Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 211730

Area Forecast Discussion
National Weather Service La Crosse WI
1230 PM CDT Wed Jun 21 2017

.SHORT TERM...(Today through Thursday night)
Issued at 320 AM CDT Wed Jun 21 2017

An active weather pattern is expected over the next 48 hours with
multiple chances for thunderstorms, possibly strong to severe.

Early this morning surface high pressure was situated over the
area. With efficient radiational cooling, some fog will be
possible early this morning before the strong June sun works to
quickly dissipate it.

Surface high pressure will then quickly move east of the area
today. Of interest will be a deepening upper trough that will
eventually shift into the Canadian prairies by tomorrow. Strong
warm advection will develop ahead of the associated frontal wave
across the plains and into the Midwest today. Some
showers/sprinkles are possible this morning with initial 700 mb
warm/moist advection. As the instability axis/steep mid-level
lapse rates approach through the afternoon with increasing mid-
level warm advection/moisture transport, chances for
showers/storms may begin to increase late in the afternoon.

There is still quite a range of solutions tonight with convective
evolution. With strong capping to the west, strong moisture
transport and warm advection around 850-700 mb near the northward
advancing warm front looks to be the focusing mechanism to get
elevated convection going. Exactly where the storms initiate and
the coverage is somewhat in question, but higher chances may occur
in western Wisconsin during the evening, with convection pushing
eastward through the night. With increasing elevated instability
Strong/severe storms are possible given 30-40 kts of deep layer
shear. With the elevated nature of the storms, hail will be the
main threat, although higher winds would be possible as well.
While locally heavy rainfall is a concern with precipitable water
values rising near 1.6 inches and deep warm cloud depths, the
stronger warm advection/moisture transport moves through pretty
quickly, which should limit the heavy rain threat.

There may be a lull in the convective activity Thursday morning as
the warm advective convection lifts away from the area. While
there is some model disagreement on the extent of storms Thursday
morning, if much of the overnight convection has shifted north and
east, this would allow for destabilization of the pre-frontal warm
sector. In this scenario, the slowly southward moving cold frontal
zone should help ignite another round of storms Thursday
afternoon and evening. A very moist environment will reside ahead
of the boundary with westerly 0-6 km bulk shear around 35-50 kts,
increasing with northward extent. Shear/instability parameters
would definitely support another severe threat over at least
portions of the area Thursday afternoon/evening with surface-based
storms possible near the low-level boundary. The highest chances
still look to be near and south of I-90. Locally heavy rainfall
will continue to be a threat with a similarly warm, moist
atmospheric profile along the slow- moving boundary. Will have to
watch for areas that may receive heavier rainfall tonight, with
the second round of storms possible tomorrow.

.LONG TERM...(Friday through Tuesday)
Issued at 320 AM CDT Wed Jun 21 2017

Dry weather should return behind the front on Friday with breezy
northwest winds and pleasant highs generally in the 70s. Northwest
flow will develop this weekend into early next week. A period of
seasonably cool temps is expected with occasional rain chances as
shortwaves rotate southeastward within the broader longwave trough.
However, there are still differences among the various models with
timing/placement of these waves, so confidence in precip chances
remains on the low side. The pattern would support periodic chances
for isolated/scattered showers and storms, especially during the
afternoon hours under peak heating, but with more dry hours than


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1230 PM CDT Wed Jun 21 2017

Good VFR expected this afternoon/evening as dry-ish lower level air
remains over the area. Moisture/instability increases into/over the
area tonight, but this generally above a rather strong inversion
around 850mb. Clouds to gradually thicken/lower tonight, but remain
VFR until around 12z Thu when a cold front pushes into the area.
Some lower clouds/potential MVFR cigs then expected Thu morning with
moisture pooled near the front. Ahead of the front for a few hours
either side of 06z, winds aloft expected to increase to around 40kts
near the 2K ft level. A generally quiet time at the airfields, but
included a LLWS mention at both sites in roughly the 04-08z time-

Main forcing/lift with the returning moisture now looks to move
rather quickly NE of the taf sites this evening, with a reduced
chance of SHRA/TSRA at the taf sites tonight as warming mid level
temps right behind the moisture keep rather strong capping over the
area. Did leave a VCTS/CB mention at KLSE 01-04z when it appears
SHRA/TSRA could fire near the MS river before moving into central WI.




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