Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 121637

Area Forecast Discussion
National Weather Service Tucson AZ
938 AM MST Tue Dec 12 2017

.SYNOPSIS...Considerable cloudiness will continue across southeast
Arizona into this evening due to an area of low pressure centered
over the Gulf of California. This low will remain to our south
through Wednesday with variable amounts of clouds and potentially
even a few sprinkles. The low will also result in breezy east to
southeast winds today. From Thursday into early next week, expect
mostly clear skies, dry conditions and less wind as high pressure
aloft builds back overhead. Above average temperatures will continue
the next seven days.


.DISCUSSION...The upper weather pattern this morning is defined by a
Rex block, or high amplitude upper ridge of high pressure centered
over the great Basin, extending into SW Canada. Beneath this feature
is a weak upper low centered over Sonora Mexico, extending into our
neck of the woods. The open orientation of this upper low will allow
it to continue to weaken the next day or two as it slowly
retrogrades to the southwest.

The latest satellite imagery indicated mid-high clouds increasing in
coverage this morning, associated with a weak mid-level feature
moving around the mean upper low. The 12z GFS depicts as much as
0.07 inches of pcpn over the Tohono O`Odham Nation by 00z this
afternoon, but given how dry we are at the surface (dewpoints in the
teens), we will likely only see in a few sprinkles. Otherwise,
expect these clouds to shift west and become more scattered in
coverage this evening.

No updates to the inherited forecast will be necessary this morning.


.AVIATION...Valid thru 13/18Z.
SCT-BKN clouds above 15k ft AGL through the period. SFC winds will
remain ELY/SELY 15-20 kts today, then diminish to less than 12 kts
this evening, becoming light and variable Wednesday morning.
Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Dry air will remain in place over the region through
the forecast period. Sustained 20-foot winds will be east-southeast
and at 15-22 mph and some gusts to around 30 mph this afternoon
before diminishing this evening. A few locations may briefly
approach critical fire weather thresholds today, but conditions are
not expected to be widespread, nor long-lived. Expect 20-foot winds
to be less than 15 mph and follow typical diurnal trends the rest of
the work week. Another storm system will track through northern
Arizona Saturday. Although we will remain dry, gusty southwest to
west winds will be present on Saturday.


.PREV DISCUSSION...A Rex block pattern continues across the western
United States, with high pressure aloft centered over the Great
Basin and a closed low over the northern Gulf of California. Both of
these features slowly retrograde west-southwest into Saturday, when
the high will be over eastern Pacific Ocean and the upper low west
of Cabo. A stronger system then dives southeast out of Canada and
into the front range of Colorado by late in the upcoming weekend,
with high pressure aloft again nosing eastward into the southwestern
United States from the eastern Pacific.

Variable amounts of mid/high clouds are expected into Wednesday as
southeast Arizona remains in close proximity to the upper low. There
may even be a sprinkle or two, although more than likely just some
"virga-ish" looking clouds. Main impact of this feature will be to
continue the elevated easterly flow across the area today. Breezy
east-southeast winds are expected this morning and afternoon at 15
to 22 mph and some gusts to around 30 mph, before diminishing this
evening. As the upper low slowly tracks southwest on Wednesday, the
forecast area should see greatly reduced wind speeds and more
typical diurnal patterns.

Otherwise, expect above average temperatures the next 7 days with
afternoon highs topping out at 7-10 degrees above normal. Clear
skies will return late Wednesday and continue into early next week,
with just the possibility of some high clouds over the upcoming
weekend. No sign of winter yet.





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