Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 212140
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
140 PM PST Tue Nov 21 2017

.SYNOPSIS...

Very warm conditions will prevail through Saturday with record
highs possible. Most areas will remain dry except very light rain
is possible Thursday as a weak disturbance moves through northern
California. Winds begin to increase Saturday, then a colder storm
system is possible by Sunday and Sunday night.

&&

.SHORT TERM...

High pressure will strengthen over northeast CA and western NV
through Wednesday, leading to further warming with highs likely
surpassing 70 degrees in some valleys. Reno`s record high is 69
and has a high probability (75% chance) of being surpassed as
cloud cover is more likely to be a cirrus deck with enough sun
filtering through. Even with less than optimal mixing, inversion
conditions are not expected as decent southwest flow aloft
prevails.

For Thursday/Thanksgiving Day, the upper ridge will flatten a bit
as low pressure moves into the Pacific Northwest, while a narrow
band of moisture pushes across northern CA and northwest NV. The
location of this moisture feed has trended a bit farther south,
possibly reaching I-80 Thursday afternoon. However, very few
impacts are expected as rain amounts will be quite light
(generally 0.10" or less) due to limited forcing, with snow
levels very high (near and above 10,000 feet). Increased cloud
cover and the weaker ridge will limit heating a bit, reducing the
probability of reaching Thursday`s record high of 69 degrees at
Reno.

The ridge rebuilds again Friday, reducing rain chances although
widespread high cloud cover and more limited mixing will likely
prevent much warming over Thursday. A weak disturbance with warm
air advection moisture may bring very light rain Friday night
into early Saturday across northeast CA and far northwest NV.
Then during the day Saturday, southwest flow increases ahead of
the next trough with gusty winds mixing down to the valleys
during the afternoon. With warm air mass still in place and the
thicker cloud shield lifting farther north, Saturday could end up
being the warmest day of the upcoming week especially from I-80
southward, with highs again above 70 degrees in some valleys and
approaching record values again. MJD

.LONG TERM...Sunday through Tuesday...

Bottom Line Up Front: Snow over Sierra passes and across
higher elevations of northeast CA could impact holiday travelers
as early as Sunday evening. Those with travel plans over mountain
passes Sunday evening/night should continue checking the forecast
for the possibility of snow. There is also a chance for travel
impacts due to snow during the Monday morning commute.

Recent computer simulations have trended a bit wetter with the
Sunday evening through Sunday night storm. It still doesn`t look
like a major precipitation producer, but wetter simulations are
showing 1 to 2 inches at the Sierra crest. The latest ECMWF model
also spreads the moisture farther south, bringing precipitation
to northern/central Mono County Sunday evening and sweeping the
cold front across Mono County Sunday night. (For southern Mono
County including Mammoth Lakes, the better chance for rain/snow
looks to be late Sunday night/Monday morning if the storm holds
together.)

The forecast challenge is how soon will snow levels fall below pass
levels? The latest ECMWF continues to be warmer keeping the snow
level above pass levels through Sunday evening. However, some colder
simulations (30% chance) bring snow levels down to pass levels
Sunday evening. The more likely scenario (70% chance) has snow
levels staying above 8,000 feet MSL through Sunday evening, and then
falling to 6500-7000 feet overnight Sunday night. Holiday
travelers are urged to continue checking the forecast for Sunday-
Sunday night.

Winds look strong with 700 mb winds between 50-60 kts Sunday-Sunday
night. The period of strongest gusts below ridge level and in
valleys depends on the (uncertain) timing and magnitude of the
cold front, but there is potential for dangerous cross-winds along
Highway 395/I-580 starting Sunday afternoon.

Monday morning is another period to keep an eye on, because light
snow north of Highway 50 could impact the morning commute down to
around 5000 feet, while heavier rain/snow could persist south of
Highway 50.

For Monday night-Tuesday, the outlook is for a high pressure
ridge to build over California and the western Great Basin,
creating benign weather conditions overall. However, a brisk
north-northeast flow could keep cool air over the region east of
the Sierra crest. JCM

&&

.AVIATION...

VFR conditions will prevail through at least Friday, except
patchy fog is again possible at KTRK early Wednesday morning. The
presence of cirrus and no new rainfall since yesterday should
reduce the fog coverage and duration compared to this morning.
Winds will be generally light through Friday, except for
periods of stronger gusts over ridges producing some lee side
turbulence. The next weather system with potential impacts to
aviation could begin Saturday with increasing winds, with even
more wind and a chance for rain and high elevation snow Sunday.
MJD

&&

.REV Watches/Warnings/Advisories...
NV...None.
CA...None.
&&

$$

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


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