Special Weather Statement, NWS Tucson, AZ

Area Forecast Discussion
National Weather Service Tucson AZ
925 AM MST Wed Jul 19 2017

.SYNOPSIS...Scattered to numerous afternoon and evening thunderstorms
will prevail through this weekend. The greatest thunderstorm
coverage should occur east to south of Tucson with lesser
thunderstorm activity west of Tucson. Below normal daytime
temperatures will also prevail into the weekend.

&&

.DISCUSSION...The large scale pattern shows high pressure aloft
centered over the southern plains across the Oklahoma/Texas
panhandle area, with southeast Arizona under southeast flow on the
back side of the high. The morning KTWC 170719/12Z sounding still
indicates ample moisture in place with a PWAT value of 1.73". Brisk
southeast steering/mid level flow noted as well on the morning RAOB,
with a consistent 20-25 knots between 600 mb and 350 mb. Satellite
imagery shows considerable clouds across the southern half of the
forecast area, especially Santa Cruz/Pima counties, with partly
cloudy skies observed over the northern half of the CWA. Water vapor
and Blended Total Precip Water imagery both noted a pocket of drier
air pushing westward across New Mexico attm, with PW values between
0.70" and 1 inch. Surface dewpoints were generally in the mid to
upper 60s across southern Arizona at 16Z. The KEMX radar indicated a
few showers over central Pima county this morning, with this area
expected to push north of the CWA in the next hour or so.

The models continue to show an active day across southern Arizona
today, with convection starting between 18Z-19Z generally along the
Rim and to the south/southeast of Tucson. With southeast steering
flow, storms will move to the northwest at 20-25 mph. The models
indicate the storms will move into the Tucson metro area mainly
between the mid afternoon and early evening hours, with the storms
continuing north/northwest into the Phoenix area later in the
evening. The drier air noted earlier pushing in from the east is
progged to overspread northeastern portions of the forecast area and
may limit some late day convection over portions of the Greenlee and
northern Graham counties, although thinking is that storms will fire
early in the day before drier air reaches this area. In addition,
the western edge of this boundary may also serve as a trigger for
storms to fire along today (dry line scenario) as it moves into the
eastern part of the forecast area. The main threats from the storms
today will again be heavy rainfall/strong wind potential. However,
the heavy rainfall potential will be mitigated by the rather fast
storm motion.

Overall, inherited forecast looks to be in good shape. Did make some
minor pop adjustments for higher mountain pops this afternoon and
bumped up pops this evening over Pinal county for expected storms
pushing north/northwest this evening.

&&

.AVIATION...Valid thru 20/12Z.
Scattered to numerous TSRA/SHRA developing again late this morning
into the afternoon. Otherwise, VFR conditions with cloud decks
generally 5k-10k ft AGL with BKN layers abv. SFC wind variable in
direction mainly less than 12 kts except vicinity TSRA where gusts
to 45 kts possible. Aviation discussion not updated for TAF
amendments.

&&

.FIRE WEATHER...Scattered to numerous mainly afternoon and evening
thunderstorms will continue the rest of the work week 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 a
bit 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.

&&

.PREV DISCUSSION...Scattered showers and thunderstorms were
occurring at this time from near Safford northeastward into portions
of the White Mountains. The showers and thunderstorms were moving
northwestward around 10-15 mph. Otherwise, mostly clear skies
prevailed across far southeastern sections with partly cloudy to
cloudy skies elsewhere.

The upper level pattern early this morning was characterized with a
trough near the Pacific Northwest coast, and strong high pressure
centered over southwest Kansas. A deep ely/sely flow regime
prevailed across southeast Arizona.

There were some initialization errors with the 19/00Z Univ of AZ WRF-
NAM and WRF-GFS as well as the 19/06Z Univ of AZ WRF-NAM (e.g.
aforementioned precip area not depicted valid present time; or,
model precip ongoing from Tucson southward into northern Sonora).
The 19/09Z HRRR initialized the ongoing precip cluster better.
At any rate, the high resolution models were similar regarding
precip potential later today and into tonight.

A gradual clearing trend from southeast-to-northwest should continue
during the next several hours as per satellite trends, then showers/
tstms are expected to develop especially near mountains late this
morning and into early this afternoon. The Univ of AZ WRF-NAM and
WRF-GFS remained fairly consistent versus their respective previous
solutions with the prospect for enhanced shower/tstm potential for
the Tucson metro area this afternoon into early this evening. This
scenario is quite similar to the SREF that depicted an axis of
highest PoPs this afternoon and evening to extend from Santa Cruz
County northward across the Tucson metro area, then arcing
northeastward toward the White Mountains.

The official forecast has a slight chance of showers/tstms area-wide
this morning, then generally 50-70 percent chance of showers/tstms
this afternoon across central/eastern sections (i.e. Tucson
eastward/ southward). Isolated to scattered showers/tstms this
evening, with the lowest PoPs across western Pima County. A slight
chance of showers/tstms continues area-wide again late tonight.

A quick glance at the 19/06Z NAM12 was quite interesting in that
this solution depicts very large rainfall amounts potentially may
occur tonight just south of this forecast area over northern Sonora.
This model may be hinting at another robust MCS to be near, or over,
this forecast area tonight.

The 19/00Z deterministic GFS/ECMWF depict a ely/sely mid-level flow
to continue Thursday, then both models and particularly the ECMWF
depict a weak upper low to be over far southeastern sections Friday
and continue into Saturday. This weak upper low or inverted upper
trough will likely enhance precip chances across eastern sections,
and only isolated coverage of showers/tstms should occur across
western Pima County.

Thereafter, the official forecast Sunday into Monday continues with
scattered to numerous afternoon/evening showers/tstms from Tucson
eastward, and isolated showers/tstms across western sections. The
ECMWF was generally more robust with moisture fields and model
generated rain amounts and coverage versus the GFS. A downturn in
showers/tstms may occur next Tuesday as the deterministic GFS/
ECMWF/CMC depict a more southwesterly mid-level flow regime to
occur. Thus, PoPs Tuesday are generally 10-20 percent lower versus
Monday, but isolated to scattered showers/tstms should still exist.

Below normal daytime temperatures will prevail for much of southeast
Arizona at least through this weekend, then a warming trend may
occur by the middle of next week if a drier regime materializes.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...None.

&&

DISCUSSION...Zell
PREV DISCUSSION...Francis

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Special Weather Statement
National Weather Service Tucson AZ
1222 PM MST WED JUL 19 2017

AZZ509-510-192015-
Upper Gila River and Aravaipa Valleys including Clifton/Safford-White
Mountains of Graham and Greenlee Counties including Hannagan Meadow-
1222 PM MST WED JUL 19 2017

...SIGNIFICANT WEATHER ADVISORY FOR CENTRAL GREENLEE COUNTY UNTIL 115
PM MST...

At 1222 PM MST, Doppler radar was tracking a strong thunderstorm 10
miles east of Clifton, or 24 miles west of Buckhorn, moving west at
10 mph.

Half inch hail and wind gusts up to 50 mph will be possible with this
storm.

Locations impacted include...
Clifton, Morenci and Guthrie.

LAT...LON 3280 10928 3303 10947 3307 10914 3292 10904
      3289 10905
TIME...MOT...LOC 1922Z 078DEG 11KT 3297 10912

Leins

Special Weather Statement
National Weather Service Tucson AZ
155 PM MST WED JUL 19 2017

AZZ507-513-192200-
Upper San Pedro River Valley including Sierra
Vista/Benson-Dragoon/Mule/Huachuca and Santa Rita Mountains including
Bisbee/Canelo Hills/Madera Canyon-
155 PM MST WED JUL 19 2017

...SIGNIFICANT WEATHER ADVISORY FOR SOUTHWESTERN COCHISE AND
NORTHEASTERN SANTA CRUZ COUNTIES UNTIL 300 PM MST...

At 155 PM MST, Doppler radar was tracking a strong thunderstorm over
Huachuca City, or near Sierra Vista, moving northwest at 25 mph.

Wind gusts up to 50 mph will be possible with this storm.

Locations impacted include...
Sierra Vista, Benson, Huachuca City, Sierra Vista Southeast, Fort
Huachuca, Charleston, Whetstone and Elgin.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

&&

LAT...LON 3159 11008 3148 11022 3175 11063 3193 11046
TIME...MOT...LOC 2055Z 129DEG 22KT 3165 11031

LEINS