Flash Flood Warning 27, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
107 PM MST Fri Sep 1 2023

.SYNOPSIS...
Widespread showers and thunderstorms will impact the region
through tonight. Locally heavy rainfall is expected. Flash
flooding will be the primary concern into this evening, with
localized amounts exceeding 2 inches, especially for southwest
Arizona and Southeast California. As a result, a Flood Watch is in
effect for much of the area through this evening. Some strong to
severe storms are also possible for southwest Arizona and
southeast California with localized damaging winds. After a few
isolated storms linger on Saturday, dry conditions will set in
with a warming trend. Temperatures will start the weekend below
normal with a return to near or above normal conditions next
week.

&&

.DISCUSSION...
Strong dynamical system across the region as an upper low digs
south across CA with a moist southerly flow entrenched across the
region with precipitable waters from 1.5 to 2 inches. Strong
upper jet dynamics in the entrance region allowing for enhanced
diffluence aloft setting the stage for strong storms and potential
flash flooding. The latest mesoanalysis showing the best
instability across southwest AZ and southeast CA where sunshine
has worked to warm temperatures. Already seeing strong storms across
Yuma County with very heavy rainfall. South central AZ, including
Phoenix, has been under plenty of debris clouds and even some
light rain keeping daytime heating limited and inhibiting
convection so far.

Strong storms will continue to increase across southwest AZ and
southeast CA into early this evening. Given the high moisture
content, flash flooding will likely be the main threat, but
localized strong and damaging winds are also possible. While the
threat for flash flooding looks to be decreasing across the
Phoenix area, not ready to rule it out. Additional convection
developing across southeast AZ is expected to lift northwest
toward the valley by late evening and would likely produce some
locally heavy downpours with strong moisture transport into the
area.

After tonight, things quiet down quite a bit. Isolated storms are
still possible on Saturday with lingering moisture, but as the
upper low across CA lifts northeast and weakens, flow turns more
west/southwest bring drier air into the region beginning Sunday.
Next week, upper heights build westward across the area and
temperatures will begin to rebound back to near or above normal
levels.

&&

.AVIATION...Updated at 1730Z.

South-Central AZ including KPHX, KIWA, KSDL, KDVT:
Main aviation focus through tonight will be the coverage and
timing of any shower/thunderstorm activity along with breezy
southerly winds. More stable conditions occurring across the
valley late this morning after last nights convection. Recovery
for storms will be questionable for much of the day. Will be
monitoring instability for better threat of storms later this
evening. For now, have included just VCSH in the TAFs for the
favored times of showers for a period this afternoon and then
again later this evening. Southeasterly winds favored for much of
the day with a slight shift to south expected by mid afternoon.
They will also become gusty later this afternoon. Any shift to
westerly winds would have to be thunderstorm driven outflows from
the west, but that chance is low at this time but will continue to
monitor trends. Overnight winds will settle in out of the typical
east. Otherwise, BKN/OVC mid/high clouds will prevail through the
forecast period.

Southeast California/Southwest Arizona including KIPL and KBLH:
Main concerns this forecast period will be thunderstorm threats
and gusty winds. Easterly winds will become southerly and gusty by
mid afternoon before shifting to the southwest this evening. Wind
gusts 20 to 30 kts are likely. Thunderstorm activity will increase
across the region by mid afternoon. Greatest threat will be from
about 22z-03z late this afternoon into this evening.

&&

.FIRE WEATHER...
Anomalous moisture will translate to cooler temperatures and greater
chances for rain today. Chances for wetting rains are highest into
the evening. Localized flash flooding is also possible. Outside
of thunderstorm winds, breezy southerly winds are expected into
the evening, particularly across southeast California and
southwest Arizona. Min RHs over the next couple of days will be
elevated around 35-45%. Rain chances linger around Saturday
before much drier air is ushered in, eliminating rain chances by
the start of next week. Min RHs are expected to fall to around
15-25% by early next week. Below normal temperatures will
gradually trend upward again next week as ridging is expected to
build back into the region.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flood Watch until midnight MST tonight for AZZ530-531-533-534-
     536>559-561-562.

CA...Flood Watch until midnight PDT tonight for CAZ560>570.

&&

DISCUSSION...Frieders
AVIATION...Frieders
FIRE WEATHER...Frieders

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