Excessive Heat in California, August 15 to 16, 2021

Area Forecast Discussion
National Weather Service Las Vegas NV
1000 AM PDT Sun Aug 15 2021

.SYNOPSIS...A strong area of high pressure centered over northwest
Nevada will lead to above normal temperatures through Monday. A
lingering moist and unstable air mass will continue to produce
isolated afternoon thunderstorms over the mountains of southern
Nevada and southeast California, with greater storm coverage over
northwest Arizona through Tuesday. Cooler temperatures and a slow
drying trend are expected to spread over the region after Tuesday.
&&

.UPDATE...Moisture and instability, along with the overall pattern,
have not changed much in the past 24 hours and the expectation for
thunderstorm development is similar to Saturday afternoon and
evening. Isolated thunderstorms that initiate over Lincoln County
and central Nye counties (SB CAPE values around 500 J/kg) will be
steered toward the southwest by clockwise flow around the 500mb high
centered over northwest Nevada. Higher moisture and instability
resides over Mohave County, and more surface heating is available
today to prime the air mass for thunderstorms moving in from the rim
country late this afternoon and evening. No need for immediate
changes to the going forecast trends for these expectations.

The high over northwest Nevada is also driving smoke from the
northern California wildfires farther south and restricted
visibilities of 4-5 have made it into the northern Owens Valley. The
weather grids will be updated with the 12Z HRRR Smoke forecast. -
Adair
&&

.PREV DISCUSSION...147 AM PDT

.SHORT TERM...through Monday night. Early morning satellite and
radar loops showed scattered to broken mid level debris clouds over
much of our CWA once again, along with isolated weak radar returns
which were likely producing brief light showers at most. Surface obs
showed light winds and seasonably warm temperatures areawide, with
dewpoints mostly in the 50s and 60s southeast of I-15 and mostly in
the 30s and 40s farther northwest. First concern will be the heat.
Excessive Heat Warning still looks good, with Heat Risk values of 3
and 4 covering most of the warned zones today and Monday. Farther
north, zones 14 and 15 are of growing concern as splotches of 3 and
4 have been increasing in coverage for today and Monday with each
passing forecast. Coverage is not great enough to warrant a Warning,
so will hoist a Heat Advisory for today and tomorrow, which will
also match with WFO LKN's Advisory for northern Nye County. Second
concern will be thunderstorms. High resolution models are showing
similarities to yesterday, with development beginning in the early
afternoon on the higher terrain, then the focus shifting to Mohave
County later in the afternoon as storms roll in from the east. 850-
300 mb bulk shear magnitude is forecast to increase over Mohave Co
in the late afternoon, so there could be an isolated severe storm
before convection fizzles around midnight. Monday looks like a
relatively down day for storms, with less coverage and less
organization than yesterday or today. By Monday night, things will
begin to change as a trough of low pressure swings into British
Columbia and the Pacific Northwest states, suppressing the big dome
of high pressure over our area. More on that in the long term
section.

.LONG TERM...Tuesday through Saturday.

The setup is now looking less favorable for deeper moisture to be
drawn into our region ahead of the aforementioned approaching trough
though lingering moisture should remain over part of southern Nevada
and northwest Arizona and maybe eastern San Bernardino County. There
should still be sufficient instability over these areas for
afternoon thunderstorms Tuesday and possibly again Wednesday and
this is reflected by the latest NBM PoPs which indicate more than
just terrain-based storms. However, the ECMWF and GEFS ensemble
means have been digging the base of the trough a little deeper
resulting a drier southwest flow which could scour moisture out to
the east during the Tuesday-Wednesday period followed by a
persistent dry west to northwest flow Thursday into next weekend.
The ensemble-based NBM PoPs are a little slow at indicating this
drying trend so far, but temperatures decrease several degrees after
Tuesday under the influence of the trough and PoPs will likely trend
downward over the next few model runs if ensembles continue to dig
the trough slightly farther south.
&&

.AVIATION...For McCarran...Light easterly winds expected this
afternoon that will once again shift to the southeast through the
evening. These southeast winds are expected to range from 8 to 10
kts, but gusts 15 to 25 kts from the southeast are possible with
storm development between 3 and 5Z. Confidence at this time is not
high enough to include in the TAF. Winds will become light and
variable overnight. Otherwise, scattered to broken mid and high
level clouds expected.

For the rest of southern Nevada, northwest Arizona and southeast
California...Light and diurnal winds are expected today across the
region with scattered to broken mid and high clouds over the Mojave
Desert and few clouds elsewhere. Gusty south-southeast winds
possible at KIGM, KEED, KIFP, KHND, and KLAS between 00Z and 05Z as
isolated storm formation moves through the Peach Springs corridor.
&&

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

SHORT TERM...Morgan
LONG TERM...Adair
AVIATION...Varian

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