Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 140927

Area Forecast Discussion
National Weather Service La Crosse WI
330 AM CST Sat Jan 14 2017

.SHORT TERM...(Today into Tuesday)
Issued at 330 AM CST Sat Jan 14 2017

Forecast concerns squarely on the winter weather maker Monday and
Tuesday. A lot of uncertainty remains in what will fall, where and
how much. Models still at odds with how much warm air they spread
into the region, and how far west. Some ice in cloud issues too.

Various forcing mechanisms going to have a hand in the pcpn chances
for Monday and Tuesday, but the focus is on 2 main swaths.

1) Warm air advection (Monday): Low level warm air
advection/isentropic upglide will spread northeast across the area
starting early Monday morning, gradually shifting to the east by 00z
tue. Models suggesting a coupled 300 mb jet structure that would
enhance the available lift.

Deep enough saturation for ice at this time, so pcpn concerns focus
on the warm layer. Bulk of the area warms quickly, likely starting
out as sleet, then full melting leading to rain/freezing rain - sfc
and/or road temp dependent (and road temps likely a factor,
especially rural/secondary roads which might not see road treatments
as often). Farther northwest profiles generally support more snow -
although the warm NAM would like a change over to liquid even in
those areas.

2) Upper level trough/sfc low (Monday night into Tue): A 500 mb
trough progged to move out of the desert southwest Sunday, spinning
across southeast WI by 12z Tue. Associated sfc low moves with the
trough, and slight differences in positioning in the various models
could/will have impacts on potential pcpn types. Good push of low
level moisture on Monday with the nose of the 850 mb moisture
transport/40-50kt jet nosing into northeast IA/southern WI 18-00z
Mon/Tue. Couldn`t rule out a rumble or two of thunder in the far
south as a result.

Temp profiles favoring liquid for the bulk of the forecast area,
with any freezing potential sfc dependent. The associated pcpn with
this lift shifts quickly east...likely away from the local forecast
area by the overnight.

3) Deformation region/second shortwave (late Mon night-Tue):
northern stream 500 mb shortwave trough slated to drop southward out
of Canada, swinging through the mid Mississippi River Valley Tue
night. Could get some enhancement in the deformation region
northwest of the sfc low as a result.

Temperatures cool enough a loft in the GFS across the west for a
return to snow. The NAM likes sleet/wintry mix. Both suggest that
there could be a loss of ice in the cloud on Tue.

4) Outcome/amounts: Complex system with a variety of forcing
mechanism working on the thermal/saturation profiles to produce a
wintry mix of precipitation. Expect the local forecast area to see
some of everything.

Letting the model blend dictate pcpn types for now, and as such the
area could see around 1/10 of an inch of ice. Snow would mostly be
confined north of I-90, with 1-2 inches.

Don`t take this as "gold" yet though as small changes in temp, etc
will have a big impact on amounts/type.

5) Headlines? For now, with these amounts, don`t think a Winter
Storm Watch is needed. If the current forecast still looks good come
tomorrow, Advisories will be needed for the Mon - Tue time frame for
the wintry mix and travel related hazards.

.LONG TERM...(Wednesday through Friday)
Issued at 330 AM CST Sat Jan 14 2017

The GFS and EC promise a break from the cold with broad upper level
ridging building in post the Mon-Tue winter weather maker. 850 mb
temps warm to 6 to 10 C above for Wed into Sat...with the EC holding
on to the warming a bit longer than the GFS. 850 mb temp anomalies
range from +1 to +3 during this period. Numerical guidance suggests
many locations will top 40 on some of these days, which looks
reasonable (cloud cover depending).

The anticipated thaw will certainly help many across the region get
rid of some ice, hard packed snow on roads/sidewalks/driveways from
the recent storms. We could use the break.

That said - depending on how quick the warm up is, and how much
rainfalls on Mon-Tue, some rises on area rivers and streams should
be expected. Ice Jams a definite possibility.

Generally looks dry into Friday of next week, with the models then
straying from each other in how to handle an upper level trough
lifting out of the desert southwest. The gfs aggressively drives
this trough northeast over the Upper Mississippi River Valley
bringing a swath of pcpn with it. The EC is about a day slower. Will
let forecast blend dictate chances for now.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1012 PM CST Fri Jan 13 2017

VFR conditions continue, though pesky clouds in the 6-8kft range
have been very stubborn to depart. Based on current trends, thinking
these clouds will hang tight through 10-11Z for RST and then 13-14Z
for LSE, waiting for a weak trough axis aloft to scour them
eastward. There is some small risk that these clouds could even
linger longer, but do expect mainly clear conditions for the TAF
sites by late morning at the latest, save for some thicker cirrus
rolling by south of both locations. Winds remain under 10 knots
through the next 24 hours, gradually shifting to a westerly




LONG TERM....Rieck
AVIATION...Lawrence is the U.S. government's official web portal to all federal, state and local government web resources and services.