Flash Flood Warning 146, NWS Las Vegas, NV

Area Forecast Discussion
National Weather Service Las Vegas NV
0830 PM PST Thu Dec 21 2023

.SYNOPSIS...An incoming system is expected to bring widespread
rainfall to areas south and east of Interstate-15 on Friday and
Saturday. Heaviest rainfall expected south of Interstate-40, with
flood concerns limited to the Highway-62 corridor and Lake Havasu
City area, where a flood watch has been issued. Drier conditions
return on Sunday while temperatures slowly warm next week.
&&

.UPDATE...Current Multi-Radar Multi-Sensor (MRMS) imagery shows
precipitation moving along the southern Californian coast, with some
precipitation penetrating inland between San Diego and Santa Barbra.
Water vapor and satellite imagery show the cut-off low responsible
for said precipitation churning right along the southern California
coast line. This system remains on track to impact our forecast area
tomorrow through Saturday. However, this system is moving slower
than previously anticipated. The latest HiRes guidance has caught on
to this trend and now does not bring precipitation into the Morongo
Basin and western Mojave Desert until around 12Z, whereas previous
guidance had it moving in as early as 06Z. Despite this slight delay
in precipitation onset time, the Flood Watched issued earlier this
afternoon continues to look good. HiRes models continue to highlight
areas south of Interstate 40 in San Bernardino and Mohave counties
as areas where the highest QPF totals are expected.

&&

.SHORT TERM...Today through Saturday.

The well-advertised low pressure system coming in from SoCal is
forecast to begin its turn east today. As it does, the plume of
moisture and precipitation ahead of it will translate eastward as
well. For today, any precipitation will be confined to our far
western areas, namely western Inyo County. Despite the abundant mid-
level and high clouds out there, high temperatures are forecast to
be above normal across the area. Not much wind to speak of on the
front side of this system, so other than some breezy east winds in
San Bernardino County, light winds are forecast around the region.

Early tomorrow morning, as the system begins to come ashore,
increased moisture and precipitation will move into our southern
deserts, spreading north and east throughout the day. Outside of the
high terrain, the best chances for measurable precipitation will
be along and south of the Interstate-15 corridor. In Las Vegas,
the highest rain chances (50-70%) will be in the late morning
through the early evening. Hi-res guidance depicts two distinct
regimes of precipitation with this system. The first being the
initial inland push from what's left of the atmospheric river
ahead of the low. The second is wrap-around/deformation zone
precipitation on the northwest side of the system. Precipitation
in this regime should be relatively light and exit the CWA by
Saturday night.

In terms of amounts, precipitation totals will generally decrease as
you move north. Many areas south and east of I-15 will see a nice,
beneficial rain. Latest HREF has 30%+ probabilities of 0.25" with
this system. Chances of seeing 1" or more are largely confined to
locations south of I-40. Given extended time period over which this
precip will fall (24-36 hours) and lack of heavy rainfall rates (10-
30% chances of 0.25"/hour), flood concerns are mainly limited to
our more sensitive areas. This includes the Highway-62 corridor
and Lake Havasu City. As a result, opted to issue a flood watch
for these areas from late tonight through early Saturday morning.
On the winter side of things, not expecting much, if any, impacts.
Snow continues to fall on the Sierra and White Mountains through
tonight, but amounts in populated/traveled areas are minimal. A
similar story is set to unfold for the Spring Mountains, with the
most impactful amounts expected to remain above populated/traveled
areas. Minor impacts possible in upper Kyle Canyon and Lee
Canyon, but shouldn't warrant any product issuance.

.LONG TERM...Sunday through Wednesday.

By Sunday, this system will be well east of us, with a northern
shortwave diving south through Utah. This will keep the area dry and
induce some breezy north winds, mainly in the Colorado River Valley.
Temperatures are forecast to be closer to normal, with 50s and low
60s across most of the CWA. Ensemble guidance keeps the region under
a northwesterly flow pattern through mid-week, resulting in dry
conditions and light northerly breezes. Temperatures gradually warm
to about 5 degrees above normal.
&&

.AVIATION...For Harry Reid...Dry conditions are expected to continue
at the terminal into Friday morning. Precipitation will begin
approaching from the south late tonight into Friday morning but
should not reach the field until after 17Z.  FEW-SCT cloud bases
will remain at 6-8kft through most of the night before CIGs of 6-
7kft develop tomorrow morning as the precipitation approaches and
occasional CIGs below 5kft are possible in and near shower activity
tomorrow afternoon and early evening.  Approach paths from the south
and southwest will be impacted earlier in the day with scattered
showers and lower CIGs.  The shower activity will continue into the
evening before decreasing after 02Z.  Winds are expected to remain
less than 10kts through the TAF period.

For the rest of southern Nevada, northwest Arizona and southeast
California...Precipitation will spread south to north tonight and
Friday, with most TAF sites seeing impacts at some point during the
forecast period. The highest chances will be across southern areas,
with KDAG, KEED, and KIFP seeing the best chance of showers starting
tomorrow morning.  Scattered showers are also possible in the Vegas
Valley by tomorrow afternoon.  SCT-BKN skies can be expected into
this evening, with CIGs less than 8kft increasing south to north
overnight. CIGs less than 4kft feet will be possible with the
heavier shower activity tomorrow morning and afternoon. Winds will
remain less than 10kts at most sites, except KDAG, where winds
gusting to 20kts are expected through this afternoon before
decreasing.
&&

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

DISCUSSION...Woods
AVIATION...Planz

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