Flash Flood Warning 136, NWS Las Vegas, NV

Area Forecast Discussion
National Weather Service Las Vegas NV
339 PM PDT Sun Sep 30 2018

.SYNOPSIS...Tropical moisture from Hurricane Rosa will spread up
from the south and across much of the region through Monday and
Tuesday. This will result in widespread rainfall, which will be
accompanied by thunderstorms and possible flash flooding. Additional
weather systems will move across the region keeping unsettled and
cooler weather in place through the week.
&&

.SHORT TERM...through Wednesday night.

Showers and thunderstorms will ramp up over areas southeast of I-15
tonight and Monday then become more extensive Monday night and
Tuesday as tropical moisture spreads up from the south. Rapidly
developing intense thunderstorms propagating north from Yuma were
accompanying a surge of moisture from the Gulf of California. This
prompted an immediate beginning to the Flash Flood Watch for
southern Mohave County and nearby sections of the Colorado River
Valley. Rainfall rates around 4 inches per hour were observed near
Yuma early this afternoon which gives an indication of the potential
of these storms over the next few days in our area as deeper
moisture moves in. The core of Rosa is still forecast to move up
across northern Baja and up through Arizona on Tuesday, but the
moisture will begin to be pulled across our region over the next 24-
36 hours. The 12Z GFS, NAM and ECMWF all indicated the potential for
showers and thunderstorms pushing up across southern Nevada Monday
night through Tuesday morning. Differential heating will be in play
Monday and Tuesday afternoon which will help enhance convection
along with broad upward forcing provided by the closed upper low off
the California Coast. Even though individual storms will be moving
rather quickly from southwest to northeast, there will be the
potential for cells to repeatedly move over areas...especially
downwind of mountains which will enhance flash flood potential where
these bands of showers/storms develop.

There are indications of a dry slot spreading in late Tuesday
through early Wednesday as the coastal closed low moves inland, then
additional moisture rounding the base may focus more over the
southern Sierra and Inyo County Wednesday. Meanwhile, there will be
lingering low level moisture to produce at least scattered showers
and thunderstorms across the region Wednesday afternoon and
Wednesday night as the system lifts over Nevada.

.LONG TERM...Thursday through Sunday.

As the zonal jet across the Pacific buckles later this week, a
highly amplified longwave is expected to develop with a highly
amplified ridge over the eastern Pacific and broad trough draped
across the western states. This pattern will keep northwest flow
aloft over the Great Basin and Mojave desert and direct a parade of
shortwaves into the region. As one trough ejects into the
Intermountain west Thursday morning another shortwave will quickly
trail behind and eject to the east by Friday morning. That second
shortwave will lack any significant forcing but may allow a few
showers across the portions of the southern Great Basin along an ill-
defined cold front Thursday into Friday; elsewhere, dry conditions
and breezy north winds are likely. With the region between weather
systems Friday, conditions will be fair outside of lingering north
breezes. Temperatures will stay steady through the latter part of
the week with valley highs in low 80s and higher elevations in the
upper 50s to lower 60s. Operational and ensemble guidance is in
fairly decent agreement on a very broad, deep upper low developing
across the northern Great Basin on Saturday. This setup has the
potential to bring a significant cool down along with showers and
gusty north winds along a cold front.
&&

.AVIATION...For McCarran...Gusts will continue to increase to
upward of 25-30 kts before subsiding around 03-04Z. Drainage
southwesterly winds between through sunrise Monday then shifting to
the southeast and increasing to 12-18 knots and gusts up to 25 knots
by the afternoon. Increasing low clouds with CIGS between 5-8 kft
and chances for rain showers and isolated thunderstorms on Monday.

For the rest of southern Nevada, northwest Arizona and southeast
California...Southerly winds will continue into the evening with
gusts peaking between 20-30 knots. Similar to Saturday evening,
gusts are expected to subside after sunset. Southerly winds will
increase to 15-25 knots again during the early afternoon Monday. By
this evening and through Monday, increasing low clouds with CIGS
between 4-8 kft along with increasing chances for showers and
isolated thunderstorms are expected at all sites but KBIH.
&&

.AVIATION...For McCarran...Gusts will continue to increase to
upward of 25-30 kts before subsiding around 03-04Z. Drainage
southwesterly winds through sunrise Monday then shifting to the
southeast and increasing to 12-18 knots and gusts up to 25 knots by
the afternoon. Increasing low clouds with CIGS between 5-8 kft and
chances for rain showers and isolated thunderstorms on Monday.

For the rest of southern Nevada, northwest Arizona and southeast
California...Southerly wind speeds are expected to increase through
morning with gusts peaking between 25-35 knots. Similar to Saturday
evening, gusts are expected to subside after sunset. Southerly winds
will increase to 15-25 knots at all sites Monday afternoon.
Increasing low clouds with CIGS between 4-8 kft and chances for rain
showers and isolated thunderstorms this evening through Monday.
&&

.SPOTTER INFORMATION STATEMENT...Heavy rainfall is expected Monday
and Tuesday, and perhaps beyond. We encourage spotters to report any
significant weather or impacts in your area.
&&

SHORT TERM...Adair
LONG TERM/AVIATION...Boothe

For more forecast information...see us on our webpage:

http://weather.gov/lasvegas or follow us on Facebook and Twitter

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