Flash Flood Watch 4, NWS Las Vegas, NV

Area Forecast Discussion
National Weather Service Las Vegas NV
1015 PM PDT Wed May 8 2019

.SYNOPSIS...A low pressure system is expected to elongate and
shift westward, eventually lingering off the California coast
through the weekend. Periods of shower and thunderstorm activity
along with cooler temperatures can be expected with possible heavy
rainfall Thursday evening across Southern Nevada and southeastern
California. Precip potential lingers through the weekend but
conditions will dry out and temperatures will climb by early next
week.
&&

.EVENING UPDATE...
No changes to the current forecast grids but confidence does
continues to increase for the potential for showers and storms
tomorrow with heavy rain potential peaking tomorrow evening along
and north of the I-15 corridor.

Models continue to show an elongated low pressure system evolving
through the course of the day tomorrow which will shift over the
Sierra tomorrow afternoon and evening and bring the region into
favorable southwesterly divergent flow. Later in the evening, 700 mb
WAA from the south will create mid level moisture convergence
coincident with this favorable divergent flow and could bring in
heavy rain and if heavy enough, flash flood concerns. The area of
highest concern looks to be areas along and north of the I-15
corridor extending from southwestern Utah to the southern Sierra
range. Timing looks to be after 8PM or so. Will pass along these
trends to the overnight crew for consideration of possible product
issuances but none were made at this time due to continued low
confidence in precip potential after 00Z Saturday.

.DISCUSSION...Rest of today through Tuesday.

Some isolated showers and thunderstorms have begun to pop-up mainly
across far eastern Lincoln county and along the AZ Strip this
afternoon. Isolated storms should mainly remain confined to the far
eastern edges of the CWA through this afternoon where the better
instability is (SB CAPE 500 J/kg). Only concern with the small
isolated convection would be lightning and gusty winds. Elsewhere,
lingering moisture has allowed for some towering cumulus to
develop but much of region should remain dry.

A more active weather pattern will begin Thursday remaining through
the weekend as multiple shortwaves move through the region. The
next shortwave will swing through the base of the long wave trough
Thursday. This shortwave will elongate and interact with a piece
of energy off the SoCal coast, creating a broad area of low
pressure by Friday. This slow moving low pressure will eventually
close off just off the SoCal coast and move eastward across the
northern California Baja. This system will create widespread storm
chances through the weekend as it evolves. With multiple days
expecting widespread thunderstorm chances and increasing PW's of
0.8-1" flash flooding could become a concern.

Focusing on Thursday, models have been consistent in developing a
broad band of precip. across southern NV, Inyo County and northern San
Bernardino County overnight tomorrow mainly between 03Z-15Z where
the best area for ascent sets up. Storm totals of 0.5"-1.00"+
will be possible with this band, creating overnight flooding
concerns and morning commute issues. With the dynamic nature of
this system, I wouldn't be surprised to see some overnight
thunderstorms tomorrow night as well. Some gusty and erratic winds
will be possible with the scattered thunderstorm activity tomorrow
afternoon. As deeper moisture moves across the region tomorrow
evening and we become saturated the gusty wind potentially will
decrease a bit but will be present with any strong thunderstorms.

Not a whole lot of confidence in the details beyond Friday morning
as the evolution of the closed low is still not fully resolved
yet. Still expecting scattered showers and thunderstorms each day
through the weekend.

By Monday, the low should begin to shift to the east, but there
will be enough lingering moisture for some scattered activity to
develop along the higher terrain Monday and Tuesday afternoons
under a broad high-amplitude ridge. &&

.AVIATION...For McCarran...There appears to be a boundary of
stronger winds west and northwest of the air terminal and lighter
northeast to east winds north, east and south of the air
terminal. Winds are expected to be predominately out of the north
and gusty for the next few hours but could occasionally fluctuate
to a light northeast to east wind. Winds should relax early this
evening becoming variable under 8 kts then increase again out of
the northeast with speeds generally around 10 kts. Scattered-to-
broken clouds around 12k feet through the TAF period.

For the rest of southern Nevada, northwest Arizona and southeast
California...Northerly favored winds 8-12 kts gusting up to 25
kts through the afternoon, decreasing this evening, then
increasing again late evening as another push of northerly winds
pushes south through the area. that will stay in place through the
majority of the afternoon. KDAG will see gusty west to southwest
winds this evening and overnight, decreasing by daybreak Thursday.
Scattered to areas of broken clouds around 10-12k feet through
the TAF period. &&

.SPOTTER INFORMATION STATEMENT...Spotter activation may be
required as an unseasonably active pattern takes hold of the
region. Thunderstorm chances look greatest Thursday and/or Friday
though sporadic activity will linger through the weekend.
&&

DISCUSSION...Kryston
AVIATION...Salmen

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