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791
FXUS65 KPSR 161757
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1057 AM MST Mon Sep 16 2024

.UPDATE...
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18Z Aviation Discussion...
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&&

.SYNOPSIS...
A weather system will pass north of the region and bring modest
chances for showers and thunderstorms across the eastern half of
Arizona through tonight. Only slight rain chances exist for the
Phoenix area, with most of the activity focused over the high
terrain east of Phoenix. Dry and seasonably mild conditions will
spread over the area Tuesday, with forecast high temperatures
hovering around 5 to 10 degrees below normal through the end of the
work week.

&&

.DISCUSSION...
Early morning WV imagery reveals an upper level trough moving
onshore over Central CA, with at least one apparent small
disturbance wrapping around its base and moving over Southern
California at this hour. Objective analysis shows surface equivalent
potential temperatures increasing just southeast of Phoenix, as
southeasterly winds within a shallow layer take hold and advect
moisture into the lower elevations of South-Central AZ. Scattered
thunderstorms had developed and tracked over Southern Gila County
overnight and may continue to do so in the very near term, aided by
the continuing isentropic ascent ahead of the aforementioned
trough and feeding off the upwards of 500 J/kg MUCAPE from the
most recent RAP analysis.

With synoptic ascent ahead of the through and small disturbances
rotating around its base, the right ingredients exist for some
isolated pop up showers over the Phoenix Metro Area, mainly in the
East Valley, within the next several hours. Otherwise, the best
chances for some isolated convective shower activity over the South-
Central AZ lower deserts will be around midday/early afternoon,
before deep southwesterly flow scours the necessary moisture to
maintain instability. The main focus of shower and thunderstorm
activity will continue to be the high terrain east of Phoenix,
including Southern Gila County, through the next 24 hours or so.
HREF mean MUCAPEs peaks over Southern Gila County this afternoon
just north of 1000 J/kg, allowing the opportunity for a few strong
storms to develop in that window, but moisture and instability drop
off rapidly heading into the evening and overnight hours. Despite
moisture rapidly scouring out overnight tonight, synoptic forcing
will be maximized as the trough axis moves through Arizona into
Tuesday morning. Remaining moisture and elevated instability over
the high terrain of southern/eastern Gila County could lead to
overnight redevelopment of fast-moving elevated convective
showers before the best forcing passes completely east of our CWA.

Aside from rain chances in our eastern CWA, widespread breeziness
can be expected today. Southwesterly 850-700 mb mean winds upwards
of 25-35 mph are likely to mix down during peak heating today across
the lower deserts. The strongest winds expected today, however, will
be in Western Imperial County this evening and into the overnight
hours. A Wind Advisory remains in effect for this area from 2 PM
through midnight PDT. The zonal jet streak wrapping around the base
of the aforementioned trough will pass directly overhead tonight;
this, coupled with a deep, strong inversion at the top of the
boundary layer upstream of the San Diego Mountains will provide the
right conditions for Sundowner-type winds to develop over Imperial
County. Gusts to between 30-40 mph will be common in the Imperial
Valley and locations just east, with stronger gusts upwards of 40
mph over some areas west of the Imperial Valley, particularly in the
southwestern corner of the County. Patches of blowing dust and
difficult driving conditions along I-8 can be anticipated.

Ensemble members are in very good agreement for the remainder of the
week maintaining deep longwave troughing over the western Conus with
a couple impressive negative height anomalies filling into the
trough base over the SW Conus. The aforementioned trough affecting
SE California Monday will lift into the central Rockies Tuesday, but
not before bringing lower H5 heights and much cooler air into
Arizona. A reinforcing shortwave with even more pronounced cool air
will spill into the region during the latter half of the week,
albeit into a very dry environment (i.e. little to no chance of
rainfall). Model guidance spread suggests only minimal uncertainty
with temperatures solidly 10F below normal for much of the week. In
fact by the latter half of the week, morning lows in the outlying
suburbs of the Phoenix metro may touch into the upper 50s; and it
would not be inconceivable for central Phoenix to fall into the
upper 60s (last time KPHX was in the 60s was May 22nd). Larger model
spread exists over the weekend regarding the potential downstream
progression of mean troughing, or maintenance of lower heights over
the west. At this time, ensemble mean output moderates H5 heights
with temperatures rebounding back closer to normal.

&&

.AVIATION...
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Updated at 1750Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
The main aviation concern for this forecast period will be gusty
S-SW winds. Winds will continue to have a southerly cross-runway
component with directions veering between 180-210 deg over the
next few hrs before finally transitioning fully to the SW by
20Z-21Z. Gusts will reach 20-25 kts at times at all metro
terminals through this afternoon and quickly diminish after
sunset. Any showers that develop this afternoon are expected to
remain confined to the higher terrain east of the Phoenix metro.
FEW-SCT cu aoa 10 kft will continue throughout much of the period.

Southeast California/Southwest Arizona including KIPL and KBLH:
The main aviation concerns will be gusty W-SW winds, particularly
at KIPL. Winds will continue to favor a W direction at KIPL and
S-SW at KBLH through tonight. Intermittent gusts upwards of 30 kts
will be possible at KIPL with gusts around 20-25 kts common at
KBLH. Winds will diminish after sunset at KBLH, but will remain
elevated at KIPL overnight. Skies will remain mostly clear at both
terminals through the forecast period.
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&&

.FIRE WEATHER...
A weather system will pass to the north of the region through
tonight, bringing scattered convective showers with isolated
embedded thunderstorms mainly over the high terrain of the eastern
districts. Best chances for wetting rainfall will continue to be
over the high terrain east of Phoenix through tonight at between 30-
50%. Widespread afternoon breezy conditions can be expected today,
with southwesterly gusts to between 25-35 mph across the lower
deserts. MinRHs will range between 15-25% through Tuesday, with
locally higher values in the high terrain of the eastern districts,
but as drier air filters in behind the passing weather system,
MinRHs could drop into the 10-15% range for portions of the western
districts on Tuesday. Temperatures will drop to between 5 and 10
degrees below normal starting Tuesday through the end of the work
week. Overnight recoveries will be fair to good through the end of
the work week mainly between 30-60%. As temperatures are expected to
heat back up this weekend, humidities will decrease once again
starting Saturday.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...Wind Advisory from 2 PM this afternoon to midnight PDT tonight
     for CAZ562-566.

&&

$$

DISCUSSION...Whittock
AVIATION...Salerno
FIRE WEATHER...Whittock