Excessive Heat in California, August 20, 2020

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
1015 PM MST Wed Aug 19 2020

.UPDATE...Updated aviation discussion.

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.SYNOPSIS...
High pressure will remain dominant over the Desert Southwest
through the next several days. This will keep temperatures well
above normal for mid to late August. Daily afternoon
thunderstorms will continue to favor the high terrain of northern
and southeastern AZ with a slight chance for activity to push off
the high terrain into the lower deserts. Lightning and strong
gusty winds, capable of kicking up dust, are the primary concerns
with any storms, but localized flash flooding and hail are also
possible.

&&

.Short-term... The Mogollon Rim early this afternoon is fairly
active, similar to yesterday. One of the main differences from
yesterday is steering flow is much weaker, but still easterly
across southern AZ. So, slower storm motions are expected. DCAPE
is still very high, 1500-2000 J/kg, which will support strong
downburst potential through this afternoon and evening.
Mesoanalysis is showing better MLCAPE this afternoon over south-
central AZ as compared to yesterday, but CIN is still very high
and may hurt chances for storms to make it into the Valley later
this afternoon. The low to mid levels of the atmosphere is also
quite dry, as evident in this morning's sounding from Phoenix.
Still a strong enough outflow may be all it takes to kick off
storms. Model soundings do suggest the cap weakens by early
evening and most look similar to last evening's Phoenix sounding,
which had no inversion to speak of in the mid and upper levels and
marginal CAPE (~500 mb). Elevated convection was able to develop
and persist through the Valley with last night's environment. So,
even if storms are unable to push into the Valley this afternoon,
there is still hope for elevated convection tonight, especially if
a weak shortwave impulse passes through the area with the
easterly flow.

.Previous discussion...
By Thursday night the high repositions further south over AZ. The
high will maintain excessive heat and very high to high heat risk
across the region through Thursday evening. However beginning
Thursday night into Friday and the early weekend model soundings
indicate a significant increase in BL and mid-level moisture and
instability. The fairly possible regional increase in moisture
from the south is supported by a forecast increase in NW MX
convection triggered by the moisture from Genevieve. In addition,
the weak wind core of the UL high reduces in size enough that
isolated convection chances will not be overly diminished for late
this week and Saturday in spite of the closer proximity of the
high, though some uncertainty remains.

The increases in cloud cover and moisture late this week and into
the weekend will also help to curb temperatures with a high of
only 107 now forecast for Phoenix for Saturday. As a result
excessive heat is now out of the forecast through the weekend
although there is a chance of another warmup for early next week
with drier air possibly moving into the region.

&&

.AVIATION...Updated at 0515Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Convection was very limited tonight; for the rest of the evening and
into the morning look for SCT to BKN mid and hi level debris decks
becoming genly FEW after 12z with little cloud into the afternoon.
There is potential for another round of late afternoon/evening
convection bringing strong gusty northeast to east outflow winds
into the Phoenix area after 02z...so TAFs have been updated to
reflect wind potential. Due to some limited confidence and model
differences, did not get overly aggressive with low
visibility/blowing dust or putting convection in the TAFs. Should
this active scenario work out updates will be made in the upcoming
packages. Otherwise there will be increasing clouds during late
afternoon/early evening with scattered high based cumulus and BKN-
OVC mid and high cloud decks with bases aoa 14k feet.

Southeast California/Southwest Arizona including KIPL and KBLH:

Look for some mid and high cloud to drift into the deserts rest of
tonight with bases aoa 15k feet; skies to become mostly FEW by
Thursday morning. Winds to favor the southeast next 24 hours at KIPL
around 10kt, and favor the south at KBLH with some increase in speed
during the afternoon tomorrow.

&&

.FIRE WEATHER...
Saturday through Wednesday:
High pressure will persist across the Desert Southwest this
weekend and  into next week, which will keep
temperatures well above normal. Highest moisture levels will be on
Saturday with Hurricane Genevieve near northern Baja California.
Afternoon min RHs will be in the 20-30% with an overnight recovery
to 45-60%, but moisture levels will then trend down the following
days as high pressure restrengthens. The increased moisture this
weekend may become a bit more favorable for thunderstorm activity,
even across the lower deserts, although primary storm chances will
stay across high terrain areas to the east of Phoenix each day.
Any storms that develop have the potential to produce strong
gusty, erratic, winds, lightning and localized flash flooding.
Min RHs will fall back into the teens by Monday with overnight
recoveries in the 35-45% range.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST Thursday for AZZ530>556-
     559>562.

     Heat Advisory until 8 PM MST Thursday for AZZ557-558-563.

CA...Excessive Heat Warning until 8 PM PDT Thursday for CAZ560>570.

&&

SHORT-TERM...Benedict
PREVIOUS DISCUSSION...Sawtelle
AVIATION...CB
FIRE WEATHER...Benedict