Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 271056

Area Forecast Discussion
National Weather Service Phoenix AZ
400 AM MST Mon Feb 27 2017

.Update...Aviation and Fire Weather Discussions.


The work week will open with continued cool temperatures as
another Pacific weather system brings clouds and precipitation to
the Southwest Deserts. Showery to rainy conditions will span west
to east today and tonight as the upper trough moves through the
region. Precip activity will mostly wrap for the area Tuesday
afternoon with skies clearing. Dry conditions and much warmer
temperatures will return for the latter half of the week.


Overnight sky conditions have remained mostly clear, but that
will change after sunrise with the arrival of Pacific moisture
from the west and troughing moving into the area from the
northwest. Split jet-stream flow across the area will merge
today, acting with incoming upper trough energy and moisture to
produce increasing clouds and precip coverage, with the peak of
activity occurring later today and into the overnight hours.
Tightening MSLP gradients ahead of and with the trough passage
will lead to breezy southwest conditions today and early tomorrow
and under increasing clouds, temperatures will maintain their
cooler than normal trends with highs in the 60s for the lower

Latest hi-res precip plots hint that scattered light showers will
begin to develop over the SE CA and SW AZ deserts by late
morning/midday. Activity on area radar scopes will continue to
blossom from midday onward, with rain likely for portions of the
Monday PM commute across the south-central AZ deserts -- including
the Phoenix area. GEFS and GFS IWT forecasts peak overnight
across south-central AZ and southwesterly oriented wind fields
with ML jet winds of 65-70kts and UL winds 125kts-plus, favorable
orographic and ascent processes will work on near 1" of PWAT in
the column. Typically favored upslope areas within southerly or
southwesterly flow regimes are expected to receive the highest
precipitation totals, ranging from 1 to 1.5" in the foothills and
mountainous locales of Gila and northern Maricopa counties. QPF
gradients decrease the farther southwest you head from the Phoenix
area (where values could range from 0.5-0.75") with 0.15-0.25"
possible through SE CA and the Colorado River Valley. Rainfall
accumulations are timed through a 12hr window or so from this
afternoon through early Tuesday AM. Cannot rule out local area
washes and drainages containing some streamflow as runoff works it
way through the area, but widespread flooding concerns are lower
than with previous rainfall events. Snow levels with this system
stay relatively high for our CWA, dropping around 6kft or so at
their lowest.

Upper trough axis will swing through the area early Tuesday and
quickly accelerate eastward, with precip chances ending nearly CWA
wide by Tuesday late afternoon. Longwave troughing will hold over
the area with additional shortwave energy brushing through the
Four Corners area Wednesday. Only notable effect on sensible
weather being to keep temperatures cool for one more day. Rapidly
clearing conditions and still cool AM temperatures could lead to
some mist/fog development for Wednesday AM, however coverage will
largely depend on rainfall amounts and if winds overnight Tuesday
go calm. Current forecasts have them maintaining some elevated
northeasterly drifts, so held off from including fog mention in
the weather grids for Wednesday AM.

UL heights and flow aloft will moderate as high pressure builds
over the central West Coast, but a still invigorated sub-
tropical jet just to our south will lag trough heights across
portions of the Southwest for the end of the week. Drier flow off
the Pacific will keep skies mostly clear and once ridging takes
hold of the area and increases thicknesses, temperatures will
undergo a healthy warming trend for the late week and weekend.
Temperatures are likely to reach the low-80s across many locales
for the first weekend of March in the Southwest.


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

Cold front slowly approaching the region for tonight, but will draw
in a considerable amount of subtropical moisture for later this
afternoon and evening. Through 18z Mon, becoming sct 6 thsd agl with
vrbl cirrus above. From 18z Mon to 00z Tue, clds thickening and
lowering, becoming 6 thsd bkn agl and 10 thsd ovc. lgt r shwrs. west
wind 5 to 10 knots. From 00z Tue to 06z Tue, continuous light rain,
moderate at times. Few 2 thsd agl with cigs 3-4 thsd ovc. Vrbl wnd
under 5 knots.

Southeast California/Southwest Arizona including KIPL and KBLH:

Cold front slowly approaching the region for tonight, but will draw
in a considerable amount of subtropical moisture for later this
afternoon and evening. Through 18z Mon becoming sct 6 thsd agl and
18 thsd bkn. From 18z Mon through 00z Tue, clds thickening and
lowering becoming cigs 5-6 thsd bkn-ovc. occasional r shwr. S to sw
wind 5 to 8 knots. From 00z Tue to 06z Tue, sct-bkn 6 thsd agl, bkn
140. Occasional lgt rain.  west wind 5 to 8 knots.  Aviation
Discussion not updated for amended TAFs.


Wednesday through Sunday...Dry northwest flow aloft. A strong
warming trend with lower desert afternoon temperatures approaching
80 degrees by Saturday and Sunday. Minimum relative humidity
decreasing into the 15 to 20 percent range. North wind generally 7
to 12 mph. Fair overnight recovery.


Spotter reports will not be needed through the weekend.




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