Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 231908

Area Forecast Discussion
National Weather Service Phoenix AZ
1210 PM MST SAT JUL 23 2016

.SYNOPSIS...Some relief in desert heat is forecast for Sunday and
next week, as more monsoon moisture and variable amounts of clouds
move into the region. There is a chance of afternoon showers and
thunderstorms each day, mainly over the central and eastern Arizona
mountains, with a slight chance of storms spreading toward the
Colorado River valley after Tuesday.




A few days ago we knew this Excessive Heat Watch, then Warning for
the lower deserts would be a little tricky because of an uncertain
amount of remnant monsoon moisture that could retard the diurnal
heating curve. And we were at the mercy of the numerical models that
were consistently forecasting a drying trend. Atmospheric thickness
values and a much warmer airmass aloft certainly support excessive
heat, however we now realize we could never dry out enough. For
example to our surprise this morning, an old and distantly traveled
convective outflow boundary moved out of Old Mexico and surfaced in
southern AZ between 4 am and 6 am mst. A weak boundary moved through
Phoenix between 530 am and 6 am (locals noted the suspended dust
this morning) sending surface dewpoints into the middle 50s.  And
between 4 am and 6 am 44 mph southerly gusts hit Yuma, with 38 mph
gusts at Blythe, all contributing to more surface humidity. This is
the last day of the Excessive Heat Warning, and higher humidity
levels with a cooler airmass should reduce afternoon temperatures
slightly and under excessive heat criteria.

Otherwise, the passing westward moving inverted trof in Mexico and
its associated thunderstorm outbreak, has contributed to a much
deeper moisture profile on the Tucson weather balloon sounding.
However with a 300/250 mb weakly convergent flow developing in the
wake of the passing Mexican inverted trof today, any thunderstorm
development should be thermodynamically driven, and focused in the
mountains north through southeast of Phoenix for later this
afternoon and evening.

Sunday and Monday...

On Sunday the pattern gets a little more interesting especially for
the evening and night-time hours. An inverted trof, weakly seen in
south Texas this morning, is modeled to move northwestward an
approach southeast AZ later Sunday evening. The 12z GFS model
(current run) is now a little slower with moving this feature into
southeast AZ, and any forcing in our forecast area in south central
AZ including Phoenix comes overnight. Therefore a slight chance of
showers and thunderstorms are forecast for a large portion of south
central AZ, including Phoenix Sunday night into early Monday morning.

Tuesday through Saturday...

This period contains alot of uncertainty as the 300/250 mb flow
pattern transitions from southeasterly (and containing a few
disturbances from Mexico), to north and northeasterly Thursday
through Saturday. With continued elevated boundary layer moisture,
afternoon thermodynamics, and small hard to time perturbations in
the upper flow, its best to go with a broad brush forecast of slight
chance showers/thunderstorms this period just about everywhere in
our forecast area.


South-Central Arizona Including KPHX, KIWA, and KSDL: A quiet period
weatherwise can be expected through the taf period as somewhat drier
air aloft and a more stable airmass keeps convective active well off
to the east of the Greater Phoenix area. Winds to mainly follow
typical diurnal trends, with little chance of any outflows from
distant thunderstorms affecting the terminals. Few-sct clouds layers
to remain mainly aoa 12k feet.

Southeast California/Southwest Arizona including KIPL and KBLH:
Little to no aviation impacts through the taf period under mostly
clear skies. Sfc winds to favor a southerly direction at KBLH and
southeasterly direction at KIPL, with the strongest winds during the
afternoon and early evening hours.

Aviation Discussion not updated for amended TAFs.


Monday through Friday... Monsoon moisture working its way back into
the region will bring chances for afternoon and evening
thunderstorms each day from Monday into Thursday. The best chances
for wetting rains will be over the higher terrain east of Phoenix,
will lesser chances over the lower deserts of South-Central and
southwest Arizona, and only slight chances over southeast
California. Some increase in convective activity is expected on
Friday, as deeper moisture begins to move into the region.
Temperatures to remain slightly above normal through the entire
forecast period. Minimum humidities in the 15-30 percent range
Monday-Thursday to rise into the 20-35 percent range on Friday.


Spotter activation is not expected.


AZ...Excessive Heat Warning until midnight MST tonight for AZZ020>023-

CA...Excessive Heat Warning until midnight PDT tonight for CAZ031>033.



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