Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 141042

Area Forecast Discussion
National Weather Service Tucson AZ
340 AM MST Thur Dec 14 2017

.SYNOPSIS...A fast moving trough will push through the Four Corners
today, bringing an increase in clouds, some breeziness and a slight
chance of showers to far southeast Arizona. Sunny and dry conditions
will then occur through Saturday. Another system may affect the area
by Sunday, with a slight chance of valley rain and mountain snow
showers. Cooler temperatures will accompany this system and persist
into early next week.


.DISCUSSION...IR satellite imagery indicated clear skies
across southeast Arizona this morning, though clouds associated with
a fast moving trough were streaming across the northeast portion of
the state. A lack of deeper moisture will generally mean just an
increase in cloud cover for our area as the trough progresses
southward. That said, cannot totally rule out a stray sprinkle or
shower across far southeast portions of Cochise County this
afternoon and evening as the system attempts to draw additional
moisture northward toward the states once it`s made it to Mexico.
Temperatures this afternoon will be noticeably cooler compared to
Wednesday due to cloud cover and falling heights.

Temperatures remain near average through Saturday as the above-
mentioned trough moves east into Texas and we watch another
potential weather maker pinch off from the mean flow along the
California coast. As a result, both the GFS and ECMWF develop a
closed low somewhere off the western coast of the Baja Saturday
afternoon and evening. At the same time, the main trough from which
this low split off, is progged to dive south through the Great Basin
towards the Four Corners region. There has been a slight improvement
in the agreement between the 14/00Z operational GFS and ECMWF with
how these systems evolve, but considerable differences still exist.
The million dollar questions is whether these disturbances will
remain independent of each other, or if the "Four Corners trough"
will subsequently re-absorb the "Baja Low" over/near Arizona. With
the thought that either outcome will increase moisture levels in
conjunction with some favorable upper level dynamics, went ahead and
increased PoPs Sunday and Sunday night, but kept them in the slight
chance category.

Thankfully, model solutions return to agreement Monday night as
this(these) system(s) depart the region. Cooler NW flow will become
zonal, then give way to slight ridging for Tuesday and Wednesday as
temperatures rebound back to slightly above average values. We`ll
then watch for another trough to dig out of the Pac NW by the end of
next week, but that is currently beyond the purview of this forecast
package, as there`s plenty to speculate on between now and then.


.AVIATION...Valid thru 15/12Z.
An increase in clouds will occur this afternoon leading to FEW-SCT
clouds at 10k-14k ft AGL mainly along a line from KSAD to KOLS.
Isol`d -SHRA possible near KDUG this afternoon and evening as a weak
trough passes through. SFC winds will follow diurnal trends ahead of
the trough today before increasing out of the NE-SE at 12-15 kts
after 15/03Z. Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Except for a possible shower in the far
southeast corner of the state this afternoon as a weak trough passes
by, expect dry conditions through Saturday. Winds ahead of the
trough will increase out of the northeast-southeast late this
evening into Friday morning for eastern locales and especially along
ridgetops. Another system will pass through Sunday into Monday
bringing with it near- or below-normal temperatures, increased wind
speeds and the potential for a more widespread chance of
precipitation. Drier air and warmer temperatures will then return
through the remainder of the forecast period.






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