Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 170015

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
715 PM CDT Sat Sep 16 2017

.SHORT TERM...(This evening through Sunday)
Issued at 315 PM CDT Sat Sep 16 2017

The main forecast concern is when and/or where thunderstorms begin
to develop during the late afternoon/early evening along the
instability gradient in the southeast 1/4 of MPX forecast area.

Earlier morning convection was all elevated and the latest batch
was moving rapidly northeast across central Minnesota. Regional
radar does show a few showers in northeast Nebraska which could be
the first start of thunderstorm activity beginning to build
across the Upper Midwest. Not a lot of support upstairs as the
main upper wave was across the western Dakotas. However, as this
area of showers moves eastward into the axis of greater
instability, thunderstorms could develop. But again, these showers
have no upper level support at the current time, and most of the
higher chances of precipitation occurs once it moves into the
instability axis late tonight. Therefore, if nothing happens as
the showers move northeast, then it may be mostly dry this evening
until the main upper wave begins to move eastward and allows
greater upper level support, and hence, better shower and
thunderstorm development along this axis of instability.

Will continue highest percentages east of the Minnesota/Wisconsin
border, but later forecasters need to watch south central
Minnesota if thunderstorms develop earlier along the area of
showers moving northeast into northwest Iowa. Otherwise, Sunday
will have clearing skies with temperatures closer to normal highs
for mid September.

.LONG TERM...(Sunday night through Saturday)
Issued at 315 PM CDT Sat Sep 16 2017

A more active pattern is expected as the upcoming week progresses,
mainly due to the formation of a trough in the western CONUS with
a ridge in the eastern CONUS, putting the Upper Mississippi River
Valley region in pronounced southwest flow aloft which will allow
several organized systems to impact this region.

The first will be a weaker system for Monday, which will feature a
shortwave trough axis aloft while a low pressure center and its
associated warm front in advance of it shifts from the central
Plains into the Ohio Valley. Enough isentropic lift to the north
of the warm front plus the buckling aloft from the trough within
deep enough moisture will bring a few isolated to scattered
showers to the area.

A more organized system is expected for Tuesday night into
Wednesday but will be fairly quick-moving as the flow aloft
becomes more amplified and a potent trough aloft kicks the system
through the region. However, even with the passage of the
shortwave trough, the longwave pattern will continue to become
more amplified by the end of the week, such that nearly meridional
flow is expected by next weekend. This will result in a deep plume
of moisture being shunted into the region in advance of the next
well-developed frontal system for the end of the week. Showers and
thunderstorms already look likely with this end-of-week system.

As for temperatures, with a more southwesterly flow taking place,
this will allow for day-by-day warm air advection for the region.
Temperatures will go on a gradual warming trend back to above-
normal levels, including highs near 80 degrees by midweek.


.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 715 PM CDT Sat Sep 16 2017

There is a swath of IFR ceilings lagging the front, with drizzle
bringing visibilities down below 2sm in some spots. This should
continue to move along as front treks east, so any area that has
low visibility in drizzle should have it for just 2-3 hours.

Am a bit worried that the broad shield of low clouds across ND,
northern into east central SD, and much of MN will persist longer
than models project. There is some cellular look from central ND
into eastern SD, but not much. And certainly no breaks in the
overcast until western SD. And this is where a west-northwest wind
could bring dryer air into southwest MN later tonight, just later
than forecast. With cold advection continuing in wake of front,
most of our area may well not scatter out until late tonight, and
central MN might not until Sunday morning. Have trended TAFs to
show the MVFR persisting much longer, but confidence is lower than
average due to models insisting on clearing out.

Lastly, narrow zone of instability from south central MN into
western WI could help with thunderstorms the next few hours. Have
mentioned thunder at KEAU through the evening, but only til 01Z
at KRNH.

KMSP...low MVFR clouds should move into KMSP after 01Z and persist
for a few hours. May be some drizzle bringing visibility down to
MVFR as well. Would not be shocked if vsby drops to IFR, but the
chance of that is not great for now.

Otherwise, thunder potential this evening has mvoed off to
Wisconsin. Once the drizzle and lower MVFR move through this
evening, there may well be a little higher MVFR or low end VFR that
remains much of the night.

Mon...VFR with SHRA/MVFR possible. Winds SE 5-10 kts.
Tue...VFR. Winds SE 5-10 kts.
Wed...VFR with TSRA/MVFR possible. Winds SW 5-10 kts.




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