Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 210250

Area Forecast Discussion
National Weather Service Reno NV
750 PM PDT Mon Mar 20 2017


Precipitation is slowly edging east this evening but has not made
it into most of our forecast area as of 730 pm. The few spots that
have received rain are from western Lassen County south into
parts of central Plumas County. The latest model run coming in at
this time backs up this slightly slower eastward movement as
southerly flow in the lower to mid levels keeps the precipitation
from moving east. That should all change by Tuesday morning as the
upper trough gets closer to the region and flow in the mid levels
becomes more southwesterly. For now we will trim pops a little
bit more early this evening except for the far western part of
the CWA. There we will increase pops for the evening hours.

This current system could present some a few problems on the
hydrologic front. The primary basins of concern are the Susan and
Middle Fork Feather. Neither are forecast to reach flood
stage...but moderate precipitation rates and relative high snow
levels will linger in the headwaters of those basins longer the
Truckee...Carson and Walker basins. Anyone in the area of
Susanville on the Susan River or from Portola downstream to
Blairsden on the Middle Fork Feather River should remain alert to
river rises by Tuesday morning/afternoon.

Elsewhere...this system will be nothing like systems earlier in
the year. But still...there are a few areas with ongoing high
water issues that could be exacerbated by additional low
elevation rain...evening as little as half an inch...or rain on
snow. Snow levels will fall late Tuesday into Wednesday and dampen
many concerns...but anyone east of the Sierra that could be
affected by brief high flows in small streams...especially those
with higher flows from recent snowmelt...and areas that are
susceptible to ponding of water from moderate to briefly rain
should be alert.

Lastly...reservoir releases on the Walker system are generating
high flows down both the West and East Walker Rivers. The West
Walker can handle even higher flows...but the East Walker may
produce some minor flooding along its reach just above the
confluence with the West Walker by Tuesday morning. There could
also be some very minor low land flooding below Yerington by
Tuesday afternoon. The Walker River Irrigation District is
managing these increased releases to provide space in their
reservoirs for expected run off from snowmelt later in the spring.


.PREVIOUS DISCUSSION... /Issued 235 PM PDT Mon Mar 20 2017/


Valley rain and mountain snow showers are expected through
Wednesday with gusty winds continuing and periods of travel
disruptions. Another spring storm is likely for early next
weekend with active weather looking possible through the end of
the month.


No significant changes this afternoon as next system will push
across the region through Wednesday. Latest radar shows a band of
moderate showers working east into the Sacramento Valley this
afternoon. Flow was backing a bit to a more southerly direction
ahead of incoming system and this will result in a slow eastward
progression of these showers into western NV. The Sierra should
begin to see rain and high elevation snow this evening with
activity spreading into western NV overnight and early Tuesday as
upper jet support and front move into the area. Snow levels will
be fairly high to start (generally in the 7000-8000 foot range. So
impacts will be at the highest passes where a few inches of wet
slushy snow may accumulate on roadways overnight into Tuesday
morning. Upper level forcing and afternoon heating/marginal
instability may be enough for isolated thunder.

As the upper trough and cold pool move overhead Tuesday night and
Wednesday, snow levels will fall to near lake level. Snow showers
will be more convective, and although not as widespread as
Tuesday, they will be capable of brief heavier snow showers which
could bring brief impacts to any of the Sierra passes. We
continued the mention of thunderstorms on Wednesday as well. Total
QPF through Wednesday will be in the 1-1.5" range along the Sierra
with 0.10"-0.35" in western NV. Snow amounts will range from a
few inches 6000-7000 feet to a foot or slightly more for higher
elevations in the Sierra.

Winds will be gusty for awhile this evening and again for Tuesday
afternoon and evening. We have raised lake wind advisories for
Tuesday. Ridge winds will gusts 70+ kts. A break in the weather is
anticipated for Thursday as the area will briefly be between
storms. Hohmann

LONG TERM...Friday through Monday...

The next in a series of storms is poised to reach northern
California and Nevada Friday into Saturday. The main concern with
this system centers on the amount of precipitation expected as it
does possess a sub-tropical moisture source.

Timing and precipitation amount uncertainties hinge on the likelihood
and magnitude of a potential split of the upper level trough. This
trough has been depicted with a stronger back-side jet which can
favor more southerly digging. 12z ensembles do show a sharper 300mb
trough with a more north-south orientation which initially favors
less orographic forcing and more initial shadowing on the lee-side
of the Sierra on Friday.

Thoughts are that even though the trough is digging a bit more, the
overall pattern from the Pacific Basin through the Atlantic Basin
has trended less amplified over the last several days, meaning that
flow will likely be progressive and having a complete southern split
is probably not too likely.

More probable would be a slower onset and more shadowing initially
on Friday followed by a bit shorter duration spillover period on
Saturday as instability increases with the progressive trough. The
QPF trend looks like a sharper gradient between the west and east
slopes of the Sierra, however seeing 1-1.5" along the crest with
0.25-0.50" for the Sierra Front across far western Nevada is still
plausible. The sharper trough may however allow snow levels to start
a bit lower on Friday (~6,500`) then lower to 5,500`-6,000` by
Saturday. Confidence remains low-medium due the splitting trend.

Saturday will see convective snow showers behind the surface front
as the cold pool aloft moves overhead. A secondary wave moves in
Sunday into Monday morning which will have less moisture but may
drop snow levels below 5,000` in scattered snow shower activity.


S-SW gusts 25-30 kts continue through early evening but winds will
increase on the ridges tonight (gusts of 65-70 knots) ahead of the
next storm with local MVFR CIGS in -RA after 09z Tuesday in the

More widespread MVFR CIGS in Sierra after 12Z with areas of the same
restrictions in Western NV from 15-00Z on Tuesday. An isolated
thunderstorm is possible during the afternoon hours as well. Winds
in the valleys could gust to 35 kts and there will be mtn wave
turbulence, but LLWS will be localized.

The showers will diminish Tuesday night, but increase again by
Wednesday morning with lower snow levels. Isolated -TSRASN are more
likely on Wednesday with quick, brief accumulation on runways in the
Sierra. Wallmann/TF


.REV Watches/Warnings/Advisories...
NV...Lake Wind Advisory from 1 PM to 7 PM PDT Tuesday for Pyramid
     Lake in NVZ004.

     Lake Wind Advisory from 11 AM to 7 PM PDT Tuesday for Lake Tahoe
     in NVZ002.

CA...Lake Wind Advisory from 11 AM to 7 PM PDT Tuesday for Lake Tahoe
     in CAZ072.



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