Dust Devil in Arizona, September 24, 2017

Area Forecast Discussion
National Weather Service Phoenix AZ
943 AM MST Sun Sep 24 2017

.SYNOPSIS...
Under sunny skies, temperatures will slowly warm over the next few
days though readings will remain slightly below average. An area of
low pressure may pull moisture back into eastern Arizona by the
middle and end of the week supporting some late season showers and
storms.

&&

.DISCUSSION...
Amplified pattern across the continental US continues to result in
cool and dry conditions across the Desert Southwest. Temperatures
as low as the upper 40s were reported this morning in the lower
deserts near Arizona City. For this afternoon, expecting high
temperatures roughly one degree warmer than those observed
yesterday and still below average.

&&

.PREVIOUS DISCUSSION...
A deep PV anomaly has propagated into western Utah early this
morning with a -20C H5 cold core descending into far northern
Arizona. With the main jet core now translated towards the eastern
side of the trough, the central vorticity core will lift through the
Rockies leaving neutral to weak warm advection in its wake over
Arizona the next several days. However with much of the flow pattern
blocked across the southern Conus, full latitudinal troughing will
linger through the forecast area keeping H5 heights hovering around
576dm and muting a more rapid and substantial warming trend.
Essentially through Tuesday, temperature forecasts progressively
warm about 2F-3F over persistence each day under sunny skies amid a
very dry airmass.

Models trends continue to indicate that a jet streak topping an
amplifying east Pacific ridge will dive into the mean western Conus
trough and induce a strengthening vorticity maximum and deepening
area of low pressure across the Desert Southwest Wednesday and
Thursday. This evolution will promote lower heights cutting off from
the northern stream and a temporary rex block across the Rockies. As
a result, operational models and their corresponding ensembles still
exhibit a large amount of variability with respect to the placement
of the low and subsequent moisture transport.

Given the apparent source region of the moisture from the east side
of the continental divide and current expanse of extremely dry air
residing in the southwest, skepticism is rather high that deep
quality moisture supporting storms outside of mountain areas will
truly come to fruition. More robust outflows may be needed to mix
moisture through the boundary layer, and current forecast soundings
are not terribly optimistic about this outcome. Regardless, the
overall pattern is supportive for convection during the latter half
of the week though could not get too aggressive with POPs given all
these uncertainties. While both the 00Z operational GFS and ECMWF
eject the closed low eastward by the weekend, ensemble members
exhibit a much larger spread in outcomes. Historically, blocking
patterns are not modeled very well so confidence is somewhat low
towards the end of the forecast period though it does certainly look
like much warmer temperatures (a return to 100F) are on the horizon
for the weekend and beyond.

&&

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

Surface high pressure building over the Great Basin will first
affect winds along the CO River today, generating gusty north
winds for KBLH for much of the day. More typical and diurnally
driven winds directions and speeds will occur for the remaining
terminals under mostly clear skies today.

&&

.FIRE WEATHER...
Tuesday through Saturday:
Troughing will persist over much of the west, receiving
reinforcing shots of jet wind and energy from the north. One batch
of energy will eventually cut off over southern UT/northern AZ
turning low level flow south-southeasterly by the early week. Warm
air advection will warm sfc temperatures while also drawing up
moisture into the region. Humidities will be on the low side with
minimum readings falling into the 10-25 percent range with fair-
good overnight recoveries through midweek. Daytime humidities will
increase back into double digit readings by Wednesday, and north
of 15 percent for most locales by Thursday. Some increase in
humidities is likely beginning Wednesday. Anticipate north and
northwesterly breeziness Monday and Tuesday over southeast CA and
southwest AZ. More noticeable easterly winds will develop over the
eastern AZ districts midweek and beyond as the upper low remains
over the region. Slight chances for precipitation return from
Maricopa County eastward for Wednesday and continue for much of
eastern Arizona through week's end.

&&

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

&&

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

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DISCUSSION...Hirsch
PREVIOUS DISCUSSION...MO/Hirsch
AVIATION...Nolte
FIRE WEATHER...Nolte