Flash Flood Warning 140, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
539 PM MST Sat Aug 13 2022

.SYNOPSIS...
The weather pattern will continue to support ample monsoonal
moisture across the region and daily chances for thunderstorms
into next week. In our forecast area, the best coverage of storms
today should exist across southwest Arizona/southeast California
and southern Gila county. For Sunday, the chances for more
widespread storms is likely to affect the south-central Arizona
deserts. Strong winds, heavy rainfall, and flooding will all be
possible with any thunderstorm activity. Flood Watches in effect
today and tomorrow.

&&

.DISCUSSION...
Less favorable weather pattern in place for south-central
Arizona, with drier southwest flow. This has dropped precipitable
water (PW) values in this area by more than a quarter of an inch
compared to yesterday. Across southwest Arizona and southeast
California, juicier southerly flow will keep PW values around
1.5-2.0 inches and keep storm chances elevated this afternoon and
evening. Plenty of clearing occurred over our forecast area this
morning, so the air will definitely be able to destabilize this
afternoon, and even more so out west where the better moisture is.
The SPC mesoscale analysis page is also forecasting the best
instability to be west of the Phoenix area, with greater than
1000-1500 J/kg of CAPE expected to develop in these areas this
afternoon. There will also be storms across north/east/southeast
Arizona. HREF continues to show outflows from activity all around
south-central Arizona progressing into the lower deserts and
converging somewhere across western Pinal or Maricopa county. Even
some hi-res models suggest that this would be the best
opportunity for lower desert storms in south-central Arizona this
evening. Regardless of location, any storm that does develop, will
have the potential to produce frequent cloud to ground lightning,
strong to severe winds, blowing dust, small hail, and heavy rain.
The chance of having greater than 35 mph wind speeds is greatest
(>50%) in southeast California where DCAPE is highest. With better
chances today across southwest Arizona and southeast California
and very slow storm motion expected once again, the Flood Watch
remains in effect.

The pattern becomes more favorable for storms across our entire
CWA tomorrow, as the flow turns more southeasterly, and HREF shows
PWs climbing back up to what they were like yesterday. Storms
will start developing in the early afternoon, also similar to what
was observed yesterday, with hints of outflows from multiple
directions interacting over the Phoenix metro. Additional storm
development looks likely in the lower deserts, with activity then
slowly progressing westward. The probability of having wind speeds
over 35 mph with tomorrow’s activity is greater than 50% across
south-central Arizona and around 30-50% in southwest Arizona and
along the Colorado River Valley. With pretty slow storm motion
expected again tomorrow and ample moisture and instability, the
flood risk will be elevated forecast-area wide tomorrow. The
chance of having greater than 1” of rain in one hour with any
storm is greatest (10-20%) across south-central Arizona and
southwest Arizona. Although coverage will be a little less across
southeast California tomorrow, any storm that develops will also
be capable of producing flash flooding. The Flood Watch across our
western CWA was extended through tomorrow evening and a new Flood
Watch was issued for south-central Arizona, which is in effect
tomorrow. Outside of flooding tomorrow, the other risks that will
be associated with storms will be frequent lightning, hail, strong
to severe winds, blowing dust, and heavy rain.

Forecast confidence then drops into Monday as additional shower
and storm potential could occur as models hint at the disturbance
stalling out over central Arizona. However, due to the anticipated
active day on Sunday, instability may be limited into Monday
leading to more of a localized heavy rainfall threat. Monday's
storm potential will likely be contingent on how widespread
Sunday's activity is and whether or not the atmosphere is again
work over.

Looking into the extended period, the weather pattern is not
likely to vary all that much with the sub-tropical high either
remaining centered to our east or shifting more across the Four
Corners area/Great Basin. Either way, it should remain in a good
position for continued southerly moist flow into much of the
region. There are some indications Tuesday may see some drier air
briefly work into eastern Arizona and this may keep storm activity
more limited. Model ensembles then indicate a potential eastern
wave tracking off the Gulf of Mexico close to our region by
Wednesday or Thursday. Since this potential disturbance is around
5 days out, we can't infer much, but it will definitely be
something to watch for later next week.

&&

.AVIATION...Updated 0039Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
West to southwest winds will continue through the rest of the
afternoon with typical breezes. Storms are not anticipated in the
metro area through the afternoon. However, an outflow(s) from the
south to southeast is projected to move through between 4-7Z with
10-30% probabilities for gusts greater than 30 kts. This outflow
could also generate isolated showers and storms, but probability
is around 10-20%. Diurnal winds will then prevail through Sunday
afternoon. Better chances for storms Sunday afternoon and evening
with an initial outflow favored from the northeast. Cloud bases
will mostly remain aoa 7-8 kft, but could dip lower in storms.

Southeast California/Southwest Arizona including KIPL and KBLH:
Storms have already passed close to both terminals and chances for
additional VCSH or VCTS will diminish over the next couple hours.
Following westerly winds this evening at KIPL, southeasterly winds
will prevail through most of the period while a southerly
component prevails at KBLH. There will be another chance for
isolated storms Sunday afternoon with up to 50% chances for gusts
greater than 30 kts from storms. Thunderstorm gusts may also
generate blowing dust. Cloud bases will mostly be aoa 8 kft.

&&

.FIRE WEATHER...
Ample monsoonal moisture is expected to persist through at least
the first half of next week keeping the fire danger threat low.
This moist airmass will continue to yield elevated humidities with
afternoon minimum RH values mostly in a 30-35% range, locally
higher, and good overnight recovery. Chances for wetting rains
will continue for most areas almost daily, especially through
Monday. Winds for the most part will remain light, with the
exception of thunderstorm outflows generating stronger gusts.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flood Watch through Sunday evening for AZZ530>533-535-536.

     Flood Watch from Sunday morning through Sunday evening for
     AZZ534-537>563.

CA...Flood Watch through Sunday evening for CAZ560>570.

&&

DISCUSSION...Feldkircher/Kuhlman
AVIATION...Benedict
FIRE WEATHER...Kuhlman

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