Area Forecast Discussion

FXUS65 KREV 192039
Area Forecast Discussion
National Weather Service Reno NV
139 PM PDT Mon Mar 19 2018
Dry weather is expected through early Tuesday with light winds. 
A period of light valley rain and higher elevation snow is expected
Tuesday evening and Tuesday night. A much stronger system pushes 
into the region Wednesday night through early Friday morning. This
will bring periods of valley rain and heavy Sierra snow along 
with gusty winds. The weekend will be colder with snow showers 
possible down to all valley floors.
The only significant changes during the short term part of the
forecast cycle were made to Wednesday. The model guidance is 
pointing toward a drier period east of the Sierra Wednesday before
spill over begins in earnest Thursday. 
Ahead of the main system...the first period of precipitation 
develops Tuesday afternoon/overnight. There are really two processes
going on at this time. 
North of I-80 there is an area of warm air advection that brings 
light precipitation to northeast California down to about the 
Tahoe area and into far western Nevada. The uplift is stronger and
persists longer near the Oregon border. There is some question 
about snow levels...but at this time it appears there is enough 
warm air near the surface...with limited evaporational cooling due
to increased limit snow accumulations to areas 
above 5000 feet from Highway 70 north. There will likely be some 
light accumulations above 6500 feet in the Tahoe basin. The 
forcing decreases enough by early Wednesday to remove POPs from 
western Nevada.
Farther south into Mono County Tuesday night...the initial push 
of moisture associated with a deep moisture plume begins to affect
the area. The forcing is not overwhelming and much of the 
precipitation is due to upslope flow and confined near the crest.
A definite break develops Wednesday east of the Sierra but 
upslope in the Sierra and weak forcing in far northeast California
should keep precipitation going through the day in those 
locations...albeit not very heavy. Breezy winds are likely in 
western Nevada with strong gusts over the ridges.
A more significant deep moisture plume begins to push into the
area Wednesday night into Thursday. This is subtropical air so 
snow levels climb above 7000 feet in northeast California...above 
7500 feet around Tahoe...and above 8000 feet in Mono County. This 
will confine snow to the highest elevations into Thursday 
QPF was increased a bit with this storm as the moisture plume is
spread a bit farther north now and there is a period of jet 
coupling in the Sierra and northeast California that could lead to
increased precipitation rates Thursday morning/afternoon. Snow 
levels begin to fall late Thursday night...but by then the 
moisture plume is sliding south. The biggest snowfall totals 
should be above 7000 feet for Tahoe and 7500 feet for Mono County.
We will issue watches for these areas. Moderate to heavy rain is 
possible below these levels until Thursday evening. That could 
cause minor flooding in urban areas 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.
In the lower valleys this event will be all rain with totals
approaching 0.50-0.75 inches in the valleys and foothills of
western Nevada, and for areas of northeast California east of 
Susanville. Much of the snow in the valleys should melt or sublimate
Tuesday and Wednesday...but there remains a slim possibility that
minor flooding of poor drainage areas could occur in some urban 
.LONG TERM...Friday through Monday... 
The cold front which finally drives snow levels down to all valley 
floors is still not due to move through until Friday (northeast CA, 
Tahoe area, western Nevada) or Friday night (Mono, Mineral, eastern 
Churchill Counties). The best shot for lower elevation snowfall and 
road impacts will be Friday night into Saturday morning as an upper 
low off the Pacific Northwest coast comes inland. At this time, the 
Sierra from Alpine County northward and western NV north of 
Interstate 80 stand the best chance at picking up a light snowfall 
from the final upper disturbance. For the Basin south of I-80, the 
forcing is currently simulated to remain too far north with snow 
showers dissipating before reaching those areas.
Chances for showers diminish Sunday and Monday, with temperatures
beginning to rebound towards normal in the GFS with temperatures 
holding below average in the EC with a weak residual trough 
hanging around. -Snyder
VFR with light winds below ridges for most of northeast California, 
the Sierra, and western Nevada through the day Tuesday. The 
exception is at/near KTRK where there is a 30-40% chance for FZFG 
from about 3-9 AM Tuesday, much higher if skies become clear; 
however, high clouds are expected tonight and a 045-060 AGL deck is 
possible by around sunrise Tuesday.
A weak system will bring light rain and snow Tuesday evening into 
early Wednesday morning for the Sierra, northeast CA, and far 
northwestern NV...with substantially lowered ceilings and terrain 
obscuration likely. For western NV, some higher terrain obscuration 
is possible Tuesday evening with light rain.
A much stronger storm for Wednesday evening through Thursday night 
will bring periods of gusty winds, moderate to heavy mountain snow, 
and valley rain. 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 areas of 
wind shear, with local studies indicating LLWS possible for KRNO 
with southerly winds Wednesday into Thursday. Snow showers are 
possible for all airports Friday afternoon into Saturday, with the 
potential for minor accumulations overnight and in the morning. 
.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.
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