the main slug of colder low level air well north of I-80 in the
EC, concern for a period of valley snow into Tuesday morning is
limited to far northeast CA and the Basin and Range from Fernley
north and east. However, the GFS allows colder air to stay over much
of the region with precipitation focused farther south near the I-80
and hwy 50 corridors. This could lead to a period of snow for the
northern Sierra and western NV into Tuesday as a moisture feed rides
up and over the colder low level air. As ensembles seem to support
their own operational models, uncertainty is rather high so stay
tuned as simulations work out their differences.

Tuesday night, the issues continue as the GFS brings in the upper
disturbance for a widespread threat for snow and lowest elevation
rain and snow showers while the EC keep a moisture feed pointed at
northeast CA for residual rain and high elevation snow showers.
Wednesday, the GFS brings a colder and drier period into the region
with zonal flow aloft over the region while the EC keeps the upper
low off the coast for continued showery conditions.

Thursday and Friday, the train really flies of the rails as the EC
carves a deep, cold and dynamic trough over the West with widespread
precipitation and low snow levels. The GFS separates the flow and
holds off precipitation until Friday night and Saturday...although
it does eventually bring in a very dynamic and wet system. Either
way, models depict another round of substantial precipitation
between Thursday and Saturday. Snyder

&&

.AVIATION...

Wind, rain, and high elevation elevation snow will move into
northeast California and western Nevada tonight and Saturday. The
main concern is for moderate or even severe turbulence and areas of
LLWS (within 020 AGL) in the lee of the Sierra crest into Saturday
morning.

For terminals in far western NV including KRNO/KCXP, winds from just
off the SFC (or even at SFC in foothills) to around 050 AGL near and
west of hwy 395/I-580 are expected to increase suddenly and rapidly
for a high shear potential. Therefore, even if there are rotors with
variable/light winds directly over terminals, approaches and
departures will face significant directional and speed shear. Once
rain intensifies by early Saturday morning (likely before sunrise),
the potential for downslope and strong shear in western NV will
diminish although it is still likely to be turbulent.

As far as CIGS/VIS and precipitation at terminals (Sierra and
western NV) through Saturday afternoon, rain is expected with snow
levels above 8000 ft MSL. For western NV, CIGS to deteriorate after
midnight with occasional MVFR CIGS/VIS well into Saturday with
-RA/RA. For Sierra (KTRK/KTVL) and northeast CA (KSVE) terminals,
CIGS and/or VIS expected to be lower with below landing minimums
likely for KTVL (less than 3 SM VIS) with near mins for CIGS (015)
at KTRK for most of Saturday. Snyder

&&

.HYDROLOGY...

Main areas of concern for flooding are the creeks along the west
shore of Lake Tahoe, Donner Creek, and the Truckee River along
Highway 89 to the Martis Valley, this includes the city of Truckee.

Atmospheric river will bring a burst of heavy precipitation, with
snow levels near and above 8000 feet, starting around 3 am Saturday
morning and persisting to about midday Saturday. This short but
potentially high intensity rainfall event combined with saturated
soils will lead to rapid rises on small creeks, streams and the
mainstem Truckee River.

Depending on the amount of shadowing, there could be some minor
flooding of creeks along the east shore of Lake Tahoe and creeks
leading out of the Carson Range and into the foothills west of Reno,
Carson and Minden. Rock and mud slides could also impact highways
near steep terrain, especially I-80, to go along with the usual
ponding of water on roads and poorly drained urban areas.

Rainfall intensity is forecast to decrease after midday though the
flood threat may persist into Saturday evening in the Truckee area.

For the Truckee River downstream of the Martis Valley to Reno-
Sparks, the water levels will rise Saturday evening/Saturday night
but no flooding is forecast.

Elsewhere rain will increase flows, but no flood issues are expected
for the mainstem Susan, Carson, and Walker rivers.  Brong

&&

.REV Watches/Warnings/Advisories...
NV...Wind Advisory until 4 AM PST Saturday NVZ002-003-005.

     Flood Watch from late tonight through Saturday evening
     NVZ002-003.

     Lake Wind Advisory from 4 AM Saturday to 4 AM PST Sunday for
     Lake Tahoe in NVZ002.

     Wind Advisory from 7 AM to 4 PM PST Saturday NVZ001-004.

CA...Wind Advisory until 4 AM PST Saturday CAZ070>072.

     Wind Advisory until 4 PM PST Saturday CAZ073.

     Flood Watch from late tonight through Saturday evening CAZ072.

     Lake Wind Advisory from 4 AM Saturday to 4 AM PST Sunday for
     Lake Tahoe in CAZ072.

&&

$$

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


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