Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
317 PM MST Tue Sep 8 2020

.UPDATE...Updated Fire Weather Discussion.

&&

.SYNOPSIS...
A strong storm system dropping through the Rockies and Great Basin
into the Southwest will bring much cooler temperatures, breezy to
windy conditions, and a fleeting chance of showers and thunderstorms
mainly over the high terrain east and north of Phoenix, with slight
chances for Phoenix as well. Drier and warmer conditions gradually
move into the region beginning later in the week.

&&

.DISCUSSION...Short term...
The center of mid level closed low pressure storm system has pushed
south into E Utah with a pronounced northwesterly and westerly H5
jet streak cutting through the N half of AZ. In the early afternoon
the associated cold front extended from Imperial Cty CA to NW
Maricopa Cty. Dewpoints behind the front are being reported in the
20s and even upper teens out west.

The stronger frontal and cold air wind gusts, where a wind
advisory remains in effect through tonight, have so far been
near 28-38 mph with a a few in the 40s. The strongest wind gusts
are still expected to reach as high as 50-55 mph before the end of
the evening. In addition to the wind advisory, a Red Flag warning
remains in effect from the Phoenix area west into tonight for
critical fire danger due to strong gusty winds and plummeting RH
values. Areas of locally dense blowing dust have also been
reported with a blowing dust advisory in effect into the afternoon
for parts of SE CA to SW AZ.

A moist and unstable airmass across the area ahead of the much
cooler and drier airmass behind the cold front is helping to kick
off a line of showers and thunderstorms north of Phoenix, and a few
small showers west of Buckeye as revealed by early afternoon
radar. High-res guidance/CAMs continue to favor more scattered
shower and storm coverage in the areas north and east of Phoenix
with much more isolated coverage for Phoenix through the
afternoon and early evening, when the cold front sweeps through
the area. Momentum transfer of strong frontal winds aloft, with
any showers and thunderstorms that do develop, in the already
gusty environment could produce severe wind gusts near the
storms. Although a widespread outbreak of severe weather is not
anticipated due to the more robust dynamics remaining confined
well to the north with the main low pressure storm system.

The widespread smoke ceilings in the upper-mid levels from the
intense fires in California may tend to persist in spite of this
afternoon's cold frontal passage. However much will depend on the
state of the fires.

&&

.DISCUSSION...Extended...
The coolest weather we've experienced in quite some time is on
track to occur Wednesday and Thursday. The forecast high in
Phoenix has now fallen to 91 degrees on Wednesday, with low 90s
extending all the way into southeast California. The overall
pattern evolution will feature a large closed low over northern
Arizona, so if this trends farther southward in position than
currently projected, there's an outside chance high temperatures
may stay in the 80s on Wednesday - which seems nearly impossible
given the relentless streak of excessive heat. Even better, the
low temperatures are going to be very pleasant for early
September. Most areas will see lows in the 60s on Wednesday and
Thursday morning, with 50s even forecast for the Globe area.

Thursday will feature a slight warmup by about 5 degrees or so,
but still comfortable, given the proximity of the upper low. Winds
should weaken some on Thursday, but still be breezy across
southwest Arizona and the lower Colorado River Valley.
A warmup is in store later next week as the upper high rebuilds over
the Southwest. Fortunately, this feature will be nowhere near as
strong as last week's mega-high, but still significant enough to
bring temperatures back into the 105-108 range with the warmest
temperatures over southeast California. This should generally remain
shy of excessive heat thresholds.

&&

.AVIATION...Updated at 1725Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Increasingly gusty winds will be the main surface weather issue
through tonight though reduced slantwise visibilities will also be a
concern with expansive FU and BLDU streaming into the area from the
west. Sfc winds will steadily veer ahead of a cold front this
afternoon, and confidence is good that gusts nearing 30kt will be
likely along the front mid/late afternoon. Given the amount of
FU/BLDU in upstream sfc observations, it would not be out of the
question if a period of MVFR vsby is observed but still a low
probability.

Otherwise, a few brief SHRA may be possible around the PHX vicinity
though coverage, duration, and confidence in formation within any
terminal footprint precludes any TAF mention. Wind speeds should
weaken while directions attempt to turn northerly later tonight
before finally obtaining an easterly component by sunrise Wednesday.

Southeast California/Southwest Arizona including KIPL and KBLH:
Very strong winds behind a cold front and reduced visibilities due
to upstream BLDU and FU will be the main weather hazards this
afternoon and evening. Confidence in wind shift timing and post
frontal wind gust speeds 30-45kt is good. Based on observations
across S NV/NW AZ, expansive FU and BLDU are being mixed and
creating MVFR (and locally IFR) visibilities. Feel a period of MVFR
vsby is likely though confidence is lower with respect to how long
these restriction persist into the late afternoon/evening.
Conditions should improve markedly by late evening/overnight.

&&

.FIRE WEATHER...
Friday through Tuesday:
A warming trend will last through the period as slightly below
normal highs in the upper 90s to 100 through Friday warm to a few
degrees above normal during the weekend. Daily min RHs will stay
in the teens through the rest of the period. Winds will generally
remain light with directions favoring diurnal/terrain patterns.
Nighttime max RH will generally range in the 25-40% range.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Red Flag Warning until midnight MST tonight for AZZ131-132.

     Blowing Dust Advisory until 4 PM MST this afternoon for AZZ530-
     533.

     Wind Advisory until 10 PM MST this evening for AZZ530>533-536.

CA...Red Flag Warning until midnight PDT tonight for CAZ230>232.

     Blowing Dust Advisory until 4 PM PDT this afternoon for CAZ569-
     570.

     Wind Advisory until 10 PM PDT this evening for CAZ560-561-564-
     565-568>570.

&&

DISCUSSION...Short term...Sawtelle
DISCUSSION (Extended)...Rogers
AVIATION...MO
FIRE WEATHER...Sawtelle

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