Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 130847

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
347 AM CDT Wed Sep 13 2017

.SHORT TERM...(Today and tonight)
Issued at 347 AM CDT Wed Sep 13 2017

Weak high pressure at the surface combined with a building ridge
axis aloft will be just enough to prevent the wavy quasi-stationary
front from the southern tip of Hudson Bay to far northern MN to
southwestern SD from making any appreciable progress east today. The
ridge from the southwestern CONUS will keep unseasonably warm
temperatures in place once again today as highs will hit the 80s
area-wide, with potentially some 90-degree readings in far
western MN. Dewpoints will also remain on the high side for the
season, reaching the mid 60s, so muggy conditions can again be
expected. The ridge will break down a bit tonight into Thursday
morning, just enough such that the surface front will drop to near
a KFAR-KDLH line in northern MN while deep moisture increases
from the west. Shower chances increase in northern MN overnight
and potentially a few isolated rain showers may reach a few
counties in the northwestern portion of the WFO MPX coverage area.
Temperatures will again run warm tonight with lows ranging from
the upper 50s to mid 60s.

.LONG TERM...(Thursday through Tuesday)
Issued at 347 AM CDT Wed Sep 13 2017

The forecast reasoning from yesterday afternoon`s discussion
remains largely intact. A large scale pattern shift will take
place Thursday with a deep trough developing across the western
U.S., and the semi-permanent western ridge of late finally
focusing over the east. Southwest flow aloft will prevail into the
weekend, with the baroclinic zone nearly stationary from the
central Rockies northeast to northern Minnesota. Multiple waves of
energy embedded in the southwest flow will bring periodic
thunderstorm chances Thursday through Saturday night. Capping will
limit the activity with eastward extent due to a dry boundary
layer, and focus it where the deeper moisture tongue will be found
across central and western MN. Generally convection that fires in
that zone through Friday night will weaken and diminish as it
heads eastward.

The highest PoPs will be Friday evening into the early overnight
hours with a quickly moving mid level wave ejecting from Colorado that
will track to the northwest of the CWA and exit northern MN by
early Saturday morning. The best overlap of forcing, moisture,
and instability will be west of I-35 where likely PoPs continue.
Convection will lose everything it has supporting it by Saturday
morning and thus quickly weaken as it approaches Wisconsin.

Another good chance of storms will come late Saturday and Saturday
night with the surface front and mid level trough axis. Robust
instability due to steep mid level lapse rates and a moistening
boundary layer may develop Saturday afternoon across MN.
Sufficient shear and forcing along the front could bring a severe
thunderstorm risk through the mid evening hours. A cooling and
stabilizing boundary layer will diminish the severe risk in
Wisconsin from late evening onward.

The biggest change to the long term were temperatures. We were
able to reach the mid 80s area wide Tuesday with 925 mb temps of
+22 to +24C. By Thursday and Friday, those will reach +24 to +28C.
Increased highs to around 90 Thursday and upper 80s Friday from
the Twin Cities southwestward away from potential convective
development. The NAM and GFS seem to be a bit overzealous with
storms Thursday night which may be impacting their temperatures
Friday. Nonetheless, held back from the full potential scenario
given this threat. An absence of convection and resultant
considerable mid/high clouds, with the presence of a well mixed
boundary layer should bring us into the low 90s easily both days.

Cooler air follows for Sunday and Monday before the trough/ridge
pattern reloads during the middle of the week. This should resume
the warm and wet pattern into late September.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1054 PM CDT Tue Sep 12 2017

VFR conditions across Minnesota, with patchy dense fog still
expected to develop overnight across Wisconsin. Areas in eastern
MN, such as KMSP and KMKT could see some fog development also,
but confidence is higher in visby restrictions at KRNH and KEAU.
High clouds will stream across western Minnesota. Later today
expect VFR conditions and southerly winds.

Could still see some fog development again overnight, but not
anticipating any visby restrictions. Winds will be southerly and
around 05 kts.

Fri...VFR. Slight chc of SHRA/TSRA. Wind S S 10G20 kts.
Sat...VFR. -SHRA/TSRA likely late. Winds S 10G20 kts.
Sun...VFR with MVFR clouds possible. Winds WNW 10G15 kts.




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