Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 170324

Area Forecast Discussion
National Weather Service Tucson AZ
824 PM MST Fri Mar 16 2018

.SYNOPSIS...Another system will increase winds again this weekend
and bring a better chance of valley rain and mountain snow to
southeast Arizona Saturday night into Sunday. Cooler temperatures
will prevail through this weekend, followed by a warm-up next


.DISCUSSION...It was a seasonable but fairly cloudy day across much
of southeast Arizona as cirroform cloudiness streamed across the
area from the southwest. Latest IR satellite imagery shows the back
edge of the cirrus is over the western deserts and will gradually
shift eastward through the overnight into Saturday morning. Latest
forecast accounts for this very well. Otherwise, another seasonable
day is expected on Saturday with considerable sunshine before a
shortwave trough and associated cold front moves through Sunday
morning with a chance of showers. Current forecast is in good shape,
no updates this evening. See previous discussion below for
additional information.


.AVIATION...Valid through 18/06Z.
BKN clouds above 15k ft AGL into early Saturday morning, with a
period of clearing late Saturday morning and Saturday afternoon. SFC
wind variable in direction mainly less than 10 kts overnight,
becoming SWLY 12-16 kts with gusts to around 25 kts Saturday
afternoon, then tapering off to less than 10 kts Saturday evening.
Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Dry conditions will prevail through Saturday. There
is a chance of valley rain and mountain snow showers late Saturday
night into Sunday morning. Showers will end east of Tucson Sunday
afternoon, with dry conditions Monday through next Thursday. Expect
some gusty southwest winds this afternoon and again Saturday
afternoon. Gusty west to northwest winds will occur Sunday, then 20-
foot winds will mainly be less than 15 mph Monday into Wednesday.
Elevated 20-ft winds may return late next week.


.PREV DISCUSSION...High clouds continue to move through the area
this afternoon although water vapor imagery suggests some drying and
clearing near southern California. With the relatively cool start to
the morning and the cloud cover, temps have been somewhat slow to
rise today with most lower desert locales finally eclipsing the 70
degree mark as of 20Z. Should end up with highs in the low/mid 70s
across the deserts by late afternoon. Look for tonight`s lows to be
in the ballpark as what was seen this morning with 40s across the
deserts and 30s east of Tucson.

For Saturday, low pressure presently over the Pacific Northwest will
begin to drop southward with a shortwave trough expected to move
through Arizona late Saturday into Sunday morning. Most of the day
Saturday should be sunny but clouds will gradually thicken as deeper
moisture is imported into the area. Latest runs of the deterministic
GFS and ECMWF, as well as probabilistic output from the NAEFS and
GEFS suggest increasing chances of valley rain and mountain snow
early Sunday morning. Given the fast moving nature of this system,
it looks to be a high prob/low QPF event with most locales only
forecast to see a few hundredths of an inch. As for snowfall, snow
levels will gradually fall to around 5000ft as the trough sweeps
through with a quick 1-2 inches possible. By Sunday afternoon skies
should begin to clear as drier and subsidence takes hold across the
area. The drier air and clearing skies could result in a chilly
night in the higher terrain east of Tucson where growing season
concerns could be realized. Looks like temps could fall to around
freezing or even into the upper 20s in the more protected valley
locations early Monday morning.

Looking beyond the weekend trough, weak ridging is expected to take
up residence across the area next week with gradually increasing
temperatures. Highs will easily be in the mid 80s each afternoon
across the deserts. Hints of a late week trough moving through
California and into Arizona continue to be evident in the latest 12Z
guidance. At this point it`s hard to be too certain that it
will/won`t rain late next week but I trended PoPs upward slightly
toward climatology next Thursday into Friday.  If nothing else the
trough will temporarily put the brakes on the warmup and should
knock temps back into the upper 70s.





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