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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

FXUS65 KPSR 220024

Area Forecast Discussion
National Weather Service Phoenix AZ
524 PM MST Sat Sep 21 2019

Updated Aviation section.


Dry and mostly clear conditions will affect much of the region 
through today with temperatures within a few degrees of seasonal 
normals. As early as late Sunday, likely lasting through the first 
half of next week, rain and thunderstorm chances greatly improve, 
especially across Arizona. This next weather system will have the 
potential of dropping heavy rainfall across portions of the area 
early next week. In addition, strong thunderstorms are also 
possible next Monday into Tuesday mainly over south central 


As for today, conditions are looking similar to yesterday with a 
broad trough sitting over the region with dry westerly flow 
giving the area one of the most beautiful Saturdays in a long 
time. Hurricane Lorena is still easily visible on satellite south 
of Arizona as the system continues its mostly northerly track near
southern Sonora. Recent trends suggest the bulk of the moisture 
from Lorena may go east of Arizona. However, whatever tropical 
moisture we see from Lorena's remnants will combine with a gulf 
surge increasing low level moisture beginning late Sunday. 
Moisture interaction with a Pacific trough will produce conditions
favorable for showers and thunderstorms capable of producing 
heavy rain and possibly severe weather. Our area will likely 
remain quiet through most of Sunday as activity remains over 
southern Arizona.

There are many key components for this forecast and even the
slightest shifts in any one detail could greatly alter the 
outcomes for Monday. There are three basic scenarios that look 
plausible with the first, and most likely, with a period of many 
strong (severe) storms and showers across the area. The next 
scenario contains fewer strong thunderstorms with more rain/showers.
Scenario three is one in which the trough and moisture never 
quite line up greatly limiting any activity. As of now, this 
scenario looks the least likely.

Model trends favor some isolated to scattered showers Monday morning
with the majority of the activity occurring Monday afternoon and 
evening as the best moisture and lift is expected to occur after 
max daytime heating. Moisture levels should increase adequately 
for convection with surface dew points returning into the 50s/60s 
and PWATs over 1.00" by midday. Concern for a severe weather 
threat is growing as the trough approaches western Arizona providing
an environment with instability, shear and lift. Model soundings 
show MUCAPE reaching upwards of 1000 j/kg by the afternoon. Bulk 
shear peaks between 40-50 kts. Storm development will be 
accentuated by jet streak dynamics and steepening lapse rates as 
cooler and drier air aloft moves in. Stronger storms may initially
develop over western Arizona along the moisture gradient and then
progress eastward towards the Phoenix area. Thunderstorms that do
develop will be capable of producing strong gusty winds and possibly
large hail. 

Aside from the severe risk, thunderstorms and strong showers will also
be able to produce heavy rain. The collaborative rainfall forecast
paints a large swath of 1-2" QPF possible across much of Arizona
with high terrain features, such as the Rim and the mountains north
and east of Phoenix, more likely to see the highest precip totals.
However, some of the guidance continues to have some sort of 
discontinuity with the QPF outputs. The GEFS plumes QPF continues
its downward spiral with the latest mean value showing around 
0.40" of rain for Phoenix, a stark drop from nearly 1.50" just a 
few days ago. Meanwhile, the EPS mean rainfall is still a bit 
wetter at nearly 1.00" through Tuesday, but this value is also 
following a downward trend. While the potential for heavy rainfall
is present, together, both of these model trends cast some doubt 
on the coverage of heavy rain. Nevertheless, nearly all members, 
as well as other guidance, show measurable rainfall across much of
the forecast area. Areas that experience strong storms or multiple
rounds of convection could realize those higher rainfall totals.

Activity should decrease somewhat into Tuesday morning with 
lingering chances for showers and storms Tuesday through Thursday.
Otherwise, cooler air from this system will keep daytime 
temperatures 5-10 degrees below normal all next week. The overall 
upper air pattern looks to remain amplified keeping opportunity 
for active weather to develop late in the period. 


.AVIATION...Updated at 0025Z 

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT;
Southeast California/Southwest Arizona including KIPL and KBLH:

Expect light surface winds over the region through Sunday
afternoon. Winds in the lower troposphere above the surface will
also remain relatively light. Skies will be mostly clear except
for some minor cirrus. During the afternoon Sunday, there will
also be an increase in altocumulus but mostly that will be
southeast of metro Phoenix. 

Tropical Storm Lorena, currently centered roughly near 26.7 
North/111.1 West will continue to weaken with time as it moves 
northward. In the process, upper portions of the storm will begin 
tracking northeastward as lower portions of the storm track 
northward. Through Sunday afternoon, there will be little
discernible impact to the TAF sites. An approaching trough in the
Westerlies will pull remnant moisture northward Sunday night and
Monday leading to an upward trend in storm chances - most notably
during the daytime and evening hours Monday (mainly east of the
Lower Colorado River Valley). 

Aviation Discussion not updated for amended TAFs.


Sunday through Saturday: 
A large portion of the state is likely to see thunderstorms and
showers Monday with lingering activity through much of the work.
These storms will produce lightning, strong winds and possibly
heavy rain. RH values will also improve greatly and taper off
through the week. Otherwise, expect some stronger afternoon
breezes, especially on Monday, but then again later in the week.


Spotters should follow standard reporting procedures.