Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 260441

Area Forecast Discussion
National Weather Service Tucson AZ
940 PM MST Fri Nov 25 2016

.SYNOPSIS...Above normal high temperatures and dry conditions
will continue on Saturday. A fast moving storm system will then
bring a good chance of valley rain, mountain snow mainly Sunday
night followed by colder temperatures Monday. Expect the coldest
overnight lows of the season so far Tuesday night, then dry
conditions with a slow warming trend the middle of next week.


.DISCUSSION...Satellite imagery this evening showed scattered thin
high clouds moving across the area. Thicker cloud cover was off Baja
California and these should overspread the area over the next 12-18
hours in advance of our next weather maker which was off the west
coast. This system will bring gusty winds to the area on Saturday
and then unsettled and cooler weather to the region Sunday and
Monday. See previous discussion for further details.


.AVIATION...Valid through 27/00Z.
Expect BKN-OVC clouds above 20k ft AGL thru the forecast period.
Surface wind will be sely/sly at 5-15 kts overnight into Saturday
mid-morning then becoming SW 10-20 with gusts of 25-30 kts
in afternoon. Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Dry conditions will continue through Saturday.
A few mountain rain and snow showers will develop Saturday night
into Sunday, mainly in the White Mountains. More widespread valley
rain and mountain snow will occur Sunday night, with lingering
precipitation east of Tucson Monday into Tuesday. Expect breezy
south to southwest winds over the weekend. West to northwest winds
will then occur early next week as the system exits the region.
Relative humidity values will increase substantially in the
Sunday through Tuesday time frame, due to increased moisture and
considerably cooler temperatures. A strong low pressure system
may move into the region late next week and next weekend for another
chance of precipitation.


.PREV DISCUSSION...Dry conditions will continue through Saturday
followed by a slight chance of showers from Tucson northeast to the
White Mountains Saturday evening. A chance of showers continues
mainly across the White Mountains late Saturday night and Sunday.

The 25/12Z GFS/ECMWF/CMC were in very good agreement with depicting
an impulse embedded with the deepening longwave trough over the
western CONUS to move rapidly eastward across the area Sunday night.
The 25/12Z GFS/ECMWF MOS guidance reduced PoPs versus previous
solutions. However, believe that fairly widespread albeit brief
precipitation will occur Sunday night.

Thus, a good chance of showers continues from Tucson eastward Sunday
night. For Tucson, the best time period for measurable precip
appears to occur from about 10 PM MST Sunday until about 5 AM MST
Monday, give-or-take a couple of hours either side. Snow levels
briefly early Monday morning should lower to around 4000 feet from
Tucson eastward, but no significant accumulations are expected below
5000 feet.

Liquid amounts should mainly range from one tenth of an inch to one
quarter of an inch. Snow accumulations up to 5-6 inches may occur
across the highest peaks (e.g. White Mountains, Mt. Graham and Mt.
Lemmon). Would not be surprised that dry conditions occur area-wide
Monday. PoPs were reduced most sections but maintained chance-
category PoPs across the White Mountains.

The next impulse will move southeastward across the Four Corners
region Tuesday. Have added a slight chance of showers Tuesday as far
west as Tucson, but chance-category PoPs continues across the
eastern mountains. Dry conditions return area-wide Tuesday night-
Thursday night. Thereafter, there were considerable differences
between the GFS/ECMWF regarding the position and path of the a
strong upper low to develop over southern California or Arizona.
Given the differences, forecast confidence is quite low at this
time. For this issuance, a slight chance of showers exists across
much of the area next Friday.

High temps Sat are forecast to range about 3-8 degs above normal. A
significant cooling trend will occur Sun-Mon, and high temps Monday
are forecast to be nearly 20 degs colder at lower elevations versus
Saturday. A very gradual warming trend is on tap Tue-Thur, although
daytime temps will remain below normal.





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