Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 250733
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
233 AM CDT Fri Sep 25 2020

.SHORT TERM...(Today and tonight)
Issued at 233 AM CDT Fri Sep 25 2020

Starting to see some improvement in the visibility on the
southern edge of the dense fog advisory as the winds aloft have
picked up. Expect this to work north with time as the stronger
winds continue to spread north and will make changes to the dense
fog advisory once the visibility improves and looks to remain up.

Water vapor satellite shows a short wave trough over southern
Saskatchewan this morning that was moving to the east. The
southern portion of this wave is expected to strengthen during the
day but is not expected to dig and should move across northern
Minnesota, northwest Wisconsin and Lake Superior. This is close
enough to bring some weak pv advection in the 500-300 mb layer
across the far northern portions of the forecast area late this
afternoon. At the surface, the area of low pressure over South
Dakota will lift rapidly northeast today with the short wave
trough, moving across central Minnesota and northwest Wisconsin.
As it does, it will lift the stalled front across central
Minnesota and Wisconsin back to the north as a warm front with
this expected to lift into the U.P. of Michigan by late afternoon.
The low level moisture transport axis will increase during the
afternoon and become focused on the warm front by late afternoon
where convection is expected to develop. Most of the hi-res meso-
scale models suggest the bulk of the convection will remain along
and north of the warm front. However, there are a few that suggest
there could be some trailing development farther south in the
moisture transport axis as the cold front approaches. The RAP is
suggesting ML CAPE around 1000 J/kg will be present in the warm
sector along with good deep layer shear of 40+ knots in the 0-6km
layer. With the strong low level winds, there should also be
around 25 knots of shear in the 0-1km layer. However, forecast
soundings suggest this may not come into play with the convection
as the warm air advection may cap off the lowest 2 km or so. As a
result, would expect any activity that gets into the far northern
sections of the area today to be elevated with hail as the main
threat and possibly some strong winds. The window for this
activity looks to rather short and generally between 6 and 9 pm
before quickly pushing off to the northeast of the area.

.LONG TERM...(Saturday through Thursday)
Issued at 233 AM CDT Fri Sep 25 2020

The cold front that moves into the area early Saturday morning
will stall/wash out a bit across northern portions of the
forecast area. Shortwave energy and an associated deepening
surface low will then move just northwest of the forecast area
Saturday afternoon, with the boundary moving back northward as a
warm front. The deepening low will lead to strengthening moisture
transport into said front and lead to additional shower and storm
chances. Instability still looks pretty meager with maybe 1000
J/kg of MUCAPE possible. In addition, we do look to stay pretty
capped owing to persistent cloud cover and a warm nose in the 800
to 700 mb layer. Even so, should be enough forcing for some
showers and storms before the warm front and nose of the moisture
transport advance north of the forecast area. Also, shear still
looks to be very strong with 60+ kts in the 0-6 km layer, so can`t
rule out a few strong storms despite the lack of instability.
Otherwise, with fairly strong southerly flow and lack of rain
across most of the area through the day, Saturday looks to be the
last day of temperatures in the 70s to lower 80s for the near
future.

Another cold front will then pass through Saturday evening
followed by a large scale upper level trough beginning to dig into
the eastern half of the CONUS. This will serve as the transition
to a wetter and much cooler period through next week. There`s a
little model spread on how far southeast said front makes it by
Sunday, with the 25.00Z ECMWF being a bit slower and hanging it up
over southeast portions of the forecast area, bringing us more
precip chances during the day. The 25.00Z GFS, on the other hand,
has it well southeast of the area (but still produces precip along
it). Bottom line, will need to keep an eye on its progress, as
it`ll serve as the focus for rain.

Beyond Sunday, the main feature of note will be a surface low
that looks to develop near the Missouri/Arkansas border Sunday
night/Monday as some shortwave energy rounds the base of the
larger scale trough. Said low is progged to deepen and quickly
advance northward, phasing up with a shortwave north of the Great
Lakes on Tuesday and with a fairly potent cutoff low resulting.
Various bits of shortwave energy will rotate around the low and
slide through the northwest flow over the forecast area giving us
additional shower and a few storm chances throughout the week.
Also, the deepening low will provide a tight pressure gradient
across the region and gusty northwest winds will be the result.
Tuesday into Wednesday looks to be the windiest, with 35-40 mph
winds at the top of the mixed layer, so could see some gusts near
30 mph at times. That all said, most guidance has trended downward
on winds. In addition, low confidence in exact timing of
rain/increased cloud cover doesn`t help with the wind forecast, as
less mixing will occur during the cloudier/wetter periods. Still,
an overall cool, wet, and windy week looks to be on tap, with
high temperatures in the 50s to lower 60s and lows in the 30s and
40s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1045 PM CDT Thu Sep 24 2020

With the light winds, decreasing clouds, and wet ground, both KLSE
and KRST have seen their low level relative humidity approach 100
percent. KLSE has been bouncing around around 1/4 and 1/2sm while
KRST has yet to see any visibility restrictions. For now did keep
KLSE down in fog until the winds aloft can scour out the low level
moisture Thursday morning. Not sure how far west the dense fog
will spread, thus have IFR/LIFR condition at KRST. As a warm front
lifts north, low level winds around 925mb increase to 25kts and
this should help increase visibilities 12-15Z. South winds
increase to 15 to 20 with gusts 25 to 30kts after 15z.


&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Dense Fog Advisory until 8 AM CDT this morning for WIZ017-029-
     032>034-041>044-053>055.

MN...Dense Fog Advisory until 8 AM CDT this morning for MNZ079-
     086>088-094>096.

IA...Dense Fog Advisory until 8 AM CDT this morning for IAZ011.

&&

$$

SHORT TERM...04
LONG TERM...CA
AVIATION...Zapotocny


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