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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

FXUS65 KPSR 230527

Area Forecast Discussion
National Weather Service Phoenix AZ
1027 PM MST Sun Sep 22 2019

.UPDATE...aviation discussion.


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. 


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 

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 0527Z 

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

Light diurnal winds will continue through the early part of the
evening with speeds mainly below 6 kts. As we head into the 
overnight hours, increasing tropical moisture will result in 
increasing low and mid-level cloud cover with BKN-OVC cloud decks
currently near 8-10 kft, dropping closer to 5 kft after 10Z. 
Going into tomorrow morning, shower and storm chances will 
continue to ramp up and hi-res models are now indicating a 
slightly earlier onset over terminals and this has been reflected
in the most recent TAF package. Latest model guidance is 
now suggesting that isolated showers could begin to develop 
near terminals as early as 9Z, with better coverage after 12Z. 
There then seems to be a hint at a break during the early part of
the afternoon, however there very much so could be showers and 
thunderstorms still lingering near or over terminals throughout
the afternoon hours. Thereafter, models continue to indicate a 
line of strong thunderstorms approaching the Phoenix terminals 
from the west after 22Z. These storms will be the strongest storms
of the day and could become severe with gusty winds, frequent 
lightning, and severe hail possible. CIGs may get as low as 4 kft 
with any storms that pass through.

Southeast California/Southwest Arizona including KIPL and KBLH:

Winds will continue to be light and at times variable through 
the morning hours before becoming gusty out of the south-southwest
in the early afternoon. In addition, low-level cloud cover is 
expected to increase through the TAF period with FEW-SCT decks 
around 5-8 kft. There is a little bit more confidence that 
showers and storms could be possible over KBLH in the early to 
mid afternoon. Any storm that develops will have the potential to
produce gusty winds and frequent lightning. Due to this, it is now mentioned in the latest TAF

Aviation Discussion not updated for amended TAFs.


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. 


Spotters should follow standard reporting procedures.




FIRE WEATHER...Deems/Hopper