Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 230421

Area Forecast Discussion
National Weather Service Tucson AZ
920 PM MST THU SEP 22 2016

.SYNOPSIS...A strong low pressure system moving north of the area
will bring a chance of showers and/or thunderstorms, cooler
temperatures and gusty winds at times through Friday. Mostly dry
conditions will then prevail this weekend and into the latter part
of next week.


.DISCUSSION...Showers and a few thunderstorms associated with the
tail end of a low pressure system passing through the Great Basin
had dissipated late this evening. Latest HRRR solution continued to
suggest the possibility of a flare up in showers late tonight and
more so Friday afternoon mainly northeast of Tucson. The current
forecast resembled this solution which still looked plausible. Thus,
no updates anticipated this evening. Please refer to the additional
sections for more details.


.AVIATION...Valid through 24/06Z.
A few showers and/or thunderstorms will continue mainly from the
Tucson area north and westward as a cold front slowly moves through
the area into Friday. Expect a band of SCT-BKN clouds at 3-7 ft AGL
to gradually make its way across the region from west to east as
well. Surface wind becoming terrain driven less than 10 kts
overnight. Breezy westerly to southwesterly winds are expected again
Friday afternoon.


.FIRE WEATHER...Isolated showers and storms are possible through
tonight and again on Friday, especially from Tucson north and
westward as a cold front moves across the area. Southwest winds of
10-15 mph and occasional 20 mph gusts will subside this evening. A
few west to southwest breezes on Friday will continue for eastern
areas. Otherwise, mainly light winds this weekend before increased
easterly breezes move in for much of next week. Minimum relative
humidity values will begin to lower considerably as early as Friday
for far western areas, then for the remainder of the region this
weekend into early next week.


.PREV DISCUSSION...A low pressure system was moving through the
Great Basin region this evening. A small threat for showers will
linger through Friday before the system moves generally over the
northern Rockies with the tail end of the system into the four
corners region by late Friday. As this system swings through, the
associated cold front will result in a significant decrease in
temperatures with highs only expected to be in the mid to upper 80s
for Tucson during the entire forecast period.

The trough axis associated with the aforementioned low pressure
system will linger just to our east into the weekend before an
upstream vort max dives south on the back side of the trough across
eastern Utah and Arizona Saturday into Sunday. As this occurs,
models are showing some significant differences beyond Sunday. The
ECMWF wants to develop a cut-off low over the general vicinity of
southeast Arizona, southwest New Mexico, northeast Sonora and
northwest Chihuahua on Sunday, while the GFS just keeps the trough
to our east over northeast New Mexico through west Texas and
northern Chihuahua. By late Monday the Euro indicates the low will
retrograde over northern Sonora, while the GFS has the tail end of
the trough sliding farther south into north central Mexico.

Meanwhile, the models develop a tropical system near the equatorial
Pacific and the GFS eventually takes it to the north and it becomes
absorbed into the low over southern Baja by the middle to latter
part of next week and moves this whole system northward. On the
other hand, the Euro takes the low over Sonora and moves it
northwest to a position over western Arizona by mid week, but then
kicks it east in response to an upstream trough. The bottom line is
that during the early to latter part of next week, the Euro starts
out wet and then becomes dry, whereas the GFS starts the week out
dry and then ends it wet. So, with this much uncertainty I just have
single digit to slight chance type POPs in the grids/forecast until
models can come to some consensus.

For Tucson, high temperatures will be 8 to 10 degs below normal
Friday, then will range from 3 to 6 degs below normal each day
thereafter, although the coolest days will be Saturday through
Tuesday. Low temps will be near normal Friday morning, then 6 to 8
degs below normal this weekend and within a degree or two of normal





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