Severe Thunderstorm Warning 224, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
357 PM MST Sun Sep 11 2022

.UPDATE...UPDATED 00Z AVIATION DISCUSSION.

&&

.SYNOPSIS...
The threat for showers and thunderstorms over the next couple of
days will gradually shift eastward through Arizona with the most
impactful, stronger storms occurring Monday. However, even this
evening, strong winds, locally dense blowing dust, and isolated
areas of heavy rainfall and flooding will all be concerns. Much
drier air will push into the region during the middle and latter
half of the week, essentially ending the storm chances. Temperatures
will hover slightly below normal through the first half of the week
before rebounding near to slightly above normal by late week into
the weekend.

&&

.DISCUSSION...
Early afternoon WV satellite imagery shows the remnant subtropical
circulation of Kay still wobbling off the southern California coast
undergoing another transition to an extratropical cyclone as dry
midtropospheric has completely cutoff any tropical influence.
Meanwhile, a progressive upstream trough and associated strong zonal
jet will act as the impetus to open and absorb this system into
westerly flow dipping towards the SW Conus. With this system
starting to move and attempting to dislodge a blocking ridge, a
seasonally robust, meridional 35kt H5 jet has developed on its
eastern periphery through Arizona. One shortwave can be seen lifting
north through the Lower Colorado River Valley while other vorticity
centers appear to be sheared along the jet axis riding north from
Mexico providing favorable ascent structure across much of the CWA.

Recent objective analysis depicts modest drying through the
environment since yesterday as both total column PWATs and low level
mixing ratios have retreated slightly. However, a solid 11-14 g/kg
are still locked into lower elevations, and more than sufficient to
support storm maintenance off higher elevations. The key may be the
vertical distribution of moisture towards the top of the boundary
layer where recent dry SW flow has eroded the higher quality
moisture; and models only partially mix above 800mb such that CinH
may remain on the order of 25 J/kg at peak heating. Nevertheless,
the combination of both MLCape and DCape 1000-1500 J/kg coincident
in an environment favorable for ascent should result in scattered
thunderstorm clusters.

Aside from a few showers and embedded storms lifting across
southeast CA in moist upslope flow this afternoon, the main emphasis
of hi-res modeling has consistently been semi-organized storm
clusters congealing over Pima/Pinal counties and belching deep
outflow boundaries north towards the Phoenix metro. The majority of
HREF membership suggests a singular large outflow (likely replete
with blowing dust) surging through the central CWA, but with varying
amounts of post-outflow storms erupting. Given the inhibition
likely present, it might take more than one outflow boundary to
breach the capping around the Phoenix area, however rarely is there
just one outflow as depicted by most CAMs. Thus, 40-60% storm
coverage as suggested by NBM POPs seems adequate, though with the
boundaries advancing at a good clip, many locations may be limited
on the total amount of rainfall.

Another facet of the forecast with a recurring similar solution
among modeling groups is the combination of outflows propagating
into western AZ, some semblance of northward advancing low level
moisture from NW Mexico, and shortwave energy encapsulated on the
nose of a 35kt jet punching north and east producing additional
clusters of elevated storms overnight near or just east of the
Colorado River. Though speed shear may keep these clusters advancing
north our of the forecast area, these types of synoptic setups have
occasionally resulted in expansive flooding due to backbuilding
convection training over the same area. HREF members hint at this
evolution, but offer a wide variety of possible outcome leading to a
lower confidence forecast; and with national centers highlighting
only minimal excessive rainfall risk, will hold off on any
headlines.

Monday forecasts continue to trend towards greater storm coverage in
the eastern part of the forecast area as synoptic scale forcing and
increasing jet energy become juxtaposed with a favorable
thermodynamic environment. While there is some question as to the
impact of overnight storms and ability for the atmosphere to
recover, forecast BUFR soundings continue to show steady cooling
aloft (H5 temperatures falling around -8C) resulting in H7-H5 lapse
rates rebounding closer to 7 C/km. As a result, both MLCAPE and
DCape values should once again range between 1000-1500 J/kg.
Overall, soundings this afternoon and Monday do not look dissimilar,
with the exception of far more robust 0-6km bulk shear of 25-30kt.
While marginal for organized or supercellular storms, this
shear/instability combination should easily support isolated strong
to severe thunderstorms from Maricopa County eastward. Strong wind
gusts, expansive blowing dust, and small hail would be expected
along with brief heavy downpours. Localized flash flooding is also
possible, but may be conditional on the requirement of cluster
interactions given the bulk shear and forward propagation tendency.

By Tuesday, pronounced westerly flow begins to settle over the
majority of the forecast area drying things considerably such that
the storm activity will be mostly confined to higher terrain north
and east of Phoenix. Since it will take more than one day to
completely scour moisture, can not rule out a few showers/storms
popping over valley terrain features and foothills, but with little
to no major impacts. During the middle to latter half of the week,
broad troughing and a strong 75kt zonal jet sweeps across the SW
Conus dislodging any remaining quality moisture. There are excellent
indications boundary layer mixing ratios fall below 8 g/kg and PWATS
hover in a 0.75-1.00 inch range essentially precluding any deep
convection throughout the entire region. During this drying regime,
H5 heights will build back near 590dm yielding temperatures back
into a normal to slightly above normal range by the end of the week.

&&

.AVIATION...Updated 2300Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Southeasterly flow is attempting to switch to southwest. This
should continue for the next couple of hours. Sometime between
2-4Z thunderstorms are expected to move out of the southeast and
into the Phoenix area bringing gusty winds, areas of blowing dust,
and isolated heavy rain/lightning. Storms should clear out of the
area by 6Z. For now VCTS is maintained in the TAF due to low
confidence in the exact timing but may be upgraded to TS with
time. Another slight chance of showers develops by daybreak but
will likely remain west of the area. Winds during the overnight
hours will mostly be out of the southeast. Winds will switch back
to southwest during the day Monday with scattered showers and
storms developing by the early afternoon. Clouds should mostly
remain at or above 8 kft.

Southeast California/Southwest Arizona including KIPL and KBLH:
Current thunderstorm activity in southeast California is expected to
remain too few and far between to impact TAFs. Later today, storms
over the San Diego County mountains and far northern Baja could
possibly produce some outflow that reaches KIPL early this evening
but anticipate wind speeds to remain light if it happens. Late
tonight, an outflow boundary from the east will likely cause a
easterly wind shift at KBLH. Additional shower and isolated storm
development is possible at KBLH as this outflow pushes through. Best
time frame would be between 8Z and 12Z tonight. Otherwise,
anticipate light winds with some late morning and afternoon cumulus
(bases AOA FL050) at both terminals.

&&

.FIRE WEATHER...
The threat for showers and thunderstorms will shift towards central
and eastern Arizona during the next couple of days, with Monday
being the day with the greater coverage. Thereafter, by the middle
and latter of the week into at least next weekend, considerable
drying is expected, which will essentially end the storm chances
everywhere. MinRHs will range between 30-45% through Tuesday and
then fall to between 20-30% by the middle to latter half of the
week, potentially falling below 20% by next weekend. Winds for the
most part will generally remain light through the period with the
exception of thunderstorm outflows generating stronger gusts.

&&

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

DISCUSSION...18/Lojero
AVIATION...Hodges/Feldkircher
FIRE WEATHER...Lojero

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