Special Weather Statement, NWS Twin Cities, MN

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
327 PM CDT Tue Jun 13 2017

.SHORT TERM...(This evening through Wednesday)
Issued at 302 PM CDT Tue Jun 13 2017

Potentially very active short term is in store, with severe
weather possible this evening/tonight, followed by what could be
our fourth day in a row of severe weather Wednesday.

We'll start with this afternoon. First, when looking at forecast
shear/instability parameters along with forecast soundings for
today, the first reaction we have been hearing around the office
is wow. The type of environment forecast to be over MN today is
something that we only see maybe a couple of times a decade up
here. 3000-5000 j/kg of MLCAPE, 40-50 kts of deep shear, h85 jet
increasing to between 40 and 50 kts, and effective storm relative
helicity north of the warm front forecast to get up between 300
and 600 m2/s2. That warm front shows up well on the 0.5 degree
reflectivity from MPX just into the far southern Twin Cities metro
area and by most accounts, it is forecast to get up to or a
little north of I-94.

So that leads to the next question of what's a miss to keep us
from having a major severe weather outbreak? There are three
problems. First, there is strong subsidence in the wake of the
this mornings MCS as evidence by the rapidly clearing skies and
wake low across northern MN and rising h5 heights across the
entire area. Second, we have a very warm EML overhead, with a
pronounced warm nose and capping inversion noted on forecast
soundings at h7, with temps at this level around 14C. This warm
air around h7 also shows up well on 7.34 micron GOES-16 low level
water vapor imagery, with very warm values coming up out of the
central plains. The other problem deals with the orientation of
our shear and horizontal vorticity vectors, which is pretty much
parallel to predicted storm motion. The shear vectors parallel to
storm motion diminishes the tornado threat, as this favors rapid
upscale growth to linear systems, while discrete cells end up
ingesting primarily cross-wise as opposed to stream-wise
vorticity.

SPC did nudge their 10% tornado probability east toward St. Cloud,
which was likely in response to the HRRR this morning, which had
been developing discrete cells this afternoon ahead of the warm
front in west central MN. However, since 17z, the HRRR has
abandoned that idea and instead has joined the rest of the CAMS in
saying the capping wins out today, with the MPX area awaiting the
arrival of a line of thunderstorms out of Dakotas, that will form
in response to falling heights and a weakening cap associated
with a short wave currently working out of southeast Wyoming.

For tonight, current expectations is that the greatest severe
threat will reside across the northern half of the MPX coverage
area, with storm coverage across the southern half of the area
more questionable. Moisture transport on the RAP at h85 shows the
low level jet splitting as it work across MN, with one enhanced
area going up across northeast MN and another area coming up out
of KS into central IA. The 18z HRRR follows this forcing pretty
close taking one MCS northeast across western MN toward Lake
Superior, with it's south end swinging through the Twin Cities,
while a second MCS moves into southern IA associated with the
southern LLJ enhancement.

Of course, we aren't done with this tonight as the cold front will
be slow to move east. There is still uncertainty with
if/when/where storms develop Wednesday, but we have seen a slowing
in the cold front in the models, most pronounced with the GFS.
This slowing explains the western shift in the day 2 convective
outlook. CAMs and deterministic models are all over the board with
how convection evolves tomorrow. The problem is upper forcing is
more in question with the front Wednesday. The main upper low will
be up west of the Winnipeg lakes, with a subtle short wave being
the main forcing for storms. The GFS is slowest with the wave,
bringing it through our area during the afternoon, which allows
storms to develop along the I-35 corridor. The ECMWF has this same
wave pushing into northeast WI by the afternoon, with storms
largely east of our area. The severe threat is definitely
conditional, but severe parameters would support all modes of
severe weather if we see convection, though once again, we would
like see a quick transition to a more linear convective mode.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 327 PM CDT Tue Jun 13 2017

Still some uncertainty on where the cold front will be located
Wednesday evening but current consensus suggests it being
oriented roughly along the MN/WI border, keeping thunderstorm
chances constrained to west-central WI & far-southeast MN. MLCAPE
values of 1500-2500 J/kg and deep shear values of 30-40 kts would
certainly be supportive of some strong to severe thunderstorm
segments & clusters across western WI, with damaging wind gusts
and large hail the main threats. The cold front clears the area by
Thursday morning with clearing & drying behind the front leading
to a warm but pleasant day.

Unsettled weather returns Friday & through the weekend as a 100 kt
upper level jet & deepening trough over the western Great
Lakes provides forcing for at least some scattered precip chances
each day. Instability looks to be low each day as the warm front &
greatest CAPE looks to set up further to the south over Iowa so
strong/severe thunderstorms are not expected. The heaviest rainfall
is expected Friday night into early Sunday morning where PW values
potentially over 1.5" could result in locally heavy rainfall,
primarily over southern MN. Decent rain chances will continue
through the day Saturday into Sunday morning as the strong forcing
aloft will continue to kick off scattered showers & thunderstorms,
but we will be looking at on & off showers & thunderstorms vs an all
day washout.

The upper level trough moves east of the area by Sunday afternoon
ushering in NW flow with drier weather & much cooler temperatures,
though still close to normal. Models begin to diverge towards
Tuesday with the Euro placing us in a drier pattern stuck between
the broad trough off the east coast, & a developing ridge over the
west. The GFS returns to zonal flow across the northern tier of the
US & brings another strong upper level jet into the area, which
would result in a more unsettled pattern for midweek.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 615 AM CDT Tue Jun 13 2017

Main issues will be gusty southeast wind and thunderstorms. A few
storms will be scattered this morning across central MN into WI,
then lift northward and/or diminish. More storms will develop late
afternoon or evening in western MN and move eastward. Some storms
could easily gust over 40 knots along with hail.  For wind, the
gradient increases this morning, first in western MN where gusts
will be around 30 knots. The gradient does translate eastward,
but not as intense as in western MN, so gusts there will be 20-30
knots.

KMSP...
Morning storms have been developing at 11Z on a wnw/ese line
across MN into WI, with the line being very close to the airfield.
These should move north and east of KMSP around 14z-15z. Next
round will wait until late evening with incoming cold front.

/OUTLOOK FOR KMSP/
Wed...VFR/MVFR with TSRA possible. Wind S-SW 5-10kt.
Thu...VFR. Wind W 5-10kt.
Fri...VFR. Chc MVFR/TSRA. Wind VRB05kt.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
&&

SHORT TERM...MPG
LONG TERM...ETA
AVIATION...TDK

Hazardous Weather Outlook
National Weather Service Twin Cities/Chanhassen MN
1242 PM CDT Tue Jun 13 2017

MNZ041>045-047>070-073>078-082>085-091>093-WIZ014>016-023>028-
141745-
Douglas-Todd-Morrison-Mille Lacs-Kanabec-Stevens-Pope-Stearns-
Benton-Sherburne-Isanti-Chisago-Lac Qui Parle-Swift-Chippewa-
Kandiyohi-Meeker-Wright-Hennepin-Anoka-Ramsey-Washington-
Yellow Medicine-Renville-McLeod-Sibley-Carver-Scott-Dakota-
Redwood-Brown-Nicollet-Le Sueur-Rice-Goodhue-Watonwan-Blue Earth-
Waseca-Steele-Martin-Faribault-Freeborn-Polk-Barron-Rusk-
St. Croix-Pierce-Dunn-Pepin-Eau Claire-
1242 PM CDT Tue Jun 13 2017

This Hazardous Weather Outlook is for portions of central and
southern Minnesota...and west central Wisconsin.

.DAY ONE...This Afternoon and Tonight

There is an Enhanced Risk of severe thunderstorms this afternoon
through tonight generally west of Interstate 35 and a Slight Risk
to the east into west central Wisconsin. A line of thunderstorms
is expected to move into western Minnesota between 6pm and 8pm. Large
hail and damaging winds will be possible with this line thunderstorms
as it moves east across the area tonight.

A tornado or two cannot be ruled out either, especially for areas
along and north of a St. Cloud to Morris line in central
Minnesota.

Prior to the storms, peak heat indices between 95 and 100 degrees
will be common across southern Minnesota this afternoon.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

There is a Slight Risk of severe thunderstorms again Wednesday,
mainly along and east of Interstate 35 corridor. Large hail and
damaging winds will be possible as scattered thunderstorms develop
during the afternoon.

Thunderstorm chances return Friday night and Saturday.

.SPOTTER INFORMATION STATEMENT...

SKYWARN spotter activation will likely be needed.

Special Weather Statement
National Weather Service Twin Cities/Chanhassen MN
507 PM CDT TUE JUN 13 2017

MNZ054-056-132245-
Chippewa-Lac Qui Parle-
507 PM CDT TUE JUN 13 2017

...A STRONG THUNDERSTORM WILL AFFECT SOUTHEASTERN LAC QUI PARLE AND
NORTHWESTERN CHIPPEWA COUNTIES...

At 506 PM CDT, a strong thunderstorm was located near Boyd, or 14
miles southwest of Montevideo, moving north at 25 mph.

Winds in excess of 40 mph and half inch hail are possible with this
storm.

This storm will be near...
  Dawson around 515 PM CDT.
  Watson around 535 PM CDT.

This includes the following highways...
 U.S. Highway 212 between mile markers 19 and 32.
 U.S. Highway 59 near mile marker 107.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

LAT...LON 4510 9605 4508 9566 4489 9582 4489 9585
      4486 9584 4481 9589 4480 9607 4481 9608
TIME...MOT...LOC 2206Z 196DEG 24KT 4486 9598

BORGHOFF