Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 201020

Area Forecast Discussion
National Weather Service Tucson AZ
320 AM MST Fri Apr 20 2018

.SYNOPSIS...A strong low pressure system moving north of the area
will bring gusty winds into this evening. Cooler temperatures will
also occur today with a few showers possible across the White
Mountains late this afternoon and evening. A strong warming trend
will then prevail Saturday into Monday with above normal
temperatures continuing into the middle of next week.


.DISCUSSION...Clear skies across southeast Arizona early this
morning except for a few lower level clouds across the White
Mountains. These clouds were associated with a strong low pressure
system that was centered over southeast Utah. A very tight mid-level
pressure gradient was around the southern periphery of this low
pressure system. Surface winds at lower elevations valid 3 am MST
were generally from the west to northwest at 5-15 mph. However,
gusts from the northwest to near 30 mph were ongoing at Safford.

The upper low will move eastward into south-central Colorado by this
evening. A series of vort lobes rotating around the backside of this
system will bring partly cloudy to perhaps mostly cloudy skies to
the White Mountains this afternoon and evening. Fairly decent height
falls may induce a few rain/snow showers across the White Mountains
late this afternoon and evening. Clear skies or just a few
cumuloform clouds elsewhere today.

Otherwise, the main impact from the system will be cooler
temperatures today as well as gusty west to northwest winds into
this evening. The strongest winds will be east of Tucson, but wind
speeds are forecast to remain below Wind Advisory criteria. Winds
will diminish shortly after sunset this evening with the loss of
daytime heating and as the pressure gradient loosens across the area.

The focus of attention Saturday will turn southwestward toward a
broad area of weak low pressure aloft centered west of northern Baja
California. Clear skies across much of the area Saturday morning,
then mainly cirriform clouds to increase especially across southern
sections Saturday afternoon and evening. Moisture mainly in the 700-
500 mb layer is progged via the deterministic 20/00Z GFS/ECMWF to
increase Sunday.

These solutions produce at least a few hundredths of an inch of
rainfall to occur near the international border adjacent Cochise
County Sunday afternoon and evening. However, the 20/00Z NAM12
depicted precip-free conditions to occur as did the 20/00Z Univ of
AZ WRF-NAM. The 20/00Z Univ of AZ WRF-GFS was faster with measurable
rainfall potential, and suggests that measurable rain may occur just
southeast of this forecast area around daybreak Sunday. Have noted
that WPC keeps measurable rainfall southeast of the area or across
northern Chihuahua in the 24 hour period ending 12Z Monday. At any
rate, for this forecast issuance have maintained single-digit PoPs
Sunday, though would not be surprised if virga actually occurs.

The models depict upper ridge amplification over the area Monday,
then southwesterly flow aloft Tuesday into Wednesday in response to
a low pressure system west of the California coast. These solutions
depict another upper ridge to strengthen over the area next
Thursday. Expect dry conditions to continue, and this pattern will
also likely translate into relatively light winds for the latter
part of April.

After cooler temps today, a strong warming trend will occur Saturday
into Monday followed by only very minor daily temp changes Tuesday
and Wednesday. High temps will be several degrees above normal
Sunday and are forecast to be about 10 degrees or so above normal
Monday into mid-week.


.AVIATION...Valid through 21/12Z.
Scattered to broken clouds at 6k-10k ft AGL with isolated -SHRA/
-SHSN across the White Mtns northeast of KSAD this afternoon and
evening. Otherwise, mostly clear skies. Winds will be variable less
than 12 kts through mid-morning then begin to pick up. This
afternoon will be gusty again with winds out of the northwest at 14-
22 kts with gusts up to 30 kts. The highest wind speeds will be at
KSAD and KDUG. Winds will then diminish starting a little after
sunset. Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Winds will pick up again this afternoon, but not as
strong as yesterday, and they are not expected to reach widespread
Red Flag levels. Mainly east of Tucson, 15-20 mph with occasional
higher gusts. Winds then become predominantly light and terrain
driven over the weekend and beyond with the usual afternoon gusts.
No precipitation is expected aside from a few showers in the White
Mountains late this afternoon and evening.






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