Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 181655 AAA
AFDTWC

Area Forecast Discussion...updated
National Weather Service Tucson AZ
955 AM MST Tue Jul 18 2017

...Updated watch/warning/advisory line for issuance of the Flash
Flood Watch from 1 pm MST this afternoon through 2 am MST
Wednesday...

.SYNOPSIS...Conditions will remain favorable for widespread showers
and thunderstorms into Wednesday. Some drying may then reduce
thunderstorm coverage west of Tucson Thursday into Friday, but
locales east of Tucson will remain active. Widespread showers and
thunderstorms should return by early next week. Expect below normal
daytime temperatures for much of the area into early next week.

&&

.DISCUSSION...The 15Z visible satellite imagery shows considerable
cloudiness west of a Casa Grande to Tucson to Nogales line, with
relatively clear skies over much of the eastern portion of the
forecast area, except along the Mogollon Rim/White Mountains. Water
vapor imagery indicates a weak disturbance pushing west across
central/western New Mexico attm. The KTWC 170718/12Z sounding came
in a with a very juicy 1.86 PWAT value which is near record levels.
The morning sounding also indicates a very unstable atmosphere with
unmodified CAPE of ~1500J/kg and LI`s of -4. Surface dewpoint
readings were in the mid to upper 60s. Still a few weak echos noted
on the KEMX radar over portions of far western Pima county in the
Ajo vicinity.

The GFS/ECMWF model runs/guidance values, along with the AZ WRF-NAM
model all depict a rather active day, with the first cells over the
higher terrain and along the Rim around 19Z. Numerous to widespread
convection is then expected during the afternoon/evening hours
before winding down from east to west late tonight. Given the
disturbance moving in from the east, high PW values, unstable
atmosphere and clear skies over eastern portions of the CWA, feel
model runs have a very good handle on weather scenario today.
Therefore, have bumped up pops rather significantly (10-20 percent)
across the eastern two-thirds of the forecast area this afternoon
and western two-thirds of CWA this evening (except far western Pima
county). Tucson metro area now sits with a solid 60 pop this
afternoon/evening, with higher values for the surrounding mountains.
The main threat from the storms today will be very heavy rainfall
potential/flash flooding, although will also have a wet microburst
potential as well. The storms will move to the west at 10-15 mph
today.

Updated grid/text package with higher pops out shortly.

&&

.AVIATION...Valid thru 19/12Z.
Scattered -TSRA/-SHRA developing KTUS vicinity east and south late
this morning, then scattered to numerous TSRA/SHRA this afternoon
into this evening. Expect -TSRA/-SHRA mainly west of KTUS late
tonight into early Wednesday morning. Otherwise, VFR conditions with
cloud decks generally 5k-10k ft AGL, and surface wind variable in
direction mainly less than 12 kts. Aviation discussion not updated
for TAF amendments.

&&

.FIRE WEATHER...Expect fairly widespread coverage of showers and
thunderstorm into Wednesday. Isolated to scattered mainly afternoon
and evening thunderstorms will then prevail Thursday into Friday. A
favorable pattern for more widespread coverage of showers and
thunderstorms is expected to return by early next week. Otherwise,
20-foot winds will mainly remain below 15 mph with below normal high
temperatures continuing into this weekend.

&&

.PREV DISCUSSION...IR satellite imagery depicted warming cloud tops
associated with a decaying MCS over south-central Arizona and over
portions of this forecast west-to-northwest of Tucson at this time.
Isolated showers and thunderstorms were moving westward across
western Pima County. Otherwise, mostly clear skies were noted east
of Tucson with mostly cloudy to cloudy skies elsewhere. Based on
satellite/radar trends as well as various high resolution models,
expect these showers/tstms to dissipate or move west of this
forecast area by 7-8 a.m. MST or so.

Several HRRR solutions as well as the 18/06Z Univ of AZ WRF-NAM were
in good agreement with the depiction of showers/tstms to redevelop
later this morning generally from the Tucson metro area eastward to
the New Mexico border. This is a similar scenario that occurred
Monday. At any rate, fairly widespread coverage of showers/tstms
should then prevail this afternoon into this evening.

Although widespread flash flooding is not expected to occur, the
ample available moisture should translate into the potential for at
least locally heavy rainfall. A threat also exists for a few
thunderstorms to produce locally damaging wind gusts. As has been
the case the past few nights, showers/tstms should favor locales
west-to-northwest of Tucson late tonight followed by dissipating
showers/ tstms early Wednesday morning. A similar scenario exists
late Wednesday morning into early Thursday morning.

The 18/00Z GFS/ECMWF/CMC and their respective ensembles continued to
depict a generally easterly mid-level flow regime to prevail
Thursday into Friday. These solutions depict the best coverage of
showers/tstms to generally prevail east-to-south of Tucson with
somewhat limited coverage of showers/tstms across western sections.

Thereafter, a pattern conducive to more widespread coverage of
showers/tstms appears in the offing starting Saturday and continuing
into early next week. The GFS and especially ECMWF depict the
presence of a pronounced inverted upper trough to approach from the
southeast this weekend, then become quasi-stationary over the area
next Monday. Thus, the official forecast continues with scattered to
numerous showers/tstms from Tucson eastward/southward Saturday thru
Monday mainly during the afternoon/evening hours; and, isolated to
scattered showers/tstms west of Tucson. Locally heavy rainfall and
brief damaging wind gusts should also occur.

Below normal daytime temperatures will prevail for much of southeast
Arizona into early next week.


.TWC WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch in effect from 1 pm MST this afternoon through 2
am MST Wednesday for AZZ501>515.

&&

$$

DISCUSSION...Zell
PREV DISCUSSION...Francis

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