Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Tucson Arizona
815 PM MST Wednesday Dec 11 2013
Synopsis...low pressure will bring a chance of valley rain and
mountain snow to Southeast Arizona Thursday into early
Friday...although most precipitation should be east of Tucson.
After slightly cooler temperatures Thursday and Friday...expect a
warmup and dry conditions from this weekend into early next week.
Discussion...upper low currently centered just west of the northern
Baja California/Southern California border. 03z satellite imagery shows clear
skies across the western and northern portions of Arizona (west and
north of Tucson attm) and a deepening moisture plume stretching from
the southern half of Sonora...northeast across southern New Mexico
and into far West Texas. Precipitable water values of more than an inch are noted
across the southern half of Sonora Mexico in this plume.
As the upper low begins to eject eastward later tonight...this plume
will be pulled further northward across the eastern half of the County Warning Area.
With middle/high clouds already across far southeastern portions of the
forecast area (mainly southeast of a kols...to ktus...to ksad line)
have made adjustments to sky cover grids and zone text products for
increased cloud cover this evening and especially overnight. And
with the increased cloud cover...have also bumped up overnight low
temperatures 1-4 degrees over southeastern portions of the County Warning Area...
southeast of a line from Nogales to Safford.
For more information on the latter periods of the forecast please
refer to the previous discussion section below.
Aviation...sky clear conditions west of a line from ktus to ksad through
12/09z...with middle/high clouds on the increase east of this line this
evening and especially overnight. Scattered-broken clouds at 6-10k feet above ground level and
broken-overcast clouds at or above 15k feet above ground level expected by 12/12z. Isolated-scattered valley
rain showers and mountain shsn developing Thursday morning...mainly east of
a line from kcgz...to ktus...to kols. Rain showers could be accompanied by
ts from ksad-kdug late Thursday. Surface wind generally less than 10 kts
through 12/23z. Aviation discussion not updated for taf amendments.
Fire weather...a low pressure system will move across the area...
resulting in a chance of valley rain and mountain snow showers from
Tucson eastward Thursday through early Friday. Expect near normal
daytime temperatures Thursday through Sunday. Afterward...dry
conditions and above normal temperatures are expected early to middle
next week. Overall...winds will generally be on the light side with
the passage of this system through the upcoming weekend...with
sustained winds mainly less than 15 miles per hour.
Previous discussion...low pressure off the upper Baja California coast still
expected to move across Arizona Thursday and Thursday night. The low
pressure area should generate enough lift to generate light
precipitation. Added moisture availability from the southern
stream being drawn northward should add to coverage and amounts.
Therefore have raised probability of precipitation across the board although confidence from
Tucson westward is still fairly low.
While most precipitation amounts will be fairly light the potential
for heavier amounts exists mainly in the mountains of Graham and
Greenlee County where up to a half inch of rain at the middle slopes
and a few inches of snow above 8000 feet are possible. These heavier
amounts could be accompanied by thunder as models advertising
sufficient middle level instability late Thursday. Inserted slight
chance for thunder in accordance with Storm Prediction Center
Timingwise...southern stream moisture is likely to work northward
into border area of Southeast Arizona by dawn with a slight chance of showers in
the Douglas area. Expect most of the rain...and mountain snow above
8000 feet...to fall from middle morning through Thursday evening but
precipitation will linger Thursday night...and possibly into Friday as a
northern stream trough follows behind the initial low pressure.
After a mild day today...a bit of a cool down can be expected with
the next couple of days until this low and a secondary northern
stream Low Pass. By Sunday high temperatures should be back to above
normal as high pressure dominates into midweek before the next
trough advances from the northwest.
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