Area Forecast Discussion
Issued by NWS La Crosse, WI

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221
FXUS63 KARX 201125
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
525 AM CST Tue Feb 20 2018

.SHORT TERM...(Today and tonight)
Issued at 330 AM CST Tue Feb 20 2018

Main short-term fcst concerns are the -FZRA/-FZDZ/wintry mix and its
exit today, colder lows and sub-zero wind chills by Wed morning.

Data analysis at 06z had a wave of low pressure over southern WI,
along a broad trough front from lower MI to the TX panhandle. This
under broad SW flow aloft over the Upper Midwest, with weak
shortwaves rippling thru. Colder BL air spilling southward north of
the sfc wave and boundary, while shortwave energy rippling NE
combined with some weak mid level instability responsible for a
developing area of SHRA and embedded TSRA, lifting NE across IA
toward the ARX fcst area. All of this on the cold side of the system
where temps were in the 20s to around 30, for another round of FZRA
to spread across much of IA into western WI early this morning.

Model runs of 20.00z initialized well. Models offering similar
solutions for today as a stronger piece of shortwave energy out of
the Rockies trough lifts toward western Ont today then continues to
shear NE toward James Bay tonight. Short-term fcst confidence is
generally good this cycle.

In the short-term, bulk of the wave of -FZRA showers and embedded
TSRA should to exit east of the fcst area by 12z, with more sct to
isolated FZRA showers or -FZDZ lingering along/east of the MS river
into the 12-16z period this morning. Conditions be very icy/
treacherous on untreated surfaces this morning. Make sure to check
the latest road conditions before leaving for school or work.

After that, yet one more weak shortwave ripple and some mid level
FGen lift NE across mainly the SE half of the fcst area thru the mid
morning into early afternoon hours. This as the sfc-850mb cold
advection surges SE across the area behind the passing lower level
trough axis. For now will continue with RA/FZRA chances mainly over
the SE 1/3 of the fcst are thru the late morning into early
afternoon hours, but some signal among a few CAMs models this may
need to be expanded as far NW as a KCCY-KLSE=KISW line before we`re
done with the wintry mix/icing precip today. It is likely the winter
weather advisory thru today will be able to be canceled early for
portions if not all of the fcst area. Given the scope of the -
FZRA/icing and the impacts, will leave the entire advisory go to 00z
Wed for now and leave day crew to assess impacts and when conditions
improve. Clouds, low level cold advection and increasing NW winds
behind the departing trough, little upward movement of temps
expected today.

Mdt low level cold advection continues tonight, under deep
subsidence aloft and with a sfc-850mb ridge axis nosing in. Clouds
to decrease overnight with 925mb falling into the -11c to -16c range
by Wed morning. Blend of guidance lows in the 0 to 15 range as the
next Can/arctic airmass builds in look good. These temps by Wed
morning combine with NW winds around 10 mph to produce wind chills
of 0 to -15 by Wed morning, putting that January bite back in the
air.

.LONG TERM...(Wednesday through Monday)
Issued at 330 AM CST Tue Feb 20 2018

For Wednesday thru Thursday night: main fcst concerns this period
are returning precip chances and types Thu/Thu night.

20.00z models offering decent agreement for hgts to rise a bit in SW
flow aloft Wed/Wed night. Weak shortwave ridging aloft to then move
across the region Thu, ahead of a stronger shortwave to lift out of
the Rockies trough into the Upper Midwest Thu night. Plenty of lower
level detail differences by Thu night though. Fcst confidence is
good Wed/Wed night, then average by Thu night.

This period starts out with cool, dry Can high pressure drifting
across the region. A cool start Wed morning with NW winds thru much
of the day. Under what should be plenty of sunshine, mixed 925mb
temps supporting highs mainly in the 20s. Lower level warm advection
quickly returns Wed night/Thu as pressures fall in the plains, ahead
of that next stronger wave to eject out of the Rockies trough.
Moisture returns as well Thu, with PW values of around 1/2 inch to
lift north ahead of the ejecting wave. By later Thu, Thu night, when
the deeper moisture/saturation and stronger lower level thermo-
dynamic forcing is arriving, 925-850mb temps over the SE side of the
fcst area progged to climb above 0C and create precip type issues.
Initially moisture column is looking deep enough for ice in the
clouds Thu/Thu evening, but near/after 06z Fri, reasonable signal
for drying above 700mb and loss of ice in the clouds, especially
near/south of i-90. Introduced some wintry mix/-FZRA in the far
south end of the fcst area Thu night, and spread a -FZRA chance
north to near I-90 later Thu night. Otherwise, looking like central/
north parts of the fcst area to see 1-2" of snow centered on Thu
night. Blend of guidance highs/lows this period quite reasonable.

For Friday thru Monday (days 4-7): main fcst concerns this period
are precip types/chances with another system Sat/Sat night,
temperatures thru the period.

Medium range model runs of 20.00z offering improving agreement for a
major portion of the Rockies trough to lift out and across the
mid/upper MS valley Sat/Sat night, with some developing signal for
another stronger shortwave trough to move across the north-central
CONUS centered on Monday. Fcst confidence is average Fri/Sat then
below average by Mon.

Friday trends to be a `between systems` day, with the ejecting
Rockies trough/energy to potentially bring another stronger wintry
system to the Upper Midwest Sat/Sat night. Plenty of detail
differences with storm track and lower level thermal profiles at
this time, but the consensus blend precip chances increasing into
the 60-70% range Sat afternoon/evening appears reasonable at this
point. Precip types with this system are far from certain but
appears we will again have to contend with a wintry mix across at
least the SE half of the fcst area. Storm track and any precip to
have large impacts on high temps Sat, as would any potential
wintry system Mon. Blend of guidance temps for Fri-Mon reasonable
for now, but those of Sat and Mon may well be too warm if the
potential systems come to pass with thick clouds and wintry precip.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 526 AM CST Tue Feb 20 2018

IFR conditions continue across the area along with a few remaining
showers of freezing rain or sleet. Those showers will wind down
through the morning, with conditions gradually improving to MVFR
and eventually VFR later today into tonight as high pressure works
into the area. Will have to keep a close eye on the potential for
one last batch of rain or freezing rain to clip LSE later this
morning or early afternoon, but confidence in that is quite low at
this time. Winds will increase from the west to northwest, gusting
to 20 knots later today into this evening.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory until 6 PM CST this evening for WIZ017-
     029-032>034-041>044-053>055.

MN...Winter Weather Advisory until 6 PM CST this evening for MNZ079-
     086>088-094>096.

IA...Winter Weather Advisory until 6 PM CST this evening for
     IAZ008>011-018-019-029-030.

&&

$$

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



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