Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 090412
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
812 PM PST Thu Dec 8 2016

.UPDATE...

Winds are still breezy, but have dropped substantially in valley
locations with most peak gusts in the 25 to 40 mph range.
Therefore have cancelled the wind advisory early for the eastern
Sierra front. Across Sierra ridges, gusts of 70 to 100 mph are
still being observed. Gusty winds and choppy conditions are still
surfacing on Lake Tahoe and the Lake Wind Advisory remains in
effect through Saturday afternoon.

Have also reduced precipitation chances this evening based on
latest radar and satellite trends. Most likely places for light
showers will be in the Sierra and mainly north of I-80 in western
Nevada, but with little forcing, precipitation totals will be
light--a tenth of an inch or less. The warm air mass is keeping
snow levels around 8000-8500 feet this evening, so any snow
impacts will be limited to the highest Sierra passes.

Attention is now turned to the next moderate atmospheric river
event Friday night into Saturday. The Sierra is going to receive
significant precipitation, but the big question remains for
western Nevada. Previous model runs were shadowing the area out
with strong winds anticipated, but latest guidance suggests better
spillover potential. Based on forecast soundings, winds are
increasing in the 700-500mb layer and are 50 to 80 kts, which
typically would lead to efficient spillover. Bufr soundings still
show a ridgetop inversion layer Friday evening, which is favorable
for downsloping winds, but this signature is quickly eroded and
the vertical wind profile isn`t typical for downsloping. The other
indicator for less wind lies in forecast model cross-sections
which aren`t showing winds ducting down to the surface and also
keep higher moisture in the lee of the Sierra. However, with 700
mb winds pushing 50 to 70 kts, periods of gusty winds are still
likely Friday night into Saturday morning and could be more
erratic depending on how much moisture spills over. In any event,
mountain wave activity is likely with turbulence, wind shear, and
rotor activity possible for aviation. -Dawn


&&

.PREVIOUS DISCUSSION... /Issued 237 PM PST Thu Dec 8 2016/

SYNOPSIS...

Periods of heavy rain, high elevation snow, and strong gusty
winds will impact the Sierra Friday and 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 midweek.

SHORT TERM...

Gusty winds have mixed to the surface this afternoon with
localized gusts up to 70 mph in Washoe Valley. Winds are expected
to stay gusty through this evening with gusts up 65 mph in wind
prone areas and up to 50 mph elsewhere in the lee of the Sierra.

Main changes to the short term forecast were increasing QPF
especially in the Sierra, raising snow levels around 500 feet,
and slowing the onset of the precipitation Friday.

Moderate atmospheric river with PWATs around 1.5" will impact the
Sierra for about 24 hours from Friday night through Saturday night.
This is expected to bring up to 2-4 inches of precipitation to
the Sierra crest. Snow levels will be relatively high due to the
sub-tropical origin of the moisture, and the fact that the jet
stream remains near the Oregon border during the bulk of the
precipitation, keeping colder air locked up north of the region.
Snow levels could fluctuate quite a bit, dropping to around
7000-7500 feet in heavier precipitation and then raising as high
as 9000-9500 feet as warm moist air moves in from the south. Most
of the snowfall is expected to accumulate above 8000-8500 feet
where around 2 feet of heavy snow could accumulate by Saturday
night. Small creeks and streams are expected to rise, but no
flooding is expected.

Northwest Nevada could be shadowed during a good portion of the
precipitation event, though predicting this is always a significant
challenge. The cold front finally starts to push south near the
end of the event, with the best chance for precipitation occurring
in Reno on Saturday.

Strong winds up to 100 mph will impact the Sierra crest Friday and
Saturday. During Friday afternoon and evening, strong winds could
mix down to the surface in the lee of the Sierra. There is some
downslope signature in the temperature profiles, but the winds are
expected to peak overnight. This will create a case where widespread
strong winds are less likely, however, occasional wind gusts of
60 mph could locally mix to the surface. The highest chance for
more widespread winds on Friday will be along the 395 corridor in
Mono County. Another round of gusty winds could impact the lee of
the Sierra again on Saturday. -Zach

LONG TERM...Sunday through Thursday...

Main changes in extended were reducing precip chances Sunday-Monday,
then going more aggressive with precip chances Tuesday-Thursday,
with lower snow levels especially from Tuesday to early Wednesday.

For Sunday and Monday, the latest guidance is favoring a break in
the active weather pattern with zonal flow keeping temperatures
near seasonal averages (mid 40s-lower 50s in lower elevations and
upper 30s-lower 40s near the Sierra). We can`t rule out some light
showers near the Sierra or northeast CA at times, but impacts will
likely be minimal. The zonal flow aloft will keep gusty ridge
winds both days, but without significant shortwave energy the winds
will not be very efficient in mixing down to lower elevations.

From Tuesday through Thursday, another atmospheric river pattern is
projected to impact the Sierra and northeast CA, leading to
increased precip chances across the region. The most significant
change is that the guidance is less amplified with ridging Tuesday
and Tuesday night, keeping a cooler air mass in place with model
soundings showing isothermal profiles. This could bring snow even
down to lower elevations fur much of Tuesday and Tuesday night,
before some warming works in to lower elevations Wednesday. The
orientation of the moisture feed has varied with the past few model
runs, so we kept quite a wide range of potential snow levels in
the current forecast.

Wednesday and Thursday still look to be milder than Tuesday, until a
trough passes through the region, most likely during Thursday
afternoon or evening. We kept some precip chances in for most of the
region but pushed the more favorable chances north for Wednesday-Wed
night, then spread higher pops back south again Thursday. Winds
should also increase and become gusty in lower elevations by
Wednesday night and Thursday.

The bottom line is that anyone with travel plans from Tuesday through
Thursday (including lower elevations) should prepare for winter
driving conditions and keep posted to the latest forecasts. MJD

AVIATION...

Winds will become the main issue tonight and Friday, with mountain
wave turbulence continuing. Occasional rotor activity and LLWS will
remain possible through this evening, especially at KRNO-KCXP.

Warmer air mass has ended the snow threat at the main terminals
for the next couple of days. Light rain showers will be mainly
confined to near the Sierra through Friday, with increasing chances
of heavier rain in the Sierra Friday night-Saturday producing MVFR
CIGS/VSBY and possible brief IFR conditions. For lower elevations,
only brief very light showers are expected tonight and Friday, with
a period of steady rain and brief MVFR conditions by late Fri
night/early Sat. MJD

&&

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

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

&&

$$

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


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