Area Forecast Discussion
Issued by NWS Reno, NV
FXUS65 KREV 261025
Area Forecast Discussion
National Weather Service Reno NV
325 AM PDT Wed Oct 26 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
Focus for the next few days remains the influx of subtropical
moisture and periods of heavy rainfall expected late Thursday into
Friday morning. A splitting low across the eastern Pacific will be
responsible for drawing a substantial amount of subtropical
moisture northward which may approach the max observed column
moisture (PWAT) values for late October. Model soundings indicate
PWATs close to 0.75" by early Friday morning with some of that
moisture originating from what is now Hurricane Seymour located
about 750 miles southwest of the southern tip of Baja Mexico.
The subtropical origins of this moisture will result in a system
characterized by very high snow levels(9,000-10,000 feet) and a
favorable dynamic and isentropic setup to see little mountain
shadowing with plenty of spillover rainfall into western Nevada.
The main changes in the forecast were to make sizable increases
in rain amounts Thursday night into Friday morning particularly
across western Nevada where good spillover is expected.
The latest model runs are showing better consistency but the GFS has
still been behaving oddly in timing/QPF in contrast to the EC which
has been steadfast in maintaining a very wet system Thursday evening
into Friday morning. In particular, the GFS has shown conflict with
its own ensemble runs as the deterministic QPF is well below the
ensemble mean and even below the lowest GEFS ensemble member. As
such, have weighted the forecast towards the EC and a wetter system.
Timing still favors the heaviest rainfall across the Sierra and
western Nevada 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 roughly a third to two-thirds of an inch across western Nevada to
up to 1.25" through the foothills. The high terrain of the Sierra,
especially the west side, will have the best chances for larger
precipitation amounts with totals from 1.25-2.25 inches with this
storm. The very high snow levels with this storm will limit snow
impacts to areas 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 in these areas along with river and creek rises, as
well as rock falls, especially in areas of steep terrain.
.LONG TERM...Saturday through Tuesday...
Unsettled weather will continue through the weekend and into early
next week with additional rain, mountain snow, and breezy winds. For
Saturday and Saturday night, there are still some differences
between the timing in the GFS and ECMWF. The GFS showing an
initial band of isentropic lift moving into the Northern Sierra,
while the ECMWF continues to show little to no precipitation
during this time. Snow levels will be around 7500-8000 feet in the
Northern Sierra Saturday night, so the major passes in
Northeastern CA should be spared by accumulating snow through
Sunday looks to be the most likely timing for the main moisture
plume to move into the Sierra and western NV. We have continued to
raise POPs and QPF values for Sunday through Sunday night, with QPF
amounts up to one inch possible in the Sierra during this time. Snow
levels will come down to around 6000-7000 feet from Lake Tahoe
northward, with snow levels in Mono County around 7000-8000 feet.
This will mean that accumulating snow will be likely around Tahoe,
including Donner Pass and Echo Summit. Light snow may be possible
even down to Lake Tahoe level, although little to no accumulation is
expected at this time below 6500 feet. There is some potential that
snow levels could be lower than we have in the current forecast, as
the GFS is showing colder air as compared to the ECMWF. Anyone
traveling in the Sierra Sunday through Monday morning should be
prepared for winter driving conditions, especially over the passes.
Monday and Tuesday, there will be a bit of a reprieve from the
heavier precipitation, although light showers are still possible
from Lake Tahoe northward. Another shortwave may move through the
region on Tuesday, although due to model differences over the last
several runs, forecaster confidence is low beyond Monday. Hoon
Lighter winds and drier conditions expected for today with a few
high clouds. Wind gusts up to 15-20kts likely this afternoon. A
wet/warm storm system will move in late Thursday through Friday with
increasing clouds, moderate to heavy rainfall and restrictions to
CIG/VIS likely. Another storm is set to move in this weekend with
additional impacts to aviation as well. Hoon
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