Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 261702

Area Forecast Discussion
National Weather Service Tucson AZ
1000 AM MST Thu Apr 26 2018

.SYNOPSIS...Dry conditions with unseasonably warm daytime
temperatures today. Expect isolated to scattered showers and
thunderstorms east of Tucson Friday into Saturday. Dry conditions
will return Sunday and continue across much of the area into the
middle of next week. However, a cooling trend will also occur with
gusty winds at times, especially Monday and Tuesday.


.DISCUSSION...The main focus will be on fire weather concerns as
the dry lightning potential increases tomorrow and Saturday. This
will be followed by a windy spell Sunday afternoon into Tuesday as a
relatively dry storm system impacts the region. The additional
sections in this product have a wealth of information on the
upcoming weather changes. Please refer to them for more details.
That said, the current forecast looked on track so no updates
necessary this morning.


.AVIATION...Valid through 27/06Z.
SCT-BKN clouds AOA 20k ft AGL becoming BKN-OVC clouds above 20k ft
AGL by late this morning. FEW-SCT clouds at 10-15k ft AGL developing
this evening with isolated -TSRA/-SHRA possible mainly near the New
Mexico state line by daybreak Friday. Surface wind less than 10 kts
and variable in direction this morning and again late tonight.
Surface wind this afternoon and evening SWLY/WLY at 7-12 kts with
some gusts to near 20 kts. Aviation discussion not updated for TAF


.FIRE WEATHER...Dry conditions through tonight. There will then be
sufficient moisture for isolated to scattered mainly afternoon and
evening thunderstorms east of Tucson Friday and Saturday.

The main concern Friday into Saturday is thunderstorms capable of
starting fires and producing strong outflow winds, especially in the
higher terrain where development is more likely. The potential
exists Friday and Saturday for an LAL 6 event across far eastern
portions of the forecast area. This will be highly dependent upon
moisture availability. That is, will moisture levels be sufficient
to generate wetting rains with the storms that develop? At this
time, wetting rains appear unlikely.

Dry conditions will then prevail Sunday into Tuesday. A slight
chance of shower exists next Wednesday across the White Mountains.

20-foot winds of less than 15 mph will occur today. Gusty east to
southeast winds are expected from Tucson eastward Friday morning.
There is the potential for gusty thunderstorm outflow winds Friday
afternoon into Saturday. Gusty south to southwest winds during the
afternoon and evening hours will prevail Sunday through Tuesday.
Critical fire conditions may return Monday and Tuesday, particularly
from Tucson eastward.


.PREV DISCUSSION...A highly amplified upper pattern prevailed over
the CONUS and eastern Pacific early this morning. A deep upper low
was centered west of the northern California coast vicinity
38N/135W, and deep low pressure was also centered over eastern
Arkansas. Meanwhile, upper ridge axis extended from southern
California northward into the Pacific Northwest.

Precip-free conditions will occur across southeast Arizona through
tonight as the upper ridge axis moves eastward into western New
Mexico. However, mid and high level clouds are expected to increase
today into tonight, with partly cloudy to mostly cloudy skies across
much of the area by daybreak Friday. As the upper ridge axis moves
into western New Mexico, lower level flow by Friday morning will
become increasingly ely/sely, especially east of Tucson. This lower
level flow regime should translate into increased lower level
moisture primarily east of Tucson.

Based on various 26/00Z model solutions, have continued with a
slight chance of showers and thunderstorms mainly near the New
Mexico state line Friday morning. Isolated to scattered coverage of
showers/thunderstorms exists Friday afternoon and evening east of
Tucson. With this forecast issuance and based on the various 26/00Z
models, have opted to remove earlier depictions of a slight chance
of showers/tstms for much of the Tucson metro area.

However, have maintained a slight chance of showers/tstms Friday
afternoon/evening for the Catalina/Rincon Mountains just northeast
and east of Tucson. The 26/00Z Univ of AZ WRF-NAM and WRF-GFS were
most notable at depicting these mountains for potential shower/tstm
initiation Friday afternoon. Isolated to scattered coverage of
showers/tstms should also occur Saturday generally east of a Mount
Graham (just southwest of Safford) to Bisbee line. A slight chance
of showers/tstms continues mainly across the White Mountains
Saturday evening, then dry conditions area-wide late Saturday night.

By midday Sunday, the deterministic 26/00Z GFS/ECMWF and their
respective ensembles were quite similar with depicting a dry wly/
swly flow aloft regime across this forecast area. The models then
depict a tightening mid-level pressure gradient over the area Monday
into Tuesday as low pressure aloft deepens over the Great Basin and
southwestern CONUS.

At this time, appears that precip-free conditions will continue
Monday into Tuesday. However, the main impacts will be gusty mainly
southwest winds especially during the afternoon and evening hours.
These winds in combination with low relative humidity may create
critical fire conditions. Please refer to the Fire Weather section
in this product for more information. A cooling trend that begins
Sunday will continue Monday into Tuesday.

Thereafter, the GFS/ECMWF were nearly identical by midday next
Wednesday with depicting an upper low to be centered over northeast
Arizona. These nearly identical solutions is fairly impressive given
next Wednesday is Day 7 of this forecast package. At any rate,
although the bulk of precip at this time associated with this upper
low should be north of this forecast area, a slight chance of
showers exists next Wednesday across the eastern mountains, and
mainly across the White Mountains. Further cooling next Wednesday
would also translate into high temps that would be several degrees
below normal.





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