Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 252127

Area Forecast Discussion
National Weather Service Phoenix AZ
227 PM MST Sat Feb 25 2017

Updated Aviation section.


Slightly cooler than average temperatures will persist through
early next week as two fast moving weather disturbances move into
the region Sunday, then again Monday night and Tuesday. The
weather system Monday night and Tuesday will have best potential
for spreading rain across the area. Dry conditions and much warmer
temperatures will return by the middle of next week.


High clouds continue to stream eastward across southern and
central California within predominantly westerly flow established
over the Southwest. Slightly below-normal temperatures will
continue today and tomorrow as midlevel heights remain suppressed
with an active storm track to the north. A series of upstream
shortwave troughs, noted as two features evident on water vapor
off the northern California coast and near the Alaska Panhandle,
respectively, will move through the region through mid-week. The
first will be a fast-moving system that will deamplify while
rapidly moving eastward tomorrow. Forcing for ascent associated
with this feature will be rather focused, and could produce some
showers across central and northern portions of Arizona, but the
lack of moisture will limit the potential for any widespread or
substantial precipitation. Slight chance PoPs exist tomorrow
morning and afternoon across roughly the northern half of the CWA,
including the higher terrain of Maricopa and southern Gila
counties, and Joshua Tree National Park. An increase in mid-to-
high-level clouds is also expected.

Attention then turns to a more substantial upper trough which will
deepen along the Pacific coast through Monday. This feature will
phase with a narrow southern stream plume of moisture, and bring
increased rainfall potential beginning late Monday and Tuesday.
Similar to the last major system, the trajectory of the preceding
moisture plume should favor the most widespread precipitation
across central and northern portions of Arizona through early
Tuesday, before shifting into eastern Arizona by late Tuesday. As
a result, PoPs have been adjusted upward again for Monday evening
and Tuesday. QPF amounts have also been increased to be more in
line with the latest WPC guidance, with roughly 0.5-1 inch
expected from near Phoenix and areas to the north/east. A few of
the higher elevation locations may locally receive over one inch
of rain. Given the orientation of the moisture plume relative to
the positively tilted system, snow levels should remain high
enough to preclude snowfall over the higher elevations of southern
Gila and northeast Maricopa counties. A second, drier disturbance
is forecast to move through the area Wednesday, but will be
moisture-starved, before more substantial midlevel ridging builds
over the western CONUS and ushers in a return of above-normal
temperatures and dry conditions late next week.


South-Central Arizona including KPHX, KIWA,and KSDL:
Southeast California/Southwest Arizona including KIPL and KBLH:

A low pressure system centered off the northern California Coast
will weaken as it moves inland. It will spread increasing
cloudiness over the forecast area from west to east tonight
through Sunday morning with ceilings getting progressively lower.
However, anticipate ceilings to remain above FL080. There will be
a lot of virga with some spotty very light showers/sprinkles
reaching the ground (mainly after 10Z over southeast CA and after
14Z over south-central AZ). Westerly winds aloft will strengthen
with the passage of this system. Westerly surface winds will
strengthen over southeast California tonight with occasional gusts
of 20-25 kts (especially over, and west of, the Imperial Valley).
Those breezes will slowly expand eastward during the day Sunday.

Aviation Discussion not updated for amended TAFs.


Monday through Friday...
A Pacific low pressure system will bring good chances of wetting
rains mainly Monday night through Tuesday morning focused
especially across the high terrain of Arizona. Below normal
temperatures are expected through next Wednesday with a
significant warming trend likely late next week. Breezy conditions
for Monday and Tuesday are expected in most areas with breezy
conditions persisting down the Lower Colorado River each afternoon
through at least Thursday. Increased humidities with minimum
readings between 25 to 35% through Tuesday will dry out starting
Wednesday with afternoon readings closer to 15% over the deserts
for Wednesday through Friday.


Spotter reports will not be needed through the weekend.




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