Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 211117
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
417 AM MST Sun Jan 21 2018

.UPDATE...Updated aviation and fire weather discussions

&&

.SYNOPSIS...
An exiting storm system will leave behind and cool and dry conditions
with high temperatures well below normal. Low temperatures Monday
morning will be cold with lower deserts in the 30s and coldest
deserts dipping below freezing. Temperatures rebound by the
middle of the week with desert highs well into the 70s by
Wednesday. Another cold front moves through during the latter part
of the workweek leading to windy conditions Thursday and much
cooler temperatures on Friday. A warming trend begins over the
weekend.

&&

.DISCUSSION...
Today through Wednesday...
The frontal boundary is well into New Mexico early this morning
with the trailing upper low centered roughly near the 4 corners.
As of this writing, shower activity over AZ was associated with a
vort max in the bottom of the trough and was trending down. Held
on to a low end slight chance of showers over a sliver of our
easternmost areas (eastern Gila County) for this morning but
otherwise, precip is over for us. A lingering gradient and fast
flow in the lower troposphere is helping to keep some lingering
mixing going. This is helping to slow down the radiative cooling
and in turn keeping temps a bit warmer than they might otherwise
be. Even still, it will be chilly this morning with a lot of lower
desert locations bottoming out in the 30s. With leftover cold air,
highs today will be several degrees below normal with little
change from yesterday.

The coldest morning of the week will be Monday morning due to
less mixing and even lower dewpoints which is favorable for
radiative cooling. There are some potential mitigating factors
though. One is some thick cirrus that moves in Sunday night.
Another is a t tightening surface pressure gradient over south-
central AZ plus some lingering northerly breezes this evening over
SW AZ and SE CA which could slow down the decoupling. That being
said, anticipate that the warmest low deserts will struggle to
stay above 40 with coldest deserts dipping below freezing. The
sub-freezing areas are not widespread enough to merit a freeze
warning at this time. The overall airmass will be warming as a
Pacific ridge slowly builds in so Monday highs warm a little bit.

As the ridge builds in, highs climb well into the 70s by Wednesday.
This warm up will be aided by enhanced mixing due to a relatively
tight north and northeast oriented pressure gradient which will
also drive some breeziness (most noticeably over the Lower
Colorado River Valley).

Thursday through Saturday...
Another low pressure system and associated cold front moves
through the forecast area Thursday and Thursday night. Pretty good
model agreement that this one will be even less of a precipitation
producer than what we just had. So much so that we`re keeping the
forecast dry. This is because it takes more of a northerly track.
However, it will bring considerably cooler temperatures - most
noticeably on Friday with desert highs in the 60s. At this time,
it appears the front will move through SE CA Thursday morning and
across AZ Thursday afternoon and evening. In the process, it will
lead to windy conditions across the forecast area. Lingering
mixing may keep Friday morning lows from bottoming out as much as
they could. Saturday morning will be slightly cooler followed by
the beginnings of a warming trend as strong high pressure starts
to build in.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL: No aviation
concerns today as light westerly flow prevails at the TAF sites
with easterlies returning tonight. Skies will remain clear through
Sunday afternoon before high clouds gradually become BKN by Monday
morning.

Southeast California/Southwest Arizona including KIPL and KBLH: No
aviation concerns as winds remain light and follow typical diurnal
tendencies.  FEW to SCT high clouds will move in this afternoon but
will remain above 20 kft.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Tuesday through Saturday: Temperatures will gradually climb through
Thursday with 70s expected in the lower deserts Wed/Thur.  Winds
will generally be light but a system passing to our north on
Thursday will bring some breezy conditions to SE California. Cooler
air rushes in behind Thursday`s system with near to slightly below
normal temperatures Friday/Saturday.  Throughout the extended
forecast minimum humidities will remain quite low, teens and low
twenties being a common occurrence, although overnight recoveries
will remain fair to good.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

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DISCUSSION...AJ
AVIATION...Wilson
FIRE WEATHER...Wilson



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