Heavy Rain in Arizona, September 1, 2016

Area Forecast Discussion
National Weather Service Phoenix AZ
225 PM MST THU SEP 1 2016

.SYNOPSIS...
Increased monsoon moisture will continue to lead to a chance of
showers and thunderstorms across parts of southwest and south
central Arizona through tonight. Westerly flow on Friday will lead
to drier and stable conditions by late afternoon, with only a slight
chance of thunderstorms over the mountains east of Phoenix. A very
dry airmass will move into Arizona from the west Saturday through
Tuesday, for clear skies and slightly below normal temperatures.
Monsoon moisture will return in abundance next Wednesday and
Thursday, leading to an increase in thunderstorm activity.

&&

.DISCUSSION...
Regional weather balloon soundings showed a dramatic increase in
monsoon moisture over the southern half of Arizona. In fact boundary
layer moisture is near excessive, with its axis from just west of
Prescott southeast to Tucson.

Upper level winds are forecast to change again as the weak former
Mexican trof in southeast AZ exits the state eastward. Weak ridging
at and above the 300 mb level will develop in southern AZ for the
first half of the day following the southeast AZ trof, with
increasing southwesterly flow aloft trending more laminarly oriented
the second half of the day.  Therefore storms today will more
thermodynamically driven, first in the mountains then a few in the
deserts. However with boundary layer moisture near excessive levels,
the main threat will be hydrology, meaning excessive local rainfall
under each storm.  Also with moisture so excessive, it wont be hard
for convective outflows to generate secondary thunderstorms as they
travel across the deserts.

At 2 pm most thunderstorms had developed over the mountains of
northern and eastern AZ, with a few showers over the deserts just
east of the Colorado River.  There is a threat of thunderstorms
across most of southwest and south central AZ the remainder of the
afternoon and evening, with a slight chance overnight.

On Friday, boundary layer moisture is forecast to decrease from the
west especially by late afternoon.  This combined with warm
advection the middle atmosphere should stabilize the airmass for
very minimal convective possibilities over the higher mountains
of south central AZ.

Saturday through Tuesday...A fairly deep Pacific low pressure system
is forecast to move into the Pacific northwest. Numerous smaller
westerly shortwaves at its southern extent are forecast to move
through Nevada, Utah, and parts of northern AZ this weekend. As a
result, increasing westerly winds dramatically scour out all the
moisture in AZ with 850 mb dewpoints going negative.  Therefore
mostly clear skies with slightly below normal temperatures are
forecast.

Wednesday and Thursday...Models continue the idea of surging a
significant amount of Mexican monsoon moisture back into AZ.  At the
same time a couple of westerly shortwaves are forecast to move
through Utah and far northern AZ.  The Increasing jetstream wind
orientation suggests entrance region jet-dynamics across central and
eastern AZ for a renewed threat for afternoon and evening
thunderstorms.

&&

.AVIATION...

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

Chances for storms exists for this afternoon through the evening for
all the terminals. Expect storm development over the mountains to
the north and east of Phoenix and to the south towards Tucson. The
possibility for outflow winds from any storms is the biggest concern
for aviation. In addition to the gusty winds, blowing dust may also
impact aviation. However, the timing of any storms or winds is to
uncertain to include in the TAFs at this time.

Southeast California/Southwest Arizona including KIPL and KBLH:

There are lingering showers for KBLH through the early part of the
afternoon. There is only a very slight chance of showers or storms
in the area through the afternoon. The winds will generally be out
of the south but with some gustiness from time to time. Otherwise,
do not expect any major impacts to aviation.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...

Sunday through Thursday:

A drying trend will take place this weekend before moisture begins
to increase later in the week. Thus storm chances will be almost non-
existent through Monday and then very slight chances return and are
confined to the higher terrain east of Phoenix Tuesday and
Wednesday. Minimum humidities will drop below 30 percent over the
weekend before increasing to 25-35 percent by Wednesday. Breezy
conditions are also expected during the afternoon through Tuesday
across much of the area and on Wednesday along the Colorado River
Valley, however critical thresholds are not expected to be reached.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

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

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DISCUSSION...Vasquez
AVIATION...Deemer
FIRE WEATHER...Deemer/Hernandez