Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 182113

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
413 PM CDT Fri Aug 18 2017

.SHORT TERM...(This evening through Saturday)
Issued at 413 PM CDT Fri Aug 18 2017

Wave that brought rain to southern MN much of the day is now over
southeast MN, but the cool cloud tops associated with it are all up
by Lake Superior, well north of where the precip is around southeast
MN, so this area of precip is expected to continue it`s diminishing
trend the rest of the afternoon. Right on the heels of this wave is
a much more compact, but stronger wave over northeast South Dakota.
This SD wave will be the source of strong to severe storms this
afternoon/evening in southwest MN. Expect these storms to more or
less follow the CAPE gradient (where sbCAPE maxes out around 2000
j/kg) southeast toward northwest IA, more or less paralleling the
Buffalo Ridge. Farther east, subsidence on the backside of the first
wave, along with more stable air in its cloud cover will limit
storms from the SD wave from making it much farther east of a
Willmar to Mankato line. Not only will the first wave limit eastward
coverage of storms from SD, but it will also keep storms in northern
MN, which went up just north of the cloud shield, from making much
progress into central MN. In the end, followed a blend of the CAMs
pretty close for PoP/QPF grids through this evening, which has the
best chances running from west central into southwest MN, with
pretty small chances outside of that area.

This activity will move south of the area pretty quick this evening,
with high pressure expected to slide over Iowa tonight. This will
leave us with our biggest issue for tonight, the potential for dense
fog. Given where the high is going and where the rain has fallen
today, it is looking increasingly likely that we will see dense fog
across at least southern MN, with the HRRR developing dense fog
pretty much everywhere west if I-35. Beefed up the fog mention in
the grids, and based the fog description (patchy, areas, widespread)
off of the visibility forecast from a blend of the short term models.

For Saturday, once the fog is gone, there will be no issues, with
mainly clear skies and highs in the low 80s expected. The only area
where we could see things play out a bit different is in west
central MN, where highs could get a bit warmer if the thermal ridge
coming across the Dakotas gets here faster than currently

.LONG TERM...(Saturday night through Friday)
Issued at 413 PM CDT Fri Aug 18 2017

The main focus in the extended will be a slow moving front that
will stall over the area Sunday night into Monday night, providing
multiple chances of showers and thunderstorms.

High pressure over the Mississippi Valley will be sliding
southeast into the Ohio Valley Saturday night. Meanwhile, a
boundary will approach from the northwest as the next trough
advances eastward across Canada. A few showers and thunderstorms
are possible along it over western MN late Saturday night, but the
coverage should be limited due to deeper low moisture not
arriving until Sunday afternoon and a strong cap in place with the
EML overhead. The front will be orphaned from the Canadian trough
while another system developing over the western High Plains
transitions it into a quasi-stationary boundary Sunday. Deeper
moisture with pwats rising above 1.5 inches and continued steep
mid level lapse rates will make it easier to break the cap and
shear is strong enough to organize storms into line segments,
possibly into an upscale growing MCS by Sunday night.

Beyond Sunday night, it`s difficult to determine where the front
will be given convective activity from the preceding 24 hours
which lends to a higher level of uncertainty with subsequent
development. But, it does appear the front won`t be surging too
far south and even deeper moisture will pool along and south of it
with pwats around 2 inches Monday. The MCS Sunday night may
progress enough eastward for some breaks in the cloud cover here
in time for the eclipse around 1 pm, before more development
occurs later in the afternoon. Continued steep lapse rates and
very high moisture content in the lower atmosphere will allow the
pattern to repeat Monday. The system will shift east Monday night.

Cooler and drier air following next week will solidify a below
average month for temperatures regionwide with a deep trough
developing across the northeastern half of the CONUS.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 128 PM CDT Fri Aug 18 2017

First of two upper waves is moving across southern MN this
afternoon, with a stronger one now over northeast SD. Cold cloud
tops with the MN wave have now pushed off into northern WI, well
north of the precip, so expect this precip shield to continue it`s
weakening trend over the next couple of hours. The SD wave will
be what supplies us with our main TS threat today, but with it
tracking toward northwest IA this afternoon, agree with what the
HRRR shows in that this is a problem for western MN. RWF/AXN have
the greatest threat for thunder, though it can not be completely
ruled out everywhere else. Confidence in thunder occurring
outside of AXN/RWF is to low to include anything more than a vcsh.
Tonight, skies will clear out with a surface high settling over
IA. This, given recent rains will set the stage for dense fog.
Like the TS, FG can`t be completely ruled out at any terminals,
though chances for FG look greatest at RWF/MKT due to their
proximity to the high. Once the fog burns off, there are no
weather concerns for Saturday.

KMSP...Though can`t completely rule out more showers later this
afternoon, confidence is highest in a dry scenario the rest of the
TAF period once the rain at the onset moves off. Fog tonight may
become problematic down in the river valley and parts of the
airport closest to the valley, but out at the ASOS, fog should not
be an issue.

Sun...VFR. Chance MVFR/TSRA overnight. Wind SW 5 kts.
Mon...Chc MVFR/-TSRA. Wind SE 5 kts.
TUE...Chc MVFR/-TSRA early, bcmg VFR. Wind N 5-10 kts.




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