Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 201610

Area Forecast Discussion
National Weather Service Tucson AZ
909 AM MST Wed Jul 20 2017

.SYNOPSIS...Scattered to numerous afternoon and evening thunderstorms
will prevail into early next week. The greatest daily thunderstorm
coverage should occur east to south of Tucson with reduced
thunderstorm activity across the western deserts. Below normal
daytime temperatures will also prevail into early next week.


Another day of anomalously high atmospheric moisture content is upon
us with today`s 12Z KTUS sounding revealing just shy of 1.7" of
precipitable water earlier this morning. Thermodynamic indices were
relatively similar to those seen on Wednesday`s sounding but the big
difference is the noteworthy decrease in mid level wind speeds. No
longer are we looking at 700-500mb speeds around 25kts, rather mean
winds are closer to 5-10kts today. Not entirely surprising since
nearly all the large scale models indicated a decrease would occur

Water vapor imagery reveals a weak circulation south of the border
(south of Sasabe, AZ) as of 16Z that is slowly drifting northward.
This is more than likely the trigger for today`s storms...assuming
we can clear out some of the lingering low clouds that have been
stubborn to erode. All signs point to clearing later this morning
with a focus of storms mainly from Tucson eastward although with so
much moisture in the area, I can`t rule out a stray shower as far
west as Ajo. Given the weak steering flow, storms will likely be
strongest/most numerous in the mountains with somewhat less coverage
in the deserts. Localized heavy rainfall is certainly a possibility
in the mountains although widespread flooding is much less certain.

Otherwise much of the same as we`ve seen lately in terms of temps
(upper 90s). Forecast grids updated mainly for observational trends
and to increase PoPs in the mountains east of Tucson, otherwise no
noteworthy changes are necessary.


.AVIATION...Valid thru 21/18Z.
Scattered to numerous TSRA/SHRA developing again this afternoon and
continuing well into the evening. Otherwise, VFR conditions with
cloud decks generally 5k-10k ft AGL with BKN layers above, and
surface wind variable in direction mainly less than 12 kts. Winds
vicinity TSRA could gust 40+ kts. Aviation discussion not updated
for TAF amendments.


.FIRE WEATHER...Scattered to numerous mainly afternoon and evening
thunderstorms will continue into Saturday with locally heavy rains.
A bit of an uptick in the number of storms is likely later this
weekend into early next week. Temperatures will remain below normal
with a continuation of elevated humidity levels. 20-foot winds will
mainly remain below 15 mph with the exception of strong outflow
winds in the vicinity of thunderstorms.


Abundant moisture continues across southeast Arizona early this
morning, with strong high pressure aloft centered over western
Oklahoma, and an inverted upper trough over far west Texas and
northeast Chihuahua Mexico. Moderate sely flow in the 700-400 mb
layer as per the 20/00Z KTWC will continue today. The 20/00Z Univ of
AZ WRF-NAM and WRF-GFS were initialized much better versus 24 hours
ago, and were definitely favored into this evening versus several
HRRR solutions that had ongoing initialization issues with respect
to precip.

Expect showers/tstms to develop late this morning mainly near
mountains east of Tucson, then scattered to numerous showers/tstms
should occur this afternoon and evening especially from Tucson
eastward to the New Mexico border. There were differences between
the WRF-NAM and WRF-GFS regarding shower/tstm potential for a larger
portion of the Tucson metro area later today.

While both solutions depict showers/tstms to occur near mountains,
particularly the Catalina/Rincon Mountains, the WRF-GFS keeps the
bulk of showers/tstms east-to-south of Tucson. Meanwhile, the WRF-
NAM suggests that outflow will be sufficient to produce showers and
thunderstorms late this afternoon and/or evening across a larger
portion of the Tucson metro area. At any rate, mainly dry conditions
should occur across western Pima County, and the bulk of showers/
tstms will dissipate by daybreak Friday.

The 20/00Z deterministic GFS/ECMWF/CMC were quite similar versus
their respective solutions with depicting a weak upper low or
inverted upper trough to reside over far southeastern sections
Friday into Sunday. This system will likely enhance precip chances
across eastern sections, and the official forecast continues with
scattered to numerous showers/tstms from Tucson eastward during the
afternoon/ evening hours. Adequate moisture warrants a slight chance
of showers/tstms west of Tucson.

The models suggest that nly/nely mid-level steering flow will likely
prevail, especially this weekend. The Univ of AZ WRFs suggest that
storm motions will potentially be more sly/swly as a result. The mid-
level flow is then progged to become sely again Monday. This flow
regime will likely promote a somewhat further westward migration of
showers/tstms Monday afternoon and evening versus this weekend.

Thereafter, there continued to be some differences between the GFS/
ECMWF regarding shower/tstm potential next Tuesday and Wednesday.
The GFS/ECMWF both suggest that shower/tstm coverage will be reduced
versus what has occurred recently, and what is expected to occur
later today through Monday. However, the ECMWF continues to be more
aggressive with this decline in shower/tstm coverage versus the GFS.

This is not to imply in any form that this forecast area will
shutdown the development of showers/tstms completely. However, the
official forecast shows a gradual daily reduction in PoPs Tuesday
into Wednesday. Thus, expect isolated to scattered showers/tstms
from Tucson eastward next Tuesday and Wednesday, and a slight chance
of showers/tstms across the western deserts.

The above normal available moisture will also translate into a
continuation of below normal daytime temperatures likely through
Monday. A modest warming trend with high temperatures returning
closer to seasonal normals is on tap next Tuesday and Wednesday.






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