Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 192112

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

.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

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

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 18Z TAFS through 18Z Wednesday afternoon)
Issued at 104 PM CDT Tue Sep 19 2017

Fog & low ceilings have been slow to dissipate this morning over
central & western MN but have risen above IFR. VFR conditions
will prevail this afternoon at MKT, MSP, RNH, & EAU, but not
expecting too much in the way of clearing out west where MVFR
ceilings are expected to remain throughout the day. Aviation
concerns tonight will be the line of thunderstorms along a cold
front moving in from the Dakotas this evening. Expecting TSRA to
first impact AXN around 04Z but begin to weaken somewhat as it
moves east. In addition, wind shear will be a concern tonight
ahead of the cold front passage with southerly winds of 50-60 kts
expected around 2000 feet. Gusty SE winds over 20 kts through this
evening, becoming westerly tomorrow morning after the cold front

A few stubborn patches of BKN MVFR ceiling may impact the terminal
this afternoon but otherwise expect VFR conditions. Line of
thunderstorms is expected to impact the terminal around 09Z
tonight. These storms should weaken as they move east so not
expecting much in the way of severe weather at MSP, but isolated
wind gusts over 50kts can`t be ruled out. A 50 kt jet at 2000 ft
is expected to develop around midnight & last until the storms
exit around 11Z. Winds become westerly tomorrow morning with
lingering MVFR ceilings becoming VFR by late morning.

Wed...Mainly VFR. Winds W 5-10 kts.
Thu...VFR. Winds SE 10-15 kts.




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