Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 251644

Area Forecast Discussion
National Weather Service Tucson AZ
944 AM MST Sat Feb 25 2017

.SYNOPSIS...Dry conditions through tonight followed by a few showers
mainly north of Tucson Sunday. A Pacific weather system will bring a
slight chance of showers Monday, then a good chance of valley rain
and mountain snow showers by Tuesday. Below average daytime
temperatures will continue into Tuesday before dry conditions and a
strong warming trend occurs late next week.


.DISCUSSION...Clear skies across Southeast Arizona, with a dry
zonal flow in between systems. Precipitable water under 1/5 of an
inch on the 12z KTWC sounding, and surface dew points in the teens
across most of the area. The below average temperatures left
behind by the last system will moderate a bit over the next 48
hours. Still looking at our next significant weather maker Monday
night into Tuesday. Please see the previous discussion below for
additional details.


.AVIATION...Valid through 26/18Z.
Clear skies this morning, then increasing clouds mainly above 15k
ft AGL this afternoon into Sunday morning. Surface wind this
afternoon and early evening wly/nwly 10-16 kts with gusts of 20-25
kts. The strongest speeds should be in vicinity of KSAD. Surface
wind variable in direction less than 10 kts at other times.
Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Dry conditions through tonight followed by isolated
to scattered showers mainly northeast of Tucson Sunday. Dry
conditions will prevail most sections Sunday night into Monday
morning. A Pacific weather system will then bring a slight chance of
showers Monday afternoon, then a good chance of valley rain and
mountain snow showers by Tuesday. Dry conditions will prevail
Wednesday into next Friday. The most significant winds will occur
Sunday and Tuesday, with the strongest gusts expected southeast of
Tucson. Markedly less wind is on tap Wednesday before some gusty
east winds occur Thursday and next Friday.


.PREV DISCUSSION...IR/Water vapor satellite imagery depicted a
cyclonic circulation representing a mid/upper level low pressure
system centered just west of the northern California coast early
this morning. This upper low is progged via the 25/00Z
GFS/ECMWF/CMC to weaken and become a positive-tilted open wave
while moving southeastward into southern California Sunday
morning. This positive- tilted mid-level open wave will then move
eastward across southeast Arizona Sunday afternoon.

Dry conditions will continue across the area ahead of this system
through tonight, and likely remaining dry across much of the area
Sunday morning. Expect clear skies across much of the area this
morning, then mainly cirriform clouds should increase from west-to-
east by this evening. Cloud bases should continue to lower to
around mountaintop level by Sunday morning.

There is a slight chance of valley rain and mountain snow showers
Sunday afternoon mainly from Tucson northeastward to the White
Mountains, where chance-category PoPs prevail. Given the dry near-
surface regime, liquid amounts should mostly be just a few
hundredths of an inch, and mainly sprinkles or virga should dominate
at lower elevations. Dry conditions will then occur across much of
the area Sunday night into Monday morning.

The broad scale upper trough is progged to deepen over the Great
Basin and California Monday night, then move eastward across Arizona
Tuesday. This system will promote deeper moisture to move into
southeast Arizona. Have noted that the 24/12Z GFSX MOS PoP for
Tucson Tuesday and Tuesday night was just 11-14 percent; but, the
25/00Z GFSX MOS PoP for Tucson Tuesday increased to 78 percent.
Meanwhile, the 25/00Z ECMX MOS PoP for Tucson Tuesday is 58 percent.
At any rate, based on these solutions and guidance as well as
coordination with neighboring WFO`s, PoPs were raised especially for
late Monday night and Tuesday.

Thus, a slight chance of showers exists Monday afternoon, then a
chance of valley rain/mountain snow showers occurs particularly west-
to-north of Tucson Monday night. There is a good chance of valley
rain/mountain snow showers across much of the area Tuesday. Snow
levels Tuesday are forecast to be near 6000 feet across the White
Mountains to around 7000-7500 feet southeast of Tucson. Preliminary
storm total liquid amounts range from 0.10"-0.50" area-wide with
snow total accumulations of 2-5 inches above 7000 feet.

A few lingering showers may occur mainly near the New Mexico border
Tuesday evening before dry conditions prevail by Wednesday morning.
The GFS/ECMWF were similar with depicting a weak upper low to
develop southwest of the area by next Friday. However, the progged
moisture fields are very dry, and so precip-free conditions should
continue Thursday into next Friday.

Below normal daytime temps will prevail through Tuesday, and for
some locales into next Wednesday. A strong warming trend with high
temps about 5-10 degrees above normal is expected by next Friday.






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