Heavy Rain in Arizona, August 1, 2016

Area Forecast Discussion
National Weather Service Phoenix AZ
205 PM MST MON AUG 1 2016

.UPDATE...UPDATED AVIATION AND FIRE WEATHER DISCUSSIONS.

.SYNOPSIS...
A very humid monsoon airmass over the region will keep chances for
showers and thunderstorms in place  across the region through
Wednesday. Later in the week, drier westerly flow coming off the
Pacific will push the chance of showers and thunderstorms into
eastern Arizona. After a period of below normal temperatures through
Wednesday, warmer temperatures are forecast to return later in the
week and into the upcoming weekend.

&&

.DISCUSSION...

Tonight through Wednesday...

Although available moisture levels remains extremely high across the
region (PWATS in the 1.70-2.10 inch range), leftover debris clouds
from yesterday's convective activity is holding down high
temperatures today. This cooler boundary layer, although with a weak
subsidence inversion in the mid-levels of the atmosphere will likely
inhibit convective activity somewhat this evening and tonight. Also,
widespread rainfall over Pima and southern Pinal counties
yesterday/last night will likely inhibit convective development down
there, which, given the southerly steering flow aloft, is where
storms would likely have to form in order to move into/affect our
cwa. The most likely areas for storm activity are expected to be
over southern Gila County and over northern La Paz County, where
storms developing over higher terrain to the north could backbuild
into, which is being hinted at on the latest HRRR/RGEM high-res
model runs. Given the high moisture values and slow storm motion,
the main threat from any storms that do develop will be locally
heavy rains and flash flooding.

Late Tuesday into early Wednesday looks like it could be a more
active period. The combination of very high PWATS (aoa 2.00 inches
at some locations), less clouds during the day on Tuesday, along
with a weak shortwave feature that is being forecast by the GFS to
move into SE AZ from the south during the day on Tuesday will likely
allow more showers/storms to develop across the lower deserts of
south-central and SW AZ than what is expected this evening/tonight,
with lighter shower activity possibly even lasting into early
Wednesday as the afomentioned shortwave only slowly moves northward
into east-central AZ. Again, the main impacts from any storms that
develop during this period will be heavy rain and localized
flooding, with the strongest cells also producing locally gusty
winds. Although storm activity will likely persist into Wednesday
afternoon, a turn to more westerly winds aloft will likely begin to
shift the greatest amount of this activity off into southern Gila
County.

Thursday through next Monday...

The latest GFS model suite is even more aggressive in developing much
drier westerly flow over our region than on earlier runs, as it
develops a rather deep upper trof over northern/central ca, starting
on Thursday and continuing into the upcoming weekend. It now looks
like shower/thunderstorm chances will be mainly confined to the
higher terrain east of Phoenix during this period (with just a few
storms over the lower deserts). Although temperatures are expected
to warm somewhat during this period as the airmass dries out and
skies clear, excessive heat is not expected to return at this time
as the upper trof over northern CA keeps the warming aloft in check.

&&

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

Expect mainly light westerly component winds with some afternoon
gusts near 20 kts possible, mostly due to outflows generated from
any distant afternoon thunderstorms that happen to form to the west.
Additional outflow winds this evening are expected to shift the
winds to an easterly or southerly component along with some breezes.
Partly cloudy skies with occasional high cloud cigs will become
mostly cloudy this evening and tonight with cigs near 10 to 12K ft.

Southeast California/Southwest Arizona including KIPL and KBLH:

Winds will be mostly southerly and occasionally breezy throughout the
period. The winds at KBLH are expected to increase out of the east
this evening with gusts up to 26 kts and reduced visibilities of 2 to
5 sm in patchy blowing dust due to outflows from distant
thunderstorms. Skies will be mostly clear to partly cloudy with
periodic few to sct mid and high clouds.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Thursday through Monday...

A gradual drying and warming trend beginning Thursday will reduce
rain chances and keep most thunderstorms confined to the Tonto
Foothills and southern Gila county on Thursday through Monday.
Expect near normal high temperatures through Friday to increase to a
bit above normal beginning Saturday as conditions become drier.
Minimum humidities of 19 to 27 percent on Thursday will steadily
decline each day until they reach 9 to 18 percent on Monday. Good to
excellent overnight recoveries on Thursday will steadily fall,
becoming fair in the lower deserts, to good at higher elevations, by
Sunday.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation could be possible on Tuesday.

&&

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

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DISCUSSION...Percha
AVIATION...Sawtelle
FIRE WEATHER...Sawtelle