Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 150942

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
442 AM CDT Tue Aug 15 2017

.SHORT TERM...(Today and tonight)
Issued at 428 AM CDT Tue Aug 15 2017

fog is becoming more widespread this morning with patches across
west central and south central MN and northwest WI expanding into
central MN. The Dense Fog Advisory will Will likely need to
expanded into these areas by 430 AM. The fog will dissipate during
the mid morning.

Extensive stratus shield associated with a decaying MCS over SD
will spread across the region today, although it may break up a
bit as it does so. Cannot rule out a few showers and storms
reaching western Minnesota later this afternoon on the leading
edge of aggressive moisture advection. Could be seeing pwats of
nearly 2 inches already reaching this area by early evening, and
the rest of the area late this evening or overnight. Poor mid
level lapse rates yielding long, skinny CAPE and high pwats point
to a predominately heavy rain scenario. Adding to this threat is
MBE velocities slowing to less than 5 kt late tonight. A potent
wave lifting northeast from the central Plains will be the focus
for development tonight, beginning over southeastern SD and
spreading toward eastern MN and western MN after midnight.
Continued with the heavy rain wording and increased PoPs to
categorical as far east as I-35. It appears the best corridor for
heavy rainfall will stretch from Sioux Falls to the Twin
Cities/St. Cloud, where an inch or two may fall before 12Z

.LONG TERM...(Wednesday through Monday)
Issued at 428 AM CDT Tue Aug 15 2017

Wednesday and Thursday...The prolonged rainfall event will
continue through Wednesday night, winding down early Thursday
morning as the center of low pressure shifts from northeast
Nebraska into southern Minnesota and into north-central Wisconsin
during this time. Efficient moisture advection due to the upper
level flow tapping into the subtropical jet will usher in PWATs
rising to around 2". There will also be an effective upper level
shortwave trough, with a cutoff low developing within it, ushering
the low pressure center on its way ENE. With the upper level low
remaining offset to the W of the surface low, this setup will
allow for more than adequate precipitation generation for much of
Wednesday through Wednesday night. While there may be a lull in
the action late Wednesday, the movement of the deep low plus even
a modest amount of insolation will help reload instability with
the deep atmosphere to increase precipitation coverage and
intensity for Wednesday afternoon and evening. There is also a
small chance (hence Marginal Risk) of severe thunderstorms due to
strong jetting aloft and bulk shear around 40kt, leading to
possible bowing segments. Yet, the deep moisture and weak lapse
rates will likely mitigate such a severe threat, making heavy rain
a more prominent problem. Rainfall rates may well easily exceed
3"/hr at times, and despite the progressive nature of the storms
(as opposed to training), the widespread coverage of such heavy
rain may cause localized flooding concerns. The rainfall will
gradually wind down late Wednesday night into Thursday as the deep
low shifts off to the east, allowing for some drying out during
the day Thursday into Thursday night. Temperatures will run close
to 10 degrees below normal on Wednesday due to the expansive
cloud/rain complications (though some pockets of mid 70s highs
cannot be ruled out) followed by warmer highs on Thursday with
decreasing clouds and lack of cold air advection behind the
departing system.

Friday through Monday...Fewer chances for rain for the end of this
week into next week, along with lesser intensity/coverage due to
the lack of any strong systems moving through the area. With an
unimpressive surface pattern in place Friday into Saturday, a weak
shortwave aloft will slide through the region Friday afternoon and
evening which may bring isolated to scattered showers and
thunderstorms. Another similar system looks to move through the
region late Sunday into Monday, again possibly bringing scattered
showers and thunderstorms. Thus, the benefit here is that Saturday
looks to be a fine day with a mix of sun and clouds and highs
around 80 degrees.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1132 PM CDT Mon Aug 14 2017

Concern remains the development of low clouds and fog tonight,
with widespread MVFR and IFR. This looks most favorable for
southern MN into the KEAU area, where lingering moisture is most
prevalent, but areas of central MN to WI will also have some dense
radiational fog with light winds. This should persist to 13Z-14Z,
then gradual improvement through the morning.

KMSP...Mostly clear skies and light northeast winds will continue
tonight. There is some potential for radiational fog, but it is
uncertain how dense it will be. This is due to light ne winds
persisting with a little bit of a gradient. Therefore kept a
period of MVFR visibility, but left the clouds as low scattered.
Then for Tuesday, some midday cu are likely, and mid/high clouds
should spread across the area later Tuesday.

Wed...MVFR/IFR with -TSRA. Wind S-SE at 5-10 kts.
Thu...VFR. Wind N-NW 5-10 kts.
Fri...VFR. Wind W 5 kts.


WI...Dense Fog Advisory until 9 AM CDT this morning for WIZ014>016-

MN...Dense Fog Advisory until 9 AM CDT this morning for MNZ042>045-



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