Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 081152
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
352 AM PST Thu Dec 8 2016

.SYNOPSIS...

An unsettled weather pattern will persist over the next several
days. Our current system will continue a transition from snowfall to
rain later this afternoon with freezing drizzle possible early this
morning.  More rain and high elevation snow is likely Friday through
Saturday with additional systems possible into early next week.
Periods of gusty winds are possible Friday into Saturday.

&&

.SHORT TERM...

Snowfall continues across the Sierra and portions of western Nevada
early this morning. Numerous chain controls are in place across the
Sierra and far northeast California along with snow covered roads
evident on NDOT and CalTrans webcams. The trend for the morning will
be a change-over to rainfall through the morning as a warm layer
aloft begins to move across the region. A few areas near Lake Tahoe
have already made the change-over to rainfall but it looks like this
is more wind driven than warming aloft at this time. The South Lake
Tahoe Airport has been reporting light rainfall since about 11pm Wed
with melted snow and wet roadways evident on the CalTrans Hwy 50 at
Meyers roadcam.

At this time, no reports of freezing rain and no bright banding
evident on radar (indicator of melting snowfall) but timing still
looks to favor the 4am-7am window this morning for western Nevada
Valleys where we would see the best conditions of warming aloft and
cold surface temperatures where freezing drizzle would become
favorable. Chances for freezing drizzle are less for the Sierra as
increased winds will warm the surface temperatures to above
freezing. Strong winds aloft will begin to mix to the surface later
this afternoon with breezy conditions between 30-40 mph possible
across the Tahoe Basin and Sierra Front. As such, dangerous boating
conditions can be expected across Lake Tahoe today and have issued a
Lake Wind Advisory as a result.

The active pattern will continue into the weekend as the next system
will provide rounds of gusty winds Friday into Saturday followed by
a weak to low-end moderate atmospheric river (AR) system Saturday
into Sunday. This system has shown high variability over the last
several days and has yielded low confidence, but latest guidance has
shown some better agreement and trends, so have introduced a trend
of higher precipitation chances and amounts along with higher snow
levels over the weekend.

After breezy winds this afternoon, the next round of gustier winds
looks to arrive Friday afternoon into evening with a potential for
gusty conditions persisting into Saturday afternoon. Wind gusts
during these periods may reach into the 40-50 mph range. Downslope
signatures have shown up in model cross-sections but are much more
defined in the NAM vs the GFS which is commonplace so not yet sold
on that stronger scenario especially with a weaker trend in the GFS.
Regardless, winds may create problems for high profile vehicles and
also continue the hazardous boating conditions on Tahoe.

In regards to the weak-mod AR, trends over the last several days
have shown a farther south progression with the main axis of
moisture. As such, the northern Sierra could see precipitation
amounts in the 1.5-2.0 inch range right along the crest with up to
an inch to an inch and a half down through the basin. These AR
system are inherently warm systems and snow levels could be as high
as 8,000-9,000 feet Friday and Saturday so only the highest
elevations should see snowfall with mainly rainfall for lower
elevations and quite a bit of shadowing in the lee of the Sierra
with much lower rainfall amounts. Areas across northern Nevada may
see a bit higher amounts however as a stalled warm front could
introduce some isentropic lift and better potential for additional
rainfall. Fuentes


.LONG TERM...Sunday through Thursday...

Weather pattern will remain active through much of the long term as
persistent zonal flow keeps the storm door open. After a brief break
on Sunday evening, we could see more precipitation by midweek.
Atmospheric river tools are also showing a break in the moisture tap
briefly on Sunday before the next slug of moisture approaches the
West Coast. Few changes were made to the long term forecast tonight.

Aside from residual showers across the Sierra, the operational GFS
is showing a relatively quiet afternoon on Sunday, but then model
simulations begin to diverge for Monday. Previous model runs kept
westerly flow across the region which would allow more moisture to
be pushed across the Sierra and western Nevada. Latest simulations
are showing a more amplified flow that would result in fluctuations
of the jet and moisture across the West Coast. Weak ridging across
the eastern Pacific is allowing the flow to be a bit more amplified
for Monday, which will cut off the moisture tap into the West and
bring down another reinforcing shot of cold air down from the north.
GFS is showing a cold front dropping down through Nevada in the
afternoon with breezy conditions ahead of and just behind the
frontal passage. On the other hand, the ECMWF is warmer and keeps
the westerly flow across the West, which would promote a more mild
and moisture-laden flow into the Sierra.

By midweek, there ia a little better consensus for another
atmospheric river to bring more precipitation to the area. Models
have varied with where the best moisture will make landfall along
the coastline, anywhere from the Pacific Northwest down to the
central California coast.

&&

.AVIATION...
FZRA/FZDZ the main threat this morning, especially for
KTRK/KRNO/KCXP. Some -FZRA already occurring near KTRK and will
continue thru 15Z before it warms above freezing for RA. KRNO/KCXP
are still snow and will likely see theis thru 15Z and it is very
light. Expect a changeover to -FZDZ around 15Z and possibly last
thru 18Z before precip ends and it is above freezing. KNFL/KLOL
another 3 hr delay from KRNO.

Until 21Z, still expect areas of MVFR CIGS/VIS, but it will
gradually wane as precip ends. Sierra terminals, particularly
KTRK/KTVL could see occasional MVFR conditions overnight with some
mtn obscuration. After 21Z for areas east of the Sierra will see
VFR, but CIGS will remain 4-5000 feet. Winds will increase from the
SW today as precip ends with peak gusts 25 kts. There will be
mtn wave turbulence, although winds aloft aren`t quite strong enough
for significant LLWS, but it will be monitored. Winds will wane
overnight.

Winds will be a bigger factor on Friday than initially indicated
with peak gusts to 35 kts possible at many terminals from the SW
along with mtn wave turbulence and some lcl LLWS. Then precip moves
baack in after 03Z Sat with MVFR CIGS/VIS for many areas.

&&


.REV Watches/Warnings/Advisories...
NV...Winter Weather Advisory until 10 AM PST this morning NVZ005.

     Lake Wind Advisory from 1 PM this afternoon to 4 AM PST Saturday
     for Lake Tahoe in NVZ002.

     Winter Weather Advisory until 10 AM PST this morning NVZ002.

     Winter Weather Advisory until 10 AM PST this morning NVZ003-004.

CA...Winter Weather Advisory until 10 AM PST this morning CAZ070-071.

     Winter Weather Advisory until 10 AM PST this morning CAZ073.

     Lake Wind Advisory from 1 PM this afternoon to 4 AM PST Saturday
     for Lake Tahoe in CAZ072.

     Winter Weather Advisory until 10 AM PST this morning CAZ072.

&&

$$

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


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