Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 161533

Area Forecast Discussion
National Weather Service Tucson AZ
832 AM MST Sat Dec 16 2017

.SYNOPSIS...Dry conditions will continue across southeast Arizona
today. A storm system will then bring colder temperatures, as well
as valley rain and mountain snow showers late tonight into early
Monday. Dry weather and warmer temperatures will return mid next
week before another storm system moves into the area late next week.


.DISCUSSION...Upper trough currently dropping south through southern
California with high clouds spreading out ahead of this feature
across the western half of Arizona attm. Developing low pressure
centered near Hanford, CA will drop into the base of this trough as
it pinches off over the northern Baja peninsula this evening. The
area of low pressure will then strengthen and move across southern
Arizona as a closed low Sunday into Monday, before shifting east of
the forecast area on Tuesday. This system will bring valley rain and
mountain snow to much of southeast Arizona starting late tonight and
continuing into early Monday. The midnight shift issued a Winter
Weather Advisory for Sunday for the higher terrain above 6500 feet
from Mount Lemmon to Mount Graham and points southward.

Otherwise for today, high clouds will continue to spread from west
to east across the forecast area. Afternoon temperatures will top
out 1-5 degrees above seasonal averages. Southeast winds will turn
southerly this afternoon in advance of the trough and become breezy
at times. Current forecast package looks to be in good shape. No
updates this morning. For more information on the latter periods of
the forecast, including the Winter Weather Advisory and expected
snowfall amounts, please refer to the Prev Discussion section below.


.AVIATION...Valid thru 17/12Z.
Clouds AOA 18k ft AGL increasing from west to east into this
afternoon. Lower cloud decks around 8-12k ft agl will develop this
evening with isolated to scattered valley SHRA and mountain SHSN
increasing into Sunday morning. MVFR cigs/vsbys esp near terrain
possible late in the period. Sfc wind ely/sely at mainly less than
12 kts through midday. Speeds increasing out of the south at 10 to
15 kts and a few gusts to around 25 kts. Speeds decrease to less
than 12 kts after sunset. Aviation discussion not updated for TAF


.FIRE WEATHER...Easterly 20 foot winds with speeds of 10 to 15
mph and gusts to 25 mph at times this weekend. Less wind will occur
for the first half of the new work week. Mild and dry weather today
will give way to a weather system bringing cooler temperatures and
valley rain and mountain snow to the area Sunday and Monday. Drier
air and warmer temperatures will then return Tuesday through
Thursday next week.


.PREV DISCUSSION...The weak low meandering slowly across northwest
Mexico is now out of the picture as a shortwave ridge pushes into
our area from the west with the remains of the low now in southern
Chihuahua. One thing we might still be able to get out of it is
moisture if we can manage a southerly flow.

The trough currently digging through the Pacific Northwest will
ultimately be able to tap into some of that moisture. An upper level
jet splitting into and south of our area will help guide this system
quickly into southern California and northern Baja over the next 24
hours. As this happens, our northeasterly and easterly flow will
become southerly late today. With a favorable trajectory, it will
also tap into some northern Gulf of California moisture to moisten
our atmosphere up fairly quickly later today and tonight.

Good agreement among most operational and ensemble models with a
trajectory pushing the main low through our area Sunday, with an
associated cold front Sunday afternoon. Best dynamics and moisture
couplet will be Sunday morning into Sunday evening, with the main
event in our corner of the state for a change (southerly moisture
intrusion not expected to be much of a factor for central and
northern portions of the state). Storm total QPF values in the .1 to
.3 inch range in valleys with .4 to 1 inch in mountains. Some
enhancement possible with convection Sunday afternoon and early
evening. Snow levels between 7k and 8k feet Saturday evening,
dropping to the 5500 to 6000 ft range Sunday morning. As low as 4500
feet possible with convection in the afternoon and the main low
Sunday evening. 1 to 3 inches between 5000 and 6500 ft, with 4 to 8
inches above 6500 ft. A few spots above 7500 or 8000 ft could see 8
to 10 inches, but that should be fairly limited in coverage.
Right now we have more of a focus for Mt Graham, the Huachucas, and
the Chiricahuas, but we`ve also included the Catalina, Rincon and
Santa Rita mountains in a winter weather advisory.

Drying from the west Sunday night, but some lingering showers in
eastern areas Monday. Cooler temps and gusty winds at times, but
nothing out of the ordinary for December.

Weak ridging with a warming trend most of the rest of next week,
with another system digging toward our general direction late in the
week. GFS is slower and deeper, we`ll see how it turns out.


Winter Weather Advisory above 6500 ft from 6 AM to 11 PM MST
Sunday for AZZ511>514.



Prev Discussion/Aviation/Fire Weather...Meyer/Carpenter

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