Excessive Heat in California, August 20 to 21, 2019

Area Forecast Discussion
National Weather Service Phoenix AZ
1042 AM MST Tue Aug 20 2019

.UPDATE...Updated Aviation Discussion.

&&

.SYNOPSIS...
Temperatures will remain several degrees above normal as moisture
and thunderstorm activity will be very limited through Wednesday.
Another excessive heat episode with daily record high
temperatures is expected today and tomorrow across southern
Arizona and southeast California. Moisture will return by Thursday
beginning a gradual cooling trend and bringing isolated
thunderstorms back into the forecast for south central Arizona
during the second half of this week, with the best chances on
Friday and Saturday.

&&

.DISCUSSION...
Clear skies and dry conditions with dew points in the 30s and 40s
except over Yuma County where they are in the 50s has allowed
temperatures to cool into the upper 70s to mid 80s once again. The
region remains sandwiched between a weak trough over the east
Pacific just off the California coast and a mid-to-upper level
ridge centered near the Texas-New Mexico border. This ridge will
expand westward and become more zonally-oriented today as H500
heights increase to 593-594 dm over the region before decreasing
slightly tomorrow as the ridge shifts further west into the east
Pacific as a weak shortwave moves through the central Plains.
Forecast H850 temperatures will increase to 30-32 deg C today and
31-33 deg C on Wednesday over Arizona, with 1-2 deg cooler values
over the lower elevations of the Colorado River and Imperial
Valleys. This will allow temperatures to warm up into the 111-114
deg F range today and 112-116 deg F range tomorrow across most
lower desert locations with isolated hotter areas. This will break
daily records tomorrow and will challenge them today, so we have
maintained our Excessive Heat Warning for the lower deserts and
even higher terrain areas in Gila County today and tomorrow.

An easterly wave that is currently moving into the Bay of Campeche
will on Wednesday will move across Mexico on Wednesday to help
trigger a pronounced increase in convection over the Sierra Madre
and Baja California. This will help trigger a Gulf surge that
should move into the lower deserts of Arizona and southeast
California by Thursday morning, generally ending our excessive
heat episode as most locations "cool" below 110 degrees for their
high temperatures on Thursday. In addition, most models develop a
tropical cyclone just off the coast of the Mexican state of
Jalisco tomorrow or Thursday that will most likely track northwest
about 100-200 miles parallel to the west coast of Baja California
Sur based on the model ensemble. An inverted trough associated
with this circulation will also move up the Gulf of California to
help support a northward progression of convection on Thursday and
Friday, with increasing confidence that convection will develop
over southern Arizona by Thursday with greater coverage on Friday
once midlevel stability left behind by our current ridge erodes.

Although thunderstorms will probably not develop over northern
Pinal and Maricopa County on Thursday due to even stronger
midlevel stability than over southern Arizona, better chances
for thunderstorms over the Phoenix metro occur by Friday. The
region will be sandwiched between the inverted trough associated
with the east Pacific tropical cyclone and a relatively deep, but
weak shortwave trough moving into the Colorado River Valley.
Although the westerly flow associated with the shortwave would
ordinarily cause drying aloft, in this case it may help advect
additional moisture into the region or at least maintain it if the
tropical cyclone remains in close proximity to Baja California Sur
instead of taking a more westward track. Although there is
significant uncertainty there, the weak shortwave trough should at
least provide some dynamic lift through weak differential PVA that
could give us an extra boost in thunderstorms for Friday.
Therefore, we continue to maintain 10-20 PoPs for much of south
central Arizona for Friday, generally following the median of
model guidance instead of the more pessimistic NBM. Considerable
cloud cover that would limit our instability and southerly
steering flow aloft are reasons for pessimism on rain chances.

Saturday's forecast is even more uncertain than Friday's as it
will depend on how much the inverted trough is able to influence
our weather along with what becomes of the westerly disturbance.
Regardless, we should have relatively deep moisture in place for
Saturday, and a more northerly to easterly steering flow appears
to be more likely assuming the westerly disturbance is overtaken
by a stronger inverted trough. Of course, this will largely depend
on how strong the tropical disturbance in the east Pacific
ultimately becomes and where it tracks as a weaker disturbance
might allow the westerly trough to push through. Nevertheless,
rain chances remain in the 10-20 PoP range for Saturday for the
Phoenix metro, with decreasing chances further west. Models are
generally in agreement that the ridge will build back in from the
northwest on Sunday once the inverted trough moves into the
Pacific, giving us a break in convection through the first half of
next week. However, the monsoonal moisture is not expected to
clear out as it did with this last prolonged two week break,
potentially setting up a busier convective period for the second
half of next week as another inverted trough slides west under the
southern edge of a subtropical ridge displaced to our north.

&&

.AVIATION...Updated at 1740 UTC.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
There are no aviation weather concerns through the rest of the TAF
period. The winds are slowly becoming westerly and should complete
the switch within the next two hours across all TAF sites. Wind
speeds will remain mostly at or below 10 kts, except for a few
afternoon breezes. The winds are expected to become easterly
tonight but will be very light and variable at times.

Southeast California/Southwest Arizona including KIPL and KBLH:
There are no major aviation weather concerns through the TAF
period. The winds will favor a more westerly direction across the
region through the afternoon and evening with some stronger gusts
possible west of the Colorado River, including KIPL. The winds
will become light and variable again tonight.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Thursday through Monday:
Moisture will return by Thursday to begin a gradual cooling trend
and bring back isolated thunderstorms into the forecast for the
higher terrain of Gila County and possibly the lower deserts
including the Phoenix metro on Friday and Saturday. Thunderstorm
chances will begin to decrease on Sunday and Monday, but elevated
moisture levels should keep temperatures within a few degrees of
normal. Minimum relative humidity values will increase from the 10
to 20 percent range on Thursday to the 20 to 30 percent range by
the weekend. Winds will follow typical diurnal patterns through
Friday before westerlies generally prevail across the south
central Arizona zones for the weekend with afternoon and early
evening gustiness each day.

&&

.CLIMATE...

Record High Temperatures

Date        Phoenix         Yuma
----        -------         ----

Aug 20    112 in 1986    114 in 1982
Aug 21    110 in 2007    115 in 1969
Aug 22    113 in 2011    115 in 1969
Aug 23    114 in 2011    115 in 2011

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST Wednesday for AZZ530>546-
     548>556-559>562.

CA...Excessive Heat Warning until 8 PM PDT Wednesday for CAZ561>570.

     Excessive Heat Warning from 11 AM to 8 PM PDT Wednesday for
     CAZ560.

&&

DISCUSSION...Hopper/Hernandez
AVIATION...Deems
FIRE WEATHER...Hopper/Hernandez
CLIMATE...Rogers/Kuhlman/MO