Severe Thunderstorm Warning 12, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
504 PM MST Wed Mar 15 2023

.UPDATE...Updated Aviation Discussion.

&&

.SYNOPSIS...
Light rain this afternoon will redevelop as scattered to numerous
showers are expected this evening through early tomorrow morning
with isolated thunderstorms likely. A cooling trend starting
today will continue through the work week. Conditions expected to
improve going into Thursday, but temperatures will remain below
average over the next several days. Uncertainty this weekend
regarding precipitation chances and temperature fluctuations.

&&

.DISCUSSION...
The trough located over the west coast will continue it's eastward
progression, reaching the forecast area today. Scattered to numerous
showers will follow the leading edge of this trough. Associated low
pressure has caused a cooling trend to begin today, that will last
through the work week. Afternoon high temperatures are expected to
be in the upper 60s to low 70s across south-central AZ, while
southeast CA/southwest AZ will reach the mid 70s. The biggest
weather concerns will be from a storm system arriving this evening
that will bring widespread impacts, including: numerous showers,
gusty winds, small hail/graupel, and thunderstorm chances.

The SPC convective outlook has increased our region to a marginal
risk for severe weather. After this afternoon's showers pass
through, another more prominent line of showers will develop with
the frontal passage. Greater instability will be associated with
these showers as the main band of storms is expected to hit this
evening through early tomorrow morning. Thunderstorm chances have
risen to 15-35%, which may lead to small/hail graupel across the
area. In addition, the pressure gradient will continue to increase
today, causing breezy to windy conditions with gusts up to 40 mph
across high terrain areas of southeast CA this afternoon. The
incoming storms will contribute to this effect, leading to more
widespread breeziness across the region this evening.

A deep moisture plume currently situated over Arizona will allow for
moderate to occasionally heavy showers today and tonight. The latest
NAEFS shows IVT values in the 99th percentile, near 300 kg/m/s
indicating that rich moisture advection is anticipated over the
forecast area. In addition, GEFS PWAT values range from 0.75 to
1.00, further proving the amount of available moisture to pull from
is sufficient enough to produce at least moderate precipitation. The
latest NBM shows PoPs between 80-90% and QPF ranging from 0.4-0.6
inches across the Phoenix metro area. The highest QPF amounts remain
confined to the high terrain areas north and east of Phoenix,
reaching just over an inch in some parts. If this moisture plume
drys out aloft, lapse rates may steepen causing stronger updrafts
and more impactful weather. However, forecast soundings continue to
show a deep saturated profile at this time.

Since this is a warmer system, snow is not a concern as snow levels
will be above 6,000 feet. However, the rainfall combined with
snowmelt will contribute to the elevated flow at Tonto Creek and the
Salt River. This was cause flooding due to significant runoff into
waterways. Flood Warnings are currently in effect through Saturday,
but are likely to be extended. Salt River Project continues to
release water at Granite Reef Dam to account for the extra
runoff.

As the trough exits Thursday, a clearing trend will follow as dry
conditions take over the region through the end of the work week.
Model spread increases as we reach the weekend, where there is
uncertainty on whether zonal flow or ridging will build over
Arizona. Occasional shortwave passages may lead to precipitation
chances on Saturday and Sunday. If the GEFS and Canadian members
solution comes to fruition, ridging will build and temperatures will
see a warming trend. However, if the Euro ensemble members
prevail, temperatures will remain fairly stagnant. Regardless,
temperatures are expected to remain below seasonal normals but
will warm several degrees heading into early next week.

&&

.AVIATION...Updated at 0004Z.

South-central Arizona including KPHX, KIWA, KSDL, and KDVT:
CIGs have temporarily improved in parts of the Phoenix area, with
the lower CIGs having become more FEW to SCT, but VCSH with
periods of -SHRA will continue for the next couple hours. The
greatest aviation impact will be later this evening with VCTS
moving west to east the Phoenix area, between 3-7Z. Brief very
heavy rain, isolated severe hail (1" in diameter), lightning, and
stronger wind gusts will all be potential impacts. There is a
10-30% chance for storm wind gusts exceeding 30 kts. CIGs this
evening are expected around 5-6k ft, but could dip to MVFR with
the storms. The heavy rain could also lead to brief IFR/MVFR
visibility. Storms will move east of the area by 8-9Z with
improving conditions. Though SCT to BKN 5-6k ft clouds may persist
through the night. Winds will favor the S/SE ahead of the storms
and shift W/SW with them. Following the storms, a period of S/SE
again is expected through early Thursday before shifting back
westerly later Thursday morning.

Southeast California/Southwest Arizona including KIPL and KBLH:
Very breezy W/SW winds will prevail through this evening, with
gusts in excess of 30 kts at KIPL and up to 25-30 kts at KBLH.
VCSH is expected at KBLH for the next hour or so, with potential
for brief VCTS, before activity shifts further east. Winds
gradually subside overnight and then shift more NW/N at both
terminals Thursday. SCT to occasionally BKN high clouds will
prevail, while FEW to SCT cumulus and wave clouds should clear
later this evening. FEW cumulus will likely develop again Thursday
afternoon.

&&

.FIRE WEATHER...
A low pressure system moving through the region today and tonight
will result in breezy to windy conditions, especially across SE CA
with winds gusting 20-30 mph across the lower deserts Wednesday
evening. Best chances for rain will be east of the Lower Colorado
River Valley today, with chance of wetting rains 50-70% across
southcentral AZ, with higher chances in higher terrain areas. The
thunderstorm threat remains, with the highest chances this evening
mainly across La Paz and Maricopa Counties. Temperatures will dip
below normal from through the rest of the week and into this
weekend. MinRH's will remain elevated in the 40-60% range today,
before a drying trend going through the end of the week.

&&

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

DISCUSSION...Leffel
AVIATION...Benedict
FIRE WEATHER...Young

Special Weather Statement
National Weather Service Phoenix AZ
405 PM PDT Wed Mar 15 2023

CAZ564-565-567-569-152345-
Chuckwalla Mountains CA-Imperial County Southeast CA-
Palo Verde Valley CA-Imperial Valley CA-
405 PM PDT Wed Mar 15 2023

...A strong thunderstorm will impact portions of northeastern
Imperial and Riverside Counties through 445 PM PDT...

At 405 PM PDT, Doppler radar was tracking a strong thunderstorm 16
miles northeast of Brawley, moving northeast at 25 mph.

HAZARD...Winds in excess of 40 mph and half inch hail.

SOURCE...Radar indicated.

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

This storm will remain over mainly rural areas of northeastern
Imperial and Riverside Counties.

This includes CA Route 78 between mile markers 62 and 65.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a beuilding.

&&

LAT...LON 3306 11539 3322 11549 3351 11494 3322 11480
TIME...MOT...LOC 2305Z 243DEG 23KT 3318 11536

MAX HAIL SIZE...0.50 IN
MAX WIND GUST...40 MPH

Heil

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