Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 170523

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1223 AM CDT Sun Sep 17 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 06Z TAFS through 06Z Sunday night)
Issued at 1221 AM CDT Sun Sep 17 2017

Cold front finally cleared EAU back at 4z, but the moisture and
instability lag the wind shift by a few hours and as the HRRR
shows, EAU will continue to be at risk for bouts of showers
through 9z until this moisture plume moves off to the east as
well. Behind this moisture plume, strong subsidence is resulting
in a rapid clearing of low clouds across western MN. Biggest
change to the previous TAFs was to speed up return to VFR
conditions quite a bit at all terminals. We will have sunny skies
today, with that subsidence keeping any cu at bay. Mid level
clouds will quickly return from the southwest tonight. These
clouds will save western WI from seeing fog, who otherwise would
with the surface high overhead.

KMSP...MVFR cigs will be gone by 9z, with no additional concerns
with a high confidence TAF for MSP.

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




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