Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 242123

Area Forecast Discussion
National Weather Service Phoenix AZ
223 PM MST WED AUG 24 2016

.UPDATE...To Aviation Discussion...


A drier and more stable airmass will slowly move into the region
through Thursday. However a slight chance of afternoon and evening
thunderstorms will remain over the mountains north through east of
Phoenix. On Friday, a surge of monsoon moisture will return to
southern Arizona resulting in a threat of afternoon and evening
thunderstorms over portions of south central Arizona, while
southwest Arizona and southeast California remain dry and stable. A
strong high pressure system will subsequently build into the region
Sunday through the middle of next week for dry weather and much
warmer afternoon temperatures.



Tonight through Thursday...

Northwesterly flow aloft is expected to prevail through this period,
which is expected to gradually push drier air into the region.
However, a weak shortwave wave embedded in this northwesterly flow
(as seen on latest WV imagery) is now beginning to trigger shower
and thunderstorm activity over northern AZ this hour. Although the
drier and more stable airmass over the region will likely not allow
most of the convective activity to survive the trip into the lower
deserts, there is a potential for outflows from these storms to make
in into the lower elevations of south-central AZ during the late
afternoon/early evening hours, with this outcome being supported by
the last several HRRR high-res model runs. At this point, do not
believe that these outflow winds will be strong enough to produce
significant blowing dust. However, have decided to raise pops a bit
over extreme northern Maricopa county to reflect the possibility of
convection skirting the northern part of our CWA. Once these
storms/outflows dissipate later this evening, expecting mainly clear
skies through the rest of tonight into Thursday night as the
afomentioned shortwave moves off to the east and drier air moves into
the region.

Friday and Saturday...

Another unusual trof is forecast to slowly deepen into Utah and far
northern AZ Friday and Saturday. This trof has origins in western
Canada. What is so unusual about this system is its vertical
structure by late Friday. For example, the 300/250 mb trof is over
Salt Lake City. Its 500 circulation center is progged to be just
northwest of Las Vegas, and the 850/700 circulation center develops
along the San Diego coast. Its very unusual to have a vertical trof
axis tilted toward the northeast.

Since the 850/700 mb circulation center will develop along the San
Diego coast, increasing low level south wind will import monsoon
moisture back into AZ, almost surging Friday afternoon and night. At
the moment it appears that any thunderstorm activity will be
thermodynamically driven, i.e. heat of the day storms mainly in
south central AZ. A chance of afternoon convection will develop over
the mountains with outflows possibly triggering secondary desert
convection by evening. Some lighter showers/virga could persist into
Saturday morning over south-central AZ as PWATS rise to near 1.50
inch and the main upper trof axis approaches the region from the
west. More storms could develop on Saturday afternoon as the the
trof moves across south-central AZ, but they will likely, for the
most part, be confined to the easternmost part of our cwa, with the
possibility of outflows making their way westward into eastern
Maricopa/Pinal Counties.

Sunday through Wednesday...
A strong high pressure system is forecast to build into the region
from the southwest direction. The monsoon moisture will be pushed to
the other side of the Mexican border, wavering across far southeast
AZ at times. Much warmer afternoon temperatures are also forecast,
between 107 and 111 on the lower deserts.


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

Moisture regionally is on the decline, but enough remains to support
storms north and east of Phoenix. Very small chances remains of a
shower/storm developing southward over the Phoenix Metro, more than
likely favoring the northern and far western fringes of the city.
Chances look much higher and better for wind shifts and gusts driven
by storm outflow with initial estimates from the north (possibly
northeast where the largest volume concentration of storms currently
is) around or after 25/02z. Confidence remains low in temporal/areal
of storm development in the Metro, so continued making no mention in
the TAFs this evening. SCT to BKN mid-level cigs will blow off and
develop from the northern storms that should dissipate in overall
thickness but linger somewhat into the Thursday morning hours.
Afternoon westerly winds to prevail before any outflow winds work
their way into the terminal airspace, before transitioning into
typical easterly overnight and into Thursday AM.

Southeast California/Southwest Arizona including KIPL and KBLH:
Clear skies and southerly oriented winds (variant between southwest
to southeast at times) to prevail for the terminals for the TAF
period. Some afternoon gustiness is possible for KBLH of 15 to 20kts
at times.

Aviation Discussion not updated for amended TAFs.


Saturday through Wednesday...
Southerly flow over the region will allow some monsoon moisture to
return by the weekend. This increase in monsoon moisture will lead
to an increase in convective activity over South-Central AZ, with
the best chances for wetting rains over the higher terrain east of
Phoenix. On Sunday a drying trend will commence, with minimum
humidities expected to drop from 15-45 percent on Sunday to 10-35
percent on Tuesday. Winds each day will favor a south to southwest
direction during the day with the gustiest winds expected on Friday,
especially along the Colorado River Valley. In this area sustained
winds will be up to 15 mph with gusts up to 25 mph.


Spotters should follow standard reporting procedures.




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