Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 200943

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
443 AM CDT Wed Sep 20 2017

.SHORT TERM...(Today and tonight)
Issued at 443 AM CDT Wed Sep 20 2017

The line of severe thunderstorms has weakened considerably early
this morning into just a line of showers and scattered
thunderstorms. Some redevelopment may occur along the cold front later
this morning and early afternoon mostly south and east of Eau
Claire before drier air reaches the entire CWA by mid afternoon.
Severe weather is not anticipated.

A surface ridge will pass overhead early this evening. Clear
skies and calm winds into the early overnight hours should allow
for some fog development over western Wisconsin. Believe return
flow will develop around midnight further west and keep fog from
forming there.

.LONG TERM...(Thursday through Tuesday)
Issued at 443 AM CDT Wed Sep 20 2017

Southeast flow will strengthen Thursday. Low level moisture will
return rather quickly with the cold front washing out over
southern Iowa and southern Wisconsin tonight. The daylight hours
should be dry with mid level height rises and the lack of a
trigger, but by Thursday night with a strengthening LLJ and
deeper low level moisture establishing itself locally, some
elevated storms may fire. Capping should limit the overall areal
coverage of activity. Believe the best chance for convection will
be north of a line from Willmar MN to Rice Lake WI. GFS continues
to be too widespread with development, although I suppose some
ACCAS isn`t out of the realm of possibility. Given MUCAPE in
excess of 2500-3000 J/kg and enough wind shear, could see a few
severe storms capable of large hail across central MN and
northwest WI. The warm front lifts north early Friday morning
with any convection focusing more across northern MN and Lake

Friday still looks quite hot and humid. Thicknesses reaching 580
DM, and thermal ridging characterized by 850 mb temps between
21-24C and 925 mb temps of 25-28C with deep mixing should send
temperatures into the lower 90s over much of the area (possibly
middle 90s in spots) if we mix as deeply as forecast soundings
indicate. Given unusually high pwats, this deep mixing will likely
not result in a decline of afternoon dew points and if anything
they may continue to rise with continued moisture advection. Some
dew points may rise into the lower 70s by peak heating with heat
indices reaching the mid to upper 90s. We may need to eventually
issue a Heat Advisory. Highs were inched upward again given latest
guidance, but are still a couple degrees shy of records. Lows
Friday will likely set new records at MSP, STC, and EAU in the low

Capping will hang on over much of the area into Friday night,
except it should break over western MN Friday afternoon or early
evening. Scattered to numerous thunderstorms are likely to develop
as the front approaches. A very unstable atmosphere in conjunction
with impressive wind shear will lead to a heightened risk of
severe weather, with all modes possible. As the SPC Day 3 notes,
shear parallel to the boundary may limit the tornado threat, but
will likely increase coverage of convection on the boundary.

This front has slowed on much of the guidance for the weekend,
resulting in a slight shift westward with the heavy rain threat
and an increase in temperatures further east. This isn`t too
surprising given the ridge to the east and possible interactions
with Hurricane Maria. Nonetheless, it still appears to be a very
good setup for excessive rainfall from west central to northeast
MN with widespread totals of 3 to 6 inches possible. By Monday
the front will finally begin making decent progress eastward.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1124 PM CDT Tue Sep 19 2017

Main issue is timing of convection later this evening into the
early morning hours. An organized line of thunderstorms is
expected to move into western MN around 03z-04z tonight then
gradually progress east, impacting all terminals. Northern
terminals have a better shot at dropping to IFR levels, mainly for
visibility in the thunderstorms but also possibly for ceilings,
while southern terminals are better poised to remain in MVFR
levels although IFR cannot be ruled out. TSRA activity should be
complete around 12z tomorrow morning with gradual clearing,
resulting in VFR conditions by midday for all sites. Other
significant issue will be winds, both at the surface and slightly
aloft. Strong jetting will make for winds at the surface to
persist at speeds around 15G25kt through the overnight hours.
Aloft, winds of around 50 kt can be expected close to 2 kft later
this evening through the overnight hours. The speeds aloft will
diminish some close to daybreak but these strong winds aloft may
still be a problem for departing aircraft in the morning.

KMSP...Clouds will continue to fill in the mid-levels this evening
then conditions will deteriorate quickly with the arrival of the
line of thunderstorms. Timing may be a bit tricky so radar will
need to be monitored to see if timing needs to be adjusted. Brief
heavy downpours and frequent lightning can be expected so this may
impact ground operations if not solely flight ops. MVFR ceilings
are then expected to linger thru the morning push then improvement
to VFR is expected by midday.

Thu...VFR. Winds SE 10-15 kts.
Fri...VFR. Winds S 10-15 kts.
Sat...Mainly VFR. Chance MVFR/TSRA. Winds SW 5-10 kts.




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