Excessive Heat in California, August 1 to 3, 2020

Area Forecast Discussion
National Weather Service Phoenix AZ
1048 PM MST Fri Jul 31 2020

.UPDATE...
Updated 06Z aviation discussion.

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.SYNOPSIS...
Strong high pressure aloft will persist over the desert southwest
the into next week. As a result, very hot temperatures can be
expected area wide today and into early next week. Hotter deserts
will easily exceed 110 degrees each day with many locations
exceeding 115 degrees into the weekend. Thunderstorm chances will
remain minimal through early next week with any isolated activity
remaining over the higher terrain of southern Gila County. A modest
return of moisture during the latter part of the week will bring a
limited but noticeable return of storm chances during the mid to
late part of the week.

&&

.DISCUSSION...
The strong and very well established UL ridge remains stationary
over AZ with anomalous H5 heights observed near 599 dam. Its ridge
axis extends far north into W Canada and forms a blocking pattern
between NE PAC and E-central CONUS low pressure systems. After
setting a second high temperature record this week of 118 degrees in
Phoenix yesterday, highs today are expected to push up to 115-117
degrees in and around Phoenix and upwards of 117-119 degrees
out west.

With regional strong high pressure and the resulting very high to
high heat risk remaining in the forecast into early next week,
excessive heat warnings will now continue for the Phoenix, Yuma and
Imperial CA areas until Monday night, and all remaining areas through
Saturday night. As we near the end of July, this month is shaping up
as the hottest month and July on record in Phoenix. For more
information please visit NWS Phoenix on Twitter and Facebook.

During the weekend models are now in agreement on repositioning the
UL high center slightly further west which will bring a moisture
field on the eastern flank of the high more into play for far
eastern AZ. As a result CAMs indicate a slight chance for very
isolated thunderstorms every evening through early next week for far
eastern, southeastern and northeastern AZ including far eastern S.
Gila Cty.

Then by the middle of the week the UL high center repositions itself
again further east of the region near NM and W TX. At the same time
an EPAC trough sets up along coastal CA. The resulting return in
moist southerly flow into the region along with a regional cooling
trend will result in a better uptick in storm chances for the
eastern high terrain areas including S. Gila Cty. In addition a
fleeting 10 percent chance of precipitation can not be ruled out for
Phoenix and the lower deserts after the middle of the week. Some
early indications also hint at a late week drying trend associated
with the trough nudging in from the west.

&&

.AVIATION...Updated at 0548Z.

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

Minimal aviation impacts are expected through Saturday evening
aside from gusty westerly winds and very hot temperatures during
the afternoon and early evening hours. Very isolated showers and
thunderstorms during the afternoon hours will continue to remain
well east and southeast of the Phoenix terminals over the higher
terrain of Gila County and southeast Arizona as high pressure
dominates over the region. Westerly winds will become easterly or
light and variable overnight at most TAF sites, likely holding off
until after 12Z at KPHX after which there is low confidence on
wind directions but high confidence they will be weak. Regardless,
westerly winds should return by late morning at all sites with
gusts up to 20 knots from mid-afternoon to early evening.

Southeast California/Southwest Arizona including KIPL and KBLH:

Minimal aviation impacts are expected through Saturday evening
aside from gusty winds during the afternoon and evening hours at
KBLH. Westerly to southwest winds will become more southerly by
08-09Z at around 10 knots before veering to west-southwesterly
late tomorrow afternoon at KBLH and by early evening at KIPL. Wind
gusts up to 20 knots will be possible during the late afternoon
and evening hours, particularly at KBLH. Very isolated convection
that may form over the higher terrain of northern Baja California
is not expected to move into southwest Imperial County.

&&

.FIRE WEATHER...
Monday through Friday:
High pressure will continue across Arizona, resulting in
relatively dry conditions and well above normal temperatures with
lower desert highs near 110-115 almost everyday through early next
week. Wetting rains are generally not anticipated through the 5
day period, although an isolated afternoon thunderstorm is
possible mainly east of Globe over the weekend and early week.
Better chances for isolated showers and thunderstorms could come
into play across parts of south-central Arizona by Wednesday or
Thursday. Min RHs will generally fall into the teens each
afternoon with single digit values over portions of the lower
western deserts. Overnight max RH should range around 25-45
percent. Meanwhile, winds will be mostly light and diurnal each
day along with afternoon breezes.

&&

.CLIMATE...

Record High Temperatures

Date     Phoenix          Yuma         El Centro
----     -------          ----         ---------
7/31    115 in 1996    119 in 1957    118 in 1934
8/1     116 in 1972    117 in 1993    118 in 1993

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST Monday for AZZ532-537-
     540>544-546-548>551.

     Excessive Heat Warning until 8 PM MST Saturday for AZZ530-531-
     533>536-538-539-545-547-552>556-559>563.

     Heat Advisory until 8 PM MST Saturday for AZZ557-558.

CA...Excessive Heat Warning until 8 PM PDT Monday for CAZ565-567.

     Excessive Heat Warning until 8 PM PDT Saturday for CAZ560>564-
     566-568>570.

&&

DISCUSSION...Sawtelle
AVIATION...Hopper
FIRE WEATHER...Sawtelle/Smith
CLIMATE...MO