Heat in Arizona, August 22, 2019

Area Forecast Discussion
National Weather Service Phoenix AZ
555 AM MST Thu Aug 22 2019

.UPDATE...Updated Aviation Discussion.

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.SYNOPSIS...
Weakening high pressure aloft and increasing moisture today will
begin a cooling trend while bringing isolated thunderstorms back
into the forecast for mostly high terrain areas of southeast
Arizona through Saturday. Highs will drop back to around normal
levels for Friday through the weekend, but a warming and drying
trend early next week will bring highs closer to 110 degrees once
again.

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.DISCUSSION...
Relief from the excessive heat should begin today as high pressure
aloft continues to weaken and boundary layer moisture increases.
However, temperatures this afternoon will be far from pleasant as
highs range from 110-112 across the warmer southeast California
deserts to 107-110 across the south-central Arizona deserts.
Models also show a shift in the low level flow beginning later
this morning with 850mb winds increasing up to around 20 kts out
of the south across far southern Arizona. This will lead to a very
noticeable increase in boundary layer moisture by late morning
with surface dew points climbing well into the 50s, possibly
briefly over 60 degrees. The increase in moisture today should be
enough to trigger some southern Arizona high terrain storms, but a
warm layer aloft (500mb temps of -3 to -4C) will most likely
prevent any storms over the lower deserts.

The low level southerly moist flow will continue into Friday,
adding to the boundary layer moisture while PWATs inch closer to
1.5" across extreme southern Arizona. Further cooling is seen into
Friday as the heights aloft continue to fall and surface dew
points climb into the lower 60s. This cooling will allow high
temperatures Friday to drop back to around seasonal normals. Storm
chances Friday are again generally confined to high terrain areas
south and east of Phoenix with only a remote chance of an outflow
induced storm into the lower deserts.

The modest moisture levels will remain steady into Saturday, but
increasing subsidence aloft from the high rebuilding over the
region will likely limit storm chances to far southeast Arizona.
Temperatures Saturday will drop another couple degrees as highs
mostly top out between 101-104. Later Saturday into Sunday, the
upper level flow begins to shift more out of east in response to
TS Ivo tracking northwestward a few hundred miles west of the
Baja peninsula. As of now, there is little indication Ivo will do
anything for our region other than shifting our winds aloft out of
the east/northeast. This shift in the flow will actually bring in
slightly drier air from New Mexico and likely end storm chances
over much of Arizona.

An elongated northwest to southeast high pressure ridge then sets
up over the Southwest U.S. Sunday into early next week. This will
bring another warming trend with highs above normal starting
Monday and possibly around 110 degrees by next Tuesday. Models
differ quite a bit on available moisture next week, but mostly
point toward fairly quiet monsoon days during the first half of
next week.

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.AVIATION...Updated at 1245 UTC.

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

Expect light to moderate winds favoring conventional diurnal
patterns along with breezy westerly winds with gusts of 17-21kt
starting early to mid afternoon and lasting into this evening. In
addition to daytime mixing, this afternoon's breezes will be
connected to a midday sea breeze outflow. Skies will be mostly
clear with FEW mid clouds near 12-14 kft today followed by FEW
low clouds near 7kft late Friday night and Friday morning. Also
expect SCT high clouds by late afternoon and evening. Most
convection today will be confined to distant locales in far
southeast AZ with only a very slim chance of fleeting, isolated
showers developing in this area during late afternoon. However,
confidence and chances were too low to mention in the TAFs.

Southeast California/Southwest Arizona including KIPL and KBLH:

Few aviation concerns are expected through the period under mostly
clear skies. Winds will favor conventional diurnal patterns
although expect some westerly wind gusts near 20kt during the late
afternoon and evening hours at KIPL and some breezy southwesterly
winds gusting near 22kt at KBLH. There will also be periods of
light and variable wind conditions during the morning and night.

Aviation Discussion not updated for amended TAFs.

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.FIRE WEATHER...
Saturday through Wednesday:
Slight chances for storms will favor the higher terrain north and
east of Phoenix, including the Tonto NF, through the first half of
next week. Humidity values will generally improve with minimum RH
values in the 15-25% range in the desert with slightly higher
values the forests. Overnight RH recovery will be in the 40-50%
range for most locations. The winds will mostly follow typical
diurnal trends with some afternoon breezes.

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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.
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DISCUSSION...Kuhlman
AVIATION...Sawtelle
FIRE WEATHER...Deems