Severe Thunderstorm Warning 68, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
255 PM MST Thu Nov 21 2019

.SYNOPSIS...
Scattered showers will taper off this evening as a low pressure
system weakens across northern Arizona. Cool air will remain in
place through at least the weekend with high temperatures mostly in
the 60s. Dry conditions are forecast for the weekend and the first
part of next week before another potential weather system brings a
return of precipitation by the middle of next week.

&&

.DISCUSSION...
The circulation of the closed low near Las Vegas is in the process
of opening up, resulting in a weakening of the cyclonic flow.
Meanwhile, conditions remain anomalously moist further east across
Arizona, where surface dewpoints in the 50s persist. Scattered
showers developed early this morning across central Arizona and are
expected to continue through the remainder of the afternoon, given
the moderate instability and insolation further upstream across
southwestern Maricopa and La Paz Counties. Given the cold
temperatures aloft, the strongest storms will be capable of
producing brief heavy rain and hail. Mean layer flow has weakened
since yesterday, which means that training cells could also produce
minor or even localized flash flooding with the potential for a quick
half of an inch of rain. Local CAMs and HREF remain in good agreement
the convection will taper off this evening as CAPE drops off sharply
after sunset.

Friday will finally bring a rain-free day to the region as the upper
low ejects eastward. In the wake of this system, temperatures will
remain below normal with highs in the middle 60s in the lower
deserts, though relatively humid conditions will linger. Latest GEFS
and ECMWF EPS both suggest a piece of the next Pacific trough will
fracture, gradually evolving into another Rex block this weekend.
Not much of a direct impact expected here, however the closed low
will help to maintain subtropical moisture west of the Baja
Peninsula. Westerly flow will persist across the Desert Southwest
into early next week with temperatures a few degrees below normal.

Latest operational and ensemble guidance is pointing towards another
period of active weather beginning the middle of next week. A broad
trough will likely descend into the Pacific Northwest, which will
help to propel the aforementioned subtropical moisture
northeastward. There is still considerable uncertainty with
regard to the position of the moisture plume, though GEFS/NAEFS IVT
magnitudes suggest it will have a significant impact with heavy rain
possible. Even if this AR remains south of the area, the
amplifying flow and upstream trough will provide another opportunity
for widespread precipitation, colder conditions and lowering snow
levels late next week.

&&

.AVIATION...Updated at 1840Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Slow-moving convection will continue to push across the Phoenix
area through this afternoon with periodic heavy rain and lightning.
Though after the current (1830Z) round, activity may shift more
to the higher terrain north and east of Phoenix. CIGs as low as
3-4 kft with FEW/SCT to 2 kft with brief lowered visibility is
possible within the convective showers. Winds are expected to
favor diurnal tendencies, likely shifting around to the southwest
this afternoon with speeds generally around 7-10 kts and limited
breezes in showers. Shower chances trail off late this evening
with lingering SCT/BKN clouds around 5-6 kft.

Southeast California/Southwest Arizona including KIPL and KBLH:

A gradual drying trend is expected through the afternoon with no
additional rain anticipated. FEW/SCT clouds aoa 7 kft should
persist through this afternoon and clear this evening as more mid
and high clouds push in from the west. Winds will remain light and
variable through the TAF period with some diurnal tendencies.

&&

.FIRE WEATHER...
Saturday through Wednesday:
A gradual drying will occur this weekend into early next week with
weak high pressure over the region for the bulk of the period. By
the middle of the week, precipitation chances return as a upper
level disturbance approaches the region. Temperatures will
continue to run below normal while winds during the period will
remain fairly light. Humidity levels for much of the period will
range from the 20s across southeast California to the 30s
elsewhere.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

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

DISCUSSION...Hirsch
AVIATION...Benedict
FIRE WEATHER...Kuhlman

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