Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 212349

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
649 PM CDT WED SEP 21 2016

.SHORT TERM...(This evening through Thursday)
Issued at 326 PM CDT Wed Sep 21 2016

The next 12 hours will feature widespread showers and
thunderstorms across a large portion of Minnesota and western
Wisconsin. While a few straight-line wind gusts and a weak tornado
are not impossible, the two greater threats this evening are
heavy rain with the potential for flash flooding and hail between
1-2" in diameter.

You don`t have to be a meteorology major to find the main warm
front this afternoon in Minnesota. North of a line from roughly
Granite Falls, to the southern Twin Cities metro, to Eau Claire
there is overcast skies temperatures in the 60s and northwest
surface winds. South of this line, the winds are southerly and
temperatures are in the 70s and 80s with dewpoints in the 70s. The
instability gradient is stretched out along this line with max
values near 3500-4000 J/kg (SBCAPE and MLCAPE) in far south
central MN and near 0 J/kg in the far northern TC metro. Good
elevated instability and steep mid-level lapse rates extends well
north of the surface boundary. In addition to the building
instability, the 925-850mb moisture transport and warm advection
have increased quite a bit over the past couple hours. The FGEN
in this area has also increased since mid afternoon. The result
has been the expansion of thunder along or north of the boundary.
Storms have been efficient rainfall producers with rain rates
between 2-3". The storms have also been efficient hail producers
as well, not too much of a surprise given the instability in the
hail growth zone, shear profile, and due to the fact that the
surface temperatures are in the 60s along and north of the
boundary. We expect the radar scope to continue to expand in
coverage this evening. The placement of the Flash Flood Watch is
still looking good, although some of the convective allowing
models are developing convection back farther to the southwest.
Though the HRRR has admittedly struggled with the short term
trends. Settle in, its going to be a busy night for weather in
MN/WI over the next 12 hours. There will likely be elevated hail
and heavy rain north of the front that move east northeast, but
there will also likely be a broken line of storms that form along
the boundary and sag south with time.

.LONG TERM...(Thursday night through Wednesday)
Issued at 326 PM CDT Wed Sep 21 2016

Highly amplified flow in the extended will evolve into a cut off low
which is a pattern that has high uncertainty. For that reason and
given the high impact weather expected in the next 24 hours, did not
stray from blended guidance in the extended period. There`s a few
highlights below.

The warm front responsible for the heavy rain will be laid out
somewhere along I-90. Storms should continue to Thursday night. On
Friday the warm front will lift northward and showers and
thunderstorm chances will be tied to the front. There will be warm
and dry conditions in the warm sector, but this will be short-lived
as a cold front moves in Saturday and brings a few showers and

Beyond this period, the upper level wave will cut off and bring a
prolonged period of clouds and light rain. As mentioned above, there
is uncertainty in this pattern, so would not be surprised if the
forecast changes. The GFS is a bit quicker with the cutoff low, but
the ECMWF catches up by the middle of next week.


.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 649 PM CDT Wed Sep 21 2016

Multiple segments of strong TSRA will occur this evening across
eastern and southern MN and west central WI. KRWF and KAXN will be
on the western/northern fringe of the activity with the Twin
Cities, KRNH and KEAU in the heart of it. Torrential downpours,
small hail and wind gusts to 35 knots will accompany many of the
storms. During the evening a new line of storms may develop across
southern MN and sag into northern IA which would be primarily
south of the TAF sites. Ceilings and visibilities will be a
problem tonight and Thursday with the TAF sites located north of a
stationary front. Expect mainly MVFR ceilings through the period
with bouts of IFR in the storms as well as late tonight/early
Thursday morning. Visibilities will deteriorate in the storms but
generally become MVFR overnight in BR/FG. Mainly ENE winds 10
knots or less tonight and Thursday, except variable and stronger
in storms.

KMSP...Main concern out the gate is the west to east line of
storms just north of the airfield. The back end of the storms in
Wright county have been trending slowly eastward but the concern
is that the line could bow southward with strong NNW winds (35
knots) impacting the airfield. Ceilings and visibilities will be
highly variable tonights going from VFR to IFR/LIFR. Overall,
expect ceilings to settle to low MVFR overnight and continue much
of Thursday.

Fri...MVFR/SHRA possible. Wind E 5-15kts.
Sat...VFR with MVFR/TSRA likely. Wind SE at 10g15kts.
Sun...VFR. Wind W-SW 10g20kts.


.MPX Watches/Warnings/Advisories...
WI...Flash Flood Watch through Thursday evening for WIZ024-026-028.

     Flash Flood Watch through Thursday morning for WIZ014>016-023-

MN...Flash Flood Watch through Thursday evening for MNZ067-069-070-

     Flash Flood Watch through Thursday morning for MNZ051>053-



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