Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 170842

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

.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 weekend.

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, temepratures 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 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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