Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 041737

Area Forecast Discussion
National Weather Service La Crosse WI
1237 PM CDT Tue Aug 4 2020

.SHORT TERM...(Today and tonight)
Issued at 251 AM CDT Tue Aug 4 2020

Early morning analysis showed a mostly quiet, cool night across the
area, as surface high pressure/ridging remained in control.
Temperatures at 2 a.m. ranged from the lower 40s in the favored cold
spots of central Wisconsin to the mid 40s and 50s elsewhere. A few
patches of clouds were seen across northwest Wisconsin (along with a
few sprinkles/light showers) and also in parts of northeast Iowa
into far southwest Wisconsin. Otherwise the rest of the area
remained clear, which should favor at least patchy fog development
in some of the river valleys. That said, latest VAD wind profile
from KARX showed some pretty decent winds aloft which may provide
just enough mixing to preclude fog development. Latest RAP guidance
does suggest these winds taper off, however, so keeping patchy fog
mention in the Mississippi River tributaries and cranberry bog areas
early this morning.

Otherwise, expect a quiet day today with highs in the upper 60s to
mid 70s and some diurnal cumulus development. Model soundings do
suggest some instability in the cumulus layer and a few CAMs do show
some returns in their composite reflectivity fields, though thinking
surface ridging and dry air in the low levels should dominate and
keep the area dry.

.LONG TERM...(Wednesday through Monday)
Issued at 251 AM CDT Tue Aug 4 2020

The models continue to indicate a decent short wave trough will
top the upper level ridge over the Rockies and drop across the
region Wednesday and Wednesday night. The models also continue to
show this wave will be weakening as it moves across the area as it
enters the confluent flow under the upper level low spinning
across Quebec. As a result, the forcing will not be all that
strong with only some weak pv advection expected in the 500-300
mb layer. Not a lot of other forcing will be in place to help
generate some convection. The low level moisture transport ahead
of the wave will be strongest across Minnesota Wednesday afternoon
and the weaken quickly Wednesday night. The frontogenesis looks
very similar with a weak band in the 1000-700 mb layer over
Minnesota that fizzles out Wednesday night as it moves into the
persistent and strong low level ridging in place over the eastern
Great Lakes into the Ohio River Valley. As a result, the best
coverage of showers and storms should be over Minnesota, west of
the local area, Wednesday afternoon with the possibility of some
of this activity surviving about as far east as the Mississippi
River Wednesday night before weakening rapidly. Another weak short
wave trough may zip across the area Thursday and Thursday night
to maintain a small chance for some rain.

Over the weekend, the models are now trending toward flattening
out the upper level ridge over the Rockies and allowing a quasi-
zonal flow to develop over the Upper Midwest. This has the
potential to allow some short wave troughs to move across the area
along with some showers and storms. Right now, none of these waves
look to be very strong and since this is a relatively new trend in
the models, will keep the rain chances generally under 40 percent.

Temperatures will be slowly climbing for the rest of the week, but
primarily staying the 70s for highs through Thursday. It still
looks warmer and more humid for the weekend with highs 80s and dew
points returning to around 70 by Sunday afternoon.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1235 PM CDT Tue Aug 4 2020

VFR conditions expected to continue through this TAF period.
SCT to BKN diurnal cu/stratocu field has expanded across the area
with light northwest winds through the afternoon. Clouds will
likely thin a bit tonight with light winds becoming more
southerly to southwesterly by Wednesday. Breezy winds aloft
overnight are still expected to preclude fog development at LSE
tonight, but some fog may develop in tributary valleys.




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