Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 190929

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
429 AM CDT TUE JUL 19 2016

.SHORT TERM...(Today and tonight)
Issued at 400 AM CDT Tue Jul 19 2016

A rather active short term period lies ahead with a couple rounds of
convection expected.

The first round, now in Nebraska, is associated with the first of
two short waves lifting northeast around the building ridge centered
over the central Plains. This cluster of showers and storms have
emitted a large outflow boundary which has since developed
additional convection over southeastern South Dakota, around the
Yankton area. With the ridge building north and the moisture
advection beginning to focus over eastern South Dakota and far
southwestern Minnesota, believe additional storms will develop in
those areas during the next few hours and shift east across southern
Minnesota through the early afternoon before diving back southeast
on the east side of the ridge. Pops were kept low over Wisconsin
through the evening, but were raised to about 50 percent across
Minnesota. Further adjustments will be needed as convection
develops. Don`t anticipate these storms becoming severe with modest
dew points keeping CAPE values low, but pwats increasing to
1.5 inches could bring locally heavy rain.

Of greater concern is the second round tonight. A warm front will
work east into Minnesota as another mid level short wave and
developing LLJ push overhead as well. General mass fields and
conceptual model would point to portions of east central MN and
southeast as a zone for thunderstorm activity developing mid to late
evening, persisting for much of the overnight hours before the LLJ
veers easterly toward dawn. Pwats increasing to 2 to 2.25 inches and
a warm cloud layer nearly 13,000 ft deep will strongly favor heavy
rainfall rates of 2 to 3 inches per hour. Believe the GFS and ECMWF
show this potential the best of all models, with the CAMs struggling
a bit which isn`t unusual for this range. They may be impacted by
low level capping. The WRFNMM does show a narrow corridor of
torrential rain along the Mississippi River (7 inch maxima) which
matches well with the GFS/EC although a bit further east. After
extensive coordination with WPC and the surrounding WFOs, think the
potential is high enough to justify a Flash Flood Watch for portions
of eastern Minnesota and far western Wisconsin. Given the expected
rates, it won`t take long for several inches to accumulate in spots
and it only takes about 1.5 inches within an hour to produce flash
flooding in urban areas.

.LONG TERM...(Wednesday through Monday)
Issued at 400 AM CDT Tue Jul 19 2016

Heat wave remains on track beginning tomorrow. Overnight
convection will likely linger into tomorrow mainly in eastern MN
and western WI, but during the afternoon clearing skies and
increasing temperatures and dew points will lead to heat index
values well over 100 degrees across western MN through the Twin
Cities Metro. This will be the beginning of a heatwave that will
last through Friday, with the worst of it being on Thursday. Made
little change to the going forecast for temperatures and dew
points. While the actual air temperatures may not be extreme for
heat wave standards, its the extremely high dew points that really
push the heat indices up in the 100 to 110 degree range. Dew
points tomorrow will only be exacerbated with the morning
convection. Continued widespread upper 70s for dew points, but
those may actually be too low tomorrow afternoon across part of
the area. Both the GFS and ECMWF indicate a pool of surface dew
points in excess of 80 degrees both tomorrow and Thursday. Another
big concern with this event is how warm the overnight lows will
be. We could be flirting with record warm overnight lows both
Wednesday night and Thursday night in the Twin Cities, which are
both in the upper 70s. So, the night time will provide little
relief from the heat. This is a dangerous situation for those with
no air conditioning.

Decided to issue the Excessive Heat Warning for much of western MN
through the metro, these areas will be dangerously hot and humid
tomorrow afternoon. Decided to hold off farther east as heat
indices won`t be quite as warm tomorrow, especially in WI due to
lingering precipitation chances and certainly cloud cover. Those
areas will likely need a Warning for Thursday. Unfortunately, the
latest guidance slowed down the cold front for the weekend, which
means Saturday will be warm and humid with temps in the upper 80s
and dew points around 70, but heat indices won`t be quite as high as
tomorrow through Friday. The GFS and EC now bring the cold front
through from late Saturday night through Sunday morning. Dew points
will fall to more comfortable levels by Sunday.

In terms of precipitation through the long term period, we`ll
start out tomorrow dealing with thunderstorms in eastern MN and
western WI from the activity left over from the overnight period.
Once that precip moves out, the thermal ridge moving in should
effectively cap the atmosphere, keeping the precip to our north
where temperatures aloft aren`t quite as warm.  The next good precip
chance will come along and ahead of the cold front this weekend.
Timing on when that moves through is difficult to nail down at this


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1055 PM CDT Mon Jul 18 2016

VFR conditions are expected overnight, with Tuesday bringing an
increase in broken mid level clouds as a warm front lifts toward
the area. Showers and thunderstorms associated with the warm front
may hold together enough to reach KRWF around or shortly after 12z
Tues before waning, but the activity is expected to stay south of
the rest of the TAF sites. Winds will be nearly calm to southeast
under 5 kts for the overnight hours, and then increase to 10-15
kts from the southeast by Tuesday afternoon (with gusts to between
20 and 25 kts) at western sites (KAXN-KRWF and KSTC).

Mostly clear skies overnight with nearly calm winds. Mid level
clouds will be on the increase by Tuesday afternoon and southeast
winds increase to 12kts as well.

Wed...VFR. Chc -TSRA. Wind S at 10g20kt.
Thu...VFR. Wind SSW at 5-10kts.
Fri...VFR Chc -TSRA late. Wind SE 5 kt.


.MPX Watches/Warnings/Advisories...
WI...Flash Flood Watch from this evening through Wednesday morning
     for WIZ023-024-026.

     Excessive Heat Watch from Thursday morning through Friday
     evening for WIZ014>016-023>028.

MN...Excessive Heat Warning from 1 PM Wednesday to 7 PM CDT Friday
     for MNZ047>049-051-054>070-073>076-082-083.

     Flash Flood Watch from this evening through Wednesday morning
     for MNZ051>053-059>063-066>070-075>078-083>085-092-093.

     Excessive Heat Watch from Wednesday afternoon through Friday
     evening for MNZ041>045-050-052-053-077-078-084-085-091>093.



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