Flash Flood Warning 38, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
451 PM MST Tue Sep 12 2023

.UPDATE...00Z Aviation Discussion.

&&

.SYNOPSIS...
Temperatures will hover close to the seasonal normal the next
several days as a series of weather disturbances pass through
northern and central Arizona. Enough moisture will remain in place
to support scattered showers and thunderstorms through Wednesday
with even some strong to severe storms possible this afternoon.
However, much drier air will surge back into the region during the
latter half of the week cutting off any additional rainfall
chances.

&&

.DISCUSSION...
Looking outside in Phoenix and on satellite early this afternoon a
cumulus field has begun developing, mainly across western Maricopa
County into parts of Yuma and La Paz counties. With ample sun
through this morning, the environment is gradually destabilizing
across the region. A few convective cells have also gone up,
including on the western edges of Phoenix. Dew points are mostly
in the middle 60s this afternoon following a push of PBL moisture
this morning. This added moisture has added to the instability of
the atmosphere that is just waiting to be realized. Mid-level
lapse rates are not ideal (6 C/km), but there is still enough
instability for robust updrafts this afternoon through this
evening. Objective analysis is showing MLCAPE values around
1000-1500 J/kg. Also of note, the kinematics are good for
maintained updrafts and isolated organized storms. Effective bulk
wind difference is analyzed around 35-40 kt, which is high enough
to even see an isolated high-based supercell or two. Initially
convection will likely struggle against the shear, but as we
continue through the afternoon, more robust updrafts are likely to
develop with peak surface heating. Organized storms will tend to
drift southeast with ~15 kt storm motion. Otherwise, storms will
push eastward. Based on the 12Z HREF, storms will mostly favor
Maricopa county eastward, but isolated activity developing over
terrain favored areas of La Paz and Yuma counties will remain
possible through the daylight hours.

Greatest risks with storms today will be strong to severe winds
and localized flash flooding. The poor to modest mid-level lapse
rates and higher LCLs (7-8K ft) are not ideal for severe hail (1+"
diameter) over the lower deserts, but there is decent CAPE in the
hail growth zone (~500 J/kg) to see larger than the more typical
pea to dime size. As has already been observed this morning, the
environment is supportive of high rain rates. A rain gauge in
Pinal county this morning measured a little over 1" in 20 minutes.
In an environment with mean mixing ratios around 11-12 g/kg and
abnormal PWATs up to 1.6-1.8", average rain rates around 2"/hr
will be common in stronger storms. While the 15 kt storm motion
could limit flooding potential, storms that lock to an updraft
source or terrain feature and/or train over the same areas could
quickly lead to localized flash flooding. The latest WPC QPF
forecast factors in the HREF which does support some areas in
South-Central AZ seeing rainfall amounts upwards of 0.5-1.0". The
HREF also has a 20-30% chance for 24 hour rainfall amounts in
excess of 1" this afternoon through tonight.

With the supercell potential, some may be wondering if there is a
threat of supercellular tornadoes as well, but the low-level wind
profile (sfc-1km SRH generally less than 100 m2/s2) and high LCLs
are not ideal. However, a couple landspouts cannot be ruled out
with the steep low-level lapse rates (dry adiabatic) and limited
enhanced stretching.

.PREVIOUS DISCUSSION...
A fairly well defined, but weakening Pacific front should move
into central AZ Wednesday as the final shortwave and upstream
positively tilted trough ensures persistent westerly flow and a
gradually subsident environment. However, sufficient residual
boundary layer moisture should still linger around and east of the
Phoenix metro with deep westerly flow aiding in orographic
ascent. Several HREF members suggest another potential round of
nocturnal/early morning elevated convective clusters Wednesday,
however this evolution may be highly dependent on the outcome of
thunderstorms this evening. Nevertheless, forecast BUFR soundings
actually indicate a more favorable thermodynamic environment
Wednesday afternoon with 10-11 g/kg mixing ratios juxtaposed with
minimal inhibition and steeper lapse rates. Based on historical
events, these situations typically result in scattered storms
initiating over local terrain escarpments and foothills, but
quickly moving into mountainous areas and exiting the CWA during
the evening hours. Would not be surprised if future high
resolution models start increasing storm potential and coverage
during the day Wednesday east of the Phoenix metro.

By Thursday morning, surface dewpoints in the 30s and mixing ratios
below 5 g/kg may have already spread throughout the entire forecast
area essentially cutting off rainfall chances. This dry regime
should persist through the weekend as the pattern becomes
temporarily blocked by a moderate amplitude quasi-omega feature,
albeit still maintaining some form of westerly flow across the
Southwest. Ensemble members show good agreement of H5 heights
holding in a 586-590dm range and yielding a temperature forecast
near to slightly above climatology. Even into early next week, the
pattern of stronger than normal dry, westerly flow will be
maintained sequestering monsoon thunderstorm activity well south
into Mexico.

&&

.AVIATION...Updated at 2345Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Current area radars show scattered SHRA/TSRA ongoing across
Maricopa County. This activity will continue to push E/SE over the
next few hrs with VCTS expected at all terminals by 02Z-03Z. The
main concerns with any storm will be gusty, erratic outflow winds
and brief reductions in vsbys due to BLDU. There is still a 30-50%
for thunderstorm outflow winds exceeding 30 kts at KPHX through
this evening. We will monitor and amd if any direct impacts are
anticipated at the metro TAF sites. Otherwise, elevated southerly
flow will continue for the next several hrs with a gradual shift
out of the ESE overnight. Another round of showers and isolated
thunderstorms may develop late Wednesday morning. SCT-BKN clouds
mainly ~8-10 kft will continue through the period.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation concerns are expected through the forecast period.
Winds over the next 24 hours will favor a westerly component at
KIPL and southwesterly direction at KBLH. Gusts up to 20 kts will
be possible at KBLH and up to 25 kts at IPL through this evening.
Gusts will diminish at both terminals by 05Z. FEW-SCT mid lvl
clouds w/bases ~10-15 kft are expected through the period.

&&

.FIRE WEATHER...
With temperatures remaining near the seasonal normal, moisture will
temporarily increase through the middle of the week aiding shower
and thunderstorm chances. Eastern districts will have the greatest
chance of wetting rainfall today and Wednesday. Minimum afternoon
humidity levels will generally fall into a 20-35% range the next
couple days, although drier air will quickly begin to infiltrate
western districts. Overnight recovery should be fair to good in a 35-
70% range. Much drier air will sweep through the region during the
latter half of the week with rainfall chances becoming neglible.
Afternoon humidity levels should fall into the teens with less
beneficial overnight recovery. Gusty afternoon west winds 20-30mph
should be common the next several days with speeds relaxing later in
the week.

&&

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

DISCUSSION...Benedict
PREVIOUS DISCUSSION...18
AVIATION...Salerno
FIRE WEATHER...18

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Special Weather Statement
National Weather Service Phoenix AZ
524 PM MST Tue Sep 12 2023

AZZ532-535-536-130100-
Gila River Valley AZ-Southeast Yuma County AZ-Yuma AZ-
524 PM MST Tue Sep 12 2023

...A strong thunderstorm will impact portions of central Yuma County
through 600 PM MST...

At 524 PM MST, Doppler radar was tracking a strong thunderstorm near
Ligurta, or 8 miles southeast of Fortuna Foothills, moving northeast
at 15 mph.

HAZARD...Winds in excess of 40 mph and penny size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
Fortuna Foothills, Wellton, and Ligurta.

This includes AZ Interstate 8 between mile markers 19 and 37.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with this storm and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 3249 11427 3260 11440 3280 11422 3267 11402
TIME...MOT...LOC 0024Z 224DEG 14KT 3261 11427

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...40 MPH

JL