Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 170437 AAA
AFDREV

Area Forecast Discussion...UPDATED
National Weather Service Reno NV
937 PM PDT Sun Apr 16 2017

.UPDATE...
Showers continue in the moist upslope flow into the Sierra. Lower
level water vapor imagery shows the drier air remaining north of
I-80 and trajectories indicate this may continue. The models seem
slow to catch onto this including the HRRR. As a result, expect
the showers to continue in the moist, relatively unstable flow
into sunrise at least south of I-80. Another 1/4 to 1/2" of precip
is likely along the Sierra Crest the remainder of the night from
Tahoe south to Mono County. This will fall as snow above 7-7500
feet with higher passes slick at times.

I have also increased the shower coverage into parts of Western
Nevada. While amounts will be light, periods of spitting rain will
occur in between some higher wind gusts.

I thought about increasing the threat of rain for tomorrow as
well. The models continue to show a drier period tomorrow morning,
and I believe it is still a bit overdone on the drying. That said,
will leave the update to the highest confidence period, that is
tonight. Wallmann

&&

.SYNOPSIS...

A system moving through northeast California and western Nevada
tonight will bring breezy conditions along with light rain and  high
elevation snow. Another disturbance will move by Monday night and
Tuesday with more significant rain and higher elevation snow. After
a final weak system Wednesday night and Thursday, a significant warm
up is expected for next weekend.


&&

.PREVIOUS DISCUSSION... /Issued 321 PM PDT Sun Apr 16 2017/

SYNOPSIS...

A system moving through northeast California and western Nevada
tonight will bring breezy conditions along with light rain and
high elevation snow. Another disturbance will move by Monday night
and Tuesday with more significant rain and higher elevation snow.
After a final weak system Wednesday night and Thursday, a significant
warm up is expected for next weekend.

SHORT TERM...

Not much change to the forecast thinking in the short term, with
two systems impacting the region between today/tonight and Tuesday
afternoon. The system/moisture feed for Monday night and Tuesday
will be the more significant one as far as precipitation.

Turning to the forecast through tonight, the main precipitation
band with an area of low pressure approaching the California and
Oregon coast is nearing the northern Sierra. The low will move off
to the northeast this evening, with the main cold pool and jet
forcing missing the northern Sierra and western Nevada. However,
the band does have help from orographic ascent up the west slopes
of the Sierra and some increased moisture from a modest moisture
feed being pushed into the region by the low pressure. This will
bring a few hours of heavier precipitation to the northern Sierra
late this afternoon and this evening, with the band sagging south
to Mono County later in the evening and overnight.

With the main cold pool and forcing missing the region, snow
levels should remain mostly above 6500-7000 feet in the northern
Sierra and across northeast CA but could lower slightly below that
in the heaviest precipitation this evening. Roads are quite warm
in the Sierra but will cool with the heavier precipitation rates
and lowering sun angle this evening. A light slushy accumulation
is still possible after about 6 PM over Sierra passes...although
it could melt off overnight after precipitation rates decrease
given temperatures in the 30s over the main northern Sierra
passes.

Out into western Nevada through tonight, with forcing weakening
precipitation amounts should remain very light (mainly less than
0.05"), with breezy conditions the main player. Late tonight and
Monday morning, the region will see a break between disturbances
although showers will continue in the Sierra with moist southwest
flow continuing between waves.

The next wave approaches Monday night and will work on moisture
and a thermal boundary draped through the Sierra and western
Nevada. This is expected to bring heavier precipitation amounts
than the system tonight. However, there is not much cold air to
inject into the system and snow levels are expected to remain
mostly above 6000 to 7000 feet through Tuesday. Still, more
travel issues are possible over I-80 and Highway 50 Monday night
and Tuesday morning with a light snowfall there.

Tuesday night, another brief break arrives ahead of another wave
for Thursday. Snyder

LONG TERM...Wednesday through Sunday...

Model solutions continue to struggle with resolving the system for
late Wednesday night into Thursday. Earlier runs had pushed this
system a bit farther north...but now the GFS has started carving the
low farther south and has it moving through slower. The ECMWF has
been slightly more consistent and has QPF south into Mono County.

So...after a brief period of short wave ridging for much of
Wednesday...we return to a fast moving area of low pressure with
brief gusty winds late Wednesday night and a period of showers with
snow levels dropping to near 5000 feet by early Thursday morning.
This system...while slower than earlier runs...is still moving fast
enough to limit the amount of precipitation expected. Total liquid
precipitation could approach a quarter-inch near the Sierra crest
north of Highway 88 with generally less than a tenth in the valleys.
Snow accumulations will be limited to areas above 5500 feet and are
not likely to exceed more than a few inches; the highest amounts
will be near the crest.

Ridging develops again by Friday and temperatures start to rise. We
are likely to see 70s in the lowest valleys by Saturday and Sunday
with 60s in the Sierra valleys. Earlier model solutions showed a
weak short wave riding over the ridge late Saturday...but the best
forcing appears to be in Oregon so no pops were included.

Warming temperatures next weekend could start to accelerate the
snowmelt process. This warming may be short lived though as some
simulations are pointing toward another trough of low pressure
developing after next weekend. Model solutions are not consistent
that far out and should be used very cautiously.

AVIATION...

Light precipitation is starting to spread into northeast California
and the Sierra late this afternoon. Gusty winds have developed in
the Sierra and to the east. Ceilings are likely to lower through the
evening and overnight with some MVFR conditions possible for
KTRK/KTVL. Wind gusts in excess of 25-30kt are a strong possibility
at all terminals west of Highway 95 this evening. This means
continued turbulence and possibly some low level wind shear.

The best chance for precipitation will be in the Sierra and northeast
California tonight...but a brief period of showers is possible along
a surface front late tonight across western Nevada.

Conditions are likely to remain unsettled with low cigs/vsbys for
KTRK/KTVL into Monday. Due to warm ground conditions the chance of
accumulating snow on the runways at those terminals is limited
tonight and Monday.

Another area of low pressure moves across Monday night into Tuesday.
This system is more dynamic and will likely create lower cigs/vsbys
with lowering snow levels by Tuesday morning. Gusty winds will
accompany this system as well. While areas below 5500 feet are not
expected to see any accumulations...there could be a brief period of
accumulating snows for KTRK/KTVL/KMMH during the heaviest
precipitation early Tuesday morning. Still...no more an inch or so
is expected.

&&

.REV Watches/Warnings/Advisories...
NV...Lake Wind Advisory until 5 AM PDT Monday for Lake Tahoe in
     NVZ002.

CA...Lake Wind Advisory until 5 AM PDT Monday for Lake Tahoe in
     CAZ072.

&&

$$

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



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