Excessive Heat in California, August 13 to 20, 2020

Area Forecast Discussion
National Weather Service Phoenix AZ
500 AM MST Thu Aug 13 2020

.UPDATE...Updated aviation discussion.

&&

.SYNOPSIS...
Strong high pressure will remain situated across the Southwestern
U.S. into next week bringing continued hot conditions. High
temperatures across the lower deserts will approach or exceed 115
degrees starting today and lasting through at least early next
week. Isolated showers and thunderstorms are possible today before
chances decrease tomorrow. Potential for a slightly better
monsoon set-up this weekend or early next week.

&&

.DISCUSSION...
Strong high pressure over southern New Mexico will continue to
strengthen and progress northwestward, positioning itself over us
Friday and near the Nevada/Utah border by Sunday. The center of
the high then appears to slowly drift back south towards the
middle of next week. This will keep very warm temperatures in
place, with highs approaching or exceeding 115 degrees each day.
The Excessive Heat Warning will expand and cover the entire
forecast area by tomorrow and will remain in effect through
Monday.

In terms of moisture, south-southwest flow on the western
periphery of the high is currently streaming in thicker mid- and
upper-level moisture. This is advecting thicker clouds, embedded
with isolated virga, into our forecast area. These thicker clouds
will likely prohibit temperatures from dropping too much
overnight, with Phoenix likely adding another 90 degree or above
low temperature to the tally. 1 inch or greater PWs will be likely
again today, with the greatest values across the western portions
of Maricopa & Pinal counties westward into the Colorado River
Valley. Best storm coverage will be in this area, as well as
across the mountainous terrain north, south, and east of Phoenix.
Any shower or storm that manages to develop will be brief and be
capable of producing lighting, gusty erratic winds, heavy rain,
and blowing dust.

As high pressure moves overhead tomorrow, storm chances will
decrease a bit due to a more stable airmass a loft. However, it
does appear that once the center of the high is north of us,
pretty much Saturday onward, there will be a more favorable
pattern aloft that could potentially drive some storms into the
lower deserts, especially on Sunday. On Sunday, there currently appears
to be better potential for a stronger outflow to push in from the
east and interact with a moist and unstable atmosphere in the
lower deserts and possibly regenerate new storms. Due to this
potential for storms on Sunday, the NBM is showing a decent spread
in the high temperature forecast. We will continue to monitor the
situation, any little change in the forecasted position of the
high, could lead to storm chances shifting to another day. Due to
the uncertainties, the Excessive Heat Warning was not extended
into the middle of next week at this time. However, if
temperatures continue to trend as warm as they have been trending,
the warning may need to be extended.

&&

.AVIATION...Updated at 0530Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Primary forecast challenge remains the potential for storms across
western Maricopa County this afternoon. Enough of a signal now for
isolated convection to develop and perhaps push an outflow across
western/central parts of the Phoenix area late afternoon and
evening. Most likely wind directions accompanying any outflow
would be southwesterly or northwesterly. Did mention potential for
a few gusts at 23Z to account for this. Otherwise, SCT-BKN high
cigs will continue, with cumulus buildups to the west/southwest
likely this afternoon. Aside from outflows, winds will follow
typical diurnal patterns.

Southeast California/Southwest Arizona including KIPL and KBLH:
No major weather concern should exist through Friday morning
though there is an outside potential for storms to get close to
KBLH late Thursday afternoon pushing an outflow boundary towards
the airport. Winds will be rather light through the period mainly
favoring a southerly or westerly component through periods of
variability are likely. Otherwise, thicker high cigs through the
morning should begin to thin during the day.

&&

.FIRE WEATHER...
Saturday through Wednesday:
Very hot temperatures and overall dry conditions will persist
through the middle of next week. Minimum humidities through the
period will drop into the teens each day with overnight recoveries
generally between 25-50%. Winds will generally follow typical
diurnal/terrain influences with speeds remaining light aside from
some typical afternoon breezes. Thunderstorm activity will be
limited with isolated storms possible but favoring the high
terrain.

&&

.CLIMATE...

Record high temperatures:

Date     Phoenix         Yuma          El Centro
----     -------         ----          ---------
8/13   115 in 2012    117 in 2012     116 in 2012
8/14   117 in 2015    115 in 2019     116 in 2015
8/15   115 in 2015    117 in 2015     118 in 2019
8/16   113 in 2013    115 in 1992     117 in 2016
8/17   114 in 2013    114 in 1992     115 in 2015
8/18   112 in 2011    116 in 1960     115 in 2015

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST Monday for AZZ537-540-
     542>544-546-548-550-551-556-560-562.

     Excessive Heat Warning from 10 AM this morning to 8 PM MST
     Monday for AZZ530>536-538-539-541-545-547-549-552>555-559-
     561.

     Heat Advisory from 10 AM this morning to 8 PM MST Monday for
     AZZ557-558-563.

CA...Excessive Heat Warning from 10 AM this morning to 8 PM PDT
     Monday for CAZ565>567-569-570.

     Excessive Heat Warning from 10 AM Friday to 8 PM PDT Monday for
     CAZ560>564-568.

&&

DISCUSSION...Feldkircher
AVIATION...Rogers/MO
FIRE WEATHER...Feldkircher
CLIMATE...MO