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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

                            
000
FXUS65 KPSR 181803 AAB
AFDPSR

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
1103 AM MST Wed Sep 18 2019

.UPDATE...Updated aviation discussion.

&&

.SYNOPSIS... 
Dry and mostly clear conditions will affect much of the region
through Saturday with temperatures within a few degrees of
seasonal normals. As early as late Sunday, likely lasting through
the first half of next week, rain chances greatly improve,
especially across Arizona. This next weather system will have the
potential of dropping heavy rainfall across portions of the area 
at some point early next week.

&&

.DISCUSSION...
The main forecast concern over the next 7-10 days is the 
potential for a significant rainfall event early next week as an 
unseasonably strong Pacific low is forecast to stall out west of 
Arizona. In the near-term, the Desert Southwest will continue to
be under the influence of southwesterly drying flow aloft as 
another Pacific low slowly tracks across the Great Basin Thursday 
into Friday. 

Boundary layer moisture currently in place across southern 
Arizona will get mixed out this afternoon and again on Thursday as
considerable drying aloft pushes through Arizona. Surface dew 
points are seen dropping into the teens and 20s across the Lower 
Colorado River Valley later today and into the 30s and 40s into 
Arizona. This scouring out of moisture will prohibit any rain 
chances with the trough passage on Thursday as skies remain clear
to mostly clear for the time being. Breezy to locally windy 
conditions are also anticipated on Thursday with gusts up to 25 
mph. Highs today will top out at or just above 100 degrees across 
the lower deserts before some slight cooling drops highs into the 
middle to upper 90s by Friday. The much drier boundary layer 
will allow overnight lows to dip well into the 60s across the 
western deserts to as low as the upper 60s Friday and Saturday 
morning across outer areas of Phoenix. 

After the quiet latter half of the week, the situation may get
quite interesting. The preponderance of models are still showing 
a deepening Pacific low diving southward along the West Coast on 
Sunday while some tropical moisture east of TS Lorena gets pulled 
northward into the Desert Southwest. Model spread with the 
incoming Pacific low remains fairly low through Sunday, but there 
are more unknowns with what will eventually happen to the tropical
systems moving northward along the Baja. No matter when the 
tropical systems end up falling apart, it seems very likely some 
tropical moisture will make its way northward somewhere into 
Arizona. Both the GFS and European show strong moisture advection
into Arizona during the day Sunday and possibly lasting through 
Monday.

Increasing uncertainty comes into play Monday into Tuesday as
model spread increases. For now the GFS/GEFS and the European all
show the Pacific Low deepening as it tracks south through 
California before getting cut off over southern California Monday 
night, potentially stalling out through Wednesday. Although
things may change, this synoptic set up does resemble the early 
October 2010 and the late September 2014 severe transition events.
The exact positioning and strength of the upper low along with 
how much moisture gets drawn northward will likely determine what 
will occur early next week. For now, PoPs have been increased late
Sunday through Tuesday with the best chances of showers or 
thunderstorms across south-central and eastern Arizona. Stay
tuned!

&&

.AVIATION...Updated at 1803Z 

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

Dry southwest flow will keep skies clear/mostly clear next 24 hours. 
There may be a bit of mid level moisture lingering over the deserts 
manifesting in FEW decks around 10k feet at times. Otherwise winds 
should follow typical diurnal patterns through early Thu morning, 
but with westerly winds persisting later into the night then usual. 
Gusts should be minimal, maybe one or two into the teens during the 
afternoon. 

A Pacific low pressure system moving into the Great Basin 
is expected to bring stronger winds during the day on Thursday, with 
mid-morning southerly crosswinds giving way to gusty westerly winds 
by afternoon. Wind gusts as high as 22 kts are possible during the 
late afternoon hours.

Southeast California/Southwest Arizona including KIPL and KBLH: 

Dry southwest flow aloft will keep skies clear next 24 hours. Winds 
will favor the west over the western deserts, with some gustiness 
expected later in the afternoon and evening. Gusts may exceed 20kt 
at times. At KBLH winds will tend to favor south to southwest during 
the afternoon and evening, unlike KIPL where winds should be mostly 
westerly. Winds should gradually diminish tonight/early Thursday.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Friday through Tuesday: 
Drier conditions and lighter winds will be seen on Friday through
Sunday with near normal temperatures. Humidity levels will mostly
range from 10 to 20% with gradually lower readings into Saturday.
Another low pressure system seems likely by next Monday,
potentially bringing decent rain chances, higher humidities, and
breezy to windy conditions through Tuesday. 

&&

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

&&

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

$$

DISCUSSION...Kuhlman
AVIATION...CB/Percha
FIRE WEATHER...Hernandez/Kuhlman