Area Forecast Discussion
Issued by NWS Las Vegas, NV

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000
FXUS65 KVEF 251020
AFDVEF

Area Forecast Discussion
National Weather Service Las Vegas NV
320 AM PDT Tue Jul 25 2017

.SYNOPSIS...Another day of fairly widespread showers and
thunderstorms is expected today. Any storms will be capable of
producing flash flooding and strong winds. Gradually drying
conditions are expected Wednesday through Friday with temperatures
warming back to above normal. Another uptick in moisture is
expected this weekend into early next week.
&&

.SHORT TERM...Today through Wednesday Night.

Showers and thunderstorms have persisted through the night, mainly
over northern Clark, northern Mohave, and southern Lincoln
counties. This activity has been courtesy of upper level
divergence associated with a jet streak extending from the Bay
Area across the Great Basin as well as a PV anomaly moving north
through Mohave County. Both of these features should combine to
push the centroid of activity from northeast Clark County eastward
across northern Mohave County through the morning.

Just as that convection moves out of the area, more general
daytime convection is expected to fire late this morning/early
afternoon. Only weakly aided by upper level divergence associated
with the aforementioned jet, this activity will be primarily
orographically/diurnally driven. Therefore storms will taper off
more rapidly during the evening hours (except perhaps over the far
northern reaches of the forecast area where stronger divergence
will exist). Current RAP/HRRR analyses indicate an axis of
significant instability extending from near Lake Havasu City north
to Lake Mead, then curving west toward Tonopah. This is a bit
further south than previously expected and both models indicate
convection firing across the southern reaches of that axis by late
morning. PW values are still in the 1.5-2.0" range in those
areas, and the 1.0" line extends north to Tonopah. This means that
any storms that do form will be capable of Flash Flooding. As a
result I opted to expand the Flash Flood Watch back south to
include southern Mohave County and the Colorado River Valley.
Areas that are experiencing storms early this morning (see first
paragraph) will see lower (but non-zero) odds of significant
thunderstorm activity later this afternoon. A secondary weak axis
of instability will develop near Twentynine Palms, but overall
coverage should be more isolated.

Drying conditions are expected Wednesday, with storms being
diurnally driven once again. The best chances will extend from
the Spring/Sheep ranges north across the southern Great Basin.
Chances will also exist over the high terrain of Mohave and Inyo
counties. &&

.LONG TERM...Thursday through Monday

Much quieter weather is expected for the end of the week into the
weekend as high pressure works is way back over the area. A
significant upper level ridge sets up over us, with increasing model
agreement with the high being centered further west and that it will
dry out for a few days. Decreased precip chances for Thursday
afternoon from previous forecast. Heights will be increasing and
with limited moisture, think precipitation activity will be
isolated.

Generally dry weather will continue on Friday before chances for
precipitation return to the forecast. Models are not in an kind of
agreement on the next push of moisture and how that will translate
to shower and thunderstorm development. Some of the models bring in
higher PWATs and theta-e values by Saturday afternoon, while others
hold off until Sunday night. Favored the slower/drier solutions
since models tend to bring in moisture too quickly. This would
introduce showers and storm chances to Mohave County Saturday
evening...and brings chance precipitation to the entire area by
Sunday afternoon. Didnt make much in the way of chances after 18Z
Sunday as confidence is too low, with general chance for precip
across the entire area each day. Without a key feature to focus on
for lift, it will be hard to determine where things will get active.

The other thing to watch will be temperatures. Set up Friday and
Saturday (and maybe Sunday depending on how quickly moisture returns
to the region) is favorable for warmer than normal temperatures. 850-
700mb temperatures jump into the 30s on many of the longer term
models- up from the middle 20s off this evening`s sounding. A lot
will depend on how quickly moisture and precip returns, which will
make or break temps. Current model guidance isn`t too impressive
with high temperatures this weekend, and it will not be anything
similar to the heatwave we saw in June, but anyone with outdoor
plans Friday through Sunday should be prepared for the heat as
above normal temperatures look likely. Temperatures will cool off
with the wave of moisture pushing into the region. &&

.FIRE WEATHER...Scattered showers and thunderstorms will occur over
much of the region again today with any storm capable of producing
very heavy rainfall. Conditions will gradually dry out later this
week, with thunderstorms decreasing in coverage each day.
Temperatures will be near normal to somewhat below normal through
Wednesday, before returning to slightly above normal late in the
week. Another uptick in monsoon moisture is expected to arrive
sometime this weekend.
&&

.HYDROLOGY...Creeks, streams and rivers in northern Inyo County
have shown decreasing flows over the last couple of days.
However, some road closures continue in the area. Bishop Creek
Bypass and Big Pine Creek Bypass continue to be utilized to
minimize flows through Bishop and Big Pine. Remember, never drive
through flooded roadways or around barricades. Also, pastureland
or farmland adjacent to creeks, streams or rivers could experience
areas of standing water.
&&

.AVIATION...For McCarran...Low confidence forecast for today as
shower and thunderstorm activity continues. Currently have showers
developing just to the north of the Valley, extending south into
eastern parts of Las Vegas. Expect this first round of precipitation
to continue the next few hours before it gets shoved eastward away
from the LAS area. In addition to the precipitation, an outflow
boundary developed and shifted southward through the airspace,
veering winds to N-NE with gusts to around 15kts. This should also
last about 2 or 3 hours at VGT and LAS before winds go variable
again.

After a brief lull this morning, more scattered shower and
thunderstorms may develop this afternoon. Hard to say if things will
get going in the Valley as the current precip may decrease
redevelopment later today, however confident enough to keep it in
the TAF forecast. If the Valley were to see more showers and storms,
the most likely time period would be between 20Z-00Z. After 00Z,
will need to watch if storms continue or quickly die off as models
show both solutions. Anything that does develop has the potential to
produce heavy rain, with lower ceilings below 100kft as well.

Low confidence in the wind forecast for this afternoon too, as wind
direction will depend on precipitation redevelopment. Current TAF
forecast would be without the influence of storms.

Quieter and drier conditions should return by late Wednesday
morning.

For the rest of southern Nevada, northwest Arizona and southeast
California...Scattered showers and thunderstorms are again possible
this afternoon, with the better chances over northwest AZ and
southern NV. Will need to watch development and wind directions,
especially in southern CA into the Colorado River Valley.

.SPOTTER INFORMATION STATEMENT...Thunderstorms capable of strong
winds and flash flooding are possible once again today. Spotters
are encouraged to report any significant weather or impacts
according to standard operating procedures.
&&

$$

SHORT TERM/FIRE WEATHER...Wolcott
LONG TERM/AVIATION........Allen


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