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Area Forecast Discussion (AFD)

Issued by NWS Tucson Arizona (TWC)

FXUS65 KTWC 251608

Area Forecast Discussion
National Weather Service Tucson AZ
908 AM MST Sun Aug 25 2019

.SYNOPSIS...Expect limited thunderstorm activity through Tuesday as 
high pressure builds overhead. The drier pattern will also bring 
hotter temperatures, especially by Tuesday. A more favorable flow 
with adequate moisture will provide scattered mainly afternoon and 
evening thunderstorms Wednesday into next weekend.


.DISCUSSION...A quick minor update this morning with the 12z KTWC
sounding. It came in with a little more moisture but also warmed
another 1.5 degrees around 500mb. Combined with weak shear, this
will still tend to inhibit support for valley storms, but a little
better environment still exists for storms south and southwest of
Tucson. Initiation in the mountains should be easy there with a
few wandering into lower elevations propagating along outflows. 
Otherwise, surface dew points will remain elevated, but we should 
see some erosion in deeper moisture across eastern areas over the 
next 24 hours. Please see the previous discussion below for 
additional details.


.AVIATION...Valid through 26/18Z.
Isolated -TSRA/-SHRA after 19Z today and into early this evening 
south to southwest of KTUS. Any -TSRA/-SHRA expected to end by 
05Z Monday. Otherwise, cloud decks mainly 10k- 15k ft MST. Surface
wind mostly variable in direction and under 12 kts. The exception
is vicinity KSAD this afternoon/evening with nwly surface wind 
12-18 kts with gusts near 25 kts. Aviation discussion not updated 
for TAF amendments.


.FIRE WEATHER...Isolated thunderstorms this afternoon mainly south 
to southwest of Tucson. Expect dry conditions across much of the 
area Monday into Tuesday, although isolated thunderstorms especially 
Tuesday afternoon near eastern and southern mountains. A favorable 
flow pattern with adequate moisture will provide scattered mainly 
afternoon and evening thunderstorms Wednesday into next weekend. 
Aside from thunderstorm wind gusts, 20-foot winds will be terrain 
driven mainly under 15 mph.


.PREV DISCUSSION... /issued 330 AM MST Sun Aug 25 2019/ 
Upper ridge axis extended from central California southeastward 
into northeast Sonora/northwest Chihuahua Mexico early this 
morning. The 25/00Z upper air plots initialized a 500-300 mb 
anticyclonic gyre over southern Pinal County northwest of Tucson. 
Various 25/00Z high-resolution models suggest shower and 
thunderstorm initiation early this afternoon will be quite limited
across this forecast area. 

Given the aforementioned upper features and based on these 
solutions, expect only isolated showers/tstms this afternoon and 
perhaps into early this evening favoring locales south-to-southwest 
of Tucson. Any showers/tstms should end shortly after sunset. Expect 
dry conditions across much of the area Monday as the upper high 
pressure is progged to remain overhead. If any showers/tstms develop 
Monday afternoon, the favored locations at this time are the 
Chiricahua Mountains followed by the White Mountains. Various 25/00Z
models then suggest isolated showers/tstms mainly near eastern/
southern mountains Tuesday afternoon and evening.

Thereafter, the 25/00Z GFS/GEFS and ECMWF/EPS were similar with 
depicting consolidation of the upper high to occur by midday 
Wednesday generally over southern California. The upper high is 
progged to remain quasi-stationary Thursday into Friday before 
moving northeastward toward the Las Vegas, Nevada vicinity by 

Have noted that these solutions generally were not quite as robust 
with the expected return of showers/tstms later this week versus the 
same solutions from 24 hours ago. However, the progged flow regime 
and various moisture fields still suggest that scattered mainly 
afternoon and evening showers/tstms should return Wednesday and 
continue into next weekend.

Regarding daytime temps, expect a warming trend to occur into 
Tuesday in response to high pressure ridge aloft and a gradually 
drier environment. The hottest temps during this forecast package
still appear most likely Tuesday, and temps Wednesday will likely 
only be just a degree-or-two lower for much of the area. There is 
the potential for an Excessive Heat product to eventually be issued 
for portions of the area, especially Tuesday. Thereafter, daytime 
temps are forecast to moderate closer to late August normals 
Thursday into Saturday.





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