Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 091156

Area Forecast Discussion
National Weather Service Reno NV
356 AM PST Fri Dec 9 2016


Periods of heavy rain, high elevation snow, and strong gusty winds
will impact the Sierra today and Saturday with periods of light to
moderate precipitation in western Nevada. Sunday and Monday will see
a break in significant precipitation before another round of storms
moves into the region midweek.



A mild start to the morning compared to 24 hours ago along the Hwy
395 corridor from roughly Reno to Gardnerville where lows are only
in the 40s with breezy conditions prevailing. Outlying valleys in
the basin and range such as Lovelock are much colder as these
areas never broke through their inversions this morning and are
currently only in the 20s.

The active weather continues into the start of the weekend as our
next system will provide strong and gusty winds beginning this
afternoon followed by a weak to moderate Atmospheric River (AR) type
system tonight and Saturday.

The winds today will be the initial concern out ahead of the
incoming moisture which is set to hit the Sierra later tonight.
Winds are already gusty across Sierra ridgetops early this morning.
Sensors in the Sierra are showing gusts around 80 mph and these
speeds will increase to 100 mph later in the afternoon as winds
aloft strengthen and peak by early Saturday morning. The main
concern has been the possibility of strong winds in the valleys
later this afternoon with some potential for downsloping wind to
occur in the lee of the Sierra. Latest guidance shows only a few
hour window to develop a standing and breaking wave signature across
the Sierra, but thinking is it will become windy today via boundary
layer mixing across the Sierra and western Nevada mainly across the
Hwy 395/I580 corridor. Wind gusts in these valleys will
strengthen with peak gusts in the 50-60 mph range with some
downslope component pushing wind prone areas to see gusts as high
as 70 mph. As such a Wind Advisory has been issued for this
afternoon and evening. Travel restrictions for high profile
vehicles, aviation shear/turbulence, and dangerous boating
conditions on Lake Tahoe can be expected today.

The next phase of this storm will be a subtropical moisture plume
that will provide a weak to moderate Atmospheric River (AR) event
tonight into Saturday. This will be a warm system with high snow
levels starting around 8,000-9,000 feet so it will be mostly a rain
event except for the highest elevations in the Sierra. Expecting
rain to begin across the Sierra around the 10pm-midnight timeframe
with the heaviest precipitation occur early Saturday morning into
early afternoon. This system will provide ample moisture to the
Sierra with decent mountain shadowing across western Nevada. Latest
model runs have weakened the shadowing a little bit and the Hwy 395
corridor could see 0.25-0.50 inches of rainfall through Sunday
morning while the Sierra could see 1.5-2.5+ inches of precipitation.
Snowfall amounts for high elevations mainly above 8,500`-9,000`
could see 1-1.5 feet of snowfall with the highest peaks seeing 1.5-
2.0 feet by Sunday morning. Most passes should see mainly rainfall
but higher passes such as Mt Rose and Carson Pass may see
significant snowfall accumulation and travel issues. Fuentes

.LONG TERM...Monday through Thursday...

Weather pattern will remain active through the long term as
persistent zonal flow keeps the storm door open along with ample
atmospheric moisture available upstream (across the Pacific
Ocean). Atmospheric river tools show a break in the moisture tap
through Monday before the next slug of moisture reaches the Sierra
Monday night. Beyond some confidence in the overall large-scale
pattern, we continue to have low confidence in details such as
snow level and the strength of the Sierra rain shadow. Therefore,
few changes were made to the long term forecast this morning.

Weak ridging across the eastern Pacific into the Gulf of Alaska
may allow the flow to be a bit more amplified for Monday, cutting
off the moisture tap into the West and possible bringing down
another reinforcing shot of cold air from the north. GFS is
showing a cold front dropping down through Nevada Monday evening
with another round of gusty winds ahead of and just behind the
frontal passage. On the other hand, the ECMWF is warmer with
ridging over the same time period. GFS ensemble also shows a wide
range of possible temperatures for the air mass Monday-Monday
night. A weak or moderate atmospheric river looks to arrive
Monday night or Tuesday but with lots of uncertainty just how
cold the air mass will be as the moisture arrives.

For the second half of the week, there is a potential for another
atmospheric river to bring more precipitation to the area but
model solutions vary a great deal from each other and from run to
run, especially regarding timing and snow level. Hopefully model
forecasts will begin to converge on a solution over the next few
days (stay tuned). JCM



Winds at Sierra ridge level around will continue at 40 to 50 kts
today and then increase to 50 to 70 kts this evening through
Saturday morning. These very strong winds will create mountain
wave activity over and in the lee of the Sierra along with
turbulence and wind shear impacts. Occasional rotor activity and
LLWS will remain possible, especially at KRNO-KCXP.

Light rain showers will be mainly confined to near the Sierra
today, with increasing chances of heavier rain in the Sierra
tonight-Saturday producing MVFR CIGS/VSBY and brief IFR
conditions. For lower elevations, only brief very light showers
are expected today, with a period of steady rain and possible
MVFR conditions by late Fri night/early Sat. JCM


.REV Watches/Warnings/Advisories...
NV...Wind Advisory from 1 PM this afternoon to 1 AM PST Saturday

     Lake Wind Advisory until 1 PM PST this afternoon for Lake Tahoe
     in NVZ002.

CA...Wind Advisory from 1 PM this afternoon to 1 AM PST Saturday

     Lake Wind Advisory until 1 PM PST this afternoon for Lake Tahoe
     in CAZ072.



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