Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 180340

Area Forecast Discussion
National Weather Service Tucson AZ
840 PM MST Sat Feb 17 2018

.SYNOPSIS...Patchy fog is possible again tonight but should not be
as widespread as Friday night. A couple of weather systems will
bring a chance of showers Sunday into Tuesday. As colder air moves
into southeast Arizona Monday, snow will fall in the mountains,
especially the Whites. After the precipitation exits the region,
freezing temperatures will return to valley locations Tuesday
night, mainly east of Tucson.


.DISCUSSION...Larger scale flow pattern has southwest flow aloft
across southeast Arizona out ahead of an upper low located near
30N/120W. Considerable mid and upper level cloudiness over the
forecast area, though some partial clearing is taking place south
and west of Tucson. This partial clearing will overspread the
remainder of the forecast area over the next several hours which
should allow enough radiational cooling to take place in a still
rather moist near surface layer to allow for patchy fog to develop
in valley locations overnight into Sunday morning. Any fog will burn
off mid to late Sunday morning, with the possibility of a few
showers developing during the day Sunday as a weak jet streak moves
by out ahead of the upper low which will shear out as it eject
across the area Sunday night. Thereafter, the focus turns to the
trough moving into the Pacific NW and will rapidly dig to towards
the area Monday. Still some uncertainty as to just how much
precipitation there will be this far south Monday into Tuesday
morning. Will keep the forecast as is at this time and let the
midnight shift take a complete look at incoming 00Z guidance. For
additional information see previous discussion below. No updates
this evening.


.AVIATION...Valid thru 19/06Z.
Expect SCT clouds 3-6 kft AGL and SCT-BKN high clouds above 15k ft
AGL through the forecast period. Due to lingering low-level moisture
patchy fog will again be possible between 18/09Z - 18/16Z, which
could create MVFR visibilities during that time period. Cigs will
also drop starting Sunday morning to BKN 3-6 kft AGL with a slight
chance of -SHRA as well Sunday into Sunday night. Winds will be
variable less than 10 kts through 18/18Z then increase out of the
southwest at 10-15 kts Sunday afternoon with gusts between 20-25 kts
before tapering off Sunday evening. Aviation discussion not updated
for TAF amendments.


.FIRE WEATHER...There will be another chance of valley rain
and mountain snow showers Sunday into Tuesday morning as a couple of
weather systems track through the southwest. The strong trough with
a tight pressure gradient will cause gusty conditions Monday with 20-
foot winds out of the west blowing near 20 mph and gusts up to 35


.PREV DISCUSSION...While the fog has lifted across the area, low
level moisture and some low-based cloudiness remain. Dew points are
still in the 40s to around 50. Expect patchy fog to form again later
tonight, although it should not be as widespread.

Looking at the big picture, an upper low west of Baja California
continues to direct moisture into southeast Arizona. The low will
kick eastward Sunday, and combined with a jet streak, act on the
moisture to develop isolated to scattered showers, mainly east of
Tucson. Snow levels will remain high with only the very highest
peaks cold enough for any snow through Sunday night.

Another area of low pressure over the Pacific northwest will dive
southeastward toward Arizona Monday before turning east Tuesday.
The associated cold front will lower snow levels Monday night and
provide enough lift for significant mountain snow. At this time, the
Rim figures to be the most favored area for accumulating snow, but
snow may also accumulate Monday night on other mountain ranges in
southeast Arizona.

As the cold front passes Monday, expect breezy to windy conditions.
Then after skies clear Tuesday night, temperatures will drop below
freezing for the first time in over a week in valleys east of Tucson
and may even approach freezing in the Tucson Metro.

Arizona will remain under the influence of low pressure aloft during
the latter half of the new work week, but at this time no
significant precipitation is expected. High temperatures for the
next 7 days will remain at or below seasonal normals. Tuesday and
Wednesday will be the coldest days, with highs 5 to 10 degrees below




Fire Weather...Lader/Hardin
Prev Discussion...Drozd


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