Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 160410

Area Forecast Discussion
National Weather Service Tucson AZ
910 PM MST Sun Apr 15 2018

.SYNOPSIS...A storm system moving north of the area will bring gusty
winds with patchy blowing dust Monday into Tuesday. Brief cooling
Tuesday followed by warmer temperatures Wednesday. Gusty winds and
patchy blowing dust will return Thursday into Friday as an even
stronger system moves through the state. Cooler temperatures behind
this system Friday, then less wind and a warming trend next weekend.


.DISCUSSION...IR satellite imagery depicted partly cloudy to cloudy
skies from western Pima County northeastward to the White Mountains.
Meanwhile, clear skies to scattered cirriform clouds prevailed from
Tucson vicinity eastward/southward into Cochise/Santa Cruz Counties.
A bone-dry regime was ongoing area-wide, and lower elevation dew
points valid 9 pm MST ranged from minus 3-5 degrees to the positive
single digits. 16/00Z KTWC sounding total precip water value was a
very dry 0.19 inch. Light to moderate westerly flow was observed
above 700 mb.

The aforementioned clouds were well ahead of a very deep upper
trough that was approaching the Washington/Oregon/northern
California coasts. This deep upper trough was depicted nicely via
IR/water vapor satellite imagery. IR satellite imagery trends as
well as 16/00Z NAM12/GFS moisture fields suggest that mainly
cirriform clouds will gradually increase from west-to-east across
southeast Arizona the rest of tonight. Some mid-level but mostly
cirriform clouds will continue to increase Monday as the upper
trough moves eastward into the Pacific Northwest/Great Basin.

High temps Monday will mainly be within a few degs of temps achieved
today; a few degs increase versus this afternoon for some locales,
and a few degs decline for other locations. Monday`s highs will be
about 5-10 degs above seasonal normals. The main impact from the
approaching upper trough Monday will be gusty mostly southwest to
west winds in the afternoon and evening. Although wind speeds are
not expected to attain Wind Advisory levels, the very dry surface
environment will contribute toward patchy blowing dust. Even more
significant is that critical fire weather conditions will occur
across all of this forecast area. A Red Flag Warning is in effect
Monday for southeast Arizona from 11 am to 8 pm MST.

The official forecast was updated for the rest of tonight for
increased sky values. Otherwise, please refer to the additional
sections for further detail.


.AVIATION...Valid through 17/06Z.
Scattered to broken clouds the rest of tonight above 20k ft AGL,
then few to scattered clouds around 15k ft AGL and broken to
overcast clouds above 20k ft AGL Monday into Monday evening. Surface
wind into early Monday morning mainly less than 12 kts. Surface wind
15Z-17Z Monday and continuing into Monday evening swly/wly 12-25 kts
with gusts near 35 kts. Surface wind swly to nwly diminishing to 5-
15 kts by end of forecast period. Aviation discussion not updated
for TAF amendments.


.FIRE WEATHER...A Red Flag Warning is in effect for all of
southeast Arizona, including Fire Weather Zones 150, 151, 152 and
153 on Monday from 11 am MST until 8 pm MST. A Fire Weather Watch is
also in effect from Tuesday morning through Tuesday evening for Fire
Weather Zones 152, 153 and the southeast portion of Fire Zone 151.

The first of a pair of storm systems will generate gusty winds
areawide Monday and across eastern zones Tuesday. Minimum RHs will
be in the single digits at all elevations. Wednesday will be a
relatively quiet day with light winds but continued very low
humidity levels.

The second system will affect Arizona Thursday into Friday. This
system has the potential to be nearly as strong as the one last
Thursday with RH levels well below 15% during the afternoon hours.
Critical fire weather conditions are likely area wide Thursday then
possibly across the eastern zones on Friday.

Next weekend will be quiet with light winds and low, but closer to
average humidity levels.


.PREV DISCUSSION /240 PM MST/...Satellite imagery showed a band of
cirrus encroaching on the area from the north and west. Area
temperatures early this afternoon were running 10 to 15 or so
degrees warmer than 24 hours ago under a ridge of high pressure.
Temperatures should top out 8 to 12 degrees above normal this
afternoon for much of the region. In contrast, area dewpoints were
in the single digits to below zero in some spots, running up to 8
degrees lower compared to 24 hours ago. This is making the relative
humidity in many spots less than 5 percent.

The ridge axis will pass to the east of the area this evening. High
clouds will then increase as upper level flow becomes southwesterly
into Monday. An upper trough approaching from the northwest will
help tighten the surface pressure gradient, kicking up gusty
southwest winds. With extremely dry air in place at the surface,
widespread critical fire weather conditions are likely to occur for
much of the day Monday, and a Red Flag Warning is in effect. On
Tuesday, the aforementioned trough will begin to pass north of the
area. Enough of a gradient should continue across eastern areas for
another round of red flag conditions, and a Fire Weather Watch has
been issued for Tuesday afternoon. More details can be found in the
Fire Weather section.

Wednesday will be a quiet day in between weather systems with light
westerly flow aloft. Another upper trough will then move toward
southeast Arizona on Thursday. The 15/12Z GFS is has trended toward
a deeper system with MEX MOS surface wind guidance trending upward
Thursday into Friday. At this time, it looks like precip will remain
north of this CWA, but winds will gust out of the west or southwest
on Thursday with red flag conditions again looking likely. The
closed low pushes through the state on Friday with winds veering to
the northwest. All that said, this system is starting to look eerily
similar to last week`s wind and fire weather event (Thurs/Fri 4/12-
4/13), so we will certainly be keeping an eye on it in the coming

Thereafter, look for high pressure to attempt to regain position
over the southwest. A shortwave trough may attempt to undercut the
high and disrupt this pattern next weekend. Plenty in the works this
week to iron out before then, though.


  Red Flag Warning from 11 AM to 8 PM MST Monday for AZZ150>153.

  Fire Weather Watch from Tuesday morning through Tuesday evening
   for AZZ151>153.




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