Area Forecast Discussion
Issued by NWS Reno, NV

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000
FXUS65 KREV 191007
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
307 AM PDT Sun Sep 19 2021

.SYNOPSIS...

Isolated to scattered rain showers are expected early this morning,
mainly north of Interstate 80 and for the Lake Tahoe area. It will
be breezy and cooler today as a cold front moves through the region.
Sub-freezing temperatures are on tap for some rural and suburban
lower valleys Monday and Tuesday morning. Warmer weather returns
for Tuesday afternoon through the rest of the week.

&&

.SHORT TERM...

Main change to the short term forecast was to raise a lake wind
advisory for Lake Tahoe this AM.

Low pressure is currently moving into Oregon with a rather ill-
defined surface cold front pushing into northeast CA and northwest
NV in response to the incoming low. Ahead of the front, winds over
Sierra ridges are peaking in the 50-85 mph range (and briefly to
around 100 mph at a sensor near the crest west of Tahoe), with
wind-prone areas near Hwy 395/I-580 peaking between 35-55 mph in
the past hour. Elsewhere, wind gusts have generally been 30 mph or
less.

The low passing by to the north today will shove a cold front
through northern/western NV and northeast CA this morning, with
the front working its way in a shallow fashion (below 700 mb)
through Mono and Mineral counties this afternoon and evening. With
winds remaining elevated until FROPA later this morning, winds on
Lake Tahoe look sufficient (15-20 mph with gusts to 35 mph) for
small boating concerns so a lake wind advisory was hoisted there
for the morning. It will remain breezy today, although peak winds
look mainly 30 mph or less. The main exceptions could be in the
Soda Spring (Hwy 95) and Chalfant (Hwy 6) valleys from mid
afternoon into the evening where winds could briefly get up to
30-40 mph immediately post-frontal.

As for precipitation this morning, it looks to remain light and
mainly north of I-80 and around the west side of the Lake Tahoe
Basin (mostly due to upslope lift west of crest) as the main area
of deep forcing/instability is over extreme northern CA/NV and up
into Oregon. Snow levels have bottomed out near 10,000 feet over
the Tahoe area and perhaps down to ~8,000 feet over far northeast
CA. As the upper low passes by to the north by mid to late
morning, any remaining showers should fall apart rapidly.

Tonight into Tuesday, an upper ridge builds back overhead. With
cooler highs and calming winds, radiational cooling should allow
lows to fall below freezing for some rural and suburban lower
valleys (and WELL below freezing for Sierra valleys) for the next
couple nights. If you have gardens still going (my tomatoes are
still cranking out plenty!), be sure to watch the forecast for
your location and consider covering your plants if the forecast is
for temperatures below 40 degrees. -Snyder

.LONG TERM...Wednesday through next weekend...

Other than a weak shortwave passage across the Pacific Northwest
bringing increased breezes (gusts 30-35 mph) Wednesday afternoon-
evening, the mid-late portion of the upcoming week will feature
warm and dry conditions with lighter winds Thursday-Friday. Highs
should mainly be in the 80s for lower elevations and mid-upper
70s for Sierra valleys, although Thursday might be a bit cooler
behind the shortwave passage as the winds shift to a northeast
direction.

As we get into next weekend, there are a couple of potential
scenarios that would yield varied results. The first keeps high
pressure in the vicinity of CA-NV which would result in continued
warm and dry conditions with generally light winds. The second
scenario is more complex with a closed low off the CA coast moving
inland toward the Sierra next weekend. This would produce cooler
and showery conditions and possibly some thunder with wetting
rainfall depending on the track of this low. While the first
scenario remains more likely among the ensemble guidance, we can`t
ignore the second possibility either. Therefore, we have trended
highs a bit lower and increased cloud cover for next Sunday but
held rain chances below 15% for now. MJD

&&

.AVIATION...

Main impact to aviation will be wind-related today as a cold
front passes through western NV with surface winds shifting to
NW-N in the afternoon, while stronger ridge level winds shift
more slowly from west to northwest through the day. This will keep
mountain wave turbulence going along with periods of wind shear.
The shear may occur at lower levels especially through midday
today. Later this afternoon into early evening, increased north
winds area expected to push across eastern portions of Mineral-
Mono counties with gusts around 30 kt. At the main terminals, wind
gusts are more likely to remain in the 20-25 kt range except
briefly stronger around the time of frontal passage.

Current shower activity mainly north of I-80 and west of Tahoe
will weaken through the morning. Ceilings could briefly dip into
MVFR category especially for KTRK-KTVL through 15Z when light
rain showers move overhead, but otherwise VFR conditions are
likely to prevail although with some mountain top obscurations.

Afterward, VFR conditions will prevail for the upcoming week. Most
days will see lighter winds, except for Wednesday when gusts
could increase to near 25 kt at the main terminals as a shortwave
trough brushes across the northwest US. MJD

&&

.FIRE WEATHER...

While the main gusty wind/low RH event of the weekend has already
passed, we will still be dealing with breezy conditions through
today. However, humidity will not be as low, with most areas
remaining above 20% through this afternoon. The only exception is
for eastern Mono County and central-southern Mineral County, where
the drier air mass will hang on through this afternoon with
minimum RH between 10-20%. As north winds increase later this
afternoon, there may be a 1-2 hour overlap of wind/RH reaching
low-end critical criteria especially in the Chalfant Valley/Benton
area and the US-95 corridor from Mina-Luning southward, but then
humidity values will rise with the post-frontal air mass.

After today, warm and dry conditions will prevail but winds will
be lighter most days through the upcoming week. The only exception
could be Wednesday due to a fast-moving shortwave passage across
the Pacific Northwest which could produce elevated fire weather
concerns. Current projections show potential for a marginal/low
end wind and RH event in some areas of western NV north of US-50.
MJD

&&

.REV Watches/Warnings/Advisories...
NV...Lake Wind Advisory until noon PDT today for Lake Tahoe in NVZ002.

CA...Lake Wind Advisory until noon PDT today for Lake Tahoe in CAZ072.

&&

$$

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


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