Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 290246

Area Forecast Discussion
National Weather Service Phoenix AZ
745 PM MST WED SEP 28 2016

A low pressure system will continue to slowly move north through the
Desert southwest into Thursday resulting in variable cloudiness and
a threat of showers and thunderstorms. Dry and calmer weather will
return Friday through early next week with seasonably warm


Two distinct middle tropospheric circulation centers were evident in
WV imagery and objective analysis: one west of Las Vegas and one
well south of San Diego. While this alignment is not the most
optimal for large scale ascent through the forecast area, a subtle
jet streak was analyzed through far SE AZ placing much of the
eastern part of the state within the favored left front quadrant.
This localized ascent became coincident with a plume of MLCape
nearing 1000 J/kg and seasonably high DCape capable of producing
deeper, more organized outflow boundaries. The main outflow has now
lifted northwest of the Phoenix metro and was spurring additional
convective activity despite growing Cinh.

Main evening updates included increasing POPs for areas along this
outflow boundary while also bumping up cloud cover this evening into
Thursday morning. Both experimental high resolution and operational
models suggest the current activity will lift into northern AZ while
a renewed batch of showers blossom Thursday morning ahead of the
aforementioned trailing shortwave ejecting north into central AZ
from the Baja peninsula. There still remains some question as to how
extensive and intense the showers may become Thursday morning, and
further upward adjustments to POPs may be necessary. Also made
rather marked increases to thunderstorm chances through Gila Co
Thursday afternoon as latest SREF and SSEO probabilities strongly
suggest concentrated convection at higher elevations with fair
instability and good dynamic support.


Otherwise, the next potent shortwave trof, currently seen in water
vapor satellite imagery 500 miles southwest of San Diego, is still
forecast to move into parts of southeast CA and southwest and south
central AZ early Thursday morning through early afternoon. Best
dynamics again appear to be over southwest AZ and Colorado River
Valley where precip probabilities are the highest Thursday morning.
A lingering threat of showers will continue over our forecast area
Thursday afternoon and early evening as this system quickly moves
into northwest AZ.

Appreciable low level moisture and high humidity will linger over
southwest and south central AZ Friday, however a more stable
westerly flow pattern is expected. Any shower threats will be
relegated to the mountains of southern Gila County Friday afternoon
and evening. Partly cloudy elsewhere.

Saturday through Wednesday...
Low level moisture will finally start to decrease from the west this
weekend under west and southwesterly flow aloft at all levels. A
much drier airmass will arrive Monday through Wednesday. Therefore
mostly clear skies and seasonably warm temperatures are expected.


South-Central Arizona Including KPHX, KIWA, and KSDL:
An outflow boundary has pushed through all terminals this evening
with residual showers lifting north and west of of the Phoenix
metro. Sfc winds should maintain an E/SE component, though some very
minor chance of a due south wind or brief variable directions exist
through midnight before becoming light easterly.

Several models suggest light SHRA passing through Phoenix later
Thursday morning with variable (or even a light north) sfc wind.
Have included a VCSH mention for the time being. Confidence is
rather low Thursday afternoon regarding renewed SHRA/TSRA
development and potential for sfc winds to finally become westerly.

Southeast California/Southwest Arizona including KIPL and KBLH:
Minimal aviation impacts through Thursday as any cigs remain above
9K ft, though sfc winds will frequently be variable in direction.
There is fairly good evidence SHRA will affect KBLH late Thursday
morning/early afternoon, though coverage may not directly affect
terminal site. Somewhat lesser chances exist for KIPL.


Saturday through Wednesday...
Chances for storms will greatly diminish as a southwesterly flow
aloft brings in drier air into the region. Minimum humidities for
the lower deserts will drop below 15 percent and down to about 20
percent for the higher terrain in Gila County. The overnight
recoveries look to be good to excellent. Conditions look to be
breezy at times, especially Sunday and Monday.


Skywarn radio Net activation will not be needed.





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