208
FXUS65 KPSR 160522
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1022 PM MST Sun Sep 15 2024

.UPDATE...Updated 06z Aviation Discussion.

&&

.SYNOPSIS...
A weather system over northern Arizona will link with moisture
returning into the southern part of the state resulting in modest
thunderstorm chances in eastern half of Arizona. Most of this
activity will be focused over the higher terrain east of Phoenix
with chances exiting the region Monday night. Dry and seasonably mild
conditions will spread into the area Tuesday with temperatures
hovering 5 to 10 degrees below normal through much of the week.

&&

.DISCUSSION...
Early afternoon WV imagery depicts a shortwave over S UT/NW AZ with
a trailing trough axis along the NM/AZ border. The resulting veered
midtropospheric flow behind this wave has trapped better quality
moisture and theta-e advection along the international border with
only a narrow corridor of 9-10g/kg mixing ratios streaming into
south-central and eastern Arizona. With another shortwave and
associated jet streak digging into central/southern California, much
of the SW Conus is now falling under anti-cyclonic subsidence with
objective analysis showing warming midlevel temperatures and
decreasing lapse rates. This scenario becomes somewhat detrimental
towards deep convective potential with forecasts suggesting MLCape
hovering under 500 J/kg around the Phoenix metro and little more
than 750 J/kg over the eastern CWA. Only the SE portion of the state
may realize MLCape above 1000 J/kg where thunderstorms would be more
prolific/expansive sending gusty winds/outflow boundaries north into
the forecast area later today.

The synoptic setup starts to become more favorable for large scale
ascent overnight as the aforementioned California shortwave digs and
intensifies along the coast. Enhanced jet energy should induce
poleward moisture advection with modest isentropic ascent combined
with orographic influences along the foothills and mountains east of
Phoenix. Models are in fairly good agreement depicting saturated
ascent through the H7 layer feeding on 500 J/kg MUCape resulting in
numerous showers with embedded elevated storms this evening and
overnight. Activity could initiate in the eastern sides of the
Phoenix metro with more robust rounds of convection streaming
through Gila County into the morning hours. While some heavy
rainfall is possible over higher terrain areas, fast forward storm
motion should limit flooding potential and any problematic areas
would be predicated on training echoes.

Favorable ascent mechanisms will continue to affect south-central
Arizona through much of the day Monday as the cold core aloft
approaches the region and a concentrated area of H8-H7 frontal
forcing begins to intersect the theta-e axis over the eastern parts
of the CWA. The moisture gradient along this frontal boundary may be
quite distinct delineating a tight gradient in POPs. In fact,
forecast BUFR soundings suggest low 60 surface dewpoints in the
Phoenix metro rapidly tumbling over 15F during the afternoon as the
moisture plume scours eastward. Nevertheless, with a few isolated
showers possible around the metro earlier in the day, more
concentrated showers and storms should be relegated to Gila County
through Monday night.

Through the western portions of the CWA, H5 height falls will be
rather impressive for mid September with readings descending into a
576-580d range. This will result in temperatures some 10F below
normal with a steepening pressure gradient leading to widespread
gusty winds. The timing of height falls coincident with a deepening
marine layer on the windward side of the San Diego mountains and
zonal jet energy at the trough base should promote a few rotors and
enhanced winds across western Imperial county. This sundowner effect
may cause a localized area of 40+mph gusts Monday evening, and in
collaboration with neighboring offices, have hoisted a wind
advisory for the typically most susceptible areas.

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; an 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...Updated at 0530Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Shower and thunderstorm activity continue to remain to the south
and east of the terminals this evening. Expect this trend to
continue through the overnight hours, with only KIWA having any
realistic shot at rain, which is reflected with a VCSH from
07-11z. Otherwise, dry conditions with diurnal wind tendencies
developing out of the east to southeast winds during the
overnight hours.

Gusty southerly crosswinds around 18-20 kts are expected to
develop tomorrow morning starting around 16Z before gradually
veering toward the southwest with gusts climbing to around 25-30
kts during the afternoon hours. Another round of shower and
storms are expected to develop primarily along the higher terrain
areas east of all terminals, with KIWA again with the best
chances for seeing any rain, as reflected in the TAF with VCSH
conditions from 20-00z. SCT-BKN clouds mostly aoa 10 kft will
continue throughout much of the period.

Southeast California/Southwest Arizona including KIPL and KBLH:
Winds at KIPL will mostly favor the west through the next 24
hours, while winds favor the south at KBLH. Winds speeds will be
elevated through the period with some gusts upwards of 20-25 kts
expected at both terminals. Otherwise, mostly clear skies will
persist.

&&

.FIRE WEATHER...
A weather system will brush by the region through Monday while
moisture increases across eastern districts resulting in chances for
showers and isolated thunderstorms. The best chances for wetting
rainfall will be over the higher terrain east of Phoenix where
chances peak around 50% tonight and Monday morning. Wind speeds will
increase with the passage of this weather system becoming more
southwesterly and increasing gustiness Monday afternoon. MinRHs
through Monday will range from the teens across the western
districts to 30-40% over eastern districts. Dry air will fully push
through the region Tuesday as below normal temperatures take hold
allowing afternoon humidity levels to fall into a 15-25% area wide.
Overnight recovery should improve into a fair to good range of 35-
65%. Dry conditions with temperatures 5-10 degrees below normal will
persist through the rest of the week.

&&

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

&&

$$

DISCUSSION...18
AVIATION...Young
FIRE WEATHER...18/Kuhlman