Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 162353
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
553 PM CST Fri Feb 16 2018

.SHORT TERM...(This evening through Saturday)
Issued at 241 PM CST Fri Feb 16 2018

Surface high pressure centered over the lower Missouri River Valley
has brought sunny skies but below normal temperatures to the region
today. The high will start to slide off to the east this evening as
an upper-level disturbance moves off the Northern Rockies toward the
Upper Midwest. Strong mid to low-level warm-air advection should aid
forcing ahead of the associated surface trough enough to produce
light snow as the trough passes through Saturday morning. There are
some questions as to how quickly the column will saturate, which
could limit snow amounts over our western counties. Overall, expect
around one half to one inch of snow from this system. Some lighter
snow or flurries may linger into Saturday afternoon as a secondary
vort max embedded in the upper-level longwave trough pivots
through.

.LONG TERM...(Saturday night through Friday)
Issued at 241 PM CST Fri Feb 16 2018

Still expecting an increasingly active weather period into early
next week as upper troughing to the west leads to increasing
southwest flow aloft over the Midwest early in the week. On Sunday a
lead shortwave trough will slide east across the US/Canadian border.
Models are in good agreement that much of the precip with this
system will stay mainly north of the area, possibly grazing far
north-central Wisconsin with some light snow/mix. Farther south,
much of the area will be in the warm sector with 925 mb temps rising
to 3 to 8C. Mid/high clouds streaming east within the fast zonal
flow and limited mixing may hold temps down a bit, but certainly
potential for another very mild day, with highs at least into the
40s over areas with little snow that see more sunshine.
South/southwest 925 mb winds will crank up to 30-40 kts through the
day, so expect a breezy/windy day, depending on how deep the mixing
is.

Later Sunday night into Tuesday continues to be the main time period
to watch for more impactful weather. There are still some
differences with the evolution of the longwave trough and low-
level thermal profiles, so details are still somewhat uncertain.
Either way, southwest flow will be in place by Sunday night/Monday
with good agreement that broad low pressure will develop
southwest-northeast along the low to mid level baroclinic zone
and ride northeastward. Moisture will surge northward Sunday night
into Monday with precipitable water values rising from 0.6-0.8
inches before the deeper moisture gets shunted eastward with the
approach of the upper trough. The strongest warm advection and
isentropic ascent signal looks to be during the day on Monday with
mainly lighter precipitation lingering into Tuesday with broad
synoptic lift with the upper trough. The slower ECMWF would keep
higher precip chances at least over western/central Wisconsin
through Tuesday with the frontal boundary hanging closer by.

Precip types remain problematic with this system with a mix of types
expected through the event. A strong warm nose aloft is expected to
lift northward into the area, so much of this area may see
rain/freezing rain, depending on surface temps, which will be tricky
with the baroclinic zone nearby. While the 16.12Z NAM brings the
warm nose over most of the area, some models keep cooler thermal
profiles farther north. There are still questions about how much
cloud ice will be present as well. With less cloud ice as the NAM
indicates, a mostly liquid solution would be expected, while the
GFS would also support some snow, especially farther north. Thus,
still plenty of questions, but the highest chances for pure rain
look to be across southwest Wisconsin and highest snow chances
over north-central Wisconsin, with higher chances for a mix in
between. With at least some icing/snow expected on Monday over
parts of the area, travel may be impacted. As colder air filters
in Monday night into Tuesday, expect a transition towards light
snow.

Beyond the early week system, much of the rest of the period looks
quiet with high pressure impacting the region through Thursday. Temps
will cool down through mid-week with highs in the teens and 20s
before beginning to warm later in the week with zonal flow trending
southwesterly. There`s more uncertainty late next week with the
timing of the upper level trough out of the Rockies.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 553 PM CST Fri Feb 16 2018

Main aviation concerns for the TAF period:

1) Low level wind shear developing in the 05Z to 06Z timeframe as
southwest winds increase to around 45 kts at 2kft.

2) IFR conditions in light snow developing by 14Z at KRST and by
15Z at KLSE.

The low level wind shear will subside by 14Z and look
for the IFR snow to exit KRST by 16Z and 18Z at KLSE with MVFR
conditions expected through early afternoon then VFR conditions
return.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...Hollan
LONG TERM...JM
AVIATION...Wetenkamp


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