Area Forecast Discussion
Issued by NWS Reno, NV
FXUS65 KREV 252153
Area Forecast Discussion
National Weather Service Reno NV
253 PM PDT Tue Oct 25 2016
Lighter winds and drier conditions are expected through early
Thursday. Confidence is increasing for a couple of systems to
impact the Sierra and western Nevada late Thursday into Friday and
late Saturday into Sunday. The first system looks warm and wet,
while the second system looks colder and may result in snow
Lingering light showers will taper off by late this afternoon for
areas north of Lake Tahoe. Gusty ridge winds have begun to weaken
marginally this afternoon, but expect a more considerable decrease
this evening. Light winds and dry conditions will prevail for much
of the week before a more active and wet storm pattern settles
across the Western U.S. starting late Thursday.
A warm (subtropical in origin) storm will impact the Sierra and
western Nevada starting Thursday afternoon. Considering the
subtropical nature of the system, snow levels will be very high
(~10,000 feet) with the primary concerns being associated with
rainfall. The latest simulations are showing a period of heavy
rainfall early Friday morning between 2am-7am before it tapers off
to showers by the afternoon. The system will quickly move through
the Sierra and western Nevada, with much of the rainfall ending by
early Friday afternoon. Storm rain totals for this storm for western
Nevada will range from a tenth to up to almost an inch in the
foothill locations. The high terrain of the Sierra, especially the
west side, will have the best chances for larger precipitation
amounts with totals from 0.5 to almost 2 inches with this storm.
With the very high snow levels, the main impacts will be above the
9,500-10,000 foot mark with between 6 to 12 inches of very wet
Sierra cement possible.
Localized heavy rainfall paired with steep terrain and/or recently
burned areas are of high concern. There will be an increased risk
for flooding, river rises, as well as rock falls, especially in
areas of steep terrain. Fire agencies involved in any recovery
efforts with recent burn areas (Marina, Little Valley, Emerald)
should be prepared for bursts of heavy rain along with an increased
potential for flooding. Although we are expecting rises on some of
the river systems in the region, like the Susan and Carson, we are
not expecting any flooding.
Those with interests or property near recent burn scars and/or steep
terrain should take extra caution as we go into this wetter and more
active pattern. After this storm exits the region, a colder storm
will impact the region for the weekend with potential snow impacts.
We advise that if you have travel plans across the Sierra to keep
an eye on the forecast changes each day through the weekend.
.LONG TERM...Saturday through Tuesday...
Saturday and Saturday night, timing discrepancies between the EC
and GFS are bringing increased uncertainty regarding precipitation.
The issue appears to be related to the location of a small lead
wave out ahead of the main upper trough (off the California coast).
The GFS brings the wave into northern California and shoves a cold
front into northeast California and northwest Nevada, with
precipitation totals of 0.50-0.75" in the northern Sierra and
perhaps up to 0.25" out into northwest NV on Saturday. Meanwhile,
the ECMWF (both 00Z and 12Z runs) indicates little or no
precipitation Saturday or Saturday night as its wave is closer to
the Oregon coast so the cold front hangs up well west of the
Sierra crest until the main trough moves inland on Sunday.
As the difference between the GFS and EC disturbances are on the
smaller-scale for Saturday and Saturday night, it is hard to lean
in any particular direction at this time as far as timing the cold
front. With this in mind, POP was kept fairly low to represent the
uncertainty. However, as far as impacts, snow levels appear in
both models to remain above the main northern Sierra passes
(Donner/I-80 and Spooner/hwy 50) through Saturday afternoon so
passes below 7500 feet are currently expected to remain wet.
Depending on the speed of the front, Saturday night could turn
troublesome for Sierra passes as the GFS pushes the main trough in
faster with substantial precipitation, most likely in the form of
snow near and above 7000 feet. Of course, the EC does not concur
as it is slower with the trough and produces no precipitation!
Sunday, even the EC brings in precipitation so pass issues are
quite possible then, with chances for a light snowfall down to
around 6000 feet/Lake Tahoe level per colder simulations.
Monday and Tuesday, simulations go a bit haywire with some
bringing another fast-moving disturbance through northern CA and
NV while others bring dry weather with some ridging. Left the
broad-brush low POP near and north of highway 50 for now.
Widespread CIGS 100-140 MSL near and north of highway 50 and west
of KNFL should become SCT and/or rise to above 120-150 MSL tonight.
A few -shra (-shsn above ~8.5 kft) may briefly bring obscuration
of higher terrain in the northern Sierra into evening. Wednesday
should feature dry conditions with just some higher clouds.
Wind-wise, flow aloft turns to deep south to southwest tonight
and Wednesday with gusts over ridges 50-65 kts this evening
decreasing to 40-55 kts. The direction is not favorable for major
mountain wave turbulence in the lee of the Sierra and have not
seen any PIREPs of note today. As far as below ridges, gusts to
20-25 kts (locally 30 kts) into this evening could bring some
modest mechanical turbulence. Winds are expected to slacken some
on Wednesday. Snyder
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