Dust Devil in Arizona, April 27, 2018

Area Forecast Discussion
National Weather Service Tucson AZ
942 AM MST Fri Apr 27 2018

.SYNOPSIS...There is a slight chance of thunderstorms east of
Tucson this afternoon through Saturday, primarily over the
mountains. Then dry conditions will return Sunday and continue across
much of the area into Tuesday with a chance of showers northern areas
Tuesday night into Thursday. It will be a cooler week with gusty
winds at times, especially Monday and Tuesday.

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.DISCUSSION...Skies have cleared across most of southeast Arizona
with the exception of far eastern sections where satellite imagery
showed some cumulus clouds starting to form. The morning upper-air
sounding at Tucson showed a noticeable increase in mid-level
moisture during the past 24 hours. This moisture increase combined
with daytime heating should translate to a few showers and
thunderstorms today, mainly in the mountains. The sub-cloud layer
remained rather dry despite the increase in moisture so still
looking at the potential for dry lightning. That said, the current
forecast looked on track so no updates anticipated this morning.

&&

.AVIATION...Valid through 28/18Z.
SCT-BKN clouds at 9-13k ft AGL developing this afternoon east of
KTUS and FEW-SCT clouds at 10-15k ft AGL developing west of KTUS.
Isold -TSRA/-SHRA possible from KTUS eastward mainly aft 27/18Z.
KSAD, KDUG and KALK will have elevated ELY/SELY SFC winds at 14-20
kts and higher gusts between until 27/20Z. Otherwise, SFC winds will
generally be SELY/SLY at 7-12 kts. Aviation discussion not updated
for TAF amendments.

&&

.FIRE WEATHER...The main concern through Saturday will be isolated
to scattered thunderstorms capable of starting fires and producing
strong outflow winds, especially in the higher terrain east of
Tucson where development is more likely. There will be pockets of
LAL 6 across eastern portions of the forecast area today and
Saturday. This basically means that where thunderstorms chances
exist, the majority of these storms will produce lightning and gusty
winds, but there will not be sufficient moisture to produce wetting
rains.

Dry conditions will then prevail Sunday into Tuesday. A slight
chance of showers then exists next Tuesday night into Thursday areas
north of Tucson.

Gusty east to southeast winds are expected from Tucson eastward
this morning. There is also the potential for gusty thunderstorm
outflow winds this afternoon, tonight and Saturday. Gusty south to
southwest winds will prevail during the afternoon and evening hours
Sunday through Tuesday. Critical fire weather conditions may return
Monday, particularly east of Tucson. Critical fire weather
conditions then are possible areawide next Tuesday.

&&

.PREV DISCUSSION...We remain under the waning influence of a dirty,
split ridge today with the axis to our east and the flow aloft
slightly diffluent. Closer to the surface a cool front was sliding
to the west and shortly before 2 am it was crossing the AZ/NM
border. This boundary is expected to continue pushing to the west
today ending up across eastern Pinal and Pima counties later this
afternoon. This boundary aiding and combined with weak upper level
forcing, fairly steep mid-level lapse rates and some mid-level
moisture brings about a bit of a threat of thunderstorms later today
through Saturday evening. This threat will be east of the surface
boundary, basically east of Tucson. With the dry low levels and
limited mid-level moisture, the storms will be primarily dry with a
bit of rain possible over higher terrain the next 24 hrs and with a
touch more moisture Saturday maybe a few showers in the lower
elevations. Either way, not enough moisture to help our situation,
but concern for lightning and strong winds on dry fuels is addressed
further below in the fire weather section. Temperatures this
afternoon will be very similar to yesterday west of Benson and
cooler to the east thanks to the cooler air behind the front. Then
Saturday, will have a degree or two of cooling west of Benson and
some warming to the east as the cool air erodes.

Sunday will be dry across the area with temperatures dropping back a
few degrees as heights begin to drop in response to a deep upper
trough over the Pacific NW beginning to move east and south. As the
trough continues to deepen winds will increase on Monday areawide
with a chance for patchy blowing dust eastern areas and afternoon
temperatures peaking cooler than average for the day.

Models are in good agreement with respect to a significant system
dropping down the backside of the mean trough and swinging it across
the area Tuesday into Thursday. On the front side of this system
Tuesday we are expecting stronger winds with patchy blowing dust and
some fire weather concerns for the afternoon with temperatures
cooling further. By Tuesday night enough moisture is advected across
AZ in advance of the trough axis that I raised pops for areas north
of Tucson Tuesday night into Wednesday night with some lingering
pops into Thursday across the White Mountains. This system will be
quite cool with the coolest afternoon temperatures occurring
Wednesday with highs 5-15 degrees cooler than average for the day.
Beyond that dry weather will return into the following weekend with
temperatures rebounding.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...None.

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