Excessive Heat in Arizona, August 18 to 31, 2018

Area Forecast Discussion
National Weather Service Phoenix AZ
1010 PM MST Fri Aug 17 2018

.UPDATE...Updated Aviation Discussion.

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.SYNOPSIS...
Isolated thunderstorms today will generally stay over the higher
terrain north and east of Phoenix and southeast California, but a
storm or two may move into the Phoenix metro early this evening.
Thunderstorm chances will decrease into the weekend with all
areas likely remaining dry on Sunday. Above normal temperatures
return Sunday through the first half of next week, with better
thunderstorm chances returning by next Tuesday.

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.DISCUSSION...

The 12Z sounding from Phoenix this morning still shows PWs exceeding
two inches with ML CAPE near 1400 J/kg. Except for a small layer
around 700 mb, light westerly flow exists throughout the profile.
With this flow, models show progressively drier air moving into
our area not only this afternoon but throughout the weekend. This
is all in response to the upper level pattern with high pressure
centered just to our south and a trough of low pressure moving
through the Pacific Northwest.

The majority of models show moisture decreasing this afternoon to
around 1.6 to 1.7 inches with ML CAPE around 900 J/kg.
Consequently, mesoscale models show thunderstorms mainly staying
confined to the elevated terrain where orographic lift can trigger
convection. This means areas of northern Maricopa and Gila
Counties would see the best potential of precipitation today
however, isolated storms are not out of the question for the lower
elevations. Currently, thunderstorms are already firing up across
the higher terrain while storm motion remains light. Because of
this, any storms that continue to form this afternoon will have
the possibility to produce localized flooding.

Throughout the weekend, the aforementioned high pressure system will
wobble around the Desert SW with the steering flow changing from
the west, to the northwest, and then eventually to the north.
This will continue our drying trend into the weekend with
precipitation chances decreasing each day. By Sunday, we are
expecting our entire CWA to be dry as the GEFS ensemble mean shows
PWs lowering to 1.2 inches.

High pressure then shifts to our east on Monday and turns the
steering flow to a more moist SE flow. It is not until late
Monday into Tuesday do we see a strong moisture push, an increase
in upper level SE flow, and shear. If this does come to fruition,
storms chances will return to the lower elevations, mainly for
south-central AZ late Monday into Tuesday. By Wednesday, the
forecast becomes a bit more murky as the ECMWF brings excessive
moisture into our area under southerly flow aloft while the GFS
reverts to a drier, low grade monsoon pattern. For now, have gone
with a blended solution with a continued chance of lower
elevations storms on Wednesday but confidence remains very low for
this extended period.

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.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:

Light flow across the Phoenix area tonight with no showers or
storms anticipated. Overall a quiet night. The quiet weather is
expected to continue into Saturday. Storms will likely develop
during the afternoon well north/east of Phoenix over the higher
terrain, but remain well outside of area terminals through the
evening hours. There is an outside chance an outflow boundary
could come down from the northeast in the 23Z-03Z window.

Southeast California/Southwest Arizona including KIPL and KBLH:

Light winds with just few/sct mid and high clouds through
Saturday. No significant impacts expected to area terminals.

Aviation Discussion not updated for amended TAFs.

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.FIRE WEATHER...
Monday through Friday:
An increase in available moisture during the first half of next
week brings a chance of isolated thunderstorms back to the lower
deserts in addition to increasing minimum relative humidity values
back above 20 percent over the lower deserts. The best chances
for storms across the entire region currently appears to be on
Tuesday, but thunderstorms will be possible over higher terrain
locations nearly each afternoon next week. Wind speeds will be
typical for mid-summer with the usual afternoon breezes.
Temperatures will remain slightly above normal through the
extended period.

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

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.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

DISCUSSION...Wilson
AVIATION...Iniguez
FIRE WEATHER...Wilson/Hernandez