Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 180341

Area Forecast Discussion
National Weather Service Tucson AZ
841 PM MST Mon Jul 17 2017

.SYNOPSIS...Conditions will remain favorable for periods of
widespread showers and thunderstorms into Wednesday. Some drying
in the west may limit thunderstorm development in western areas
late in the week, while eastern areas remain active. A return of a
more favorable pattern should push storms deeper into the state
once again next weekend.


.DISCUSSION...The 00Z KTWC sounding isn`t as worked over as I
expected. Saturated at 1.84 inches precipitable water with a
modest mid level easterly flow and respectable elevated
convective potential. How to get at it? Mid-level water vapor
imagery is hinting at a weakness in the flow near far southwest
New Mexico. The latest GFS rotates this feature into southeast
Arizona in the early morning hours and subsequently (along with
the downscaled NAM), keeps a 30-40 percent chance of valley
showers with isolated embedded thunderstorms overnight. It`s a
very subtle feature, but for now we`re going to keep the forecast
going as is with the chance of storms redeveloping in the early
morning hours.

Please see the previous discussion below for details beyond


.AVIATION...Valid thru 19/06Z.
Isolated to scattered TSRA/SHRA continuing overnight. Isolated to
scattered -TSRA/-SHRA will re- developing mainly east-to-south of
KTUS Tuesday morning, with widespread TSRA/SHRA expected areawide
Tuesday afternoon and evening. 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...Widespread showers and thunderstorms will prevail
through Tuesday. Thereafter, the greatest daily coverage of
thunderstorms Wednesday into this upcoming weekend is expected to
generally occur east to south of Tucson. Enough moisture will remain
for isolated coverage of thunderstorms west of Tucson. Below normal
daytime temperatures will also continue into the weekend.


.PREV DISCUSSION...A busy afternoon with just about any
thunderstorm of substance dropping flash-flood level rains. Deep
moisture remains in place across the area with precipitable water
values 1.5 to 1.7 inches and surface dew points in the mid to
upper 60s. Each day we continue to fight through convective
inhibition left over from overnight activity, and each day the
flow is enough, in combination with the influence of our
persistent inverted trough, to reset the atmosphere relatively
quickly. The afternoon is going about as expected with initial
development in the partially clear areas well away from the
morning MCV over southwest Arizona. Several areas of flash
flooding this afternoon with rainfall amounts between 1 and 3
inches over large areas.

With a little less activity expected upstream in New Mexico, we
might actually have an overnight period without many storms,
however with this much moisture in the atmosphere we extended the
Flash Flood Watch until 5 AM just in case.

We can`t sustain this much monsoon activity forever. As long as we
don`t get clobbered tonight, tomorrow should be above average as
well. We will keep a favorable easterly to southerly flow with
high pressure shifting into the central plains, but our long fetch
of deep moisture may slow a little with less focus. Inverted
trough trying to set up again through Sonora later in the week
into next weekend. That, and we will remain open to any easterly
impulse that comes along.


Flash Flood Watch until 5 AM MST Tuesday for AZZ501>515.




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