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230
FXUS65 KPSR 162027
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
125 PM MST Mon Sep 16 2024

.SYNOPSIS...
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A weather disturbance will shift northeast of the region by Tuesday,
but not before yielding a chance for showers and thunderstorms
across eastern Arizona tonight. Dry and seasonably mild conditions
will spread over the area through the middle and end of the week
with forecast temperatures hovering 5 to 10 degrees below normal.
Readings should moderate closer to normal over the weekend.
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&&

.DISCUSSION...
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Afternoon WV imagery and objective analysis depicts a seasonally
deep negative height anomaly spinning over central California with
an impressive jet streak punching inland towards the forecast area
through the trough base. While the primary cold core and most
pronounced height falls will remain north of the CWA as this system
ejects eastward, the initial midtropospheric front was leaking into
the area, and along with jet energy was aiding in large scale
synoptic ascent. Within the boundary layer, ample moisture has
surged north with 12-13 g/kg mixing ratios per in situ aircraft
sampling resulting in MLCape 500-1000 J/kg within an environment of
only weak inhibition. However, this area favorable for convective
development will be battling drier air filtering eastward and
steadily eroding the low level moisture. The best chances for
showers and storms will align over higher terrain east of Phoenix
where orographic effects of strong, moist S/SW flow will be
optimized this afternoon and evening. However, even a few fast
moving showers will be possible (~25% chance) through the central
and eastern portion of the metro mostly tied to assistance from
terrain escarpments.

Through SE California, the combination of onshore flow and a
deepening marine layer along with veering H8-H7 zonal flow crossing
the San Diego mountains will promote mountain rotors and sundowner
winds through western Imperial County. Areas of blowing dust will
need to be monitored originating from source regions in recreation
areas east of Borrego Springs. A gradual weakening of gusts should
materialize later overnight as thermal and pressure gradients relax.
Well farther east into southern Gila County, scattered showers may
persist well into Tuesday morning as moist ascent persists ahead of
the incoming midlevel front tapping lingering modest instability.
HREF membership depicts the potential for multiple round of fast
moving showers impacting the far eastern CWA before the trough axis
and substantially drier air finally propagates through the state
late Tuesday morning.

Ensemble members remain in very good agreement 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. Unusually low H5 heights in a 578-
584dm range will support an extended period of below normal
temperatures beyond the middle of the week. In fact, a reinforcing
shortwave with even more pronounced cool air and H5 heights as low
as 572-576dm will spill into the region late in the week. Despite
the passage of these weather disturbances, a very dry environment
will remain in place resulting in no chance of rainfall. Model
guidance spread suggests only minimal uncertainty with temperatures
solidly 5F-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 over the weekend
though temporal trends suggest a slightly delay in this warming.
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&&

.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...
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A weather system will pass to the north of the region tonight
bringing scattered showers and isolated embedded thunderstorms
mainly over the high terrain of the eastern districts. Thereafter, a
much cooler and drier airmass will spread across the region with
temperatures 5F-10F below normal for the majority of the week. As a
result, minimum afternoon humidity levels in the teens will be
common in lower elevations and closer to a 20-30% range across
higher terrain. Overnight recovery will range widely from fair to
good at 30-70%. As temperatures warm closer to normal this weekend,
humidity levels will decrease further with single digit readings
more likely in lower desert communities. Winds should not be
unusually strong with only some afternoon upslope gustiness.
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&&

.PSR WATCHES/WARNINGS/ADVISORIES...
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AZ...None.
CA...Wind Advisory until midnight PDT tonight for CAZ562-566.
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&&

$$

DISCUSSION...18/Whittock
AVIATION...Salerno
FIRE WEATHER...18/Whittock