Dense Fog Advisory 2, NWS La Crosse, WI

Area Forecast Discussion
National Weather Service La Crosse WI
310 PM CST Thu Jan 19 2017

.SHORT TERM...(This evening through Friday)
Issued at 310 PM CST Thu Jan 19 2017

The widespread fog has gradually lifted through the late morning and
early afternoon. Meanwhile, low clouds along with light rain have been
spreading northward across eastern Iowa within region of enhanced low-
level moisture transport ahead of an upper low slowly lifting northeastward
out of the central Plains. The light rain will continue to spread northward
into the area late this afternoon and evening. While most areas are
expected to be above freezing, areas north of I-94 stand the best chance
for some freezing rain/icy roads with temps near freezing and
cold roads. Have issued a Freezing Rain Advisory this evening into
Friday morning for Clark/Taylor counties to account for this. Will
have to watch surface/road temps closely this evening for any
possible adjustments to this area.

In addition, outside of the precip, there will likely be fog again and
some drizzle as well into Friday with NAM soundings indicating deep
saturation to around 1.5 km in depth along with some weak warm advection
and light boundary layer flow. As a start, have included northeast Iowa
counties in a Dense Fog Advisory from midnight tonight until noon Friday,
where precip is expected to be lighter and confidence is highest. This
could need to be expanded through the evening or overnight. Areas of
Clark/Taylor counties that have been slow to clear, and thus have warmed
little today, may see fog return quickly this evening, even before the
precip arrives, so that will be another area to watch.

.LONG TERM...(Friday night through Thursday)
Issued at 310 PM CST Thu Jan 19 2017

Another upper shortwave trough will lift northeastward into the Upper
MS Valley Fri night in response to amplified upper level ridging/blocking
downstream into eastern Canada. The strongest mid-level forcing/Q vector
convergence will shift from Iowa into northwestern WI bringing some
light rain into the region, with boundary layer temps largely above
freezing. With the relatively weak forcing based on the placement
of the upper wave via the GFS/ECMWF, it may be a very light rain/drizzle
scenario. Also, with plenty of boundary layer moisture and modest
low-level warm advection, may still have some fog issues to deal
with as well. Not much change is expected into Saturday with low
stratus persisting along with some residual light rain/drizzle
from the slowly moving upper wave. Temps on Saturday will be mild,
but how warm we get likely will be impacted by the degree of low
stratus/fog across the area.

The disjointed blocking upper level flow pattern will continue into
early next week with little change in air mass. As a result, temps
will remain seasonably mild with generally cloudy under weak surface
ridging.

The next time frame to watch will be later Tues/Wed with both the 19.12Z
GFS/ECMWF indicating an upper shortwave trough/surface low ejecting
out of the Rockies towards the Upper MS Valley. It's too early to have
confidence in the track, amounts, and thermal profiles/precip types
yet, but this will be a time to watch. For now based precip types
on surface temps as a start.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1124 AM CST Thu Jan 19 2017

Widespread dense fog remains intact at RST and will very slowly
lift over the next several hours, though with IFR conditions
remaining. KLSE has improved in visibility but will still see a
lower ceiling for a while yet. An area of rain arriving through
late afternoon and this evening should actually help visibilities
improve for a time, but behind that rain shield, expect to see
another round of lowering ceilings and visibility, such that LIFR
conditions or lower return for most locations along with some
drizzle at times.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Freezing Rain Advisory from 6 PM this evening to 9 AM CST Friday
     for WIZ017-029.

MN...None.
IA...Dense Fog Advisory from midnight tonight to noon CST Friday for
     IAZ008>011-018-019-029-030.

&&

SHORT TERM...Lawrence/JM
LONG TERM...JM
AVIATION...Lawrence

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Special Weather Statement
National Weather Service La Crosse WI
112 PM CST Thu Jan 19 2017

WIZ017-029-192100-
Taylor-Clark-
Including the cities of...Medford...Neillsville
112 PM CST Thu Jan 19 2017

...Areas of Dense Fog Lifting into Mid Afternoon...

Areas of dense fog continue to linger for areas near and north of
Interstate 94, with visibility reduced to near zero in spots. This
fog is expected to gradually lift through 3 pm. Those traveling
are urged to slow down and be prepared for rapid fluctuations in
visibility.

Lawrence