Weather Service Report


043 
FXUS65 KPSR 230553
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1053 PM MST Wed Sep 22 2021

.UPDATE...
Updated Aviation Discussion.

&&

.SYNOPSIS... 
Warm and dry conditions will continue through Thursday afternoon 
with slightly above normal high temperatures.  A low pressure system 
and an increase in moisture by Friday will bring highs back down to 
near or slightly below normal along with a slight chance of storms. 
The slight storm chances and near to slightly below normal highs may 
persist through the weekend and early next week.

&&

.DISCUSSION... 
As we enter the Autumn season (Exact time of the Autumnal Equinox 
was at 12:20 MST/PDT today), strong high pressure aloft over the 
region is giving us another day of above-normal temps, with many 
lower desert locations already at/above 100 degrees at this hour. 
The airmass is also quite dry (PWAT's well under 1.00 inch), 
resulting in mainly clear skies over the region.

The latest GEFS/EPS model suites (and most of their respective 
members) have finally come in to a solid consensus on the details of 
a change in the weather over the next few days. They all are now 
showing an upper trof dropping southward across the central Great 
Basin tonight/Thu, then developing/retrograding a closed low SSW-
ward across western AZ on Friday, then take it slowly eastward along 
the International Border from late Friday night thru early Monday. 
The track of this upper low center is expected to enable it to pull 
a moderate amount of mid-level moisture northward out of NW Mexico 
and even a bit of moisture NE-ward from a tropical disturbance that 
is now located several hundred miles southeast of the southern tip 
of Baja. Current indications are that PWAT's will rise into the 1.30-
1.60 inch range during the late Thu-Sun period. 

The combination of this moisture along the mid/upper-level 
cooling/dynamics associated with the upper low center is expected to 
increase precip chances across the region from Thursday onward into 
the upcoming weekend. It now appears the best chances for measurable 
rain (20-30%) will be during the Thursday night/early Friday period 
across SW AZ and SE CA as the upper low passes directly over that 
region then again during the late Saturday-early Monday period (20-
40% POPS) as moisture is wrapped northwestward around the upper low 
center as is tracks off to the south and east. At this point, it 
appears that most of the rainfall will be in the form of light 
showers, with a lack of instability keeping TS activity to a 
minimum. Chances for 30 kt or greater wind gusts look quite low at 
this point, with the best chances (10-40%) being across SE 
CA/extreme SW AZ Thu afternoon/evening. Total QPF for the entire 
late Thu-Sun period appears it will be quite low (0.25 inch or 
less), although heavier amounts cannot be ruled out in a few spots. 
The main impact from this system will likely be cooler temps, 
especially during thw afternoon, with most lower desert locations 
seeing highs only getting up into the mid-upper 90's during the 
entire Fri-Mon period. 

It appears a short-lived return to dry conditions will occur during 
the late Monday-Wednesday period before perhaps another Pacific trof 
brings unsettled weather back to the region later next week.

&& 

.AVIATION...Updated at 0553Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
A large upper trough is moving through western North America with
the axis over the Canadian Rockies and extending south-
southwestward to the Sierra Nevada mountains. There is a much
smaller disturbance over northwest Sonora (remnants of a
thunderstorm complex from Tuesday night). The disturbance will
slowly move northeastward and track through southeast AZ on
Thursday. The bulk of the associated cloud cover is expected to
remain outside of the Phoenix metro area. Meanwhile, the large
trough will continue tracking eastward and expand southward. The
southern end of the trough will track through southern California
and into AZ Thursday and Thursday night. Shower and thunderstorm
activity Thursday afternoon will most likely remain well outside
of the Phoenix (mainly over Pima County and possibly La Paz
County) and will tend to be relatively weak. Otherwise, no
significant cloudiness except for increasing high clouds. But,
there is potential for modest outflows to reach the Valley floor
KIWA by late Thursday afternoon or evening. However, confidence in
a distinct wind shift is too low at this time to reflect in the
TAFs. Overall, surface winds will follow familiar diurnal
patterns.


Southeast California/Southwest Arizona including KIPL and KBLH:
A large upper trough is moving through western North America with
the axis over the Canadian Rockies and extending south-
southwestward to the Sierra Nevada mountains. There is a much
smaller disturbance over northwest Sonora (remnants of a
thunderstorm complex from Tuesday night). The disturbance will
slowly move northeastward and track through southeast AZ on
Thursday. Meanwhile, the large trough will continue tracking
eastward and expand southward. The southern end of the trough will
track through southern California and into AZ Thursday and
Thursday night. Thunderstorms Thursday afternoon will favor the
higher terrain San Diego and Riverside Counties and far northern
Baja. There may also be some isolated storms over La Paz County. A
stray shower at KIPL and/or KBLH couldn't be ruled out but
confidence too low at this time to reflect in the TAF. Otherwise,
anticipate increasing high clouds during the morning with
additional scattered to broken mid clouds in the afternoon and
evening. Overall, anticipate surface winds to follow familiar 
diurnal patterns. 

&&

.FIRE WEATHER... 
Friday through Tuesday: 
A weather system will influence the area beginning on Thursday.
Uncertainty remains with respect to the impacts of this system, but 
at a minimum it should increase moisture/cloudiness and bring slight 
storms for the lower deserts and high terrain. The chance of wetting 
rains is slight at 10-20% Fri and through early next week and up to 
30% by Sun-Mon for the E high terrain. Slightly below normal highs 
expected through the period. Min RH around 22-30% is favored through 
the period. With the exception of locally gusty winds near 
thunderstorms, winds will be fairly light each day, with typical 
afternoon breezes that will favor local diurnal/terrain influences.

&&

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

&&

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

$$

DISCUSSION...Percha
AVIATION...AJ
FIRE WEATHER...Sawtelle

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Western Regional Climate Center, wrcc@dri.edu