Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 201152 AAA

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
450 AM MST Tue Feb 20 2018

.UPDATE...Updated aviation and fire weather discussions.


Today will feature some of the coldest air of the winter, with
freezing temperatures possible in some areas Wednesday morning. A
gradual warming trend is expected through early next week, though
temperatures will remain below average.


Latest water vapor imagery and streamline analysis suggest that
the broad trough axis to our west has slowed. Consequently, mid
clouds continue to stream northeastward ahead of the trough from
Maricopa County eastward. Embedded within these clouds are
scattered light rain and snow showers across the higher terrain,
though little to no accumulation is expected, even across the
highest peaks. Meanwhile, further west across central Arizona, dry
air continues to scour out the residual moisture, with surface
dewpoints as low as the lower teens.

Models remain in good agreement and indicate the aforementioned
trough will weaken as it lifts to the northeast today. However, in
its wake, an anomalously cold air mass will settle across the
Desert Southwest. Latest NAEFS indicates 700 mb temperatures will
fall below the 5th percentile (relative to climatology). Despite
the sunny skies, a high temperature of only 58 degrees in Phoenix
is expected, which would be the coolest day since December 21st
(57 degrees). A reinforcing shot of dry air will also overspread
the forecast area, with dewpoints falling into the single digits
this afternoon. The dry air combined with the light winds and
clear skies will provide a favorable setup for efficient
radiational cooling tonight. It is likely that some spots in the
outlying Phoenix suburbs will observe a few hours of freezing
temperatures early Wednesday morning, though coverage will not
necessitate a Freeze Warning.

Longwave trough across the western CONUS will persist into the
weekend, resulting in a continuation of the below normal
temperatures. GFS/ECMWF are in good agreement that a series of
short-wave troughs will move through the region over the next
several days. The strongest of these systems is likely to impact
Arizona Friday. While some GEFS and ECMWF ENS members indicate
that light precipitation will reach as far south as Phoenix, the
general consensus is that precipitation will remain confined to
northern Arizona. Behind this system, gradual height rises will
yield a discernible warming trend into early next week.


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

In the wake of a passing cold front, lingering low/mid level cloud
decks persist across portions of the greater Phoenix area, with CIGs
running 8-10k feet as of 5 am. Expect clouds to thin/clear shortly
after sunrise but there may be CIGs lingering til 14z or so before
skies become mostly sunny. After that expect genly clear skies thru
tonight with maybe a few higher based CU persisting into the
afternoon. Winds to favor the west in the Phoenix area today,
finally returning to the east after midnight tonight.

Southeast California/Southwest Arizona including KIPL and KBLH:

No significant aviation impacts expected over the next 24 hours.
Skies to be mostly clear at the TAF sites. Winds to favor the north
at KBLH with a few gusts to near 20kt possible during the mid-day
period. Winds favor the west at KIPL thru late morning...and again
this evening, otherwise turning more towards the north at speeds
mostly 10kt or less.

Aviation Discussion not updated for amended TAFs.


Thursday through Monday:
A series of low pressure systems will move through the Desert
Southwest through at least Friday. This will help to reinforce the
well below normal temperatures across the region and will keep
conditions a bit unsettled with the main precipiation threat across
south central Arizona Friday and Friday night. Humidity will stay
elevated Thursday into Friday with minimum RH mostly between 20 and
30 percent over the deserts. Drier westerly flow aloft returns
Saturday although high temperatures stay below normal. Continued dry
weather with a modest warming trend is expected Sunday into Monday
as minimum RH values drop into a range mostly between mid teens and
low 20s. Expect breezy to locally windy conditions Thursday and
Friday with winds favoring the west; strongest winds are likely to
be over far southeast California and in the favored higher terrain
locations. Rather light winds are on tap for the weekend and into
early next week.


Spotters should follow standard reporting procedures.




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