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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

                            
000
FXUS65 KPSR 222155
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
255 PM MST Sun Sep 22 2019

.SYNOPSIS... 
A stark change to the weather is expected as conditions favorable
for showers and storms improve across the area. Scattered showers
and thunderstorms may develop late tonight or early Monday 
morning across portions of south-central Arizona with an increase
in activity by Monday afternoon. Isolated severe thunderstorms 
capable of producing damaging winds and hail are expected along 
with locally heavy rainfall capable of producing flash flooding. 
Although rain chances will begin to decrease on Tuesday, elevated 
rain chances will linger through Thursday as the upper level 
disturbance remains near Arizona. This disturbance should finally 
move east of the region by Thursday night, with drier conditions 
expected by Friday into the weekend. 

&&

.DISCUSSION...
Weak ridging over our area today is allowing temperatures to run 
a few degrees warmer than yesterday. To our northwest, the long 
awaited and well discussed Pacific trough is digging south along 
the west coast and should continue that trajectory over the next 
few days. Meanwhile, surface dew points have reached into the 50s
and low 60s across southeastern Arizona as tropical moisture 
surges northward. Visible satellite shows the strongest convection
deep over Sonora but some showers are developing over Pima 
County. Showers and thunderstorms will be mostly confined to 
southern Arizona this afternoon and evening; however, an isolated 
storm or shower cannot be ruled out in our forecast area, 
especially if moisture surges north quicker than anticipated.

Finally seeing much better model agreement and consistency regarding 
the evolution for the previously mentioned Pacific trough. Most 
guidance depicts the trough digging into the southwest and
reaching southeast California/western Arizona Monday. Meanwhile, 
the aforementioned moisture will spread into Arizona later today 
through Monday morning. Light to moderate showers and isolated 
storms are possible over south-central Arizona, particularly over 
the higher terrain north and east of Phoenix, as early as Monday 
morning.

Later Monday, the ingredients for strong thunderstorms, some 
possibly severe, will be present as the trough provides atmospheric
forcing via upper level diffluence and a strong jet streak aloft.
Surface dew points will climb into the 50s/60s by midday Monday 
across much of Arizona and into southeast California. Model 
soundings show rich moisture from the surface up through the H7 
layer with much drier air aloft. The moisture gradient will be 
steep with that moisture boundary setting up somewhere close to 
the Colorado River, whether just on the Arizona side or even into
southeast California is still uncertain. Instability will be more
than adequate with MUCAPE values in excess of 1000 j/kg and 
steepening lapse rates as cold air aloft from the low moves 
overhead. With veering winds and bulk shear values upwards of 50 
kts, some supercells may develop.

Altogether, these conditions will be conducive for convection with
initiation beginning late morning/early afternoon along the 
moisture gradient. Thunderstorm activity will spread eastward 
through the afternoon with the threat persisting well into Monday 
night and possibly through Tuesday morning. The threat from storms
that initialize early will focus on large hail before shifting 
towards damaging winds later. The latest Day 2 SPC outlook carries
a large portion of western and central Arizona in a slight risk 
for severe weather with a broad marginal risk including southeast 
California and most of Arizona.

In addition to the severe threat, heavy rain will also pose a 
flooding concern with rain rates possibly exceeding 1.00" per hour
with any thunderstorm. Orographic lift will also enhance any
convective showers/storms making the higher terrain features 
north and east of Phoenix as places most likely to see the most 
rainfall. As such, WPC has a slight risk in their Day 2 Excessive 
Rainfall Outlook centered over a large portion of central Arizona.

Activity should wane in our area by late Tuesday morning although
enough moisture will be present that lingering showers will be 
possible. Some hi-res guidance shows isolated showers and storms 
over Gila and eastern Maricopa Counties Tuesday afternoon as a 
cutoff low takes shape. Residual moisture and a second Pacific 
trough later in the week justify elevated PoPs in the forecast, 
primarily for Maricopa County eastward, late Wednesday and 
Thursday. As for temperatures, the persistent troughing and 
shortwaves will keep a cooler airmass in place resulting in 
daytime temperatures below seasonal normal values through the 
forecast period. 

&&

.AVIATION...Updated at 1840Z 

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

Visible satellite this morning shows mostly clear skies over the 
Phoenix metro with some mid and high level clouds beginning to 
push into the region. Light diurnal winds will continue through 
today with wind speeds generally remaining below 10 kts. 

As we head into tonight, increasing tropical moisture will result
in increasing cloud cover with SCT-BKN decks around 8-10 kft and
25 kft along with a few decks developing as low as 5 kft. Going 
into Monday morning, isolated showers will begin to develop across
south-central Arizona so VCSH has been mentioned in the TAFs. 
Can't rule out a few thunderstorms in the morning but confidence 
is pretty low. The best chances for thunderstorms look to occur 
during the early afternoon/evening timeframe. Went ahead and add 
VCTS for KPHX but confidence in exact timing is low and is subject
to change in future TAF updates. Some of these storms could be 
strong to severe with gusty winds, frequent lightning, and severe 
hail possible. CIGs may get as low as 5 kft with any storms that 
pass through.

Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation concerns across southeast California at this
time. Winds will continue to be light and at times variable
through much of the period. Cloud coverage is expected to increase
with FEW-SCT decks around 9-10 kft and SCT-BKN clouds around 25
kft developing later today and into tomorrow. Can't rule out a
vicinity shower or thunderstorm developing early tomorrow
afternoon near KBLH but confidence is too low to mention in the
TAF.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Wednesday through Sunday: 
No changes to the outlook forecast...Lingering isolated shower
Aside from gusty thunderstorms, winds should generally remain
below 15-20 mph until Friday and Saturday when stronger winds are
expected with the approaching disturbance. Minimum relative
humidity values will remain in the 20 to 30 percent range for most
locations except a few spots in southeast California that may drop
into the upper teens. Overnight recoveries will remain decent as
long as the moisture remains in the area. 

&&

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

&&

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

$$

DISCUSSION...Deems
AVIATION...Smith 
FIRE WEATHER...Deems/Hopper