Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 150700

Area Forecast Discussion
National Weather Service La Crosse WI
200 AM CDT Sat Jul 15 2017

.SHORT TERM...(Today through Monday)
Issued at 200 AM CDT Sat Jul 15 2017

Early this morning, water vapor satellite showed a short wave
trough extending from southwest Ontario into northeast North
Dakota with a cold front at the surface in roughly the same
location. The short wave trough will continue to move southeast
through this evening with the main portion of the wave staying
north of the Great Lakes and traveling across southern Ontario.
There is the suggestion that a weak portion of this wave will
break off and take a more southern track coming across northern
Wisconsin this afternoon. Both the 15.12Z NAM and GFS indicate the
best pv advection in the 500-300 mb layer will stay over Canada
with the main short wave trough and any weak pv advection with the
more southern wave, looks to actually come through early in the
afternoon, well ahead of the front. With this timing, the weak
short wave trough may not provide much, if any, forcing to help
form convection as the front moves in. As the front moves in, a
band of weak frontogenesis in the 1000-700 mb layer should
accompany it along with weak moisture convergence in the boundary
layer. The low level jet still does not look to be much of a
player as the moisture transport continues to look like it will
start post-frontal and come into the back side of the front. A
band of 2 to 3 ubar/s of up glide on the 310K isentropic surface
should be in place right ahead of the front. Forecast soundings
continue to show a moderate low level cap will be in place ahead
of the front and the concern becomes if the low level forcing with
the front will be enough to break the cap and if it is, will
there be enough time for convection to develop before the front
moves through and starts to stabilize the atmosphere again. Both
the NAM and the 17.04Z RAP suggest the cap should break to allow
at least some scattered convection to form over parts of
Wisconsin. Farther west, the prospects of the cap breaking do not
look as good and the front could pass through southeast Minnesota
and northeast Iowa dry. For now, will continue with a 20 to 40
percent rain chance as the front comes through for the entire
area, with the highest chances early this evening over central

If some storms can form, they will have the potential to be
strong to possibly severe. Models are likely overdoing the dew
point forecasts ahead of the front and thus the CAPE as well.
Would expect some lower 70s dew points to be able to return ahead
of the front which should yield ML CAPE values of 2000 to 3000
J/Kg. The shear profile does not look all that impressive with
30 to at most 35 knots expected and with that primarily in the 0-3
km layer. Forecast soundings do show a rather fat CAPE profile
above the cap along with some dry mid level air, so would expect
large hail and damaging winds to be the main threats with any
potentially severe storms.

.LONG TERM...(Monday night through Friday)
Issued at 200 AM CDT Sat Jul 15 2017

The latest GFS and 15.00Z ECMWF continue to indicate there will be
a lot of rain chances for next week without much upper air
support. The upper level ridge currently over the Rockies should
gradually shift east into the central part of the country and get
flattened some as well. This looks to place the main band of
westerlies along the Canadian border or farther north through the
period. At the surface, both models still want to bring a cold
front into the region Monday night into Tuesday and then stall
this feature over the region for the remainder of the week. If the
front does indeed remain over the area, it will be the focus of
the on and off rain chances for next week.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1144 PM CDT Fri Jul 14 2017

Skies have cleared under high pressure and are expected to remain
cloud-free through the night. One caveat will be the potential
development of some patchy fog near sunrise Saturday morning,
especially within river valleys. For now, will keep KLSE fog free
as model guidance suggests near surface winds will increase just
enough to preclude restrictions to visibility.

Some cumulus development is expected through Saturday, mainly in
the 3000 to 5000 ft agl layer. A cold front will then drop south
across TAF airfields in the early evening. This could result in
some scattered showers/thunderstorms, especially at KLSE, but not
enough confidence in timing/coverage to include storm mention at
this time.

Light and variable wind overnight will increase slightly from the
south-southwest after sunrise and then eventually shift to the
northwest after cold frontal passage late in the period.




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