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fxus63 karx 171914 

Area forecast discussion
National Weather Service La Crosse WI
214 PM CDT Sat Aug 17 2019

Short term...(this evening through sunday)
issued at 213 PM CDT Sat Aug 17 2019

Plenty of features to note in early afternoon observational
analysis. The weak ridging at the surface and aloft that has kept US
dry today continues to push off to the east while a weak shortwave
trough approaches. To the south, a warm front was draped across
northern Illinois westward into Iowa with a fairly sharp
accompanying wind shift. To the northwest, a much stronger shortwave
trough and surface cold front were moving through the Dakotas. Each
of the aforementioned features will play a key role in the weather,
namely precip chances, through the short term period.

The first concern will along the warm front late this afternoon as
the weak shortwave moves through. A few of the cam solutions
(hrrr/esrl hrrr) along with the rap hint at some isolated activity
developing along the front sometime in the 20-23z timeframe. Latest
Storm Prediction Center mesoanalysis shows around 1000-1500 j/kg of MUCAPE along the
front, so it will just be a matter of how much convergence will be
seen. In addition, around 40 kts of deep layer shear could be enough
to yield some degree of organization to some of the storms, though
veer-back-veer wind profiles won't help.

Attention then turns to further development along and near the warm
front late tonight as a fairly strong low level jet/moisture
transport signal are progged to point into southwestern portions of
the forecast area. Overall trend in the rap through the day has been
slightly weaker with this feature, but it still remains plenty
strong as of the 17.17z run to get storms to develop. In addition,
the cold front will be approaching the area around the same time,
with convection well underway ahead of it. While deep layer shear
will be on the downward trend through the night, low to mid level
shear will increase as the low level jet strengthens, and the rap
has consistently progged 2000-3500 j/kg of MUCAPE to build into the
forecast area prior to convection developing/arriving. While all of
this seems to point toward strong to severe storms, timing won't be
until after midnight (probably closer to 8-9z), so storms would
likely be elevated, any severe weather reports /hail, wind/
likely being fairly isolated. In addition, while the airmass will
be supportive of heavy rain, the cold front moving in should keep
things fairly progressive and preclude any widespread flooding

The front will continue eastward on Sunday with perhaps a few
lingering showers during the day, especially southwest WI. Plenty
of cloud cover is expected for much of the day, with temperatures
topping out in the mid 70s to mid 80s.

Long term...(sunday night through saturday)
issued at 213 PM CDT Sat Aug 17 2019

A glorious Monday is followed by return flow from the south as
high pressure departs to the east. Instability builds during the
day based on consensus of the deterministic 17.12z models...with
MLCAPE values in the 1500-2500 j/kg range. This is ahead of a cold
front shifting in from the west. However, warming mid-level
temperatures look to provide a capping inversion that may be hard
to break given the degree of forcing in the low-levels. Low-level
moisture convergence varies along the front, and the moisture
transport signal does as well in the lowest 2 km. Wind shear is
marginal as well but there are suggestions of a mid-level wind Max
slightly upstream of the front. So, Tuesday will need to be
monitored for thunderstorm potential as there are at least a few
ingredients that are in play to favor storm development. Forecast
rain chances are in the 30-40% range.

after a cold front moves through, Canadian high pressure builds
into the region. This will bring a refreshingly cooler airmass
into the region as well as drier weather for Wednesday. This dry
period is expected to last through Friday. Later in the week, the
high pressure moves to the east and warmer air returns. Although
low precipitation chances creep back into the region (~20%),
Saturday is questionable regarding precipitation as the flow
becomes more southwesterly.

Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1229 PM CDT Sat Aug 17 2019

VFR conditions expected through much of this evening with
scattered cumulus in the 3000-3500 agl layer this afternoon and
winds from the south/southeast in the 5 to 10 kt range. Widespread
showers and storms are then expected during the overnight hours
ahead of a cold front, though confidence remains a bit low on
exact timing at each taf site, so will need to refine that in
future issuances. That said confidence is high on storms
occurring, so introduced thunderstorms and rain tempo group with IFR visibility at
both sites. May need to eventually add some gusts associated with
storms, but will hold off for this issuance as there are still
some questions if the strongest winds will make it to the surface.
Most storms should exit by Sunday morning with the front not far
behind, though there are some hints of a lingering MVFR cloud deck
with perhaps some lighter showers. Winds behind the front will
become westerly/northwesterly at around 10 kts.

Arx watches/warnings/advisories...


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