Flash Flood Warning 103, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
412 PM MST Thu Aug 11 2022

.UPDATE...UPDATED 00Z AVIATION DISCUSSION...

&&

.SYNOPSIS...
A 15 percent chance of widely isolated to scattered mostly
distant and high terrain storms continues this afternoon in
Phoenix, with better 30 to 50 percent chances across the high
terrain areas. Storm chances then gradually increase this evening
through tomorrow and the weekend with locally heavy rainfall,
localized flash flooding, gusty winds and patchy blowing dust
possible. Near to slightly below normal highs will fall further
below normal over the weekend and through early next. Storm
chances for next week remain moderate.

&&

.DISCUSSION...
The midlevel anticyclone remains favorably located near the S-Cent
Rockies with moist SE-E undercutting flow through the mid-upper
levels aloft. WV imagery reveals a very moist airmass across the
region with upstream E wave disturbance(S) advancing W through NM.
Early afternoon radar revealed convection sparking E-NE of Phoenix, N
and SE AZ. Afternoon ACARS soundings showed an anomalously moist
airmass with PW maintaining near 1.7->1.9" along with a near moist
adiabatic, slightly worked-over and conditionally unstable
profile. Mesoscale analysis indicated PW of 1.5->2" and mean W of
12->14 g/kg across the region. MU and SB CAPE readings were to
~1.3K->1.5K j/kg around Phoenix and >2-2.5k j/kg across the W
deserts.

Under mostly clear skies, morning heating across Phoenix and the
lower deserts has been stronger today than recent mornings. Models
also indicate temporary subsidence around Phoenix and the lower
deserts this afternoon. A 15 percent chance of widely isolated to
scattered mostly distant and high terrain storms continues this
afternoon in Phoenix, with better 30 to 50 percent chances across
the high terrain areas. Recent HRRR runs have latched onto a late
afternoon E outflow pushing into the Phoenix Metro with the
potential for locally gusty winds. This is in good agreement with
current radar trends.

Storm chances then gradually increase this evening through tomorrow
and the weekend with locally heavy rainfall, localized flash
flooding, gusty winds and patchy blowing dust possible. The WPC day
2 ERO for tomorrow/Fri also features a large "Slight Risk" area
of excessive rain fall for most of AZ. This is associated with the
next E wave out of NM.

HREF and members are fairly bullish on pronounced convection with
locally heavy rainfall developing tomorrow/Fri afternoon across S
Gila Cty and near/E of Globe and burn scar areas (and the
Mogollon Rim and SE AZ) before descending into the lower deserts
and parts of the Phoenix Metro during the evening hours. HREF wind
speeds for Fri evening feature a pair of fairly robust converging
outflows, one from the NE and the other from the SSE. The
resulting low level convergence through Pinal and Maricopa Cty,
coupled with the UL disturbance, would support pronounced
storms/convection for the Phoenix Metro and lower deserts
spreading west of Phoenix, including into parts of W-SW AZ and SE
CA, well into the night. The preferred WPC storm total QPF for
Phoenix on Fri is ~0.10-0.15" and ~0.3-0.5" for both the E AZ high
elevations and parts of the western deserts. The HREF shows a
50-70% chance for 3-hr QPF >1.0" for the E AZ high terrain for
Fri evening and a corresponding 10-30% chance for Phoenix and the
lower deserts. This represents a good signal for locally heavy
rainfall and additional localized flash flooding in the high
country and the lower deserts.

For the weekend, ensembles are latching onto a slightly larger E
wave on Sat evening and Sun moving from NM to AZ in general. The
preferred NBM and WPC solutions feature an increase in POPs while
maintaining healthy storm total QPF for Sat evening and Sun
morning comparable to the aforementioned Fri evening/Sat morning
storm total QPF.

At some point early next week, the pattern will begin to shift
with the sub-tropical high to our northeast weakening, while a
weak trough possibly sets up off the southern California coast.
This scenario may not bring much of a change to our area with
models continuing to show adequate moisture levels for storms, but
overall forecast confidence is low due to the change in the upper
level pattern and some model differences. Temperatures are
expected to cool off slightly at some point over the weekend into
early next week with forecast highs closer to 100 degrees due to
anticipated more active monsoon conditions.

&&

.AVIATION...Updated 2310Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Winds are variable across the metro area, with areas of southerly
winds in the southeast and north valley areas. Closer to KPHX
winds have been southwest. Wind directions will shift between 0-1Z
across the metro area as an outflow from the northeast moves in.
Winds may shift again to the southeast later this evening due to
additional storm activity. Thunderstorms are expected to stay out
of the valleys, but there is a 10-20% chance a storm could develop
close to a terminal. Due to the expectation of multiple outflow
boundaries, wind directions will be gusty and erratic at times
with periods of relatively low predictability. Overnight winds
will become southeasterly with another small 10-20% chance of
virga showers near dawn. Additional thunderstorm activity appears
likely Friday afternoon, with a 30% chance of spreading into
valley locations.

Southeast California/Southwest Arizona including KIPL and KBLH:
Winds will generally favor the south-southeast. KIPL will have a
westerly wind shift for a few hours later this evening but speeds
will mostly remain below 10 kts. Chances for showers and
thunderstorms this afternoon and evening are 15% or less at the
terminals. However, any thunderstorm that develops would be
capable of gusty erratic winds, blowing dust, and locally heavy
rainfall. Skies will remain mostly cloudy through the TAF period
with ceilings mainly aoa 10 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. Winds for the
most part will remain light, with the exception of thunderstorm
outflows generating stronger gusts.

&&

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

DISCUSSION...Sawtelle/Kuhlman
AVIATION...Hodges/Feldkircher
FIRE WEATHER...Kuhlman

Special Weather Statement
National Weather Service Phoenix AZ
504 PM PDT Thu Aug 11 2022

CAZ560-561-564-568-570-120045-
Chuckwalla Mountains CA-Chuckwalla Valley CA-Chiriaco Summit CA-
Joshua Tree NP East CA-Joshua Tree NP West CA-
504 PM PDT Thu Aug 11 2022

...A strong thunderstorm will impact portions of Riverside County
through 545 PM PDT...

At 504 PM PDT, Doppler radar was tracking a strong thunderstorm 9
miles southwest of Eagle Mtn, or 9 miles west of Desert Center,
moving northwest at 5 mph.

HAZARD...Winds in excess of 40 mph.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects.

Locations impacted include...
Chiriaco Summit.

This includes CA Interstate 10 between mile markers 85 and 103.

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.

&&

LAT...LON 3376 11540 3360 11547 3362 11578 3387 11562
TIME...MOT...LOC 0004Z 118DEG 5KT 3374 11557

MAX HAIL SIZE...0.00 IN
MAX WIND GUST...40 MPH

18

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