Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 250915

Area Forecast Discussion
National Weather Service Phoenix AZ
215 AM MST Sat Feb 25 2017

Somewhat 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.


An extremely convoluted blocking pattern persists over the eastern
half of the Pacific basin while downstream quasi-zonal flow across
the southern Conus has supported seasonably typical temperatures and
tranquil weather. While heights aloft will remain nearly steady,
continued warming in the H9-H8 layer will allow afternoon highs to
be realized some 4F-8F warmer than Friday. A compact PV anomaly near
41N 131W this morning will propagate towards southern CA throughout
the day tapping and spreading thicker high level moisture inland.
Based on satellite imagery and model forecasts, feel the thicker
cirrus capable of inhibiting daytime insolation will be delayed
until very late afternoon and evening creating minimal impacts after
daytime highs have already been reached.

The aforementioned PV anomaly/shortwave will rapidly stretch and
shear eastward through northern AZ into a downstream jet entrance
region Sunday. Any appreciable moisture and saturation still appears
to be trapped in a H7-H5 layer while deep layer ascent will be
waning with the loss of more amplified wave structure. Based on
forecast soundings, have no doubt virga and sprinkles will be common
throughout much of the forecast area Sunday morning. However, the
better upslope trajectories and saturated orographic ascent will
favor light rain at higher elevations north and east of Phoenix
before a brief respite of more expansive drier air and deep
subsidence spreads back into the area Sunday afternoon/evening.

It is quite possible global models and their associated ensemble
members are finally converging on an accurate solution with respect
to a larger trough axis and stronger moisture flux flowing through
the CWA late Monday into Tuesday. The combination of a shortwave
descending from the Pacific NW and absorption a subtropical wave and
Pacific moisture plume will form a pronounced longwave trough axis
along the West Coast. Trends in model output point towards a more
full latitudinal trough, but more importantly a slower evolution
capable of pulling better quality moisture north and east into AZ
before a deep frontal boundary sweeps through the forecast area
Tuesday afternoon.

Certainly something appears to have been identified in the GFS
initialization routine as not only does the operational member
indicate more robust moisture advection, but almost every single
GEFS member now advects subtropical moisture into central AZ Monday
night into Tuesday. Outside of an occasionally dry ECMWF or GEM
model run, most of these global models and associated ensembles were
already showing some measure of moisture advection. So, the trend in
GEFS members only reinforces the prior conceptual thinking of good
theta-e advection and water vapor transport ahead of deep layered
ascent and frontogentical forcing. In fact, GEFS members now show a
good clustering of QPF values in a 0.50-0.75 inch range at lower
elevations (when prior iterations could barely single out more than
a few members with any precipitation). Unless something abruptly
changes with the next initialization scheme, the primary forecast
challenge remaining will be to identify the most likely time frame
for rain as models are still somewhat uncertain with specific
timing. Regardless, have boosted POPs rather substantially for
central Arizona Monday night into Tuesday morning while starting to
trends QPF amounts towards the ensemble mean.

A blocking pattern will quickly become reestablished across the
central Pacific basin by the middle of next week leaving the general
fixed downstream longwave features of west coast ridging and east
coast troughing. All operational models show some measure of a weak
cutoff circulation into northern Mexico albeit completely devoid of
available quality moisture. This feature may be sufficient to keep
temperatures somewhat in check as H5 heights only peak in a 570-
576dm range. However given the stronger March sun angle and good
downslope compressional heating potential, feel highs in the 80s are
a good bet by the end of next week under sunny skies.


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

Northwest flow aloft will persist across the region, promoting
typical diurnal wind shifts with speeds mostly 8kts or less for
all the terminals. Overnight skies to remain clear before high
level cirrus moves in from the west and increasing to BKN coverage
by Saturday aftn.

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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