Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 210820
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
320 AM 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
not.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1147 PM CDT Tue Jun 20 2017

The NAM is a bit less aggressive with developing some fog at KLSE
on Wednesday morning. It is now in better agreement with the GFS
and RAP at keeping the boundary layer dry. Due to this, kept the
fog out at KLSE.

A short wave trough will bring a broken mid to high deck of clouds
to the area on Wednesday morning as it moves southeast through
Iowa. A 6 to 8K deck of diurnal clouds will develop late
Wednesday afternoon.

There continues to be quite a bit of mixed model solutions on
precipitation chances for Wednesday afternoon and evening. With
the best consensus during the evening, opted limit showers and
thunderstorms to that time period.


&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...JM
LONG TERM...JM
AVIATION...Boyne



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