Flash Flooding and Heavy Snow in Arizona, November 28 to 29, 2019

Area Forecast Discussion
National Weather Service Las Vegas NV
331 PM PST Thu Nov 28 2019

.SYNOPSIS...A massive winter storm will continue to bring
significant precipitation to the area through tonight. The system
passes over the area Friday resulting in a cool day with mainly
mountain showers through the day. The weekend will begin dry but a
disturbance will bring moisture to the Sierra later Saturday through
Monday while the remainder of the area stays dry. Atmospheric
ingredients could come together to bring more widespread
precipitation to the area by the middle of next week.
&&

.DISCUSSION...through Friday.

Tonight, precip becomes more showery over the western CWA and
accumulating amounts should be lighter. For the central and eastern
portions of the CWA including eastern San Bernardino, Clark, Lincoln
and Mohave counties, the heaviest precipitation is either continuing
or just getting underway. Add to that, isolated thunderstorms
embedded within the wider precipitation shield producing enhanced
rain rates and potentially localized flash flooding concerns over
some of the same areas where already significant precip amounts are
forecast.

Friday, the core of the system moves over Nevada and cold air aloft
will lead to instability showers throughout the day mainly over the
mountains of Nye, Clark, Mohave, and Lincoln counties. The whole
system moves east Friday night ending precip chances for us.

Saturday through Thursday...The weekend begins dry as we say goodbye
to the multi-day Thanksgiving storm. The weather pattern remains
progressive however with brief ridging passing over the area
Saturday with a eastern Pacific disturbance bringing subtropical
moisture to the central and northern California coast Saturday night
and Sunday. In the meantime, further north over the PacNW, a low
pressure system pinwheels westward and merges with a low pressure
system off the coast and their joined forces eject shortwave energy
moving around its base to the northern coast of California. The low
then sinks southward through the eastern Pacific and taps into
abundant tropical moisture that it brings into the southwest mid
week.

Among the operational models, the southern Sierra appears to be on
the extreme southern end of the initial moisture plume that
intersects the Sierra range over the weekend into the early part of
next week. The official forecast uses a blend of the NBM and WPC QPF
yielding snow amounts in the 1-2 foot range for Aspendell and
Whitney Portal with upwards of 3-3.5 feet at the peaks from Saturday
through Monday. Spillover into the Owens Valley early in the event
while snow levels are low, gives an inch or two of snow
accumulation. Snow levels are forecast to rise through the event,
with rain potentially falling at elevations that could still have
significant snow from this current event, possibly creating runoff
issues if it all comes together.
&&

.AVIATION...For McCarran...Continuous light to moderate rain
and ceilings around 1-1.5 kft AGL will persist over the terminal
area through about 06Z then rain should taper off. Ceilings could
temporarily lower to around 500-700 feet between this evening.
Ceilings should break up and lift between 07Z and 14Z. Scattered
clouds with bases around 5 kft lingering near the mountains could
move over the terminal in the afternoon causing a brief period of
ceilings broken 5 kft. Gusty southwest winds up to 25-30 knots can
be expected much of the Friday.

For the rest of southern Nevada, northwest Arizona and southeast
California...Widespread rain and snow will bring extensive MVFR to
IFR conditions along with mountain obscurement across the region
through late tonight. Precipitation will taper off from west to east
between 04Z and 12Z with VFR conditions likely returning to much of
the region Friday morning. Areas of low clouds and fog will be
possible in some valleys before clearing out late Friday morning and
afternoon. Lingering snow showers and low clouds over southeast
Nevada and northwest Arizona will gradually decrease in coverage
Friday afternoon. Widespread southwest winds gusting 25-30 knots can
be expected Friday across the southern Great Basin and Mojave Desert
region. Gusts over 35-40 knots will across the western Mojave Desert
including Barstow.
&&

..SPOTTER INFORMATION STATEMENT...Spotters are encouraged to report
significant rain and snow amounts, significant winds, and weather
related damage according to standard operating procedures.
&&

DISCUSSION...Salmen
AVIATION...Adair

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