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Area forecast discussion 
National Weather Service Tucson Arizona
815 am MST Thursday Sep 3 2015

Synopsis...there will be a good chance of showers and thunderstorms
across the area today and Friday. Some thunderstorms will generate
strong winds and heavy rain. Isolated to scattered showers and
thunderstorms are then expected over the weekend and into early next
week...mainly favoring areas from Tucson east and south with lesser
chances west of Tucson.


Discussion...water vapor imagery shows moisture plume streaming
northeast from west of Baja California/Tropical Storm Kevin across the eastern
half of the forecast area. 15z dewpoints were in the lower 60s
across much of the area...with the exception over far western Pima
County...where values were generally in the 50s. The 12z ktwc
sounding shows plenty of available moisture to work with...with a precipitable water
value of 1.51 inches. Kemx radar already indicating large area of
showers (mainly less than 45dbz) extending from Sunizona. A few lightning strikes were also noted over
far eastern Santa Cruz County in the vicinity of Elgin. This area of
precipitation was moving to the northeast at around 10 miles per hour.

Numerical guidance values continue to highlight high pop potential
across southern Arizona today/tonight. WV imagery shows two weak
impulses embedded within the southwest of Nogales across
northern Sonora...with the current precipitation across southern
Arizona on the northern edge of this feature. The other impulse was
across southern Sonora...with more impressive thunderstorm
development already occurring on the northern/eastern flank of this
feature. Ongoing forecast today for more clouds...cooler high
temperatures and increased thunderstorm threat on track. No updates.
For more information on the latter periods of the forecast...please
refer to the previous discussion section below.


Aviation...valid through 04/12z.
Isolated -tsra/-shra this morning. Scattered to numerous -tsra/-shra
will then occur across much of the area this afternoon and evening.
MVFR conditions and wind gusts of 35-45 kts will occur with the
stronger thunderstorms and rain. Otherwise...expect layered clouds mostly between 8-
15k feet above ground level and surface wind generally less than 10 kts into tonight.
Aviation discussion not updated for taf amendments.


Fire weather...a good chance of thunderstorms today and Friday.
Isolated to scattered thunderstorms will then occur over the
weekend. The least activity will occur across fire zone 150 west to
northwest of Tucson. Brief strong...gusty and erratic winds will
accompany the stronger thunderstorms. Another potential increase in
storm activity Monday...followed by a gradual drying trend leading
to less thunderstorm activity by next Wednesday. Otherwise...expect
terrain driven winds mostly less than 15 miles per hour into next week.


Previous discussion...Friday looks like a solid average thunderstorm
day as large scale features remain in place and the southwesterly
fetch from the vicinity of Kevin will keep moisture coming in and
any potential weak impulses in the flow.

The flow is weaker over the weekend and we should see more of a
westerly flow as the mean trough position shifts deeper into the
Continental U.S. North of our area. By this point Kevin will still be southwest
of the area but considerably weaker moving into an unfavorable shear
zone. Still some storms around but less than what we expect for
today and Friday (pending any more impulses in the flow that we
aren't picking up yet).

GFS and European model (ecmwf) are both picking up on a stronger inverted trough
rotating around the southern and western periphery of the high
(still to our east and southeast)...and into our area Monday. We'll
push the forecast back into a solid average thunderstorm day for
that. After that the ridge shifts westward into a blocking position
south of US while the larger scale mean trough position dominates
the country north and northeast of US. That would tend to shut
things down around Tuesday or Wednesday. It's September so we'll
continue to keep an eye on the tropics. There's already another area
of disorganized cloudiness and showers located several hundred miles
south of Manzanillo to watch for any potential impacts next week.


Twc watches/warnings/advisories...none.



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