Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 250419

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

Updated Aviation section.


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.



Large outflow came through the Phoenix area this evening, in case you
missed it. Widespread winds observed in the 30-40 mph range with the
highest measured gust going to Scottsdale Airport with 54 mph (by far
the highest, see PNS issued at 749 PM with peak values). Typically,
outflow from the northeast is not a good trajectory for dust
development as there is not significant source material like what we
see between Phoenix and Tucson. As expected, there was some minor
blowing dust with it as it moved through the urban areas, but lower
values were not reported until the outflow got toward the outer
suburban areas. Usually as the outflow fans out, it loses its punch
as the aerial extent exponentially increases. This one kept going a
bit longer than we might expect though, aided by some convective
development over the city. As of 8 PM, it has moved outside Maricopa
and Pinal counties and is likely of minimal concern.

With the cool air that spread in the outflow`s wake, we lowered
overnight temperatures a bit. Latest runs of the HRRR (which was just
operationally increased from a 15 hour forecast to 18 hours) suggests
lows in the mid 70s tonight. Nudged temps toward the forecast but
thinking we may not get quite that cool, especially with mid level
debris hanging around a bit yet tonight.



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:
Storm activity done for the night. Surface winds will favor north
and northeasterly through 05Z-07Z before becoming downvalley
favoring east. Debris clouds will continue to diminish with just some
lingering few-sct after 06Z.

Southeast California/Southwest Arizona including KIPL and KBLH:
Southerly winds will predominate through Thursday with afternoon
gusts of 15-20kts. An exception is the Imperial Valley where
westerly winds will be common between 06Z- 12Z.

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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