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FXUS65 KTWC 061001

Area Forecast Discussion
National Weather Service Tucson AZ
300 AM MST Fri Dec 6 2019

.SYNOPSIS...Dry conditions with warmer than normal daytime 
temperatures into Saturday. Another weather system will bring valley 
rain and high elevation snow Sunday into Monday. Dry conditions with 
seasonable temperatures will prevail next Tuesday into Thursday.


.DISCUSSION...Upper ridge axis extended from southeast Arizona 
northwestward into the Pacific Northwest early this morning. This 
ridge axis was ahead of deep low pressure centered west of the 
northern California coast, or near 40N/135W. Generally zonal flow 
aloft will prevail across this forecast area into Saturday as the 
upper ridge flattens. This pattern will yield variable amounts of 
mostly cirriform clouds into the first half of the weekend.

The 06/00Z GFS/GEFS and ECMWF/ECENS were quite similar with 
depicting a progressive positive-tilted upper trough to move 
eastward into the western CONUS Saturday night. A few showers 
producing virga or very light rainfall may develop as early as 
Saturday evening across western Pima County, but much of southeast 
Arizona will remain precip-free Saturday evening. However, isolated 
to scattered showers should occur late Saturday night mostly 
northwest of a Safford-Tucson-Kitt Peak line.

Have noted that the 06/00Z deterministic ECMWF has trended away from 
a closed upper low system depicted via the 05/12 ECMWF to move 
eastward across the area early next week. As such, the 06/00Z ECMWF 
was very similar to the GFS with the depiction of an eastward moving 
positive-tilted upper trough to move eastward across southeast 
Arizona Sunday into Monday. Despite the positive-tilted structure, 
moisture will be sufficient to produce fairly widespread valley 
rain. Snow levels by daybreak Sunday should be mostly above mountain 
tops, but are forecast to fall to 7000-7500 feet midday Monday.

The mid/upper trough axis is progged to move well east of this 
forecast area late Monday night. Given that the 06/00Z ECMWF has 
trended away from a slower moving closed upper low system and is 
very close to the GFS solution, precip chance were lowered Monday 
night into Tuesday. Thus, a slight chance of valley rain and 
mountain snow showers exists Monday night east of Tucson followed by 
dry conditions area-wide Tuesday.

At this time, storm total rainfall amounts will generally range from 
0.10 inch to 0.50 inch in the valleys, and from 0.25 inch to one 
inch in the mountains. Storm total snowfall accumulations should 
range from 1-5 inches across the highest peaks of the White 
Mountains, Mount Graham, and the Catalina and Rincon Mountains near 
Tucson. The bulk of precipitation should occur east of a Picacho 
Peak to Kitt Peak line with little if any measurable precipitation 
west of this line.

Although the models depict a shortwave trough will move eastward 
across the area next Wednesday night, this system is not expected to 
produce any measurable precip at this time. As a result, dry 
conditions are forecast at this time to continue next Wednesday into 

High temperatures today and Saturday will range about 3-7 degrees 
above normal. A cooling trend is on tap Sunday into Monday, with 
daytime temps Monday trending a few degrees below normal. High temps 
Tuesday into Thursday will mostly be within a few degrees of 
seasonal normals.


.AVIATION...Valid through 07/12Z.
Expect SCT-BKN cirrus clouds above 20k ft MSL persisting through the 
forecast period. Variable SFC wind less than 10 kts this morning. 
Winds then become easterly less than 12 kts this afternoon turning 
northwesterly at TUS and OLS in the evening. Winds will then be 
light and variable tonight into Saturday morning. Aviation 
discussion not updated for TAF amendments.


.FIRE WEATHER...Dry conditions through Saturday. A weather system is 
expected to impact the area early Sunday into Monday with a chance 
of valley rain and mountain snow. The best chance for precipitation 
will be from central Pima County eastward and northward, and 
particularly across the higher elevations. A slight chance of 
showers continues Monday night across eastern sections, with dry 
conditions returning Tuesday and continuing through the end of the 
work week. 20-ft winds will generally remain less than 15 mph 
through the forecast period.





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