Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 131605
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
905 AM MST Wed Dec 13 2017

.SYNOPSIS...A weakening area of low pressure over the Gulf of
California will result in partly cloudy conditions along the
International border today. Two storm systems will then move south
from the northern Rockies and across New Mexico over the next couple
of days, bringing several degrees of cooling and breezy conditions
to southeast Arizona. The first system will move to our east
Thursday into Friday, with the second system expected on Sunday into
Monday. By Sunday, high temperatures will be right around average
levels. High pressure aloft will then build back across the desert
southwest next week for a return to well above average readings.

&&

.DISCUSSION...
Low pressure continues to slowly spin south of the US/Mex border
this morning albeit it`s positioned a bit farther south today vs.
yesterday. Still some lingering cloud cover out there per regional
IR/Vis satellite imagery but significantly less than what has been
seen recently. Clouds didn`t manage to prevent temps from falling
drastically overnight with numerous protected locations east of
Tucson falling into the 20s and 30s this morning. Most locales have
rebounded into the 40s as 50s as of 16Z.

The forecast for the rest of the day today is relatively
straightforward with mostly sunny skies and above normal temps. Look
for desert highs to top out in the mid 70s later this afternoon.
Inherited forecast was adjusted slightly to capture observational
trends but no significant changes appear necessary whatsoever.
Attention will focus on the potential for two quick moving storm
systems tomorrow and again Sunday/Monday. At this point impacts look
to be minimal outside of some breeziness and a stray mountain shower
or two. Will see what the 12Z suite of ensembles have to show in the
next few hours.

&&

.AVIATION...Valid thru 14/18Z.
SKC to SCT clouds AOA 15k ft AGL through the period. Expect ELY/SELY
SFC winds 8-15 kts this morning to weaken to less than 12 kts and
become WLY/NWLY this afternoon. Terrain driven winds will then
prevail overnight tonight into Thursday morning. Aviation discussion
not updated for TAF amendments.

&&

.FIRE WEATHER...Dry conditions through the forecast period. Elevated
20-foot winds from the east-southeast this morning will generally
diminish to less than 15 mph this afternoon with typical diurnal
trends returning. Some northwest breezes are then likely Thursday
afternoon followed by easterly winds on Friday as a storm system
passes east of the region. Another storm system looks to pass to the
east of our area again late this weekend bringing breezy
southwesterly winds to a few spots.

&&

.PREV DISCUSSION...The quasi-stationary upper low which has been
parked over the northern Gulf of California the last 36 hrs or so
will weaken and move slowly south today. There should be
considerably less cloud cover across southeast Arizona with this
feature today, with varying degrees of mid/high clouds along the
International border (for partly cloudy skies) and mostly sunny
skies elsewhere. Daytime temperatures will be up 2-4 degrees (versus
yesterday), with highs generally in the upper 60s to mid 70s across
the valley locations of southeast Arizona.

Otherwise, the highly amplified ridge pattern that has been across
the western United States will begin to shift westward into the
eastern Pacific Ocean. The overall position of this ridge will be
important for upcoming weather across southeast Arizona. At this
time, the ridge retreats just far enough west to allow a couple of
systems to dive south out of the northern rockies and across New
Mexico. This will result in several degrees of cooling across
southeast Arizona starting on Thursday and continuing into Sunday.
These systems will also bring breezy conditions to mainly eastern
areas Thursday and again on Sunday. With the current track of these
systems, any precipitation will be limited to areas east of our
forecast area, over New Mexico and points eastward. However, if the
ridge were to retreat a little further westward than anticipated, we
may end up with some precipitation along the New Mexico border. This
is something to keep an eye on going forward. The cooling trend late
this week into the upcoming weekend will lower daytime highs to
right around seasonal averages by Sunday.

Next week the ridge of high pressure shifts back eastward across the
southwestern United States. This will result in a return to well
above normal temperatures. High temperatures by mid next week are
expected to be 5-10 degrees above seasonal averages.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

Discussion...Leins/Zell
Aviation/Fire Weather...Carpenter

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