Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 220332
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
832 PM MST Tue Feb 21 2017

.SYNOPSIS...
Similar weather is expected through Wednesday afternoon before a dry
cold front moves through the region Wednesday night and Thursday.
This will bring below normal temperatures for everyone with breezy
conditions to the western positions of our CWA. Another system will
affect the region over the late weekend and into early next week.

&&

.DISCUSSION...
Atmospheric conditions continue to broadly dry from the wet
weekend pattern with the only signs of moisture around the area
are any lingering surface puddles and wet neighborhood greenbelts
along with the high level and icy cirrus overhead. Quasi-zonal
flow stretches through much of the SW as the sub-tropical high is
now centered over northern Old Mexico. UL jet and storm track span
as far south as central CA and the Great Basin states, where
scattered rain and thundershower activity runs from Salt Lake
through central and northern CA. Series of upper troughs will
continue to track through the Intermountain West, with the main
trough ushering in a cool frontal passage and associated breezy to
windy conditions for the lower AZ/CA deserts beginning Wednesday.
No updates planned for the near-term/nighttime package with dry
conditions, introduction of increasing winds for SE CA and high
level cloudiness are already present in gridded forecasts.

&&

.PREVIOUS DISCUSSION /Issued at 247 PM MST/...
Satellite imagery shows mid-level clouds beginning to move over
SE California this afternoon in response to a broad upper level
trough slowly moving into our area. This band of clouds will
continue to edge eastward over the next 24 hours in conjunction
with the main trough. Despite this, the airmass over our area
isn`t expected to cool much for tomorrow and we will likely see
high temperatures similar to today. The one exception being SE
California where the troughs influence may bring a few degrees of
cooling over today.

By Wednesday evening, the leading edge of the trough will approach
the Phoenix area.  Since the best dynamics will be well to our north
and moisture will be minimal, we are only expecting high and mid
level clouds.  However, behind this dry front will be much cooler
air and 1000-500mb thickness values are expected to drop from
approximately 560dam Wednesday afternoon to 548dam by Thursday
morning.  As one could expect, this will bring a rather large drop
in daytime high temperatures (~10 degrees).  Some hi-res models also
suggest that breezy/windy conditions could exist, particularly in
the western portions of our CWA as we transition into this cooler
airmass Wednesday evening into Thursday morning.

No big changes are expected for Friday as the trough is slow to exit
our area and the upper level flow transitions to nearly zonal.
Heights build slightly on Saturday which will likely push the deserts
back into the low 70s.  By late Sunday, both the GFS and ECMWF
position another trough just offshore of California and move it into
our area during the early parts of next week.  While it is still a
ways out, this particular system is currently forecast to dig deeper
than our Wed/Thus trough which would position it to provide much
better lift and precip chances.  Since the major global models are
in general agreement, have upped PoPs slightly for early next
week.

&&

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

High level clouds becoming BKN to OVC will continue to track in
from the west overnight with CIGs remaining AOA 25kft. Winds will
continue to follow usual typical directional patterns overnight
with light speeds mostly 8kts or less. A dry but cooler weather
system will begin to move in from the west tomorrow (Wednesday),
slightly lowering high level CIGs down towards 18-20kft and
increasing west-southwesterly winds for KIPL and KBLH by midday
with gusts 20-25kts possible.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Thursday through Monday...Dry conditions are expected for most
areas through Sunday. Above normal temperatures will occur Tuesday
and Wednesday. A dry cold front moving through the region
Wednesday night and Thursday will lead to cooler temperatures and
locally breezy conditions. Anticipate slight warming Friday and
Saturday. Minimum humidities will trend downward through the
workweek (especially on Thursday) with most lower elevations in
the 15-25% range by Thursday. Another storm system will affect
the region late this weekend into early next week. The main effect
will be breezy west winds on Sunday and especially Monday.
Chances for rain also increase on Monday over the entire forecast
area.

&&

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

&&

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

$$

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DISCUSSION...Nolte
PREVIOUS DISCUSSION...Wilson
AVIATION...Nolte
FIRE WEATHER...Rogers



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