Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 132325

Area Forecast Discussion
National Weather Service La Crosse WI
625 PM CDT Thu Jul 13 2017

.SHORT TERM...(This evening through Friday)
Issued at 323 PM CDT Thu Jul 13 2017

At 3 PM, cyclonic flow associated with a low pressure area over
Northern Lower Michigan continued produce partly to mostly cloudy
skies across the Upper Mississippi Valley. With the loss of
diurnal heating tonight, these clouds should gradually break up
across much of the area. With mostly clear to partly cloudy
skies, dry dew points, and light winds, temperatures will fall
into the 50s early Friday morning.

On Friday morning, the cyclonic flow will be a bit further east,
so the greatest concentration of diurnal cumulus clouds will be in
central and north-central Wisconsin. These clouds will continue
into the early afternoon and then decrease as drier air and
subsidence builds across this area. High temperatures will be in
the mid and upper 70s.

.LONG TERM...(Friday night through Thursday)
Issued at 323 PM CDT Thu Jul 13 2017

From Saturday afternoon into Saturday evening, a Canadian cold
front will move southeast through the area. Ahead of this cold
front an elevated mixed layer will be advected into the region
from the Central Plains. This will result in a 900 to 600 mb cap
across the region. As the cold front brings slightly cooler air in
aloft, this cap will gradually erode during this time frame.
However, there is still some questions on whether there will be
enough convergence along the front and cooling aloft for showers
and storms to develop. The NAM continues to show that the area
will remained capped. Meanwhile, the GFS and ECMWF cools down
enough aloft for showers and storms to develop. Another question
is whether dew points are too high. The NAM and GFS are raising
the dew points into the mid and upper 70s and the ECMWF raises
them into lower 70s. This results in the 0-1 km mean CAPES being
1000 to 1500 J/kg higher in the GFS and NAM. With the best shear
in the 0-3 km range thinking that damaging winds are the main
threat. However, considering the possibility that the instability
may be in the excess of 3500 J/kg, large hail will also be a
possibility. All of this will be dependent on whether the cap

From Tuesday into Thursday, the GFS and ECMWF continue to struggle
on how much the 500 mb ridge will flatten. The GFS has a much
stronger wave and this result in a front stalling near the the
Minnesota/Iowa border through much of this time period. With a
very unstable and wet air mass south of this boundary. This could
result in a prolonged period of very heavy rain and severe weather
for areas along and south of Interstate 94. Meanwhile, the ECMWF
has a weaker wave initially and its first round of showers and
storms are located across central Minnesota and northern
Wisconsin. This front then slowly sinks south as additional
convection fires along this front. This is a time period that
needs to be watched closely.


.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 625 PM CDT Thu Jul 13 2017

Widespread VFR/low MVFR cloud deck across most of MN/northern WI
will move slowly eastward through the period. BKN-OVC clouds at
TAF airfields this evening should remain above 3000 ft agl, but
could at times flirt with MVFR thresholds. Will need to watch back
edge for eventual scattering/clearing, but should occur later
this evening into the overnight. Scattered VFR clouds will then
persist through the rest of the period. Winds through 15.00Z will
remain from the northwest, generally 12 kts or less.




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