Excessive Heat Warning 4, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
225 PM MST WED JUL 20 2016

.SYNOPSIS...
The dry weather across southeast California and southwest Arizona
will begin to spread eastward, with the chances for showers and
thunderstorms gradually dwindling across central Arizona the next
several days. The combination of thinning moisture and a much warmer
atmosphere building into the region, will cause afternoon
temperatures to soar, especially Friday and Saturday. Slight cooling
is expected Sunday through next week, with minimal chances of
thunderstorm activity.

&&

.DISCUSSION...

Upper air data showed monsoon moisture was firmly entrenched over
most of AZ. Monsoon moisture was not excessive, but not minimal
either. For the remainder of the afternoon and evening, most showers
and thunderstorms will be confined to northern and southeast AZ
mostly outside our forecast area. Therefore a slight chance of
afternoon and evening thunderstorms are still expected the remainder
of today and this evening.

Thursday...again, with the absence of any notable upper level
features, thermodynamics will likely control the possibility of late
afternoon and evening convective activity. Another low grade monsoon
pattern will continue in south central AZ, i.e. a slight chance of
late afternoon and evening thunderstorms. Southeast CA and southwest
AZ will remain relatively dry and hot.

Friday and Saturday...latest model forecasts of thinning moisture in
conjunction with increasing atmospheric thickness values, and much
warmer 850 mb temperatures, are helping us bring the magnitude of
warming into focus for Friday and Saturday. Model guidance, and our
experience of dealing with how the surface airmass reacts to a
decrease in humidity, has us concerned about dangerous heat. Our new
Heat Impact Level guidance tools are putting the entire CWA into a
marginal Excessive Heat Event by 1 or 2 degrees. Therefore an
Excessive Heat Watch has been posed for most of our forecast area
except for Joshua Tree National Park CA zone 30.

Sunday through Thursday...afternoon temperatures are forecast to
remain above normal...in the 110-112 degree range, but below
excessive heat criteria.  A low grade monsoon is also expected with
a slight chance of showers and thunderstorms over the south central
AZ deserts and mountains, including Phoenix.  Southeast CA and
southwest AZ will remain dry.

&&

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

Possibility of thunderstorms and any outflow winds are of concern.
Storms have already developed in the higher terrain to the north and
east of Phoenix. Confidence in the exact timing and extent of storms
or outflow winds is not high but current thinking is an outflow from
the north is quite possible given current storm development.
Additional outflows are possible but will depend on other storm
development. Otherwise, winds should be light. Vicinity showers
remains in the TAFs for the afternoon.

Less convective activity is expected Thursday into Friday as monsoon
moisture steadily thins out.

Southeast California/Southwest Arizona including KIPL and KBLH:

Little aviation concerns through with storm activity well off to the
east into central and northern Arizona. Expect mostly clear skies
and southerly winds through the day today.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...

Saturday through Wednesday...Temperatures are climbing to well above
normal territory through the weekend as moisture levels thin and
humidity values drop. Chances for thunderstorms/wetting rain dwindle
and become almost limited to the higher terrain of eastern Arizona
by Friday and Saturday. Consequently, afternoon humidity levels will
fall below 15 percent over the weekend before increasing to the 15-
20 percent range beginning Sunday and into next week as thunderstorm
chances slowly increase from east to west. Higher humidity values
can be expected in the higher terrain locations. Outside of
thunderstorms, winds will be typical with some afternoon gustiness.

AVIATION...CB/Deemer FIRE WEATHER...Iniguez/Deemer/CB

.SPOTTER INFORMATION STATEMENT...
Spotter activation may be required for this evening.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Watch from late Thursday night through
     Saturday evening for AZZ020>023-025>028.

CA...Excessive Heat Watch from late Thursday night through Saturday
     evening for CAZ031>033.

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DISCUSSION...Vasquez
AVIATION...Deemer
FIRE WEATHER...Deemer