Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 160420

Area Forecast Discussion
National Weather Service Tucson AZ
920 PM MST Mon Jul 15 2019

.SYNOPSIS...Very hot temperatures across much of the area Tuesday.
Isolated to scattered thunderstorms Tuesday afternoon and evening
mainly east to south of Tucson. A better flow regime will likely
increase thunderstorm coverage Wednesday afternoon into Wednesday
night. Scattered mainly afternoon and evening thunderstorms with
near normal temperatures to prevail Thursday into early next week.


.DISCUSSION...Partly cloudy skies prevail across much of southeast
Arizona at this time. The exception was across the central/southern
portion of the Tohono O`odham Nation where isolated showers were
producing very light rainfall amounts under mostly cloudy skies.
Cloud tops west to southwest of Tucson have been rapidly warming
during the past 1-2 hours. Several HRRR solutions and satellite/
radar trends suggest that any showers will end by midnight. Precip-
free conditions should then prevail area-wide late tonight.

However, several HRRR solutions as well as the 16/00Z NAM12 and the
16/00Z Univ of AZ WRF-NAM suggest that virga or perhaps sprinkles
may develop around or shortly after daybreak Tuesday generally
across central portions of this forecast area. As such, have opted
to depict a slight chance of sprinkles 12Z-18Z Tuesday (Tuesday
morning) generally in an area bounded from Tucson-Sierra Vista-
Nogales-Sells. The chance of measurable rainfall remains under 10
percent. Isolated to scattered showers and thunderstorms should
occur Tuesday afternoon and evening mainly east to south of Tucson.

Very hot temperatures will occur across much of the lower elevations
Tuesday. As such, an Excessive Heat Warning remains in effect until
8 pm MST Tuesday generally northwest of a Safford-Tucson-Sells line.

Please refer to the additional sections for further detail.


.AVIATION...Valid through 17/00Z.
Isolated -TSRA/-SHRA near International border southwest of KTUS
ending by 06Z Tuesday. Isolated -SHRA may occur KTUS vicinity
12Z-18Z Tuesday, but low forecast confidence of this scenario.
Isolated to scattered -TSRA/-SHRA mostly east to south of KTUS
Tuesday afternoon and evening. Brief wind gusts to 40 kts should
occur near stronger TSRA. Otherwise, cloud decks mainly 12k-18k ft
MSL. Aside from aforementioned tstm gusts, surface wind Tuesday
afternoon swly-nwly 10-15 kts with gusts to 20 kts. Surface wind
variable in direction under 10 kts at other times. Aviation
discussion not updated for TAF amendments.


.FIRE WEATHER...Excessive Heat Warning until 8 pm MST Tuesday from
Tucson westward and northwestward, and portions of the Upper Gila
River Valley including Safford. Otherwise, isolated to scattered
mainly afternoon and evening thunderstorms Tuesday especially east
to south of Tucson. Increased moisture and a better flow regime
should enhance thunderstorm coverage Wednesday afternoon into
Wednesday night. Thereafter, scattered mainly afternoon and evening
thunderstorms Thursday into early next week. Expect brief strong,
gusty and erratic winds near thunderstorms. Otherwise, 20-foot winds
will be mainly under 15 mph.


.PREV DISCUSSION /138 PM MST/...Thunderstorms have developed over
the higher terrain in the absence of any forcing and are having a
hard time moving off the terrain due to the very light mean wind
present under the upper level high. This will likely remain the case
through the day until deep enough outflows move into the valleys and
trigger new storms. We are currently at 103 at the airport and are
running neck and neck with this time yesterday which means we look
good for the 106-107 forecast high temperature today.

For Tuesday with ongoing drying due to the upper level high overhead,
convection will be reduced further with plenty of sunshine and heat.
High temperatures will be similar to or a bit warmer than today, the
reason the Heat Warning is in place through Tuesday.

Then Wednesday the high will have shifted far enough east to induce a
southerly flow again which will begin directing moisture back into
the region. The bonus to that is a small tropical wave that will be
directed north roughly up the gulf which should enhance the moisture
later Wednesday into Thursday for plenty of storms, especially south
of Tucson. This will also help to pull afternoon temperatures back
about five degrees from where they are now.

Then beyond that through the weekend varying degrees of monsoon
moisture will be over the region for daily thunderstorm threats which
will be different from one day to the next and temperatures hovering
near seasonal norms.


Excessive Heat Warning until 8 PM MST Tuesday for AZZ501-502-



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