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Area Forecast Discussion (AFD)

Issued by NWS Tucson Arizona (TWC)

FXUS65 KTWC 202222

Area Forecast Discussion
National Weather Service Tucson AZ
322 PM MST Fri Sep 20 2019

.SYNOPSIS...Tropical moisture will push toward the area over the 
weekend with shower and thunderstorm chances near the border as 
early as Saturday afternoon. A weather system moving through the 
area Monday and Tuesday will interact with this moisture for 
increasing chances of rain, and possible heavy rain. Temperatures
will remain near seasonable readings the next several days.


.DISCUSSION...As a trough pushes through the region north of our
area this morning, a sustained westerly flow has dried us out.
Precipitable water on the 12Z KTWC sounding was down to 1/2 inch
and surface dew points have dropped into the upper 30s to middle 
40s this afternoon. This will change a lot over the next 36 hours
as Hurricane Lorena continues to push deep moisture toward our 
area. The morning La Paz sounding just north of the hurricane
showed over 2 inches precipitable water with 1.5 inches as far
north as Hermosillo this afternoon. Our PW should increase back to
the 1 inch mark by late Saturday with 1.5 inches approaching the 
border Sunday morning.

As mentioned this morning, Lorena has gained some separation from
Mario and should be able to be resolved on her own. Current NHC 
track is further east and a little stronger, staying near or just 
east of the Baja spine. She should begin to weaken before mid 
Gulf, then at some point the remaining mid level circulation 
should decouple and shear into the higher latitude flow and into 
the desert southwest late in the weekend. Best guess on this is a 
track across Sonora just south of our border. However, the 
significance of this event for SE AZ is still going to come down 
to the timing and trajectory of the trough and associated low 
approaching the area from the northwest Monday-Tuesday.

Latest ensembles are lending more confidence to a slower and lower
latitude trajectory for the developing low, including most members
in a prime position near western Arizona Mon-Tue and maybe even
Wednesday. With such a strong ridge offshore it definitely has a
chance at an atypically deep track for late September. We'll 
continue to build our forecast foundation on an NBM base with 
GFS/ECMWF ensemble mean blend. WPC QPF values look solid.

A westerly flow will reassert itself sometime the second half of 
the week, and push the moisture back out of the picture.


.AVIATION...Valid through 22/00Z.
FEW-SCT clouds 8k-10k ft MSL this afternoon and evening with
clearing overnight. Cirrus level clouds will increase from the
south Saturday. Surface wind southwesterly at 10-14 kts with 
gusts to 20 kts this afternoon and early evening, becoming
variable generally less than 8 kts overnight. Isolated
thunderstorms possible near the international border late Saturday
afternoon. Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Expect dry conditions today then a slight chance of 
showers and thunderstorms along the international border Saturday 
afternoon and evening as moisture begins to move in. Deeper moisture 
moves in starting Sunday leading to a chance of showers and 
thunderstorms area wide Monday and Tuesday. Wednesday into 
Thursday storm chances decrease and stay mainly from Tucson 
eastward. Seasonably warm daytime temperatures will occur today, 
with below normal readings Saturday into the middle of next week. 
Aside from any thunderstorm wind gusts, 20-foot winds will 
generally be terrain driven and less than 15 mph.





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