Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 182045

Area Forecast Discussion
National Weather Service La Crosse WI
245 PM CST Thu Jan 18 2018

.SHORT TERM...(This evening through Friday)
Issued at 245 PM CST Thu Jan 18 2018

Looking at a quiet period through Friday. Upper level ridging will
begin to build in over the region tonight and then flatten out
Friday as a short wave trough tracks across southern Canada. This
wave will stay far enough to the north that it will only bring some
clouds to the area. Southwest flow in the low levels will continue
to bring warmer air to the region. Temperatures at 925 mb should
reach about 8C Friday, but with a large inversion in place, surface
temperatures will be much slower to respond. Still, high
temperatures Friday are expected to be about 3 to 5F warmer than
today topping out in the upper 30s to lower 40s.

.LONG TERM...(Friday night through Thursday)
Issued at 245 PM CST Thu Jan 18 2018

One more quiet day is expected on Saturday. With a mild start and
925 mb temps remaining in the 3-6C range, highs again should reach
the 30s to low 40s with some increase in clouds during the day.

The time to watch for impactful weather is Sunday/Monday as a
storm system swings up from the southwest. There are still enough
model differences to limit confidence on sensible weather impacts,
despite good agreement that a moisture-laden system will impact
the region.

By late Saturday night and Sunday morning as the depth of the
low-level saturated layer increases with developing warm
advection, a light wintry mix or freezing drizzle may develop.
Eventually any precip may switch over to just drizzle/light rain
during the afternoon, depending on how surface layer temps
respond through the day.

Sunday night into Monday, a vigorous upper level trough is
forecast to eject from the central plains into the upper Midwest.
The 18.12Z GFS continues its trend of lifting the closed low a
little farther north than the ECMWF. At the surface the GFS
remains farther west with the surface low. The mean surface low
track from the GEFS is from near Des Moines Sunday night to
northern Lake Michigan by Monday evening. The ECMWF over the past
few runs has preferred a more southern track from southern Iowa
towards Milwaukee. These differences significantly impact how much
warm air will work northward into the region and consequently
precip types. A farther north and west solution would bring more
rain over a larger part of the area until colder air works in on
the backside of the low, while a farther south ECMWF solution
would bring significant snow into at least northern parts of the
area. Not only are there placement differences, but also timing,
with the slower ECMWF holding onto some light snow into Tuesday,
compared with the faster GFS. Given these differences, confidence
remains on the low side for specifics (amounts, precip types) just
yet, despite very good large scale agreement that a significant
storm will impact the Upper Midwest Sun-Mon.

Following behind the system, a progressive upper pattern is
expected with northwest flow giving way to southwest flow aloft by
late week as an upper ridge axis shifts into the central US ahead
of another upper trough moving into the western US. A few upper
shortwave troughs may slide southeastward through mid-week, but
under the influence of dry surface high pressure, little in the
way of precip is anticipated. Fairly seasonal high temps mainly in
the 20s will continue through mid-week, with temps possibly
getting back into the 30s later in the week ahead of the next
system approaching from the west.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1101 AM CST Thu Jan 18 2018

VFR conditions are expected through the period. Ridging aloft will
be moving into the region but gradually weakening overnight into
Friday. With this ridging moving in, only expecting some
occasional high level clouds to be over the area. At the surface,
high pressure over the southeast part of the country will maintain
a south to southwest flow over the region with speeds around 10




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