Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 191703

Area Forecast Discussion
National Weather Service La Crosse WI
1203 PM CDT Tue Sep 19 2017

.SHORT TERM...(Today and tonight)
Issued at 324 AM CDT Tue Sep 19 2017

At 3 AM, weak 850 mb moisture convergence associated with the low
level jet is resulting in scattered showers along north of the
Interstate 94 corridor in Wisconsin. The 19.00z models are in very
good agreement that this moisture transport will gradually wane
during the remainder of the overnight. As a result, expect these
showers will be gone by day break.

The models are in good agreement that the clouds will gradually
dissipate this morning and that the afternoon will be partly to
mostly sunny. High temperatures will range from the mid 70s to
lower 80s.

For tonight, the 850 mb moisture transport will increase
dramatically ahead of an approaching longwave trough. In addition,
the mixed layer CAPES will climb into the 1000 to 1500 J/kg across
southeast Minnesota and northeast Iowa. A squall line will develop
over the eastern Dakotas and western Minnesota during the evening
and this will march toward our area during the overnight. As the
main short wave energy ahead of this trough ejects into northern
Minnesota, this line will slow and weaken as it moves east across
eastern Minnesota and northeast Iowa.

.LONG TERM...(Wednesday through Monday)
Issued at 324 AM CDT Tue Sep 19 2017

On Wednesday, a cold front will push east across the Upper
Mississippi River Valley. By noon, this front will be close to
the Mississippi River and it will east of the area by 00z
Thursday. Despite the short wave moving northeast into Canada,
there will be sufficient shear and instability for supercells to
develop ahead of the cold front. The main severe weather threats
would be damaging winds and large hail. However, with this said,
it may be east of our area before this occurs.

On Thursday, the GFS and GEM produce showers and storms south of
the Interstate 90 corridor. Meanwhile, the NAM and ECMWF are dry.
Looking at the soundings for this time period, the atmosphere
looks too capped for any precipitation to form. Due to this,
removed the precipitation for this time period.

From Friday into the weekend, the upper level ridge will be
building across the region. This will slow the eastward progress
of a cold front over the northern and central Plains. Due to this,
confined the rain chances to north-central Wisconsin and southeast
Minnesota. I would not be surprised that even these areas could
be potentially dry too. Southwest flow ahead of this cold front
will likely result in warmer temperatures than the MOS guidance.
As a result, the temperatures were raised anywhere from 3 to 5F.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1203 PM CDT Tue Sep 19 2017

Morning stratus finally lifting/dissipating across portions of
northeast IA and southeast MN, leaving VFR conditions for the rest
of this afternoon into this evening at both KRST and KLSE. Low
pressure and a cold front will be pushing out of the Northern
Plains into the Upper Mississippi River Valley region by Wednesday
morning. Lift out ahead of this low and front is expected to
produce some altocumulus overnight and a slight chance of an
isolated shra/ts. With chances so low due to expected isolated
nature of this convection, opted to keep out of the TAFs for now.
MVFR cloud expected to move into the TAF sites after 14z as lower
level moisture is pulled northward along the cold frontal
boundary. Otherwise, models showing bulk of any shra/ts activity
associated with the low and front remaining north of the TAF
sites, so continued dry through 18z.

Of note will be a bout of low-level wind shear due to increasing
southerly winds around 2kft in the 35-45kt range tonight into
early Tuesday morning ahead of the approaching low and front.




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