Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 150410

Area Forecast Discussion
National Weather Service La Crosse WI
1100 PM CDT Tue Aug 14 2018

.SHORT TERM...(Tonight through Wednesday night)
Issued at 204 PM CDT Tue Aug 14 2018

GOES Water Vapor imagery early this afternoon shows a mid-level
trough pushing slowly southeast across Montana and the Northern
Plains while a closed low was spinning slowly east through the
Central Plains. At the surface, a weak convergent wind field
extended from the UP through northern WI into southwest MN. Radar
showing scattered showers/storms across southwest into northern MN
ahead of the Northern Plains wave and associated weak surface cold
front.  A little more widespread convection was noted with the
closed low with northern periphery holding mainly south of I-80.
Meanwhile, our forecast area remains under weak ridging with partly
sunny skies resulting from the cirrus canopy ahead of the closed low
and associated convection. Temperatures as of 1 pm were well into
the 80s with dew points in the 60s.

Going into tonight, we are expecting that mid-level trough and weak
surface cold front to continue dropping southeast, extending from
the UP through southeast MN by morning. Meanwhile, that closed low
will budge eastward into IA. A few of the CAM`s show convection over
northern WI into MN diminishing by later this evening with loss of
heating and better moisture transport shifting east. Most of the
convection associated with the IA closed low stays south of the
area. Bottom line for our forecast area will be some low-end
shower/thunder chances across the north and south. Otherwise, look
for lows tonight in the 60s.

For Wednesday into Wednesday night, will be watching interaction of
that northern stream wave and closed low passing south across IL.
Some weak surface convergence and weak mid-level deformation forcing
should spark isolated to scattered showers and storms. With
precipitable water values hovering near 2 inches, these showers and
storms will be capable of some heavy downpours. Also, very weak
steering winds exist, so any storm will be slow moving adding to
localized heavier downpour potential. Temperatures should be a bit
cooler than last couple of days due to increased cloud cover and
scattered convection. Plan on highs in the upper 70s to the lower

After a few lingering showers in the evening, mid-level ridging
moves in later in the evening/overnight. This should shut down any
remaining convection with partly cloudy skies expected.

.LONG TERM...(Thursday through Tuesday)
Issued at 204 PM CDT Tue Aug 14 2018

From Thursday into Friday, a short wave trough will move southeast
out of the Northern Plains and then move east across Iowa and the
Mid Mississippi River Valley.  As has been the trend over the past
several model runs, this system continues to shift further and
further south.  As a result, the rain chances are now mainly
confined to northeast Iowa and southwest Wisconsin.

High pressure will then build across the region over Saturday into
Sunday morning. With 925 mb temperatures warming back into the lower
20s.  We will likely see a return to above-normal temperatures.

For early next week, the models are in good agreement that a
longwave trough will move southeast into the region from Canada.
While there is agreement with this wave, there are differences in
the strength of the surface low which is generated.  The ECMWF is by
the strongest with this low.  Meanwhile, the GFS has a weak low. Due
to the uncertainty just stayed with the model consensus.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1100 PM CDT Tue Aug 14 2018

Cigs: Variety of mostly high/mid level clouds indicated via latest
sfc obs/satellite imagery. However, a smattering of mvfr associated
with a weak sfc front across central MN. Models don`t favor bringing
these clouds south - although a few are starting to suggest mvfr
could develop on edge of main cloud shield. Potential impacts for
KRST. For now, will stick with the bkn high cigs from an area of low
pressure to the south. Likely see some afternoon vfr cu development,
with some threat for bkn cigs.

Vsby/wx: numerical guidance still bullish on widespread fog toward
12z Wed. High bkn clouds will hamper these chances - but if it
clears for a few hours, that fog threat increases. Best chances seem
to lie in the west for now, where the low clouds could also
manifest. With high clouds holding in place, and T/Td spread of 8F
at 03z, the potential for thick fog at KLSE is decreasing.

With some weak boundaries at the sfc and a sfc/upper level low
passing eastward just south of the area, some shra/ts chances for
the local area into Wed night. Higher chances for TAF sites moreso
in the late afternoon/evening hours - but not enough confidence to
add to forecast at this time.

More small chances move in later in the day Thu as an upper level
shortwave spins across IA. Whether there would be impacts at
KRST/KLSE is uncertain at this time.

Winds: generally light through the period, more southeast tonight,
favoring northerly on Wed.




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