Flash Flood Watch 9, NWS Las Vegas, NV

Area Forecast Discussion
National Weather Service Las Vegas NV
327 AM PST Mon Nov 18 2019

.SYNOPSIS...By Tuesday, a significant weather system is expected
to bring widespread rain with the possibility for the first
significant snowfall for elevations above 6000 feet. While
confidence is still very low on details such as rain/snow amounts,
confidence overall is increasing for the potential for
significant impacts associated with this system through Thursday
and possibly Friday. &&

.SHORT TERM...through Monday.
While overall, very little in the way of impacts or even sensible
changes will occur between now and early Tuesday, the upper level
flow pattern will be taking shape for the mid to late week
period. This is characterized by a well advertised and evident low
just west of the Baja peninsula working to pull up tropical
moisture from the south. This feature will be meandering around
this general area for the next 48 hours or so before becoming
influenced by deep diving energy from the Pacific northwest.

Until then, some local enhancement of the surface pressure gradient
today is already resulting in gusty north winds along the
Colorado River Valley. Expecting this to persist through much of
the day today, however mostly localized to the Laughlin/Bullhead
City areas, with lower winds elsewhere. Boaters on Lake Mohave
from Cottonwood Cove and south could see some enhanced wave
action, possibly getting to 1-1.5 feet at times. Lesser winds are
expected Monday with no weather impacts expected.

.LONG TERM...Tuesday through Sunday.
Focus in the forecast continues to be the system and potential
impacts Tuesday through Friday. Increasing confidence in a
widespread precipitation event midweek, with some details coming
into better focus. This is a difficult system to forecast,
especially as so much will depend on how the southern California
Low will interact with the incoming Pacific trough.

In general, think the overall evolution will be the following:
Thunderstorms should develop Tuesday along the leading edge of the
moisture surge and forcing, however its uncertain how much
convection will develop. Any thunderstorm activity should
dissipate Tuesday night but overall precipitation coverage will
increase as significant forcing pushes into the area. Wednesday
night the area of main forcing will shift out of the area, however
the closed cold core low will move overhead, so we will
transition to a more showery regime. Then as the low exits
Thursday evening and overnight, precipitation should end from west
to east and should be lighter as moisture slowly decreases.

Tried to fine tune precip chances and amounts based on overnight
model trends. Wanted to reflect ares of better forcing and
moisture where there could be excessive rainfall amounts. This
brought up QPF amounts for areas southeast of I-15, and decreased
them elsewhere. Areas to the north and west will be displaced from
the best forcing and with dry air being pulled in behind the
system Thursday, precipitation should shut down faster and be
lighter than over Clark, Mohave, and eastern San Bernardino
counties. Heaviest rain still should be Tuesday night and
Wednesday, outside of any possible convection Tuesday afternoon.
Concern for flooding is increasing through these areas as ensemble
probabilities show decent chances for over an inch of rain in 24
hours. Given the anomalous high PWAT values through 00Z Thursday,
this is not out of the question, and highlighted the best chance
of an inch or more of QPF in the Day 2 and Day 3 excessive
rainfall outlooks, with best chance for excessive rainfall through
Mohave county into Arizona. Continued uncertainty with the end
time of precipitation Thursday night or Friday as longer range
models still disagree on how quickly the upper level low will lift
out and exit to the east.

Did not make any changes to the current headlines. Latest model
runs brought in the colder air faster, which will play a
significant role in snow amounts if cold air gets in fast enough
to coincide with the heaviest precipitation. Will likely need
advisories or warnings at some point, but would like to continue
to watch temperature and QPF trends before issuing anything.
Biggest change from the previous forecast: snow amounts in the
Sierra came down a few inches due to lower QPF amounts and faster
end time with the latest update.

Concerned about Tuesday afternoon and thunderstorm potential.
With the leading edge of moisture pushing north into the area,
there should be enough lift to kick off at least a few showers.
However, modest instability will be in place ahead of the
moisture surge, which may result in scattered convection Tuesday
afternoon. As a strong area of diffluence aloft will be moving in
just behind the moisture boundary, could have some decent, longer
lived thunderstorms Tuesday afternoon. Hi-res models have very
different scenarios on timing and coverage of this potential
convection, so confidence in the Tuesday afternoon and evening
forecast is low. High confidence that there will be some storms
around, but how much convection that develops is the question.
Tried to focus precipitation chances Tuesday afternoon over the
most likely areas for storms in southern Mohave County, but
convection may develop through southern Clark county and eastern
San Bernardino counties. With any convection, there is a
possibility of training storms and heavy rain, which could result
in flash flooding. Will need to watch hi-res model trends for
Tuesday as this could be a time period of higher impact weather.

This is obviously a complicated and rare weather system for the
desert Southwest, so continue to watch for updates and changes as
confidence increases and details become more clear.

After the system exits, dry and cool weather looks like the most
likely scenario for Friday into next weekend.
&&

.AVIATION...For McCarran...Light and diurnal winds expected
generally below 6 kts with a northeasterly component during the day
and a southwesterly component overnight. Quiet weather will continue
through the TAF period with dry conditions and occasional high
clouds aoa 25 kft.

For the rest of southern Nevada, northwest Arizona and southeast
California...Winds will be light and diurnal with winds remaining
generally below 8 kts. The exception will be down the Colorado River
Valley where north wind gusts 20-25 kts later this morning and into
the afternoon. Winds in the Colorado River Valley will begin to
diminish late this afternoon. Quiet weather will continue through the
TAF period with dry conditions and occasional high clouds aoa 25
kft, although lower ceilings 10-15 kft may start working north
toward Needles by early Tuesday morning.
&&

.SPOTTER INFORMATION STATEMENT...Spotters are encouraged to report
any significant weather or impacts according to standard operating
procedures.
&&

SHORT TERM...Salmen
LONG TERM...Wolf
AVIATION...Pierce

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