Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 260348

Area Forecast Discussion
National Weather Service Phoenix AZ
848 PM MST THU AUG 25 2016

On Friday, a moderate surge of monsoon moisture will return to
southern Arizona resulting in an increasing threat of afternoon and
evening thunderstorms over portions of south central Arizona. A drier
southwesterly flow will develop Saturday, with storm chances mainly
confined to the higher elevations of eastern Arizona. Scattered
thunderstorms will return to the region by the middle of next week.


Isolated showers and thunderstorms developed late this afternoon,
mainly across portions of Maricopa and southern Gila counties. Some
blowing dust was observed near the Gila River Indian Community and
portions of Sun Lakes and Chandler, which prompted a Blowing Dust
Advisory. Latest Phoenix sounding shows little CAPE and activity has
subsided rapidly across the lower deserts with the loss of heating
this evening.

Models are in generally good agreement that low-level mixing ratios
will increase steadily overnight, resulting in increasing clouds
towards morning. Latest runs of the CAMs including the HRRR hint at
some light shower activity developing overnight or early Friday
morning across Central Arizona. PoPs remain near 10 percent across
these areas. Otherwise, the forecast remains on track with near
normal temperatures expected.



A trough of low pressure stretches from Idaho southwest over central
California. Models indicate this trough is likely to spin off a
closed low eventually moving down into southern California by
Saturday. This will slightly shift our steering flow to be a bit less
zonal, more from the southwest, helping to gradually increase some
moisture into our region for this weekend. This is likely to slightly
push up PoPs particularly across eastern portions of Arizona. However
we still are keeping PoPs fairly modest with only slight chances for
all but the highest terrain in our CWA for this weekend.

Weak high pressure ridge should build back over the area Monday
through Wednesday which should keep monsoon activity fairly quiet
except for higher terrain and the chance for outflow boundaries from
those storms pushing across the lower deserts.



Today and Tonight...The overall atmospheric flow regime continues
in its late September like pattern, with strong west and
northwesterly flow aloft replete with numerous trofs/shortwaves
diving into the western states from Canada. The strong high pressure
system that was suppose to build into AZ from the west late
yesterday and today stalled offshore, continuing the strong
northwest flow aloft, i.e. above 300 mb. A couple of disturbances
are poised to move into central and northeast AZ today, with a
larger former Canadian trof moving into central UT.

With marginal remnants of monsoon moisture continuing across parts
of southwest and south central AZ, the combination of these weak
shortwaves should enhance another threat of afternoon and evening
mountain thunderstorms mainly north through east of Phoenix. A few
mtn thunderstorms could send some convective outflows into the
greater Phoenix area during the evening hours.  A better chance will
exist over our southern Gila county zone 24. The threat of storms
will diminish after dark. Our forecast areas in southeast CA and
southwest AZ should be devoid of storms today as the atmosphere is
too stable.

Friday through Sunday...850/700 mb circulation centers are still
forecast to develop along the San Diego coast by early Friday
morning. Increasing low level south winds from these features, i.e.
wind below 10 thsd ft, are still forecast to transport increasing
amounts of monsoon moisture from Mexico, surging north at times
predominately into south central AZ toward Gila Bend, Phoenix, and
Tucson. At the same time, the former Canadian trof in Utah is
forecast to slowly deepen into northern AZ Friday through Sunday,
with mid/upper level flows becoming increasingly cyclonic over AZ.
In fact a 500 mb closed low center is forecast to develop over
northwest AZ Saturday.

Increasing monsoon moisture, relatively cool air aloft, and hard to
time cyclonic perturbations will result in a threat of afternoon and
evening showers and thunderstorms across much of south central AZ
Friday and Saturday, including the greater Phoenix area. Our forecast
areas in southeast CA and southwest AZ should remain fairly stable
for little or no storm threats.

On Sunday it appears that any threat of afternoon and evening storms
should be relegated to the mountains of southern Gila County zone 24.

Monday through Thursday...A strong high pressure system located off
the southern CA coast will finally build into AZ stabilizing the
airmass and shifting most of the monsoon moisture into far southeast
AZ. This setup should preclude any convection in our forecast area.



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

Quiet conditions will generally prevail across the Phoenix area
through Friday evening. However, increasing moisture will be
sufficient to generate at least isolated showers at some point
between early Friday morning and early Friday afternoon. Additional activity
late Friday afternoon will likely develop south of the area, which
will increase the threat of an outflow boundary and possibly blowing
dust reaching the Phoenix terminals.

Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation weather impacts anticipated. Winds will generally retain
a southerly component, occasionally gusting as high as 20 kt at KBLH
Friday afternoon.

Aviation Discussion not updated for amended TAFs.



Sunday through Thursday:

Southwesterly flow aloft is expected to keep any chances for
convective activity, and wetting rains mainly confined to the
higher terrain north and east of Phoenix through Tuesday, with dry
conditions prevailing across the lower elevations. A modest increase
in convective activity is then expected on Wednesday and
Thursday across south-central AZ as the flow aloft becomes
more southerly ahead of the next upper low center.  Minimum
humidities in the 15 to 35 percent range from Sunday into Tuesday
will rise into the 20 to 40 percent range on Wednesday and Thursday.
Outside of any erratic thunderstorm winds, winds to mainly follow
typical diurnal trends, except for some gusty southerly winds from
time-to-time up the Colorado River Valley.


Spotter activation is not expected Thursday.




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