Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 151744

Area Forecast Discussion
National Weather Service La Crosse WI
1244 PM CDT Sat Jul 15 2017

Issued at 1244 PM CDT Sat Jul 15 2017

Going forecast looks to be in pretty good shape heading into the
evening hours, with our cold front of interest just having
crossing into far northwest Wisconsin the past hour. Boundary
layer moisture continues to increase over in our direction, with
dew points climbing well into the lower 70s as far east of
southeast Minnesota (looking at you RST with a recent 70F dew
point). Expect that trend to continue at the same time warmth
expands with a plume of 23-26C 925mb temps expanding just ahead of
the front likely delivering highs into the upper 80s-lower 90s by
late afternoon, despite a slower start courtesy of some passing
mid/high clouds. Per the expected environment, a 90/73 parcel will
do the trick to erode initial stout capping, and per trends we may
do a little better than that, suggestive that convection may have
some chance to expand a little farther west than earlier trends.
Still unsure just how far west things will get, especially with
NAM/GFS dew point progs near 80F well overdone and some better
capping closer to the warmer temps aloft west of the Mississippi
River (not to mention less low level convergence out that way),
but we`ll just have to watch satellite trends through late
afternoon to see if we can build up a better cumulus field.

As far as severe potential, deep/fat CAPE profiles in the presence
of marginal shear for supercells do suggest a very large hail
threat, and in fact the setup isn`t too much unlike the July 6th
hail event which dropped some baseballs closer to the Trempealeau
area. Any storm that can develop some spin/supercell
characteristics could put down some pretty big stones, with
ambient mid level dryness also suggestive of some wet microburst
potential. All in all, our current forecast timing of 4-10pm still
looks pretty good for the greatest threat, though could slip by
an hour or so into late evening. Stay tuned...


.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 18Z TAFS through 18Z Sunday afternoon)
Issued at 1244 PM CDT Sat Jul 15 2017

A cold front continues to push south out of northern Minnesota,
slated to push through the LSE/RST areas roughly 01-03Z.
Increasing moisture ahead of that front is expected to lead to
scattered shower and storm development into late afternoon and
this evening, with the greatest risk locally from about LSE on
east into Wisconsin. Can`t completely rule out a shower or storm
closer to RST, though confidence in convection that far west
remains quite low given increasing capping over areas west of the
Mississippi River. Any storms could produce large hail and brief
damaging winds. As the front crosses the area, the risk for storms
will quickly end as winds shift to the north, with some
development of MVFR stratus expected into northern Wisconsin, with
a risk for that to expand southward toward LSE and eventually RST
through Sunday morning.




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