Area Forecast Discussion
Issued by NWS Reno, NV

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718
FXUS65 KREV 241107
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
307 AM PST Fri Nov 24 2017

.SYNOPSIS...

Very mild conditions will prevail through Saturday with a few record
high temperatures possible. Saturday and Sunday will be windy ahead
of a strong cold front which will bring snow to the Sierra and
northeast California Sunday and Sunday night. Expect travel delays
across the Sierra due to slick driving conditions and higher post
holiday traffic volume. Next week will be colder with more typical
temperatures for November.

&&

.SHORT TERM...

Some moderate changes were made to the previous forecast mainly
concerning timing for onset of winds and precipitation. Other
changes tended to be minor like increasing wind speeds over ridges
this morning.

Models continue the trend of slowing the approach of a moderate to
strong Pacific storm. Current projections are about 6-12 hours
slower than previous runs. Additionally, the secondary and
stronger wave of this system is trending wetter. This will have
implications for travel disruptions over the Sierra Sunday night
into Monday.

Winds:

Sunday is shaping up to be the period of highest concern for
winds. The 700mb wind vectors have shifted a little, more south-
southwest, and we could experience a quasi south-wind event for
the Sierra Front. This flow direction promotes stronger gusts, low
level wind shear for the Reno Airport, and possible damage to
outdoor decor, fences, and trees; some power outages will be
possible.

After a slight breezy morning for the foothills today, winds are
expected drop to around 10 mph today through early Saturday.
Winds become breezy by Saturday afternoon as gusts increase into
the 30 to 40 mph range.

The jet associated with the approaching system is projected to
be transverse to the Sierra a little later with the slower system
translational speed. As a result, valley winds were decreased
Saturday evening through the overnight hours. Ridge winds will
still increase, but a weak stable layer will likely blunt most
valley winds. Washoe Valley is an exception where conditions will
likely be gustier than surrounding valleys. However, as the jet
maximum of near 150kts approaches early Sunday, expect winds to
rapidly increase for valley locations...especially along the
Sierra Front.

Downsloping signatures remain present for Sunday with impressive
75kt 700mb wind speeds. Therefore, wind gust estimates remain
around 60 mph along the Sierra Front and Highway 395 corridor.
Wind prone locations could easily gust to 70 mph. Ridge wind gusts
will likely exceed 100 mph at times on Sunday. Expect travel
disruptions for aviation and for high profile vehicles.
Additionally, conditions on Lake Tahoe and Pyramid Lake will be
hazardous with waves exceeding 3 to 4 feet this weekend.

Precipitation:

Light rain is lingering along the Oregon border this morning with
the rest of the region experiencing dry conditions. There is
potential for some light, warm-air advection precipitation very
early Saturday morning for northern Lassen, eastern Modoc, and
northern Washoe counties. NAM favors precipitation in these
locations while GFS is slightly farther north. Only a trace to a
hundredth or so is possible.

For the next significant system: like winds, the onset of
precipitation and lowering of snow levels has been pushed back.
Also, with a more southerly flow component, the extent of
precipitation chances were trimmed back considerably east of the
Virginia Range. Locations east of the Virginia range will
experience the bulk of their precipitation along the frontal
boundary more into Monday.

Elsewhere in the Sierra and western Lassen County, chances ramp up
considerably later Saturday night into early Sunday morning. The
first wave moves through late Sunday afternoon bringing moderate
to heavy precipitation in the Sierra into Sunday night. Snow
levels begin to fall in earnest Sunday night after midnight;
starting around 8000 feet dropping to around 6500 feet early
Monday morning. Anticipate snow accumulation over Sierra passes
and possibly down to the west shore of Lake Tahoe overnight.

As mentioned before, QPF was increased for the first portion of
the storm with totals up to 1.5 inches along the Sierra crest
through Sunday night. These totals fall rapidly away for the crest
with Reno receiving up to 0.25 inches of rain through Sunday
night. Foothill locations will do a little better with up to 0.5
inches. As for snow through Sunday night, 4 to 8 inches for the
higher passes and up to 4 inches around Lake Tahoe are reasonable
ranges at this time. Up to 3 inches for locations west of
Susanville is also reasonable through Sunday night. The bulk of
this snow will fall after 10 pm.

Bottom line: Expect travel delays due to increased traffic volume
and degrading conditions by Sunday evening. Boyd

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

Most of the forecast problems in the extended time frame center
around the passage of the cold front Monday and how fast the upper
trough influence will leave the forecast area Monday afternoon and
evening. Then there is the issue of a secondary trough that will
need to be addressed Tuesday night into Wednesday.

Medium range model trends suggest frontal passage early Monday
morning through the northern half of the forecast area...with
passage through the southern half by early Monday afternoon. The
ECMWF is still a little slower than the GFS and that skews snow
levels...temperatures and winds a bit. For now we will trend toward
the slightly faster GFS...but keep precipitation hanging on a little
longer over southern areas in deference to the ECMWF solution. We
raised pops for Monday morning along the front into western Nevada
as frontogenetic forcing should be enough to generate showers...even
though qpf will be fairly low. Showers change to snow showers down
to about 4000 feet around sunrise Monday morning. That is just in
time for the early morning commute. It is not out of the realm of
possibility that some of the valleys in western Nevada could see 1-2
inches of snow prior to 8 am Monday morning. The details are not
certain at this point...so check back as we get closer to the
frontal passage time.

Colder air filters in Monday night and early Tuesday. Tuesday should
bring benign weather...but now there is a secondary trough showing
up in the model solutions for Tuesday night into Wednesday. Both the
GFS and ECMWF keep the bulk of the forcing north of the CWA with
only a very small chance of any precipitation near the Oregon
border. But the Canadian model and a few of the GEFS ensemble
members carve this trough much farther south...bringing another
quick-hitting round of precipitation to the region. This far out it
is hard to say which is correct; we will lean toward the drier
solution at this point.

Even if we do not see any precipitation...colder air will move into
central and eastern Nevada Wednesday night into Thursday and develop
a decent gradient across the region. This will mean increased
easterly ridge winds through the Sierra Wednesday night and Thursday
with colder highs both days.

&&

.AVIATION...

Flat ridging aloft today and much of Saturday should result in VFR
conditions with occasional wind gusts up to 20 kt at the surface.
Ridge winds today will gusts 40-50 kt and then increase late in the
day Saturday to 50-60 kt. This will mean continued instances of
turbulence over and downwind of the ridges through the weekend.

Saturday night into Sunday and approaching trough of low pressure
brings increased winds aloft and more turbulence. Winds in the
lower levels will begin to increase as well Sunday into Sunday night
with gusts near the terminals approaching 30-35 kt or more. Moisture
increases Sunday as well. Rain and high elevation snow develop
through the day from northeast California into the northern Sierra.
This will lead to lower cigs/vsbys Sunday afternoon into Sunday
evening. MVFR/IFR conditions become more widespread Sunday night and
extend into western Nevada Monday morning along a cold front that
moves through the region.

&&

.REV Watches/Warnings/Advisories...
NV...High Wind Watch from Saturday morning through Sunday afternoon
     NVZ003-005.

     High Wind Watch from Saturday morning through Saturday evening
     NVZ002.

CA...High Wind Watch from Saturday morning through Sunday afternoon
     CAZ070-071-073.

     High Wind Watch from Saturday morning through Saturday evening
     CAZ072.

&&

$$

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



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