355
FXUS65 KVEF 201900
AFDVEF

Area Forecast Discussion
National Weather Service Las Vegas NV
1200 PM PDT Sat Jul 20 2024

.SYNOPSIS...Excessive heat builds across the Mojave Desert this
weekend while thunderstorm potential becomes largely confined to the
southern Great Basin and northwest Arizona. Moisture increases
during the work week, bringing precipitation chances westward. Later
in the week, a Pacific system may provide cooler and drier
conditions.
&&


.UPDATE...Expecting another round of showers and thunderstorms
across southern Nevada, northwestern Arizona, and northern Inyo
County this afternoon, but with a lesser areal extent than
yesterday. Decreasing PWATs will aid in reducing flash flood threat
this afternoon, but cannot completely rule out the risk beneath
training or stationary thunderstorms. Convection will favor the
higher terrain today and will taper off shortly after sunset.

The monsoonal high pressure system has continued to shift in our
direction, with the center of the 500 mb high directly overhead.
Though we aren`t expecting record-breaking temperatures this week,
dew point temperatures in the 50s will make 110s feel oppressive.
Coupled with overnight low temperatures around 10 degrees above
seasonal averages, many desert valley locations will fail to drop
below 90 degrees at night. Expect similar conditions through the
week as monsoon moisture lingers through the work week. The
Excessive Heat Warning has been expanded today to include far
southern Nevada, San Bernardino County, the valleys of Inyo County,
and southern Mohave County through Sunday. Will continue to assess
the need for an extension of this headline over the next few shifts.
No changes have been made to the current forecast as it looks to be
in good shape.
&&


.PREVIOUS DISCUSSION...157 AM PDT Sat Jul 20 2024/


.SHORT TERM...this weekend.

The 500mb high continues to drift over the forecast area, becoming
centered overhead today. In the 850-700mb layer, flow has become
more westerly particularly in the lower Colorado River Valley.
Latest VAD wind profiles from KYUX show westerly or northwesterly
winds from 2000 to 8000 ft ARL, indicating that any moisture
advection from the Gulf of California has ceased. This would explain
the 0.20" drop in PWAT observed between the 12z and 00z soundings
yesterday, and the continued decrease shown in model guidance. By
this afternoon, most guidance brings PWATs down to ~1" or less for
most of the area. As a result, PoPs come down across the lower
deserts, with the 20+ percent rain chances confined to eastern
Mohave County, Lincoln County, and the Sierra. Tomorrow, low/mid-
level flow in the Colorado River Valley reacquires at least some
southerly component, bringing a slight increase in moisture. Not
sure this moisture will be sufficient to change PoPs much from
today, with the best chances remaining in the southern Great
Basin/Sierra and eastern Mohave County. However, with the 500mb high
forecast to drift just off to our west/northwest, northerly steering
flow develops over SE Nevada and NW Arizona. This has the potential
to push the convection and/or outflow that develops in Lincoln
County or SW Utah down into the Las Vegas Valley Sunday evening. A
few CAMs (and even the NBM to an extent) support this potential,
with northerly outflow of 20-40 mph and a slight uptick in PoPs
around the valley Sunday evening.

Outside of any thunderstorm influences, heat will continue to
increase across the area, particularly the lower elevations of the
Mojave Desert. Excessive Heat Warnings go into effect for the
remainder of this area at 10 AM this morning and continue through
Sunday evening. Highs are forecast to be 105-120 while lows remain
in the 80-95 range. This will yield Major to Extreme HeatRisk,
posing a threat to anyone without adequate cooling or hydration.

.LONG TERM...Monday through Friday.

Moisture will increase across the area early next week as the upper-
level ridge wobbles and we see southerly flow return in the mid-to-
lower levels of the atmosphere. As such, we will precipitation
chances increase on Monday with a 30% to 60% chance of showers and
thunderstorms in eastern Mohave County and widespread 20% to 50%
chances of precipitation across southern Nevada. These increased
precipitation chances will push further to the west on Tuesday, with
the higher terrain of Inyo and eastern San Bernardino Counties
seeing the introduction of 20% to 40% chances of precipitation.
Afternoon and early evening precipitation chances will continue
through mid-week before they begin decreasing during the latter
portion of the week as an incoming shortwave approaches the Pacific
Northwest, eroding at our ridge. As the ridge erodes and 500 mb
heights fall we will see temperatures decrease a few degrees, which
will be a welcome relief after starting the week with temperatures 5
to 8 degrees hotter than normal.
&&


.AVIATION...For Harry Reid...Light south-southwest wind the rest of
the night. Winds speeds will remain 6 kts or less but the direction
will transition from the northeast after 16Z. Still looking at the
development of an isolated shower or thunderstorm west of the valley
over the Spring Mountains/Red Rock Canyon between 19Z and 22Z.
Outflow winds from the collapsing storm near Red Rock Canyon could
shift the wind to a westerly direction after 22Z. Winds look to
remain from a west-southwest direction this evening and overnight.
Similar conditions expected Sunday with shower and thunderstorm
formation staying along the higher terrain west and south of the
valley with outflow winds potentially impacting the valley
terminals.

For the rest of southern Nevada, northwest Arizona and southeast
California...Outside of the Las Vegas Valley, isolated thunderstorms
expected over northwest Arizona, central Nevada and the vicinity of
KBIH in the Owens Valley. Erratic, gusty winds up to 30 mph at KBIH
possible between 21Z and 02Z. Ceilings will drop to 10-12 kft with
thunderstorm activity. Winds will return to diurnal trends overnight
before isolated thunderstorm chances return to the region
Sunday afternoon with similar timing.
&&


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

&&

$$

UPDATE...Soulat
SHORT TERM...Woods
LONG TERM...Stessman
AVIATION...Pierce

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