Flood Watch 1, NWS Reno, NV

Area Forecast Discussion
National Weather Service Reno NV
343 AM PDT Tue Mar 20 2018

.SYNOPSIS...

A period of light valley rain and higher elevation snow is expected
this evening and tonight. A much stronger atmospheric river type
storm pushes into the region Wednesday night through early Friday
morning. This will bring periods of heavy valley rain and heavy
Sierra snow along with gusty winds. The weekend will be colder
with snow showers possible down to all valley floors.

&&

.SHORT TERM...

Clouds will be on the increase today as a period of light valley
rain and higher elevation snow begins this evening and tonight.

However, the main focus in the short-term forecast is the incoming
Atmospheric River (AR) storm poised to provide multiple feet of high
elevation snowfall (Sierra cement) and periods of heavy rain to
lower valleys in the Sierra and western Nevada. Main change in the
forecast was to continue to increase expected precipitation totals
Wed-Thu.

*TIMING: Initial light to moderate precipitation with this storm
 begins on Wednesday in the Sierra but expecting the main period
 of heaviest precipitation to begin Wednesday night into early
 Thursday morning. Moderate to heavy precipitation continues
 Thursday afternoon before quickly diminishing Thursday night as a
 pronounced dry slot cuts off precipitation.

*SNOW LEVELS: AR storms are inherently warm systems as their
 moisture has tropical origins. As such, snow levels on Wednesday
 into Thursday morning will begin in the 7,500-8,500' range from
 the Tahoe Basin extending southward into Mono County. Snow levels
 will begin to drop in the late Thursday afternoon into evening
 time frame as the cold front nears. There is still some timing
 uncertainty on how quickly the front arrives but nonetheless,
 late Thursday afternoon-evening looks to be the period where
 travel deteriorates across most passes and snow will be possible
 at Lake Tahoe level and along portions of Hwy 395 through Mono
 County.

*RAIN/SNOW/HYDROLOGIC CONCERNS: This AR storm will provide heavy
 rains and high elevation snowfall Wed-Thu with most
 precipitation occurring Thursday. Overall, the main moisture
 plume will be concentrated through the Sierra in Mono County
 where 2-4 feet of snow will be possible above mainly 8,000' with
 localized areas up to 6 feet along the crest in southern Mono
 County. The Tahoe Basin will see up to 2-3 feet mainly above
 7,500' with localized areas up to 4 feet along the crest. Snow
 amounts below 7,500' are much lower confidence but generally
 6-12" are possible down to 7,000' feet with a few inches possible
 down to lake level by Thursday night.

This will be a rain event for western Nevada with moderate to heavy
rain expected on Thursday. 0.50-0.75" are possible across western
Nevada valleys. Rises on small creeks and streams can be expected
with some minor flooding possible in urban and poor drainage areas.

Minor flooding is also possible through the Sierra where snow has
not been removed to allow for drainage. There could also be rises on
small streams below 7000 feet that drain into the Tahoe basin and on
the east side of the Sierra. Although, main stem rivers do not look
like an issue, rises are expected on all rivers. The current
forecast takes the Middle Fork of the Feather River in Portola to
monitor stage, but at this point all other rivers are forecast to
remain below the point of concern. Fuentes

.LONG TERM...Friday onward...

The break in the precipitation between the warmer and colder
portions of the storms Friday morning is even more defined in latest
model guidance. Colder air moves into the region Friday as an upper
trough digs into northern CA/NV with a secondary cold frontal
boundary shifting through the region Friday night. This boundary
will provide the forcing to develop snow across the area with the
best chances in the Sierra, northeast California, and in western
Nevada mainly north of Highway 50. This is a fast moving feature and
there will only be a short period where the moisture and forcing
align, so am not expecting significant snow accumulations. However,
with snow levels at all valley floors, even light accumulations may
bring travel impacts across the region. A final passing shortwave
could bring one more chance for light snow showers Saturday night.
Considering we are getting into the latter part of March, most road
impacts from these lighter snows will be limited to the overnight
and early morning hours.

A warming and drying trend looks likely for next week, however
springtime is notorious for poor long range model guidance, so
nothing is a lock just yet. -Dawn

&&

.AVIATION...

Mid and high level clouds continue to stream into the region early
this morning, limiting the chances for fog formation. The one
exception is KTRK with patchy freezing fog through 16z this
morning.

A weak system will bring light rain and snow this afternoon and
evening with slightly lowered ceilings and terrain obscuration
possible. A much stronger storm is on tap for Wednesday and
Thursday with strong and gusty winds, and moderate to heavy
mountain snow and valley rain. Most precipitation should be
limited to the Sierra and northeast California on Wednesday, with
widespread spillover into western Nevada on Thursday. Terminals
should all just see rain, but expect widespread mountain
obscuration and periods of MVFR (30% chance IFR) ceilings and
visibility.

Winds will bring turbulence and wind shear, with local studies
indicating LLWS possible for KRNO Wednesday into Thursday. The
most likely periods for strongest valley winds will be Wednesday
and again Thursday afternoon. The period between will have heavy
precipitation which will dampen the wind speeds, however erratic
stronger gusts remain possible. Periods of snow showers are
possible for all airports Friday afternoon through Sunday morning
with the potential for minor accumulations, especially from 03z-
15z. -Dawn

&&

.REV Watches/Warnings/Advisories...
NV...Winter Storm Watch from Wednesday afternoon through late
     Thursday night above 7000 feet in NVZ002.

CA...Winter Storm Watch from Wednesday afternoon through late
     Thursday night above 7500 feet in CAZ073.

     Winter Storm Watch from Wednesday afternoon through late
     Thursday night above 7000 feet in CAZ072.

&&

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