Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
600 PM MST Wed Aug 11 2021

.UPDATE...Updated Aviation discussion.

&&

.SYNOPSIS...
Upper level disturbances will produce favorable conditions for
showers and isolated thunderstorms across the region most days.
Thunderstorms are favored to move through areas west of Phoenix
later today. Primary threats with any storms remains strong winds
blowing dust, and flash flooding the primary threats. Rain
chances decrease towards the end of the workweek, although
do not completely diminish, and then increase again for this
weekend. Persistent humid conditions and periods of cloudy skies
will help keep afternoon temperatures near or slightly below
normal most days.

&&

.DISCUSSION...
A weak upper low continues to meander around central AZ and helped
drive another round of morning convection in the Phoenix area. The
circulation has since lifted just northeast of the CWA this
afternoon, but there are lingering impacts from the rain that
fell. Particularly, significant runoff has collected into streams
like Sycamore creek, with flow into the Verde river, and Tonto
creek. A Flood Warning is in effect for these areas for the next
few hours.

Scattered convection has flared up in a few areas this afternoon,
including southeast AZ, the high terrain north of Phoenix, and a
cluster of storms in southern Yuma county. There is also isolated
convection in western Maricopa county. The strongest storms have
been in southern Yuma county where where MLCAPE is analyzed around
2000-2500 J/kg and PWATs are around 2 inches. Shear is pretty much
non-existent in the this area and across much of the region,
resulting in slow-moving ordinary pulse storms. Storms in Yuma
county have been aided by colliding outflows. Strong winds,
blowing dust, and torrential rain have been observed and will
remain the primary concern through the rest of the afternoon.

The central and west Valley as well as portions of western
Maricopa have missed out on storm activity the past few days and
it is no surprise that's is where some of the isolated convection
has been flaring up. It is evident in the CU field that these
areas are less stable than area that saw the rain this morning.
The environment still seems conditional, but a strong enough
outflow, outflow collisions, and/or cell mergers may be sufficient
to fully realize the available CAPE and moisture in these areas.

With the high moisture content, any storm will be capable of
1-2"/hr rain rates, and locally higher for brief periods, which
could lead to localized flash flooding. Strong gusty winds may
also generate blowing dust, in addition to lightning. The overall
severe threat is fairly low, but with a cell merger or outflow
collision, it cannot be completely ruled out. CAMs are indecisive
on whether additional convection will be possible overnight and/or
Thursday morning, but with the upper low still in close proximity
and moisture levels still elevated, it cannot be ruled out,
especially in areas that miss out on convection this
afternoon/evening.

&&

.PREVIOUS DISCUSSION...
As the influence of the aforementioned low wanes Thursday,
afternoon storms are favored to develop over the more traditional
areas, such as the Mogollon Rim, with the possibility for activity
to move off the higher terrain. Confidence for a completely dry
day or two in the CWA is low, but Friday seems poised to be the
quietest period of the forecast, as a H5 ridge strengthens over
the Southwest late this week. A moist atmosphere and embedded
upper level waves and disturbances within the synoptic circulation
will keep storm chances in the forecast with PoPs somewhere in
the 15-50% most days through at least Tuesday. Temperatures are
forecast to remain 2-5 degrees below seasonal normal across south-
central AZ while warming a bit more into the 105-110 degree range
along the Colorado River and southeastern California.

&&

.AVIATION...Updated at 0054Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
A low pressure system will continue to meander across central
Arizona through Thursday, promoting favorable conditions for
additional showers and thunderstorm activity this evening.
Numerous outflow boundaries are evident on the TDWR and will
continue to interact this evening, resulting in scattered
convection across the Valley. After 03z, uncertainty increases
considerably. Any storm that develops will be capable of producing
occasional lightning, brief, heavy rain, winds up to 45 kt and
ceilings as low as 6k ft.

Southeast California/Southwest Arizona including KIPL and KBLH:

Isolated thunderstorms will remain a possibility overnight,
particularly at KBLH. Any storms that develop will be capable of
producing occasional lightning, brief, heavy rain, and winds up to
45 kt. Probabilities are generally around 20 percent tonight
lowering to 10 percent Thursday.

&&

.FIRE WEATHER...
Friday through Tuesday:
Shower and storm activity may be a little more quiet on Friday but
conditions will remain favorable for activity most days. Any
storms or showers that develop will likely produce heavy rain that
could lead to flash flooding. RH values also remain elevated with
periodic influxes of moisture and below normal temperatures in
most places. Min RH will fall into the 25-35% range most days
while overnight recoveries 50+ percent for most areas. Winds will
favor typical diurnal/terrain influences most days.

&&

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

&&

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

DISCUSSION...Benedict
PREVIOUS DISCUSSION...12
AVIATION...Hirsch
FIRE WEATHER...12

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