Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 211739

Area Forecast Discussion
National Weather Service La Crosse WI
1139 AM CST Sat Jan 21 2017

Issued at 1139 AM CST Sat Jan 21 2017

Per trends and expectations and in coordination with surrounding
offices, have decided to simply extend the Dense Fog Advisory
through 18Z tomorrow. southern areas remain stuck in the soup at
the moment per webcams/obs while northern areas have struggled to
see much dense fog, owing to precipitation moving through those
areas. However, that is changing quickly with visibility dropping
north of I-90...a trend that should continue into the afternoon as
an occluded frontal boundary lifts north but then washes out as
the associated surface low fills with light flow and pooling of
moisture over melting snow pack along that feature. South of that
boundary, it is possible (maybe even likely?) that visibility will
improve for a bit this afternoon and early evening, but given a
return to lighter flow overnight, suspect residual stratus will
simply build back down into a fog bank for most locations. Given
the complex setup, have opted to take the simple route with the
advisory extension but of course will continue to monitor trends.

Meanwhile, us forecasters here are a bit jealous of conditions
just to our east across Illinois where widespread sunshine is
kicking temperatures into the upper 50s already! So close...yet so


.SHORT TERM...(Today through Monday)
Issued at 212 AM CST Sat Jan 21 2017

Observations this morning continue to show dense fog covering most
of southeast Minnesota, northeast Iowa and southwest into central
Wisconsin. The dense fog has recently shown some expansion into
parts of north central Wisconsin while the higher terrain of the
remainder of western Wisconsin was also likely experiencing dense
fog. Will expand the dense fog advisory to cover the remainder of
the forecast area. The 20.21Z SREF probabilities look to have the
best handle on this fog once again this morning and suggest there
should start to be a general increase in the visibility as the
rain works across the area, starting between 12Z and 15Z with what
looks to be a pretty rapid improvement after that.

Water vapor satellite early this morning shows the remains of the
upper level low that impacted the region Friday was over North
Dakota. The next short wave trough was lifting north across
southwest Iowa. This wave will continue to lift north through the
day and is expected to reach the northern Great Lakes tonight. As
it moves across the region, it is expected to produce some
moderate pv advection in the 500-300 mb layer through the morning
hours along with some weak qg convergence in the 1000-500 mb
layer. The warm air advection ahead of the wave will produce a
broad area of 2 to 4 ubar/s of up glide on the 290K isentropic
surface that will also work across the area this morning. This
forcing is already producing an area of rain across western Iowa
that is expected to lift north/northeast with the wave and across
the area. The best chances for rain should be across the western
sections of the area where the forcing will be the strongest, but
the entire area will have a chance of seeing some rain with this
system. While the best chance for rain will be this morning, there
will be some lingering chances through the night across the north
until the upper level trough axis moves past the area.

.LONG TERM...(Monday night through Friday)
Issued at 212 AM CST Sat Jan 21 2017

Both the 21.00Z ECMWF and GFS continue to show a short wave trough
coming across the central Rockies early next week. A closed upper
level low should quickly form with this wave over the Missouri
River Valley Tuesday but then just about as quickly start to
become an open wave as it begins to merge with the strong system
that is working its way up the the East Coast. The surface low
track looks to have shifted just a bit to the south with the
latest runs now taking it from Kansas across southern Iowa into
northern Illinois. Looking at the GFS forecast soundings, the bulk
of the precipitation from this system should fall as snow, except
on the leading edge when low level temperatures may be warm enough
for a change over to rain or a mix of rain and snow. With the
current timing, some of the precipitation could start to move in
Tuesday afternoon with the majority of the precipitation then
falling Tuesday night into Wednesday morning. With the current
track, accumulations will occur that would likely be in the
advisory range. Behind this system, it looks like temperatures
will return to more seasonable values with some on and off light
snow chances as the upper level flow pattern remains cyclonic with
some weak short wave troughs moving across the region.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1139 AM CST Sat Jan 21 2017

Widespread lower stratus and fog continues across the area and is
likely to remain in place through midday Sunday. There is a risk
that ceilings could briefly lift for LSE later this afternoon as
an occluded frontal boundary and some warmer air intrude
northward, though confidence in that remains lower. Any higher
ceilings and/or visibility should replaced by a return to lower
conditions tonight, with widespread IFR to LIFR conditions
expected just about everywhere. Winds will remain light through
the period (under 10 knots) shifting from the southeast now to
south later this afternoon and northwest tonight.


WI...Dense Fog Advisory until noon CST Sunday for WIZ017-029-032>034-

MN...Dense Fog Advisory until noon CST Sunday for MNZ079-086>088-

IA...Dense Fog Advisory until noon CST Sunday for IAZ008>011-018-019-



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