Excessive Heat in California, August 13 to 16, 2019

Area Forecast Discussion
National Weather Service Phoenix AZ
1030 AM MST Tue Aug 13 2019

.UPDATE...Updated aviation discussion.

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.SYNOPSIS...
Dry and much warmer conditions will affect the region through the
middle of this week under generally clear skies. Excessive heat
with high temperatures between 110 and 115 degrees are expected
through Thursday over the lower deserts. Temperatures will
decrease slightly on Friday and further into Saturday with a
brief increase in moisture, with isolated thunderstorms mainly
confined to Gila County.

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.DISCUSSION...
Unseasonably dry air is now in place across southern California
and Arizona with PWATs generally below 1". Surface dew points have
mostly dropped into the 40s with many locations even briefly
dipping into the middle 30s. This lack of moisture more so
resembles June instead of August. This amount of drying during
the monsoon season isn't all that unusual as it often happens at
least once a season. However, when we do dry out this much is
usually leads to hot conditions and oftentimes excessively hot
temperatures.

The forecast over the next few days has not changed as we are
still looking at a big warm-up with highs today likely topping
110 degrees in many locations. Highs Wednesday and Thursday will
be even warmer with many lower desert locations flirting with 115
degrees. The upper level high pressure system responsible for
this heat will shift westward over the region today with the high
center settling near the New Mexico/Arizona border on Wednesday.
Upper levels heights over the next few days will likely hit 90% of
climo and this may be enough to threaten record highs in some
locations Wednesday and Thursday. Eventually, the positioning of
the high should allow for some moisture seepage back into
southeastern portions of Arizona beginning Thursday, but
especially on Friday. By Friday, the combination of lowering
heights aloft and increased boundary layer moisture should lower
highs closer to 110 degrees. Friday's forecast highs are fairly
close to meeting excessive heat levels, so the Excessive Heat
Warning may need to be extended into Friday for some zones.

Shower and thunderstorm chances do return on Friday for high
terrain areas, but models have overall been backing off on rain
chances. Temperatures will continue to lower into the weekend as a
strong Pacific trough moves into the Northwest U.S. while pushing
the subtropical high much farther to the east. This should allow
temperatures to retreat closer to normals this weekend, but model
guidance indicates temperatures should stay above normal by a
couple degrees.

Next week looks to be a near repeat of this week as it looks like
we will dry out and warm up again during the first half of next
week. Once the Pacific trough to our north exits eastward late in
the weekend, the subtropical high will rebuild to our east and
begin shifting westward toward our region. Eventually, the high
center is seen shifting directly over the Desert Southwest by the
middle of next week. This scenario will likely bring another
significant heat episode and highs above 110 degrees.

&&

.AVIATION...Updated at 1730 UTC.

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

Minimal aviation concerns expected through the TAF period. Dry
southwest flow will keep any clouds/thunderstorms well east of the
Phoenix area, with mainly CLR skies expected. Afternoon/evening
westerly breezes likely to persist well into tonight, with perhaps a
period of light/variable winds between midnight and daybreak on
Wednesday before becoming light easterly during the morning hours on
Wednesday. Light western winds to return on Wednesday afternoon.
Winds speeds mainly to be aob 8 kts.

Southeast California/Southwest Arizona including KIPL and KBLH:

Minimal aviation concerns expected through the TAF period. Dry
southwest flow will keep skies clear. Winds will favor the south or
southwest at KBLH through tonight, with light/variable winds
Wednesday morning. Winds to remain out of the southeast at KIPL this
afternoon before switching to west this evening. Most speeds will
remain below 8 kts.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Thursday through Monday:
Very dry conditions will continue over the region through Thursday
as minimum relative humidity values will generally be in the 5-15
percent range for the lower deserts to around 15-20 percent for
higher terrain locations east of Phoenix. Overnight recovery will
be poor to fair except at higher elevations where better recovery
is expected. Moisture will increase some on Friday and Saturday
with a slight chance of isolated thunderstorms over higher terrain
locations in Gila County before moisture decreases once again on
Sunday and Monday. Winds will remain on the light side with
diurnal tendencies and occasional afternoon breeziness except for
Friday when wind gusts in excess of 20 mph will be possible.

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

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.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning from 10 AM this morning to 8 PM MST
     Thursday for AZZ530>546-548>551-553>556-559-560.

CA...Excessive Heat Warning from 10 AM this morning to 8 PM PDT
     Thursday for CAZ561>570.

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DISCUSSION...Kuhlman
AVIATION...Percha
FIRE WEATHER...Hopper