Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 202035

Area Forecast Discussion
National Weather Service Tucson AZ
134 PM MST Thu 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.


Slow moving convection continues to blossom over the mountain peaks
of southern and eastern Arizona this afternoon, but have had little
to no luck descending into the surrounding valleys and deserts. KEMX
VAD wind profile paints a very similar picture as what was seen on
this morning`s KTUS sounding - very light/erratic winds at 10 kts or
less all the way up to 30kft. Most storms that have formed have run
through their entire life cycle in about an hour or less which is
about par for the course in an environment like this.

Convection for the rest of the day will be very disorganized and
mainly confined to the mountains. PoPs in these areas have already
been elevated into the 80-90 percent range and I see no reason to
drop those values. Desert PoPs, including Tucson`s, shouldn`t be
eliminated altogether since intersecting outflows could result in
rapid storm development virtually anywhere. Low confidence forecast
overall for the rest of the evening/overnight all due to weak wind

Tomorrow will be very similar to what`s been seen today...i.e. slow
moving storms that are primarily upslope driven. Only difference is
a bit more drying is forecast across western Pima Co and it`s
entirely likely that locations from Sells to Ajo could conceivably
be dry tomorrow. Elsewhere it`ll be much of the same in terms of
areal coverage.

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.


.AVIATION...Valid thru 22/00Z.
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.






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