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Area Forecast Discussion (AFD)

Issued by NWS Tucson Arizona (TWC)

FXUS65 KTWC 212130

Area Forecast Discussion
National Weather Service Tucson AZ
230 PM MST Sat Sep 21 2019

.SYNOPSIS...Tropical moisture will continue to push into Arizona 
this weekend into early next week. At the same time a deep upper 
level storm system will dive down the Pacific Coast Sunday into 
Tuesday and remain in the area into Thursday while interacting with 
the moisture in place. The result will be a chance of showers and 
thunderstorms mainly south and east of Tucson through Sunday then 
scattered showers and thunderstorms Monday into Thursday across much
of the area.


.DISCUSSION...An interesting week ahead thanks to a combination of
tropical moisture and a very deep (for late September) upper level
low that is forecast to move into the region.  

In the near-term, Lorena is closing in on the Mexico mainland and
weakening quickly. As a tropical system, it is expected to die as it
moves north over Sonora Mexico over the next 24 hrs. As the remains
of this storm moves north, westerly flow over the region is expected
to begin pushing the bulk of this to the east Sunday, but not before
catching the international border area east of Nogales including the
southeast half/two thirds of Cochise county. There is some potential
for heavy rain across this areas Sunday into Sunday evening as a
result. I raised POPS and QPF this area as a result but this will
need to be watched for further adjustments.

Then Sunday night into Monday the upper low will move from western 
Oregon Sunday night to the southern California/Las Vegas area 
Monday night. In advance of that low, southerly flow will tap into 
tropical moisture to the south and spread it north across much of 
Arizona. This interesting setup will allow for showers and 
Thunderstorms to develop across a large portion of the state with the
current focus of the strongest storms and heaviest rainers northwest
of Tucson up through Kingman Monday afternoon into Monday evening. 
Thanks to the shear profile associated with the upper low, there is 
a chance for a few severe thunderstorms with damaging winds and hail
in the above mentioned area. 

Moving into the Monday night through Thursday period, the NAM has 
been tossed (collaboratively) as being too progressive with the upper
level system. We leaned heavily on a GFS/ECMWF blend through the 
period as like the NAM, the NBM has not caught up to the apparent 
evolution of the upper low which has significant forecast impacts. 
The expected evolution of the upper low is for it to settle in 
roughly the Yuma area Tuesday into Wednesday then opening up and 
lifting northeast Thursday into Thursday night in response to another
deep upper level system dropping into the Pacific Northwest. Based 
on that scenario, I raised POPS across our forecast area Monday night
into Tuesday which at this point looks to be the best period for 
more widespread showers and thunderstorms for southeast Arizona. With
the low to the west on Wednesday we will be in a favorable location 
relative to the low and would expect some moisture to be forced 
across the area for a continued chance of showers and isolated 
thunderstorms. If the above scenario continues to play out, another 
threat of showers and storms can be expected when the upper low lifts
to the northeast Thursday. Forecasting the movement of an upper low 
at that time range is risky at best as a lot will depend on the 
northern stream to drive it out, but at this time the above scenario
is a reasonable bet. Pretty much a wetter and cooler than normal 
week overall. Hopefully it will trim our monsoon rainfall deficit. 

Beyond Thursday we should be dry Friday through at least the first
half of the weekend. 


.AVIATION...Valid through 23/00Z. 
Isolated -TSRA/-SHRA this evening southeast of Benson with a chance 
of a storm affecting the KDUG terminal through 22/03Z. Cloud decks 
generally SCT 8k-12k ft MSL through the valid period. Tonight expect
high clouds to roll in southeast of KTUS with cloud coverage SCT-BKN
20k ft MSL. Scattered TSRA/SHRA expected again Sunday afternoon 
south and southeast of KTUS. SW wind 7-13 kts becoming variable later
tonight then SWLY 8-15 kts Sunday afternoon. Aviation discussion not
updated for TAF amendments.


.FIRE WEATHER...Expect isolated thunderstorms near the International 
border into the early evening as moisture begins to move in. Deeper 
moisture moves in starting Sunday leading to a chance of showers and 
thunderstorms south and southeast of Tucson Sunday, then affecting
much of southern Arizona off and on Monday into Thursday. Below 
normal temperatures expected through next week. Aside from any 
thunderstorm wind gusts, 20-foot winds will generally be terrain 
driven and less than 15 mph.




FIRE WEATHER...Rasmussen

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