Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 171755

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1255 PM CDT Sun Sep 17 2017

Issued at 1231 PM CDT Sun Sep 17 2017

Updated to include 18z aviation discussion below.


.SHORT TERM...(Today and tonight)
Issued at 345 AM CDT Sun Sep 17 2017

Strong isentropic downglide in the wake of the cold front
overnight has aggressively cleared out the clouds and all we are
left with is the back edge of the clouds associated with the
moisture plume over western WI. With the high this morning
centered over the Black Hills moving over southern MN, we will
keep our clear skies through the afternoon. It will also feel more
fall like today. NAM boundary layer dewpoints show us mixing down
into the low/mid 40s for dewpoints this afternoon, which is
pretty close to what the short term model consensus guidance gave
us and what was observed across the Dakotas yesterday, so went
with that for dewpoints. With 925mb temps down between 4c and 8c,
we`ll stay in the 60s today.

Tonight, the wave currently over Idaho will be moving into the
upper MS Valley. This will bring a slug of mid/upper level
moisture into the area, but the low levels will remain dry with
the high pressure moving through today still nearby. We`ll see
some isentropic lift trying to lift into southwest MN by the end
of the short term, but the better LLJ support and moisture
transport will be down closer to the surface warm front across
southern Nebraska. Any precip we see move our direction will be
fighting the dry low level air. Given expected orientation of the
LLJ and its associated moisture transport, have some low pops
working just into southwest MN late tonight. The good thing about
this cloud cover though is that it will preclude any fog formation
on what otherwise would be a good dense fog setup for western WI.

.LONG TERM...(Monday through Saturday)
Issued at 345 AM CDT Sun Sep 17 2017

The long term concerns remain the overall development of the
amplified long wave pattern and how Atlantic tropical system could
affect timing/movement of cold front late in the week and into the

Initially  we have a short wave lifting northeast across the
upper midwest and will likely clip our southeast cwa through
Monday. Models of trended a bit stronger with this feature.
Instability remains very limited with this however and will just
mention slight chance thunder into south central MN. We should see
a dry day Tuesday as high pressure retreats to the east. The
frontal system that moved through Saturday night will drift
northward again into Tuesday night. Clouds will increase with
showers/thunder possible as southerly flow/moisture increases as
well. Instability increase as well, with MUCAPES increasing to
2500 J/kg on deterministic models ahead of the front/upper trough.
SPC already has painted a marginal severe risk for western MN for
mainly late Tuesday night.

This trough/front exits to the east mainly Wednesday morning with
the models trending a little faster with this feature. this will
leave Thursday on the dry side.

The deterministic models diverge some on overall development and
timing of the next trough and its passage over the region into the
weekend. Both become highly amplified with a rather deep western
conus trough developing. The east will see interactions of the
eventual 2 tropical systems and how it develops the eastern conus
ridge will determine overall timing of the western conus troughs
progression. It seems the ECMWF usually has a better handle on
overall amplified weather patterns with a usual slower movement of
weather systems. Considering whats happening over the east, that
may be the way to go here. At the moment we bring the front into
area over the weekend and stall it over central mn By late
Saturday. By that time we see a southwest fetch of moisture from
the southwest with PW`s showing at least a 2-3 standard deviations
above normal along this boundary. This will likely promote a
potential heavy rain period over the weekend. At the present time,
it is favoring a Saturday night through Sunday night time frame.

In the meantime, temperatures will warm above normal again by
Tuesday, with reading back in the lower 80s possible by Friday
ahead of the weekend front.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1231 PM CDT Sun Sep 17 2017

Morning cloud deck around 020 is continuing to erode to the north.
Still enough patches of clouds to mention FEW/SCT in parts of
central MN into central WI, with only KAXN having a ceiling at
initialization. The clouds at 025 look to finally dissipate by
this evening, resulting in only high clouds for much of the
overnight. Lowering decks return tomorrow as a weak upper level
wave approaches. Isolated showers are possible late morning into
tomorrow afternoon for mainly the southern TAF sites but
confidence not that great so have gone no further than VCSH. Winds
will continue from the W thru this evening then go light/variable
before swinging to SE overnight thru tomorrow.

KMSP...Enough clearing to not have anything more than FEW025
coverage for this afternoon, and even that may be generous. Clouds
to return overnight and gradually lower during the day tomorrow.
Some showers are possible tomorrow afternoon but coverage is quite
questionable to will run with only VCSH at this point. Growing
potential for MVFR ceilings late tomorrow afternoon as well.

Tue...Mainly VFR. Chance MVFR ceilings. Winds SE 5-10 kts.
Wed...Mainly VFR. Chance TSRA/MVFR early. Winds W 5-10 kts.
Thu...Mainly VFR. Chance MVFR ceilings. Winds SE 5-10 kts.




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