Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 210843

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
343 AM CDT Fri Jul 21 2017

.SHORT TERM...(Today and tonight)
Issued at 343 AM CDT Fri Jul 21 2017

The next 24 hours will continue to be active as thunderstorms with
heavy rain and severe weather are expected to move across Minnesota
and Wisconsin again later today and tonight. The Storm Prediction
Center has an enhanced risk of severe weather for most of the area,
and the Weather Prediction Center has a moderate risk of heavy rain
across southeast Minnesota and southwest Wisconsin. Lastly, heat
indices could approach 100F across southern Minnesota if skies
manage to clear behind this morning`s convection and high
temperatures reach the lower 90s with dewpoints in the mid to upper
70s. Looking ahead, storms should exit the region tonight setting
the stage for a drier couple of days to start the week.

As for timing, the 21.03 HRRR Experimental and most members of the
21.00 NCAR ensemble bring showers and thunderstorms across the
southern part of MN into western WI this morning, and then develop
the main line of severe convection across eastern North Dakota and
drive the storm southeast during the evening hours. This solution
seems very plausible, and trended toward that timing with the
hourly pops. The precipitation should be over behind this line of

The magnitude of the severe weather threat will likely to be dependent
on the morning convection and its impact on the instability ahead
of the aforementioned trailing line of thunderstorms. As was the
case Wednesday, morning convection could limit the severe
potential for the rest of the day if the atmosphere isn`t able to
recover. The current thinking is this morning`s convection will
not significantly inhibit the northward advancement of the warm
front and we should manage to develop a few thousand J/kg of
MUCAPE by late afternoon given the mid/upper 70s dewpoints that
are forecast to be in place across the region along and south of
the warm front. Deep layer shear should be around 40 to 50 kts
given the veered wind profile associated with the warm air
advection. Low level helicity values will be sufficient for quick
spinup tornadoes if a QLCS does manage to develop as some of the
models indicate.

In summary, no single solution is capturing this event. Feel that
the warm air advection precip is underdone by some of the models,
and the timing may be too slow on the other models. Overall expect
morning precip currently over the NE/SD border to move into MN/WI,
and then have some recovery behind it in time for a forward
propagating line of storms to move across Minnesota this evening and
into western Wisconsin. Should see some elevated storms develop
ahead of this line, and these would pose the flood threat if the
manage to train over the same area for a couple of hours.

.LONG TERM...(Saturday through Thursday)
Issued at 343 AM CDT Fri Jul 21 2017

An upper low will descend southeastward from Manitoba to Lake
Superior Saturday and Saturday night. A belt of 40-50 kt mid level
westerlies will overspread the region during the afternoon, but
poor mid level lapse rates will limit the degree of instability
despite lingering 70 degree surface dew points. Despite these
factors, a few thunderstorms could develop over western Wisconsin
where the deeper moisture will reside. The meager instability will
limit the severe threat overall, but the degree of shear could
organize a storm or two enough to produce a large hail threat
given the dry air aloft.

Cooler and drier air will follow for Saturday night into Monday.
Could be a bit chilly Sunday night with lows possibly in the 40s
in some areas of central MN and WI.

The warm, humid, and very wet pattern is looking to make a
comeback mid to late week. A potent system tracking east across
central Canada will send a front southward into the Upper
Mississippi Valley Tuesday, which will eventually stall as forcing
shifts away from the region. Meanwhile, the ridge will make an
attempt to build back east into the Missouri Valley, with an
active MCS zone setting up nearby. Best severe threat appears
Tuesday and Tuesday night while the strongest mid level flow is
overhead, due to the short wave trough passing to the north.
Biggest threat next week is again heavy rainfall totals with
repeated rounds of convection into Thursday.


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1104 PM CDT Thu Jul 20 2017

High clouds will eventually stream in from the west/south
overnight. In the meantime clear skies, light winds, and low level
moisture will aid in fog development overnight, particularly from
central MN into west central WI. Expect occasional/patchy
visibility reductions to IFR, and possibly even LIFR at KRNH and
KEAU. The next area of showers and thunderstorms is expected to
enter southwest MN around daybreak, then spread toward central
MN/WI during the morning hours. MVFR conditions attendant with the
shra/ts are possible. More development will be possible in the
afternoon/eve. Variable winds under 10 knots are expected
overnight, then winds increase from the southeast on Friday.

Could see 4-5sm visibility reductions with patchy fog around the
area overnight through daybreak. Showers in the vicinity of the
field still look to be possible around 15z, with stronger activity
possible after 20z into the evening hours.

Sat...Mainly VFR. Chc MVFR in -SHRA/TSRA. Winds lgt and vrbl.
Sun...VFR. Wind NW 10 kts.
Mon...VFR. Wind ESE 5 kts.




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