Special Weather Statement, NWS Twin Cities, MN

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1045 PM CDT SUN SEP 4 2016

.SHORT TERM...(This evening through Monday)
Issued at 200 PM CDT Sun Sep 4 2016

Surface analysis this afternoon shows a wavy quasi-stationary
front emanating from a deep low over central Canada, snaking
southwest through the Dakotas and into WY/UT. At least one low
pressure center has developed along the front, particularly over
central SD and over SW WY. Aloft, the region is caught in
prevailing southwest flow aloft between a deep ridge centered over
the Ohio Valley and a longwave trough over the Pacific Northwest.
The ridge/trough features will have only slight eastward movements
through the next 24 hours, and hence the surface front will remain
fairly steady, remaining just to the W of MN/WI. However, a couple
of notable shortwaves rounding the large trough look to swing
through the region overnight. This is in conjunction with a long
plume of Gulf of Mexico moisture streaming into the region. The
surface low will eject toward north-central MN overnight, which
will allow rain/t-storms to develop in the Dakotas and western MN
overnight, then gradually spread eastward over much of the
coverage area through Monday morning. With PWATs increasing to
around 1.75 inches by daybreak tomorrow morning and this deep
moisture remaining in place through the day Monday, the potential
is there for pockets of moderate-heavy rain to develop from time
to time overnight before slight bubble ridging arriving from the
west by daybreak makes for a "break in the action" for much of the
day on Monday. That said, rainfall amounts of 1-3 inches for much
of the area with isolated 3-5 inch spots cannot be ruled out
should storms train. As for the severe potential, that will be
limited to far western MN this evening where effective CAPE/shear
are confined. As for tomorrow, any sunshine will only enhance
instability which is already expected to climb to the 2000-3000
j/kg range for MLCAPE. Stronger low-mid level jetting will also
aid in strong-severe storm development for tomorrow, mainly for
late in the day. Overall, much of the coverage area can expect
overnight rain and thunderstorms. As for temperatures, the diurnal
range will remain fairly small in advance of the front. Highs
that hit the mid 70s to lower 80s will only drop into the mid-
upper 60s tonight.

.LONG TERM...(Monday night through Sunday)
Issued at 255 PM CDT Sun Sep 4 2016

Several issues will unfold over the next 3 days as whether which
area of MPX forecast area will receive the bulk of the heaviest
rainfall.

There is no question that parts of the Upper Midwest will receive
a substantial amount of rainfall, with upwards of 4-6 inches
(Isolated areas) before slightly drier air filters into the
region late Wednesday/Thursday. The main question is timing and
where the frontal boundary and strong moisture advection resides
which will dictate the best area of both high rainfall rates, and
location of how convection will setup.

The setup is almost a perfect scenario of a heavy rainfall event
across the Upper Midwest. Both in terms of synoptic scale and
mesoscale which leads to a "high" end flooding event.

Models have been forecasting an Upper Ridge building across the
southeastern states with a broad trough continuing to dig across
the Rockies early this week. Strong southwesterly flow aloft will
be the rule until this pattern changes late in the week.
PWAT/Mixing ratio values are extremely high (Almost off the scale
for climatology) for this time of year, but the added affect of
synoptic scale lift will produce rounds of thunderstorms. There is
another weather element that is even more rare in terms of
moisture depth and efficiency. A tropical wave off the coast of
Mexico is forecast to become a tropical storm/hurricane in the
next day or so. As the Upper Ridge builds over the southeast, some
of the moisture from this tropical system will be entrained across
the southwest, and eventually into the Plains and Midwest. There
is some question on timing of which this tropical moisture will
surge northeastward before the frontal boundary is far enough
south of our region to not get affected by its elements.

As for timing of when the best chances of rainfall will occur
across central and southern Minnesota, and west central Wisconsin,
the best scenario is Tuesday morning through Wednesday afternoon.
This is the time period where both moisture depth, frontal
boundary location, and upper level support will be maximized.

As said before, there is no question that parts of the Upper
Midwest will receive several inches of rainfall before Wednesday
night. Forecasters need to pay attention to short wave energy and
moisture advection for adding higher/lower pops to the forecast.
This stems from the frontal boundary location which is critical
during these heavy rainfall event.

QPF amounts will be problematic due to the nature of convection,
but overall 1 to 3 inches across MPX forecast area before
Wednesday night seems reasonable.

Past Wednesday, the mean upper wave pattern will slowly become
more west-northwest leading to cooler weather, but not necessarily
dry weather. As the mean trough begins to take over by next weekend
and into the following week, several fast moving cold fronts will
affect the northern tier of the United States. The main theme will
be cooler conditions, especially the week of September 12th.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1045 PM CDT Sun Sep 4 2016

SHRA and TSRA will be on the increase over the next several hours
as strong low level moisture transport and elevated instability
shift from eastern SD into MN and western WI. The back edge of
this activity will push through western WI by mid morning on
Monday. The influx of low level moisture will lead to an expanding
area of MVFR or lower ceilings overnight and Monday morning.
Expanded out the end time on the MVFR across central MN and
included KMSP for a time during the morning as well. Due to the
strong disturbance moving out of SD and across MN/WI overnight,
it's hard to pinpoint when more convection will develop Monday
afternoon as the low level focus/wind field will be quite
perturbed and will take some time to refocus, perhaps until
sometime in the evening. Therefore, no SHRA/TSRA indicated in the
TAF's for monday afternoon/evening at this time.

KMSP...SHRA will be on the increase once again after 06z with
TSRA likely in the 08z-11z time frame. Expect brief heavy
downpours. The activity will be winding down thereafter with a few
showers through mid morning. Concern is growing that MVFR ceilings
will exist in the wake of the SHRA/TSRA and a period of BKN030 was
inserted between 12z-16z. SSE winds 10-15 knots through the
period.

/OUTLOOK FOR KMSP/
Tue...VFR/MVFR with TSRA/SHRA. Wind VRB 5 kts.
Wed...VFR/MVFR with TSRA/SHRA. Wind E 5-10 kts...becoming S.
Thu...VFR. Winds W 5-10 kts.

&&

.MPX Watches/Warnings/Advisories...
WI...None.
MN...None.
&&

SHORT TERM...JPC
LONG TERM...JLT
AVIATION...RAH

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
424 PM CDT SUN SEP 4 2016

MNZ041>045-047>070-073>078-082>085-091>093-WIZ014>016-023>028-
052130-
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-
424 PM CDT SUN SEP 4 2016

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL
MINNESOTA...EAST CENTRAL MINNESOTA...SOUTH CENTRAL MINNESOTA...
SOUTHEAST MINNESOTA...SOUTHWEST MINNESOTA...WEST CENTRAL
MINNESOTA...NORTHWEST WISCONSIN AND WEST CENTRAL WISCONSIN.

.DAY ONE...TONIGHT

THUNDERSTORMS ARE POSSIBLE TONIGHT ACROSS THE OUTLOOK AREA.
THERE IS A MARGINAL RISK OF SEVERE WEATHER GENERALLY ALONG
AND WEST OF A LINE FROM ONAMIA TO ST CLOUD TO REDWOOD FALLS.
LARGE HAIL AND DAMAGING WINDS ARE THE PRIMARY HAZARDS.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY

PERIODIC THUNDERSTORMS CAN BE EXPECTED FOR MUCH OF THE UPCOMING
WEEK. THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR MUCH OF
THE COVERAGE AREA MONDAY INTO TUESDAY. LARGE HAIL AND DAMAGING
WINDS ARE THE PRIMARY HAZARDS. IN ADDITION...A FEW STORMS WILL
PRODUCE HEAVY RAIN WHICH COULD CAUSE FLOODING. THE HIGHEST RISK
FOR FLOODING IS ACROSS SOUTHERN MINNESOTA AND WESTERN WISCONSIN.

.SPOTTER INFORMATION STATEMENT...

SKYWARN SPOTTER ACTIVATION IS NOT EXPECTED TO BE NEEDED TONIGHT
BUT MAY BE NEEDED MONDAY THROUGH MONDAY NIGHT.

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
309 AM CDT MON SEP 5 2016

MNZ068-069-076-050900-
SCOTT-CARVER-LE SUEUR-
309 AM CDT MON SEP 5 2016

...A STRONG THUNDERSTORM WILL AFFECT CARVER...SCOTT AND LE SUEUR
COUNTIES...

AT 307 AM CDT...A LINE OF STRONG THUNDERSTORMS WAS NORTH OF
MANKATO...MOVING EAST AT 60 MPH.

WIND GUSTS UP TO 50 MPH ARE POSSIBLE WITH THIS LINE OF STORM.

LOCATIONS IMPACTED INCLUDE...
BLOOMINGTON...EDEN PRAIRIE...BURNSVILLE...LAKEVILLE...SHAKOPEE...
SAVAGE...CHASKA...CHANHASSEN...PRIOR LAKE...ST. PETER...
WACONIA AND VICTORIA.

LAT...LON 4424 9403 4434 9393 4439 9396 4451 9388
      4456 9392 4462 9385 4464 9377 4468 9377
      4468 9389 4472 9390 4472 9401 4498 9400
      4498 9377 4489 9376 4489 9352 4480 9351
      4479 9333 4455 9328 4453 9353 4420 9353
TIME...MOT...LOC 0807Z 254DEG 51KT 4438 9392

JRB