Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 172120

Area Forecast Discussion
National Weather Service La Crosse WI
320 PM CST Sat Feb 17 2018

.SHORT TERM...(This evening through Sunday)
Issued at 320 PM CST Sat Feb 17 2018

Trough aloft this afternoon will move out of the region tonight,
dragging with it the lingering light snow still across portions of
western and central WI. Progressive surface ridge axis then drifts
across the forecast area overnight and with clearing skies/light
winds, could see some patchy fog for areas south of I-90. Otherwise,
temperatures tonight will drop into the single digits and teens
above zero.

For Sunday, another short-wave will slide along the Canadian border,
bringing a swath of snow to northern MN/WI. Looks as if most if not
all snow will remain north of the forecast area, but will keep a
small POP across Taylor County where any accumulation will be
minimal. Of greater concern Sunday will be strong southerly winds.
17.12Z model consensus is showing a rapid increase in winds around
1500 ft agl and above into the 50 to 60 kt range. Main question is
cloud cover and how this will impact the depth of the mixed boundary
layer. Right now, indications show it topping out just below the
strongest winds, but this could still result in frequent gusts to 40
mph or higher, especially for areas west of the Mississippi River
Valley. Right now, the forecast calls for winds just shy of Wind
Advisory criteria, but this is something to watch closely over the
next 12-24 hours as a headline may be necessary from late Sunday
morning through much of the afternoon. Otherwise, Sunday will
be partly to mostly cloudy with highs in the mid 30s to lower 40s.

.LONG TERM...(Sunday night through Saturday)
Issued at 320 PM CST Sat Feb 17 2018

All in all, no significant changes were made with the messy storm
system set to impact the region through Tuesday. Several waves of
low pressure will eject northeastward within southwest flow aloft
early this week along a well-defined southwest-northeast oriented
baroclinic zone. Strong low-level northward moisture transport
into the boundary will help increase precip water values to
0.5-1.0 inches, with standard anomalies rising to 2-3 across
northeast Iowa and southwest Wisconsin Monday morning.

Strong warm advection/isentropic ascent will lead to increasing
precip chances on Monday. There has been fairly good agreement with
the strongest forcing/highest precip chances area wide being
centered on the daytime hours on Monday. Recent runs of the
NAM/ECMWF have suggested another wave riding along the thermal
gradient could keep higher chances going across southwest into
central Wisconsin through Tuesday, while the GFS remains slightly
farther east with the boundary, and consequently would only bring
some light synoptically driven precip to the area on Tuesday.

The local area will be situated near the baroclinic making for a
very difficult forecast for precip types. Plenty of warm/moist air
will lift northward up over the surface boundary, with the extent of
low-level cooling north of the boundary playing a pivotal role in
the degree of icing. The idea of the warmest temps and best chances
for pure rain over southwest Wisconsin, transitioning to a mix to
the north and some snow up into north-central Wisconsin where
there may be less influence from the warm nose through Monday.
With the warm nose aloft hanging on into Monday night and Tuesday
precip types will remain a concern over the entire area with a
continued threat of mixed precip. It`s possible a few rumbles of
thunder could make it into southwest Wisconsin closer to the
surface boundary on Monday.

Model consensus favors around 0.25 to 0.75 inches of precip
through the event, although the potential exists for higher
amounts across southwest Wisconsin closer to the deeper moisture
and in closer proximity to the low-level boundary, especially if
the NAM/ECMWF solutions pan out. Will have to watch this area for
any hydro impacts if the higher amounts materialize. Impacts from
icing Monday/Tuesday look to be the biggest concern with some
snow accumulations potentially over far north-central Wisconsin.

The mid-week period continues to look quiet with a dip in temps and
high pressure over the area. Guidance suggests a return to southwest
flow aloft later next week and next weekend with temps rebounding.
With the developing southwest flow, another relatively active
stretch of weather is possible towards next weekend, although
confidence is low right now with details.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1148 AM CST Sat Feb 17 2018

Back edge of snow stretches from near KAEL to KEAU and will move
eastward through early afternoon. IFR visibility and MVFR ceilings
will continue in light snow through 17.19Z-17.20Z. MVFR ceilings
may then linger for another or two before a return to VFR
conditions late this afternoon and through the rest of the period.

Light west-southwest wind will be the rule through this afternoon
becoming light/variable or light from the south overnight. Then
expect big changes as winds become strong from the south by mid
Sunday morning. Sustained surface winds will increase to between
15 and 25 kts with frequent gusts at KRST topping 30 kts. Winds
aloft will also be very strong, increasing to between 45 and 55
kts around 2000 ft agl. As a result, introduced low-level wind
shear at both TAF airfields to end the period.




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