Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 200004
AFDMPX

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

.UPDATE...
Issued at 656 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 00Z TAFS through 00Z Wednesday evening)
Issued at 656 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



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