Special Weather Statement, NWS Twin Cities, MN

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1130 PM CDT Tue Sep 19 2017

.UPDATE...
Issued at 1124 PM CDT Tue Sep 19 2017

Updated to include 00z aviation discussion below.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 411 PM CDT Tue Sep 19 2017

Forecast concerns tonight focus on the potential severe weather
threat from a complex of thunderstorms expected to develop this
evening across the Dakotas.

Upstairs, a broad 500 mb low is present over western Canada.
Along the periphery of this low, a well-defined vorticity maxima
is pivoting into the Dakotas & will be the main driver for the
storms tonight. Right behind this vort max is a strong 100-120 kt
jet at 300 mb which will provide plenty of kinematic support for
this system. At the surface, a a deepening split low under the
left exit region of this jet will advance a cold front into
western Minnesota late tonight. Increasing southerly flow ahead of
the cold front & behind a relatively stationary warm front has
created a primed, but narrow, warm sector over the eastern Dakotas
& far-western MN, which will be the area of most concern for
severe weather tonight.

Main question for our forecast will be the timing of storms along
the cold front & when they move into western MN. Despite dewpoints
in the upper 60s & SFC CAPE values increasing to over 2000 J/kg
in SW MN, a strong cap remains in place thanks to 700 mb
temepratures over 10C. Thus, not expecting any storms to develop
until the stronger forcing from the surface cold front approaches
towards 10 PM tonight across far western Minnesota. By this time
surface-based instability will be starting to decrease with only a
sliver of the area still in the warm sector at this time,
generally SW of a Ortonville-Redwood Falls-Fairmont. Forcing along
the front will be increasing by the time it reaches western MN
which will likely result in any discrete storms ongoing across the
Dakotas at the time merging into a long line. Thus we're looking
at mostly a damaging wind threat tonight with the most likely area
being far western MN. While currently looking unlikely... if any
discrete storms are able to develop before the warm sector closes
off in western MN, 50-60 kts of effective bulk shear along with
steep mid-level lapse rates would be supportive of supercells or
hybrid line/supercell clusters capable of a large hail & tornado
threat.

This line of storms will continue to trek east, reaching the
longitude of St Cloud around 2 am, the Twin Cities around 4 am,
and into west-central WI by 5 am. Instability will be weakening by
the time this line reaches eastern MN so not expecting much in
the way of widespread severe weather in these areas. However
kinematic profiles will be increasing overnight allowing storms to
remain organized & along with the fast forward speed of the line,
a few isolated wind gusts have a chance to punch through the
stable nocturnal boundary layer.

Showers will linger along the cold front into early afternoon
across west-central WI but we'll see clearing skies Wednesday
morning behind the cold front. Drier air & more seasonal
temepratures move in behind the front as well with highs in the
mid 70s expected.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 411 PM CDT Tue Sep 19 2017

Continued concerns/questions for the extended outlook.

1. Excessive Rainfall
2. Abnormally high temperatures/humidity levels.

Models continued the trend toward a drier forecast for the
southeast 1/3 of MPX forecast area from Friday through Sunday.
Basically in areas from south central Minnesota to west central
Wisconsin, including some of the Twin Cities metro area. However,
at the same time period, west central and central Minnesota will
see copious amounts of rainfall, with several inches likely before
it ends late in the weekend, or early next week. This is again
based on the upper ridge building/holding across the Great
Lakes/Ohio Valley region, keeping the south/southwest flow aloft
in our area.

There has been some slight differences in the strength, and
location of the upper ridge center over the past few model runs.
But, consensus in the EC/GFS/GEM models remain that this ridge
will dominate the eastern 1/2 of the nation. Therefore, models
will struggle on holding back the surface front across the Upper
Midwest, and hence, forecasting precipitation too far to the
east/southeast.

Again, there is no reason for this front to move anymore eastward,
then from a southwest, to northeast orientation across the center
of Minnesota. Very warm and humid conditions will continue ahead
of this front with temperatures likely too cool based on current
trends on Friday/Saturday. There is only a very small chance of
outflow boundaries, associated with convection, that will lead to
some minor changes in the surface front location. So, I have kept
slight chance/chance percentages where uncertainty exists in
southern/eastern Minnesota. I wouldn't be surprised to see west
central Wisconsin dry for the entire weekend. There is no doubt
that there will be a sharp temperature gradient across the state,
with temperatures some 20-30 degree differences along the frontal
boundary. The best location of precipitation will occur 50 to 100
miles west/northwest of the surface front where upper level
support and no cap will prevail.

Due to PWAT values nearly 2-3 standard deviations above normal for
this time of year, and the added scenario of several rounds of
precipitation along and west of the cold front, I wouldn't be
surprised to see rainfall totals of 3 to 6 inches before it ends
early next week. This type of regime, with the amplified trough-
ridge over our region, is very conducive of excessive rainfall and
flooding. Please be aware of this for the upcoming weekend. The
second element is the possibility of severe weather due to the
amplified/strong upper features, combined with above normal
temperatures.

Not until the upper ridge begins to break down, and the mean
trough in the west de-amplifies, will our weather pattern change
to a cooler and less unsettled regime.

&&

.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.

/OUTLOOK FOR KMSP/
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.

&&

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

UPDATE...JPC
SHORT TERM...ETA
LONG TERM...JLT
AVIATION...JPC

Hazardous Weather Outlook
National Weather Service Twin Cities/Chanhassen MN
1239 PM CDT Tue Sep 19 2017

MNZ041>045-047>070-073>078-082>085-091>093-WIZ014>016-023>028-
201745-
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-
1239 PM CDT Tue Sep 19 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 across western
Minnesota, a Slight Risk across portions of eastern Minnesota,
and a Marginal Risk across western Wisconsin tonight. The main
weather hazard will be damaging winds later this evening in far
western Minnesota. A smaller risk of large hail, with an isolated
risk of a tornado or two across far western Minnesota this
evening as well. The risk of large hail and tornadoes is very low
in central and eastern Minnesota, but isolated damaging wind gusts
is still possible late.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

Thunderstorm chances continue over western Wisconsin Wednesday
morning, then periodic chances return area wide Thursday night
through Monday. Some strong or severe storms are possible. Of
particular concern is heavy rain potential with a slow moving
front and repeated rounds of thunderstorms. Several inches of
rain could lead to an increased flood risk across western and
portions of central Minnesota.

.SPOTTER INFORMATION STATEMENT...

SKYWARN spotter activation may be needed across western Minnesota
later today.

Special Weather Statement
National Weather Service Twin Cities/Chanhassen MN
1255 AM CDT WED SEP 20 2017

MNZ073-074-082-200630-
Brown-Watonwan-Redwood-
1255 AM CDT WED SEP 20 2017

...A LINE OF STRONG THUNDERSTORMS WILL AFFECT REDWOOD...WESTERN BROWN
AND WESTERN WATONWAN COUNTIES...

At 1254 AM CDT, radar indicated strong thunderstorms were located
along a line extending from near Seaforth to near Revere to near
Brewster. Movement was east at 45 mph.

Wind gusts up to 50 mph and dime size hail are possible with these
storms.

Locations impacted include...
Sleepy Eye, Springfield, Morgan, Walnut Grove, Lamberton, Wabasso,
Butterfield, Comfrey, Sanborn, Vesta, Lucan and Clements.

LAT...LON 4385 9486 4411 9487 4411 9509 4419 9510
      4419 9511 4420 9512 4420 9551 4454 9548
      4455 9535 4448 9491 4450 9486 4445 9478
      4443 9469 4385 9469
TIME...MOT...LOC 0554Z 261DEG 40KT 4451 9531 4416 9534 4374 9543

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
101 AM CDT WED SEP 20 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1241 AM     TSTM WND GST     CLARA CITY              44.96N 95.37W
09/20/2017  M59 MPH          CHIPPEWA           MN   MESONET

&&

BORGHOFF

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
102 AM CDT WED SEP 20 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1230 AM     TSTM WND GST     4 SSW GRANITE FALLS     44.75N 95.56W
09/20/2017  M55 MPH          YELLOW MEDICINE    MN   AWOS

&&

BORGHOFF

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
129 AM CDT WED SEP 20 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1248 AM     TSTM WND DMG     BELGRADE                45.46N 95.01W
09/20/2017                   STEARNS            MN   LAW ENFORCEMENT

            NUMEROUS LARGE TREES AND POWER LINES DOWN
            THROUGHOUT THE CITY OF BELGRADE.

&&

JPC

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
206 AM CDT WED SEP 20 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0202 AM     HAIL             SARTELL                 45.62N 94.21W
09/20/2017  E1.00 INCH       STEARNS            MN   TRAINED SPOTTER

&&

JPC

Special Weather Statement
National Weather Service Twin Cities/Chanhassen MN
212 AM CDT WED SEP 20 2017

MNZ051-059>061-200800-
Anoka-Hennepin-Sherburne-Wright-
212 AM CDT WED SEP 20 2017

...A LINE OF STRONG THUNDERSTORMS WILL AFFECT WRIGHT...NORTHERN
HENNEPIN...WESTERN ANOKA AND SOUTHEASTERN SHERBURNE COUNTIES...

At 211 AM CDT, radar indicated strong thunderstorms were located
along a line extending from Becker to near Rassat to Hollywood.
Movement was east at 45 mph.

Wind gusts up to 50 mph and dime size hail are possible with these
storms.

Locations impacted include...
North Minneapolis, Buffalo, Brooklyn Park, Plymouth, Maple Grove,
Coon Rapids, Blaine, Minnetonka, St. Louis Park, Andover, Brooklyn
Center and Fridley.

LAT...LON 4543 9351 4541 9351 4541 9322 4512 9323
      4497 9321 4498 9406 4545 9398
TIME...MOT...LOC 0711Z 264DEG 40KT 4539 9389 4516 9393 4492 9399