Flash Flood and Damaging Wind in Arizona, August 3, 2016

Area Forecast Discussion
National Weather Service Phoenix AZ
917 AM MST WED AUG 3 2016

.SYNOPSIS...
An excessively moist airmass will continue to cover much of the
region into tonight with the best chance of showers and
thunderstorms over Arizona. While some areas may not see rain,
locally heavy rainfall is possible in some locations. A drier and
more stable airmass will begin to spread into the area starting
Thursday pushing thunderstorm activity into eastern Arizona. An even
drier and warmer airmass will then cover the region over the weekend
and into early next week with very limited thunderstorm activity and
above normal temperatures.

&&

.DISCUSSION...
Storms already underway this morning near and north of Flagstaff.
Isolated weaker showers have been nibbling at the periphery of our
forecast area. Satellite imgery shows a short wave moving from our
forecast area into FGZ and VEF areas. There is also an MCV over the
Mogollon Rim. 12Z soundings have VERY high precipitable water values
(2.15 inches at PSR, 2.00 at TWC, and 1.78 at 1Y7). Winds aloft are
generally weak as well. PSR sounding indicates that it won't take
much heating for convective showers to be able to get going. 1Y7
however has very little CAPE. Latest models reflect this to varying
degrees in their boundary layer CAPE initializations. Through the
course of the day today they generally show very little CAPE over
southwest AZ and portions of southeast CA. Getting a look now at 12Z
GFS, it is the most conservative over our forecast area (compared to
RAP and NAM) though it also has a batch of CAPE west of the Lower
Colorado River Valley. Given the moisture availability and the
aforementioned flow features, went ahead and issued a Flash Flood
Watch for much of our area (except for Yuma and Imperial Counties).
A fly in the ointment is that we may be losing some of the dynamical
forcing from the short wave as it departs our area later today. Also
of note, model QPF is not impressive for our area but is still
significant over southern Gila County and northern fringes of
Maricopa County. That is where the best potential will be for flash
flooding though cannot rule out adjancent areas as we saw last night
with the valley floor getting some very heavy rain in spots. More
later.

&&

.PREVIOUS DISCUSSION...ISSUED 257 AM MST/PDT...
After a busy afternoon and early evening in many areas, storm
activity has taken a brief break, but likely not for long. A juicy
atmosphere remains across our entire CWA, but the lack of a major
trigger mechanism makes picking out when and where the next round of
storms will develop nearly impossible. Current conditions show a weak
mid level circulation centered right over the Phoenix area with
favorable cyclonic flow covering all of Arizona. Winds aloft remain
very weak with not much of a steering flow for any storms. Any
showers or storms that do develop in this highly moist environment
this morning will be efficient rainfall producers. Some modest CAPE
remains mostly over southern Arizona early this morning which will
also help in any storm development.

High-res convective allowing models have been of little use in this
moist and modestly unstable environment with high variability between
most models and poor performance. We should have some isolated to
scattered showers and a few thunderstorms through the rest of the
morning, likely focused across south-central Arizona. The localized
flood potential remains into tonight as PWATS are forecast to remain
near 1.75 inches, slightly lower than the past couple days but still
impressive. It's difficult to pin point an exact location for storm
development later today, but as the mid level circulation slowly
lifts to the northeast much of the activity should initialize over
the high terrain northeast of Phoenix. With little to no steering
flow can't rule out these storms sending outflows into the lower
deserts triggering additional storms. The extent of storm development
today will somewhat depend on any available heating and there should
be at least some to recharge the atmosphere. PoPs have not been
adjusted much over the previous forecast with mostly slight chances
over the deserts to likely PoPs over the high terrain east and
northeast of Phoenix.

The synoptic scale pattern will become more progressive starting
tonight into Thursday as a weak upper level trough nears the
California coast. By Thursday morning, southwesterly winds aloft will
move into southeast California and western Arizona bringing drier air
off the Pacific. PWATS are forecast to steadily drop on Thursday
dropping below 1.50 inches over our western areas with slightly
higher values over the eastern half of Arizona. This scenario will
greatly reduce our storm chances with only slight chances remaining
over south-central Arizona and chance PoPs over the high terrain.
This drying pattern will strengthen on Friday into the weekend with
the bulk of the moisture being scoured out by Friday. PoPs for Friday
into the weekend are confined to high terrain locations east of
Phoenix as PWATS drop down to around 1.25 inches over south-central
Arizona and less than an inch over our western areas. Temperatures
during this period will get a decent boost with highs quickly
ascending into a 105-110 range. This warm and dry pattern will most
likely persist through early next week and possibly longer.

&&

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

An extremely moist monsoon airmass will continue across the area
today with a few morning showers, decreasing to a slight chance of
thunderstorms this evening. Through 18z Wed, bkn clds 8-10 thsd agl
with isolated shwrs. Light east wind 4 to 7 knots. From 18z Wed to
02z Thu, sct-bkn clds 10-12 thsd agl, increasing west wind 8 to 12
knots. Slight chance of tstm mainly north through east of KPHX and
near the mtns.

Southeast California/Southwest Arizona including KIPL and KBLH:

Through 03z Thu, sct to locally bkn clds 10-12 thsd agl. South wind
8 to 12 knots.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Friday through Tuesday...

Drier westerly flow aloft will push monsoon moisture into far
eastern AZ this period with a slight chance of afternoon
thunderstorms over the higher terrain of southern Gila County
bordering the White Mountains. Otherwise mostly clear skies are
forecast along with warmer temperatures. Afternoon southwest winds 8
to 15 mph are expected each day. Minimum relative humidities on
Friday will range from a dry 15 percent in southeast CA to 35
percent in southern Gila County. Minimum relative humidities will
slowly decrease each day and by Sunday and Monday ranging from 10
percent in southeast CA to 25 percent in southern Gila County. Good
to excellent overnight recovery is expected.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters are encouraged to report according to best practices and
trained criterion.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch through this evening for AZZ020>024-027-028.

CA...Flash Flood Watch through this evening for CAZ030-032.

&&

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DISCUSSION...AJ
PREVIOUS DISCUSSION...Kuhlman
AVIATION...Vasquez
FIRE WEATHER...Vasquez