Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 210434

Area Forecast Discussion
National Weather Service Phoenix AZ
934 PM MST Mon Feb 20 2017

Drier and more stable conditions will develop through Wednesday
as a ridge of high pressure system builds over the region. This
will result in a significant warming trend. A mostly dry cold
front will move through the region Wednesday night and Thursday,
with temperatures dipping a bit below normal. Another system will
affect the region over the weekend.


Congested CU fields rapidly cleared with the loss of diurnal
heating and sun angle before tonight`s 6:18pm MST sunset. The
weekend storm system continues to march across the country, with
trough heights running through northern MN down through the south
TX Gulf Coast. Quasi-zonal flow is transitioning into the forecast
area as seasonal sub-tropical ridging builds over the Pacific
Ocean off the Baja Peninsula. AMDAR flights w/ Td/vapor traces
around the region still point to elevated BL and LL humidities
however urban locales have lost much of the wetted pavement and
surface ponding during the day under the partly sunny conditions.
In spite of some limited surface drying, elevated BL moisture,
mostly clear skies and light overnight winds could support
fog/mist development early Tuesday morning. Fcst soundings support
a few hour window of near saturated near-surface conditions over
the south-central AZ desert areas. Evening forecast grids are in
good shape and rest of the forecast for the week the area will
fall under a warming trend with normal temps (low 70s) quickly
being surpassed by mid-week.


Today through Wednesday: Plenty of lingering low level moisture
remains over the region and latest satellite imagery is showing
scattered to broken cumulus/stratocumulus clouds over much of the
south-central deserts. Cloud ceilings right now range primarily
between 4 and 7 kft with these ceilings expected to gradually rise
this afternoon as more day time mixing occurs. More significant
clearing will occur near sunset, with mostly clear skies expected

Tomorrow morning across the lower deserts of south-central
Arizona, dewpoints will be in the upper 40s to low 50s and low
temperatures are expected to dip down into this range as well.
This small temperature/dewpoint spread, along with clear skies and
calm winds could result in some redevelopment of patchy fog/ mist
in this area. NAM BUFKIT PHX sounding is backing up this thinking,
indicating that the best window for possible fog development will
be between 5AM and 10AM.

Ridging will continue to strengthen and build into the region
bringing warmer and more stable conditions. High temperatures
will be climbing well above normal (normal for Phoenix is 72
degrees both tomorrow and Wednesday) and will be reaching the mid-
to upper- 70s each day.

Wednesday night through Thursday:
The ridge will start to break down as a cold front starts to make
its way across the desert southwest Wednesday night into
Thursday. This system will remain relatively dry with NAEFS
probabilities of QPF only showing less than a 10 percent chance of
rain in our region, with slight chances across the higher terrain
north and east of Phoenix. The main impact with this system will
be slightly elevated winds along and west of the Colorado River
valley Wednesday night into Thursday morning and primarily east of
the Colorado River valley Thursday morning into Thursday evening.
Sustained westerly winds will be near 10-15 kts, gusting up to
20-25 kts as this front sweeps across the region.

Friday through Sunday...
Temps warm slightly Friday and Saturday before another trough
moves through the region Saturday night and Sunday. The 00Z ECMWF
is a bit faster than previous runs and thus is faster than the 00Z
GFS (even more so for the 06Z GFS). The 00Z CMC is in between.
NAEFS output and calibrated GEFS develop depict slight chance PoPs
with this system (mainly for our Arizona zones) and that is
reflected in the forecast. Temps drop on Sunday as well but not


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

Daytime CU fields dissipated quickly before sunset and skies will
remain mostly clear for the overnight hours. Some high level
clouds may intrude from the west, remaining AOA 25kft. Enough
boundary layer moisture remains to maintain some concern for fog
development in the vicinity of the Phoenix area terminals, however
confidence remains mostly low that development will occur over a
wide margin of the Phx area terminals. Current thinking is KIWA
and KSDL could see the potential in at least a vicinity form for a
few hours just before sunrise and until mid-morning Tuesday.
Overnight winds will follow usual nighttime directional headings
with very light speeds.

Southeast California/Southwest Arizona including KIPL and KBLH:
Conditions will continue to dry out from the weekend weather, with
skies remaining mostly clear and winds returning to more typical
headings. Cloud cover will continue to thin from even the CU
fields Monday, with only high level cirrus FEW-SCT expected AOA
25kft. Light, variable winds will be predominant overnight and
into early Tuesday AM as well.

Aviation Discussion not updated for amended TAFs.


Wednesday through Sunday...Dry conditions are expected for most
areas through Saturday. Above normal temperatures can be expected
Tuesday and Wednesday. A low pressure system passing mainly to the
north Wednesday night and Thursday will lead to cooler
temperatures and locally breezy conditions. Slight chances for
rain with the midweek system will be limited to the higher
terrain of south-central AZ. Anticipate slight warming Friday and
Saturday. Minimum humidities will trend downward through the
workweek (especially on Thursday) with most lower elevations in
the 15-20% range by Friday. Another storm system will affect the
region this weekend. The main effect will be breezy west winds on
both Saturday and Sunday. A slight chance for rain is also
forecast on Sunday for south-central Arizona.


Spotters should follow standard reporting procedures.




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