Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 212009

Area Forecast Discussion
National Weather Service Tucson AZ
110 PM MST Fri Jul 21 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. A
less favorable pattern may temporarily bring reduced thunderstorm
coverage around the middle of next week.


Scattered showers and storms continue to fire in the higher terrain
locations east of Tucson. After quite an early start to the
convection, things have tempered a bit in terms of intensity. This
is temporary however as much of the lower cloud cover across the
area has eroded and new (more vigorous convection) is forming across
the Chiricahua Mtns as well as along the Mogollon Rim. Tucson has
been right on the edge of storms so far and whether that trend
continues through the rest of the afternoon is the big challenge of
the day. Most runs of the HRRR and latest UA WRFs suggest convection
will continue to move into the area from the north. Even though most
of the organized storms will be along the AZ/NM border where a
remnant circulation can still be seen on satellite and where mid
level lapse rates are steeper, nearly all hi-res models depict
enough areal coverage that I feel comfortable hanging onto a decent
mention of PoPs around town through mid evening. Boundary collisions
and even the slightest bit of ascent have been all we`ve needed the
past few days and not much has changed thermodynamically across the
area today. Given the fact that we`re already seeing isolated storms
propagating off the Catalina Mtns, I think higher than normal PoPs
are still more than warranted in this case. CAPE is much more
marginal as one heads west and I think most locations across western
Pima Co will be dry.

Similar moisture and stability indices are forecast across the
region on Saturday with a bit more of an easterly component to the
flow. Storms will likely form by late morning east of Tucson and
attempt to descend into the deserts. They`ll be fighting a bit of a
battle as additional dry air (not to mention warmer air aloft) will
have moved into the western half of the state by then and how far
storms make it before they dissipate is up for debate among the more
reliable operational models. Looks like another decent shot at rain
in Tucson, with much lower chances to the west. Confidence starts to
go downhill on Sunday as the day starts off looking like a typical
late morning through early evening convective event, but the past
few deterministic GFS/NAM/ECMWF all indicate a weak inverted trough
will move through the area late Sunday night into Monday morning.
Could end up being another double shot of storms like we had a few
days ago (last Saturday) with precip lingering into early Monday
morning. I ended up raising PoPs a bit during this time period but
confidence is always low this time of year given so many variables
at play.

Big picture-wise, it looks like daily storm chances will be the rule
through much of next week. There is some suggestion in the guidance
that low/mid level steering flow will take on a bit of a
southwesterly component by the middle/end of the week which will
usher in enough dry air to put the brakes on storms from Tucson
westward. I agree with the idea of a slight reduction in storm
chances, especially from Tucson to Ajo, although there will still be
strong/moist flow across much of Sonora and I don`t see much in the
way of a large scale pattern change that would prevent that from
advecting into AZ. PoPs gradually return to a climo-like appearance
by the end of the week but that`s certainly subject to change
depending on how much convection forms over Sonora next week.


.AVIATION...Valid thru 23/00Z.
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. SCT TSRA developing after 21/18Z and lingering through
22/06Z. 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 early next week with locally heavy
rain and strong winds. Temperatures will remain below normal with a
continuation of elevated humidity levels through Tuesday. Wednesday
onward conditions will become a bit drier and warmer with somewhat
less convection. From 20-foot winds will mainly remain below 15 mph
with the exception of strong outflow winds in the vicinity of






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