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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

                            
000
FXUS65 KPSR 182356
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
456 PM 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...
Through Saturday, the Desert Southwest will continue to be under 
the influence of dry southwest flow aloft resulting in minimal 
precipitation chances. A dry Pacific low will slowly track across
the Great Basin Thursday into Friday and bring our region breezy 
to gusty conditions and cooler temperatures. The breeziest 
conditions will be felt during the mid to late afternoon hours on 
Thursday with gusts up to 20-25 mph. High temperatures will be on 
a slight decrease as well, with highs today in the low 100s, 
dropping into the mid to upper 90s Friday and Saturday. Skies 
through the first half of the weekend will remain mostly clear 
with plenty of sunshine.

The main forecast concern will be Sunday onward with the 
potential for a significant rainfall event as an unseasonably 
strong Pacific low is forecast to impact Arizona. Models are still
showing a deepening Pacific low diving southward along the 
western CONUS Sunday into early next week 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, with increasing uncertainty Monday 
night through the latter part of the week as model spread 
increases. The European shows the Pacific Low deepening as it 
tracks south through California before getting cut off over 
southern California Monday night and potentially stalling out 
through at least Thursday. The GFS track is a little further to 
the east and a little bit faster, drying out our forecast area by 
Tuesday. There are still some unknowns with what will eventually 
happen to the tropical systems moving northward along the Baja. 
However, no matter when the tropical systems end up falling apart,
it seems very likely that tropical moisture will make its way 
northward somewhere into Arizona and both the GFS and European 
show strong moisture advection into Arizona Sunday through 
Monday. 

The most recent GEFS plumes show that there is still significant 
uncertainty when it comes to how much precipitation we will get 
from this weather system. Currently, the GEFS plumes for Phoenix 
are showing that we could get as little as nothing at all or up to
5" during the entirety of this event (the mean is showing 1.95").
The EPS also has a large spread showing as little as nothing at 
all or as much as 6-8" inches during the entirety of this event. 
Hence, this could either end up bringing us nothing, or bringing 
us a record amount of rainfall. 

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 continue
to remain elevated late Sunday through Wednesday with the best 
chances of showers or thunderstorms across south-central and 
eastern Arizona. Stay tuned for the latest updates!

&&

.AVIATION...Updated at 2355Z 

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

Biggest weather impact through the TAF period will be increased 
wind speeds during the day Thursday, with afternoon southwesterly 
gusts up to 20-25 kts. Typical diurnal directions are expected, 
though winds may be slower to veer towards the southwest midday. 
This will likely result in a few hours of gusty southerly 
crosswinds for KPHX and KDVT. Speeds gradually subside after 
sunset Thursday evening. Skies should remain mostly clear with 
occasional FEW-SCT cumulus based around 10 kft.


Southeast California/Southwest Arizona including KIPL and KBLH: 

Dry southwest flow aloft will keep skies clear next 24 hours. Winds 
will favor the west to southwest at both terminals, with gustier
conditions expected Thursday afternoon. Gusts may exceed 20-25 kts
Thursday afternoon. Winds should gradually diminish late Thursday
evening.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Saturday through Wednesday: 
Drier conditions and lighter winds will be seen Saturday and most
of the day Sunday with near normal temperatures and humidity
levels between 5% and 15%. Another low pressure system seems 
likely by late Sunday night, potentially bringing decent rain 
chances, higher humidities, and breezy to windy conditions through
potentially Wednesday. Strongest winds right now look to be on 
Monday with gusts up to 20-30 mph.

&&

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

&&

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

$$

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