Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 190440

Area Forecast Discussion
National Weather Service La Crosse WI
1140 PM CDT Fri Aug 18 2017

.SHORT TERM...(Tonight through Saturday Night)
Issued at 145 PM CDT Fri Aug 18 2017

GOES Water Vapor loop shows a rather vigorous mid-level cyclonic
circulation/PV-anomaly rotating southeastward through the region
this afternoon. Radar had scattered showers and isolated storms
associated with this feature mainly confined along and west of the
Mississippi River. Otherwise, pleasant temperatures were in place
with readings in the middle 60s to the lower 70s.

For tonight, the aforementioned mid-level wave will drift southeast
of the region with rain chances tapering off and clearing skies from
north to south after midnight as high pressure build in. Will have
to watch for some fog development tonight, especially across those
areas that had rainfall today into this evening. Confidence is
fairly high there will be be patchy/areas of fog given clearing
skies/light winds/very moist lower boundary layer, but confidence is
lower on how dense it will get. Will have to keep a close eye on
this for possible fog headlines for later tonight into Saturday
morning. Plan on lows in the middle/upper 50s.

Rest of Saturday looks very nice with high pressure in control. Look
for mostly sunny skies with highs topping off in the upper 70s to a
few lower 80s. Dew points look to be in the comfortable range as
well in the lower/middle 60s.

Continued mostly clear conditions expected for Saturday night as
high pressure remains in control. A few more high clouds drifting in
from the west and increasing southerly winds should mitigate
widespread fog development. Will leave fog mention out for now but
will continue to monitor.  Otherwise, plan on lows in the upper
50s/around 60.

.LONG TERM...(Sunday through Friday)
Issued at 145 PM CDT Fri Aug 18 2017

Will be watching an approaching cold front on Sunday. Looking at
Bufkit soundings, a fairly strong cap will be in place, so better
shower/thunderstorm chances look to hold just northwest of the area
through the afternoon. Otherwise, warm air advection ahead of the
front expected to push highs into the lower/middle 80s along with
increasing dewpoints into the 65-70 degree range. So, a
warmer/more humid day on tap.

Shower/thunderstorm chances increase across the area Sunday night
into Monday as the cold front hangs up across the south and a wave
of low pressure rides along it. This increases moisture transport
across the front along with increasing CAPE and Bulk Shear. Exact
position of the front and timing of the wave still a bit off between
models but good agreement that clouds and scattered showers and
thunderstorms will be in the region for the solar eclipse Monday
afternoon. In fact, may have to watch for a few strong to possibly
severe storms given CAPE/Shear profiles. Locally heavy rainfall also
possible as precipitable water values approach 2 inches. Will be
keeping a close eye on all this.

Showers/thunderstorms continue to look likely heading into Monday
night through Tuesday morning as a strong mid-level trough drives a
cold front southward through the area.

Canadian high pressure builds into/holds over the area for Wednesday
through Friday for dry and slightly cooler than normal conditions.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1140 PM CDT Fri Aug 18 2017

Confidence is increasing that there will be periods of fog
overnight into Saturday morning. Winds have become light/calm and
besides some convective debris cirrus for the next couple hours,
skies should be mostly clear. 04Z dew point depressions have
dropped to 2 degrees at KLSE and 4 degrees at KRST. The primary
limiting factor could be the depth of the light wind layer.
Forecast soundings seem to be dropping winds aloft too quickly
based on the latest KARX VAD wind profile and if these winds
remain slightly elevated overnight, fog may not be as restrictive
to visibility. For this reason, will drop KLSE/KRST to 1SM/2SM
respectively for now, but reductions between 1/4 to 1/2SM are
possible and observations will need to be monitored closely for

Once fog lifts/dissipates Saturday morning, expect VFR conditions
for the rest of the period with mostly sunny skies and light




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