Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 181130
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
530 AM CST Sun Feb 18 2018

.SHORT TERM...(Today through Tuesday)
Issued at 335 AM CST Sun Feb 18 2018

High Impact Weather Potential: Strong southerly winds expected
today. Looking more and more like advisory-level speeds will be
achieved for parts of the area. Bigger focus is on Monday-Tuesday
with our well-advertised wintry mess of precipitation, including the
potential for a prolonged period of light freezing rain and icing
for parts of the area.

Lots to talk about early this morning, so let`s dig into it. All is
quiet currently with low level ridging sliding southeast of the
area, but a quick look west shows lee cyclogenesis underway across
western South Dakota, tied to a shortwave emerging out of the
northern Rockies at the moment. That feature will track well north
of the area today, with warm advection-driven precip mainly
occurring north of the local area, save for perhaps far northern
Taylor county where some light snow is possible this afternoon on
the southern periphery of deeper saturation. Bigger story by far
remains winds with downright impressive speeds progged aloft,
including an astounding 75+ knots at 850mb - well at the top of the
spectrum for standardized anomalies. Thankfully we won`t be able to
mix that high given time of year, but even mixing to 900mb or
slightly higher (per GFS soundings) gets us gusts to advisory levels
across mainly southeast MN and northeast IA (typical flatter/windy
locations), and probably along the bluff tops either side of the
Mississippi. Will really have to watch the sneaky but small
potential that we over-achieve on mixing today, as even a slight
bump upward in mixed depth would result in some near warning-level
gusts for open areas.

Winds quickly crash out toward sunset with development of an
inverted trough axis across the area, tied to the next lee cyclone
taking shape over western Kansas. That inverted trough feature for
days has been progged to hang out pretty close to the area, along a
very strong baroclinic zone that will serve as the focus for several
rounds of precipitation Monday into Tuesday, out ahead of a deep
western CONUS trough. From a moisture standpoint, still impressive
PWAT values progged just to our east, pushing 1+ inches, though the
heaviest rain focus thankfully appears to be just to our south and
east within the low level inverted trough axis where warm advection
ascent over the strong frontal zone and moisture transport are
maximized. Farther north, our forcing continues to look more
disjointed and broad in nature, suggesting we`re not in for any one
period of heavy precipitation, but instead will deal with various
waves of lighter precip, with actual precip type dictated by both
the boundary layer temps and a notable warm nose aloft that will
likely "wiggle" back and forth at times.

With that said, a strong signal for one to two-tenths of an inch of
prolonged freezing rain/drizzle has existed for many days now
bisecting the CWA, with sufficient lack of a warm nose aloft for
mainly snow far northern areas (looking at you Taylor County), while
farther south areas initially deal with likely just rain given a
warmer boundary layer. Surface temps will be key in this event, and
even at this closer hour, there are questions about exactly where
the freezing line will be laid out, so fully expect some updates to
the forecast over the next 24 hours as we further refine that.
However, as mentioned yesterday, this setup has all the hallmarks of
a classic freezing rain setup with persistent low level northerly
flow feeding progressively colder air southward with time in
combination with a strong over-running signal aloft. All in all,
definitely foresee higher-level impacts from this setup, but from an
ice amounts standpoint, probably looking at a prolonged advisory
versus any type of warning but will have to closely watch the
potential for a secondary batch of precipitation to add to ice
totals on Tuesday with increasing hints of a stronger frontogenetic
axis developing across the area. Should that occur, that would tip
the scales in favor of a possible Ice Storm Warning for parts of the
area. Suffice it to say higher-level messaging remains warranted for
this system, and have issued advisories along the western fringe
where confidence in lower ice amounts is high, but will opt for a
Winter Storm Watch farther east where those higher amounts could
be realized.

.LONG TERM...(Tuesday Night through Saturday)
Issued at 335 AM CST Sun Feb 18 2018

A much-deserved stretch of much quieter weather expected for
midweek, with our early week system quickly departing by Tuesday
evening, leaving high pressure along with a shot of colder air
Wednesday into Thursday. Looking beyond, there remains plenty of
hints we will stay in a more active weather regime by late week and
into next weekend (and perhaps beyond) with continued southwest flow
aloft promoting the idea for several additional shortwaves to lift
north toward the area. Of course, at this point, confidence in the
timing of any such system remains low, as does the type(s) of
precipitation that could occur, though some chance for additional
wintry precipitation does appear possible at this longer range.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 530 AM CST Sun Feb 18 2018

Main aviation concerns much of this TAF period remain the LLWS
starting around 14-15Z and continuing into the evening. Look for
southwest winds to increase to 50 to 65 kts at 2 kft after 14Z,
while sfc winds increase into the 20-25kt g29-38kt range from the S-
SE. Both sfc and 2k ft winds will diminish late this afternoon and
thru the evening. VFR conditions expected today thru this evening,
with lower level moisture increase during the late evening and
overnight hours into Mon morning. Some MVFR cigs expected after
midnight tonight with IFR cigs/MVFR vsbys possible, especially at
sites like KRST toward the end of the TAF period into Mon morning.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Storm Watch from Monday morning through Tuesday afternoon
     for WIZ017-029-033-034-041>044.

     Winter Weather Advisory from midnight tonight to 6 PM CST
     Tuesday for WIZ032.

     Winter Storm Watch from Monday afternoon through Tuesday
     afternoon for WIZ053>055.

MN...Wind Advisory from 9 AM this morning to 3 PM CST this afternoon
     for MNZ086-087-094-095.

     Winter Storm Watch from Monday morning through Tuesday afternoon
     for MNZ088-095-096.

     Winter Weather Advisory from midnight tonight to 6 PM CST
     Tuesday for MNZ079-086-087-094.

IA...Wind Advisory from 9 AM this morning to 3 PM CST this afternoon
     for IAZ008>010-018-019-029.

     Winter Storm Watch from Monday morning through Tuesday afternoon
     for IAZ009-010-019.

     Winter Weather Advisory from midnight tonight to 6 PM CST
     Tuesday for IAZ008-018.

     Winter Storm Watch from Monday afternoon through Tuesday
     afternoon for IAZ011-029-030.

&&

$$

SHORT TERM...Lawrence
LONG TERM....Lawrence
AVIATION.....RRS



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