Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 181845

Area Forecast Discussion
National Weather Service La Crosse WI
145 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 18Z TAFS through 18Z Saturday afternoon)
Issued at 1231 PM CDT Fri Aug 18 2017

A few light showers in southeast Minnesota will continue to
dissipate while moving east this afternoon. KLSE could see a quick
shower toward the beginning of the period, with remnant showers
lingering a bit longer around KRST. Otherwise, the main concern for
this TAF issuance is fog potential tonight. Model soundings in good
agreement with light winds and clear skies, especially at KRST, so
included 2SM visibility mention at KRST beginning at 9Z. However, if
showers/storms that currently look to remain west of the area do
move a bit farther east this evening, some clouds could linger at
KRST, reducing the fog threat. For KLSE, the RAP has trended a bit
windier aloft in the last few runs leading to less confidence for
fog development, so went with BCFG, but the potential is certainly
there for reduced visibility at the TAF site. Any fog should
dissipate at both sites by 14Z, giving way to clear skies and very
light winds for the first part of Saturday.




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