Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 182030
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
230 PM CST Sun Feb 18 2018

.SHORT TERM...(This evening through Tuesday)
Issued at 230 PM CST Sun Feb 18 2018

All attention on the next couple days as a complex storm system
sets-up across the Upper Midwest. Really wish the 18.12Z model
suite was in better agreement, but differences persist, making
headline decisions quite difficult.

For tonight, an inverted trough axis cuts across the forecast
area, allowing colder air to filter into northwest areas. Near
surface layer begins to saturate after midnight and with some weak
lift and freezing surface temperatures, could start to see some
light freezing rain/drizzle, although any impact looks to be a
light glaze at best. As a result, the Winter Weather Advisory
starting tonight at 06Z across the northwest forecast still looks
ok and no changes planned at this time.

Things get real tricky Monday. Saturation deepens with increasing
isentropic lift through the day, which will result in periods of
light precipitation. The good news...there is little to no cloud
ice, meaning precipitation should fall as liquid. The bad news...
whether it freezes or not will depend on surface temperatures and
there is disagreement between models with how warm it will get.
The 18.12Z ECMWF/GEM are slightly farther north with the low
embedded within the inverted trough, resulting in a warmer (and
wetter) solution. The NAM is farther south, keeping temperatures
colder, but is also drier. The GFS is somewhere in-between and
probably a good proxy for model consensus at this time. All said,
temperatures across portions of northeast IA into southwest WI
will be warm enough for all rain through tomorrow afternoon.
Farther to the north and west, freezing precipitation is expected
with some ice accumulations to around a tenth, locally up to two
tenths. Untreated roads will become slippery (especially rural
roads), making travel difficult to hazardous at times.

There then appears to be some consensus for a drying period Monday
evening into Monday night. Because of this break and noting that
ice accumulations Monday are not expected to be worthy of ice
storm verbiage, decided to issue a Winter Weather Advisory for
areas previously in the Winter Storm Watch through Monday
afternoon. Do not allow the advisory to lull you into a false
sense of security...there will still be travel impacts throughout
the day for those areas that see light freezing rain/drizzle. Just
didn`t want to "shut down" the region by issuing an Ice Storm
Warning for Monday when impacts should not be as severe. This
advisory goes into affect Monday 12Z through 00Z Tuesday.

The plot thickens late Monday night through Tuesday when there is
the potential for more widespread (and more significant icing)
across a broader swath of the forecast area as deeper forcing
moves in. Temperatures do look to be colder (more in that
preferred mid to upper 20 range for an ice storm), BUT still big
differences in QPF. The ECMWF is quite wet, while the GFS/NAM are
drier with the highest QPF just south and east of the forecast
area. If there was to be an ice storm, best chances are late
Monday night through Tuesday, but confidence is not high enough to
issue a warning at this time. As a result, will push back the
Winter Storm Watch to 00Z Tuesday through 00Z Wednesday for much
of the region. Not ideal to have this right after tomorrow`s
advisory, but this seemed most reasonable given the potential
impacts and previously issued headlines.

.LONG TERM...(Tuesday night through Sunday)
Issued at 230 PM CST Sun Feb 18 2018

Tuesday night-Sunday...high pressure will begin to build across
the area Tuesday night and Wednesday providing a break from the
precipitation along with a brief cool down back into the teens and
20s for highs. However, with the flow aloft will remain
southwesterly through the period, so a relatively active pattern
will likely continue. There are model differences in the timing of
ejecting shortwaves out of the plains so confidence in specifics
is low, but certainly potential for precipitation late this week
and during the weekend given the pattern. Temps will trend upward
back near to above average late this week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1156 AM CST Sun Feb 18 2018

This set of TAFs contains 2 main aviation hazards: 1) Strong
southerly winds through this afternoon and 2) -FZRA/RA, along with
MVFR to IFR ceilings and visibilities, beginning early Monday
morning.

Strong southerly winds at the surface around 20-30 kt with gusts
to 30-40 kt will persist through 21Z this afternoon and then
slowly diminish into early evening. The higher sustained winds and
gusts will be at KRST. Also of concern is low-level wind shear,
with 2 kft winds as high as 50-60 kt out of the southwest through
the afternoon. These winds should relax by 06Z tonight.

A low pressure system will bring a prolonged period of wintry
precipitation to the region early Monday into Tuesday. Light
precipitation is forecast to develop by sunrise Monday morning.
At KRST, mostly light freezing rain is expected for the duration
of the event. KLSE is forecast to have temperatures around 32-34 F
tonight through Monday morning, so there is some uncertainty on
whether precipitation there will fall as -RA or -FZRA. Decided to
stick with -RA at KLSE for now, but this will need to be
monitored closely. Along with the precipitation, expect
visibilities to drop to MVFR levels and ceilings to near IFR
levels. Based on station elevation, left KLSE at MVFR ceilings but
brought KRST down to IFR ceilings.


&&

.HYDROLOGY...
Issued at 227 PM CST Sun Feb 18 2018

Some model solutions are very wet for portions of the far
southeast forecast area Monday night into Tuesday. Whether this
falls as rain or freezing rain will depend on temperatures, but if
it`s all liquid, there could be some responses on area rivers and
streams. This solution is far from certain, but want to raise
awareness that higher rainfall amounts could cause some flooding
concerns.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory from 6 AM to 6 PM CST Monday for WIZ017-
     029-033-034-041>044.

     Winter Storm Watch from Monday evening through Tuesday afternoon
     for WIZ017-029-033-034-041>044-053>055.

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

MN...Winter Weather Advisory from 6 AM to 6 PM CST Monday for MNZ088-
     095-096.

     Wind Advisory until 3 PM CST this afternoon for MNZ086-087-094-
     095.

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

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

IA...Winter Weather Advisory from 6 AM to 6 PM CST Monday for IAZ009-
     010-019.

     Wind Advisory until 3 PM CST this afternoon for IAZ008>010-018-
     019-029.

     Winter Storm Watch from Monday evening through Tuesday afternoon
     for IAZ009>011-019-029-030.

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

&&

$$

SHORT TERM...Rogers
LONG TERM...JM
AVIATION...Hollan
HYDROLOGY...Rogers



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