Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 191104
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
604 AM CDT Wed Sep 19 2018

.SHORT TERM...(Today and tonight)
Issued at 435 AM CDT Wed Sep 19 2018

A busy short term period with thunderstorms moving in from the
Dakotas this morning, showers and storms throughout the day, then a
heavy rain threat tonight.  Heavy rain and flooding are the primary
hazards, but hail and gusty winds will be possible with the
strongest cells.

As of early this morning, a couple clusters of strong storms were
moving across South Dakota.  One are across the far southeastern
corner of SD has been expanding and slowly drifting eastward,
expanding northward very slowly.  The second area of storms was
located  across north central SD, but these storms are screaming
eastward at about 50 MPH. These storms should reach western MN by
6AM, with strong winds and hail possible.

For today, the morning thunderstorm activity should have no problem
continuing eastward directly across our area, from near Montevideo
through Willmar, the Twin Cities, and Eau Claire this morning. After
that, the guidance agrees in developing continuous showers and
storms throughout the day very near this same corridor.  Meanwhile,
the southeastern SD acitivity should mostly continue east through
northern Iowa, but could see some storms sneak up along I-90 and
move through this morning.  For the remainder of the daytime hours,
a band of showers and storms will likely setup west to east across
south central MN, along the northern fringe of the sustained low
level convergence/warm air advection aloft.  This area will also be
juxtaposed with favorable jet dynamics with the strong jet streak
across far northern MN into Canada, and decent vorticity advection
moving through the area.  Some hi-res guidance suggests up to an
inch or two of rain is possible during the day across this area.

For tonight, the heavy rain threat is the main concern across far
southern MN.  The main takeaway from the latest guidance was there
remains uncertainty in the placement of the mesoscale heavy
precipitation band tonight.  The heaviest precip is now forecast to
occur along I-90 and the Iowa border.  Given the placement of the
low level jet tonight, the highest winds and moisture transport look
to occur from Kansas up through eastern Nebraska and Iowa, and
possibly into far southern MN.  Very high PWAT values of about 1.75"
will pool along and south of the warm front and would expect
convection to develop along and south of the boundary.  Went with a
blend of WPC with the HREF for QPF with puts down 2-4" of rain with
locally higher amounts in extreme southern MN, with the maxima
actually occurring just on the other side of the Iowa border.  There
are some differences in the northern extent of the low level jet
tonight and also the elevated front, so still plenty of uncertainty
in exactly where the mesoscale precip band sets up.  Think that
north of the band will see mostly stratiform rain, so not as
concerning.  For these reasons, held off on any Flash Flood Watch
until placement of heavy rain threat is more assured.

.LONG TERM...(Thursday through Tuesday)
Issued at 435 AM CDT Wed Sep 19 2018

Main concern in the long term is right out the gate with heavy rain
and severe threat, including a tornado potential Thursday/Thursday
night. After that, the general idea remains the same, with one cool
day Friday, a dry and mild weekend, another cold FROPA Sunday
night/Monday with attendant thunderstorm chances, followed by a more
substantial cool down going into the second half of next week.

The system for Thursday is certainly looking more intriguing
tonight, with a deepening low expected to track from near Sioux
City, IA to the northwest Twin Cities metro and the western U.P. of
Michigan between 18z Thu and 06z Fri. This track will bring all
sorts of potential hazards to the MPX area Thursday. It looks like
our precipitation will come in two waves, Thursday morning into the
early afternoon as the warm front lifts north, then late afternoon
into the evening as the cold front sweeps through. Ahead of the low
and cold front, the GFS continues to show Pwats increasing to around
2.2 inches, which isn`t just high for September, but is getting up
into all time record high territory for any time of the year, so the
heavy rain threat is expected to continue. Given the deepening nature
of the low, the large scale pattern from the severe perspective is
quite impressive, as the LLJ rapidly strengthens to over 60 kts by
the evening.  This leads to some impressive curved hodographs in the
warm sector. Any discrete cell we get in this environment will be in
a primed tornado environment. SREF sig tor ingredients continues to
point to southeast MN up into west central WI as having the greatest
potential for seeing a tornadic environment along/ahead of the warm
front. Eventually though, storm mode will become linear as activity
develops along the cold front. Given the low level wind fields,
there will be a wind threat with this expected squall line. The
heaviest rain is expected along and east of the low track, with a
fairly widespread 2-3 inches additional inches of rain expected from
around St. James, up through the Twin Cities and into northwest WI
on east. This is only a slight southeastward shift in what the
previous forecast had.

Behind this system, high pressure drops down from the northern
Rockies. We`ll have a cooler day Friday, with highs mainly in the
50s and a cold start to Saturday morning. There`s certainly
potential for lows to be colder than currently forecast in central
WI, where the lower end of the model envelop drops lows into the
upper 20s out by Ladysmith and Eau Claire. A lot would have to go
right for it to get that cold out there, so stayed with a blended
forecast for now, though there is certainly good potential for at
least a frosty start to the day Saturday out in WI. However, during
the day Saturday, we`ll be getting back into southerly return flow,
with highs in the 70s possible out in western MN, with mid 70s
expected to be common across the area on Sunday.

During Sunday, the next cold front will be coming across the Dakotas
and this looks to move through here Sunday night through Monday.
We`ll likely have a line of weakening thunderstorms moving into
western MN late Sunday night, with storms increasing in
strength/coverage Monday afternoon along the cold front. Lapse
rates, shear, and instability all look sufficient to once again
support at least an isolated severe threat, which could very well
end up being our last threat for severe weather for the year.

As the previous discussion mentioned, we see a significant pattern
shift behind this front for the middle of next week, with a Rex
block forming out in the eastern Pac off the west coast of North
America. The will feature an anomalously strong ridge building well
up into Alaska, with downstream troughing resulting over central
North America. This will put us into northwest flow and looks to
result in a cool end to September and start to October.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 600 AM CDT Wed Sep 19 2018

LIFR/IFR cigs will persist through the period as we remain north
of the main front, with continuous showers and thunderstorms
beginning in the next few hours. Winds will generally be ENE, and
it`s hard to pin down any potential breaks in the precip, hence
the seemingly never ending VCSH at a minimum. Once the precip
moves in later this morning, very little opportunity for brief
breaks through the period.

KMSP...Thunderstorms about 2 hours away, but for the most part
thunder will be scattered through the period, with the focus
becoming far southern MN tonight.

/OUTLOOK FOR KMSP/
Thu...MVFR with IFR/RA possible. Wind ENE becoming NW at 20G25
kts.
Fri...MVFR. Wind NW 15G25 kts.
Sat...VFR. Wind S at 10G20 kts.

&&

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

$$

SHORT TERM...SPD
LONG TERM...MPG
AVIATION...SPD


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