Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 241540

Area Forecast Discussion
National Weather Service Phoenix AZ
840 AM MST Fri Feb 24 2017


Cooler than average temperatures will persist through early next
week with two fast moving weather disturbances moving into the
region Sunday and again Monday night. The weather system Monday
night will have better potential for spreading rain across the area.
Dry conditions and much warmer temperatures will return by the
middle of next week.



A very tranquil, and chilly morning across our cwa today, under flat
ridging and dry west-northwesterly flow aloft. The combination of
mostly clear skies, light winds, and a very dry airmass (0.15 in
PWAT in the KTWC balloon sounding) allowed temperatures to fall
sharply overnight. Lows this morning across the lower deserts ended
up in the upper-30 to mid-40 range (for the most part), which is
around 5-8 degrees below normal for this time of year. Today looks
like it will be very similar to yesterday, under clear skies, with
most of the warming aloft occurring aoa 500 mb. Lower desert highs
are expected to, once again, remain mainly in the 60s. AS far as the
very short-term is concerned, other than some very minor adjustments
to the hourly grids, inherited forecasts look good.


A dry, stable pattern has become established over the SW Conus this
morning with strong quasi-zonal flow trapping moisture and lift well
north into the central/northern Rockies. Persistent 564dm H5 heights
will keep temperatures in a slightly below normal category though 2C-
5C of warming in the boundary layer will allow for a nice moderation
Saturday as compared to today. Otherwise, upstream WV imagery shows
good potential for thicker cirrus to begin overspreading the region
tomorrow. Almost all guidance captures this evolution in advance of
a blocked Pacific flow pattern opening up and forcing at least 2
waves through the SW US early next week.

The first such shortwave will deamplify over cntrl/srn California
becoming sheared into a downstream jet entrance region Sunday
afternoon. Attendant moisture associated with this wave will be
extremely limited and focused primarily in the mid and upper levels
of the atmosphere (above H7). Not surprisingly, model QPF reflection
is meager at best and almost exclusive to higher elevations of
northern and eastern AZ. This scenario had already been accounted
for necessitating only minor forecast adjustments where virga and
sprinkles would be the most likely outcome for the vast majority of

On Monday and Tuesday, the trailing `kicker` shortwave will descend
southward towards the developing longwave trough base, absorbing a
piece of subtropical Pacific energy and moisture slug along the way.
Unfortunately, forecast confidence has deteriorated in the past
several model runs with frequent run to run changes in shortwave
depth, speed, and moisture transport. The 00Z operational GFS
remains much more dampened and faster with the shortwave passage
while also narrowing and displacing the subtropical moisture flux
south of the CWA. This solution has only limited support from the
ensemble majority and other operational global members. The 00Z
ECMWF is much slower with the wave passage, though has also backed
off the idea of a more organized subtropical plume surging into
central AZ. With the true outcome likely somewhere in between, have
utilized an ensemble mean which resulted in a modest reduction in
POPs on Monday and spread more into Tuesday morning.

The flow pattern will become more predominated by longwave features
during the second half of next week with mean troughing over the
eastern Conus and ridging through the NW. There is excellent model
agreement that a nearly cutoff wave will spin and meander over
northern Mexico albeit completely devoid of appreciable moisture.
While H5 heights in a 570-576dm range are fairly typical for early
March, increased sun angle and insolation should push temperatures
back into a near to above normal range. Kept forecast highs fairly
conservative in the mid/upper 70s for the most part, however would
not be shocked to see reading eclipse the lower 80s by the end of
next week.


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

Dry northwest flow will keep skies mostly clear. Variable light
winds this morning in the Phoenix area will become more dominant
out of the west through this evening. North to northwest winds
will persist for KIPL and KBLH at 6-9kts.

Aviation Discussion not updated for amended TAFs.


Sunday through Thursday...
Dry conditions should prevail for most areas on Sunday ahead of an
approaching Pacific low pressure system. This weather system will
bring chances of wetting rains mainly later Monday 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 on Monday are expected in most areas with breezy
conditions persisting down the Lower Colorado River each afternoon
through 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 and Thursday.


Spotter reports will not be needed through the weekend.




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