Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
200 PM MST WED JUL 27 2016

.SYNOPSIS...
Despite lingering moisture across the region, strong high pressure
will keep thunderstorm activity isolated for the next couple days as
well as keeping temperatures unusually hot. As this high pressure
system weakens by the end of the week, better moisture profiles will
stream north throughout the state. In addition to bringing somewhat
cooler temperatures, increased thunderstorm activity will impact the
area through at least the beginning of next week.

&&

.DISCUSSION...
Latest WV imagery shows a well-defined moisture discontinuity between
drier air across northern Arizona and moist air across southern AZ.
Recent AMDAR flights out of KPHX shows steering flow out of the east
and northeast around 10 kt. Model soundings indicate that
northeasterly steering flow around the anticyclone to the north will
increase to around 20 kt this afternoon, which will be favorable for
storms to propagate out of the higher terrain and into the lower
deserts.

Earlier trends in the CAMs including the HRRR suggested a greater
chance for convection in the lower deserts. Most recent trends are
not as bullish, however latest satellite imagery does show towering
CU developing directly upstream across the Mogollon Rim. Dewpoints
remain above average, generally in the upper 50s and lower 60s.
Another day of 110+ degrees (25th so far this year) is likely in
Phoenix, which will produce MLCAPE between 500 and 1000 J/kg. Some
CIN is evident in the latest observed soundings, though the
expectation is that this will be mostly eroded once temperatures
peak late this afternoon.

Above normal PoPs are warranted for this evening across south-
central Arizona and points to the east. The strongest storms will be
capable of producing brief heavy rainfall, strong to severe wind
gusts and small hail. Meanwhile, trajectory is not ideal for a big
dust storm, though there is a discernible threat for blowing dust
across mainly Pinal County.

&&

.PREVIOUS DISCUSSION...

Perhaps the most significant impact over the next 48 hours will be
excessively hot conditions manifesting through southeast California
and the Lower Colorado River Valley. In addition to strong heights
aloft, ensemble means depict the H7 thermal ridge nearing the 99th
percentile while best performing guidance suggests afternoon highs
peaking near 115F in the population centers of El Centro, Blythe, and
Parker today and Thursday. While this isn't the hottest weather of
the summer thus far, it certainly will be dangerous for those
outdoors during peak heating; and have issued an Excessive Heat
Warning highlighting these larger population centers.

All indications suggest Friday being a transition day where heights
and temperatures aloft begin to deteriorate while more substantial
deep moisture seeps north and west into a larger chunk of Arizona.
Forecast soundings still only depict around 9 g/kg boundary layer
mixing ratios, however the depth of this moisture may begin to reach
above the H7 layer and deeper more organized outflow boundaries may
become capable of initiating convection at lower elevations with
these type of sounding profiles. Based on SREF probabilities and
sounding appearance, not overly enthused about widespread rainfall
Friday though it certainly looks like events occurring on Friday
evening will set the stage for more activity over the weekend.

At this time, its difficult to ascertain which specific day during a
Saturday through Tuesday time frame would be more convectively
active or provide the most significant impacts. However, all model
indications point towards at least one of these days yielding well
organized thunderstorms with the potential for isolated severe
weather and flooding. Forecast soundings show sfc-H7 mixing ratios
markedly improving to 11-12 g/kg while H5 temperature cool to around
-6C yielding MLCapes 1000-1500 J/kg which would be readily released
by any good inverted trough or deep combination of outflow
boundaries. Certainly, there will also likely be one of these days
where the environment become convectively overturned and
contaminated leading to gross inactivity, though conceptually any
one of these days justifies elevated POPs.

&&

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

The best chance of thunderstorms will be this evening between 5 and
8 PM as they track in from the east. Mainly light westerly component
winds during the afternoon should turn easterly with gusts of 22 to
28 kts. with any early evening thunderstorms along with variable bkn
cigs from 8 to 10K ft. Otherwise expect periodic mid to high level
debris cloud cigs.

Southeast California/Southwest Arizona including KIPL and KBLH:

Expect mostly light to occasionally breezy southerly and westerly
component winds along with typical diurnal shifts. Otherwise skies
will be clear to partly cloudy with few to sct mid and occasionally
bkn high clouds.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Saturday through Wednesday...

A sequence of Monsoon disturbances from northwest Mexico will help
to maintain demonstrable moisture flow over the region throughout
the period. As a result, significant chances of scattered wetting
rains from thunderstorms from the lower deserts to southern Gila
county are expected every afternoon and evening. Maximum
temperatures are expected to hold near to slightly below normal.
Minimum humidities will range from 22 to 32 percent in the lower
deserts to 33 to 50 percent from the Tonto foothills to the high
country. Seasonably breezy south to southwest winds in the late
afternoons are also likely. Overnight recoveries will be good to
excellent.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation is not expected.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 11 PM MST Thursday for AZZ020.

CA...Excessive Heat Warning until 11 PM PDT Thursday for CAZ031>033.

&&

Visit us on Facebook...Twitter...and at weather.gov/phoenix

DISCUSSION...Hirsch
PREVIOUS DISCUSSION...MO
AVIATION...Sawtelle
FIRE WEATHER...Sawtelle

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