Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 171806
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1206 PM CST Sun Dec 17 2017

.UPDATE...
Issued at 1206 PM CST Sun Dec 17 2017

Visibilities have improved across much of southeast Minnesota and
northeast Iowa. There could still be a few pockets of dense fog
into the early afternoon, but conditions have improved enough to
allow the Dense Fog Advisory to expire at noon.

UPDATE Issued at 525 AM CST Sun Dec 17 2017

Dense fog has become more widespread across portions of southeast
MN/northeast IA. Not anticipating much improvement until late
morning. Hoisted a Dense Fog Advisory until noon today.

&&

.SHORT TERM...(Today and tonight)
Issued at 300 AM CST Sun Dec 17 2017

Early this morning, low stratus has overspread the entire forecast
area along light easterly flow. Also seeing some patchy dense
fog, especially across portions of southeast MN and northeast IA.
These clouds are unlikely to go anywhere today, trapped under a
low-level subsident inversion. Fog will dissipate later this
morning, but persistent cloud cover will keep temperatures in
check with afternoon highs from the upper 20s to mid 30s.

A robust short-wave currently rotating across central KS will
weaken as it lifts northeastward today, but all of its QPF is
expected to remain south and east of the local area. Meanwhile, a
secondary northern stream trough will move across the Dakotas and
western MN later this afternoon. Short-term high resolution
guidance shows a narrow band of enhanced frontogenesis out ahead
of this wave across central MN into northern WI, which may produce
a brief period of light snow this afternoon mainly impacting
Taylor County.

Clouds and cooler temperatures today should limit melting of
current snowpack, but will still have to watch fog potential
again tonight. Otherwise, expect lows in the 20s with light
southwest wind.

.LONG TERM...(Monday through Saturday)
Issued at 300 AM CST Sun Dec 17 2017

The next Canadian trough and associated cold front will move
across the forecast area Monday night. With little moisture to
work with, not expecting any precipitation with its passage.
However, strong warm air advection on Monday ahead of the wave
will result in temperatures rising into the upper 30s to lower
40s for most areas. Only a slight dip in temperatures is expected
behind the front on Tuesday with highs mainly in the 30s. Both
days will be breezy with roughly 20 to 30 kts in the shallow mixed
boundary layer. Southwest winds Monday will shift to the northwest
in the post frontal air mass on Tuesday.

A large eastern Pacific trough is still on track to move into the
intermountain west on Wednesday. Latest model trends now have the
heaviest warm advective snows across ND, northern MN, and into
northern WI. Could still see some of this precipitation clip the
northernmost forecast area late Wednesday and Wednesday night, so
POPs in the 20-60 percent range seem reasonable at this time.

Still uncertainty in the surface low track Thursday into Friday.
However, looking at past runs of the GFS and EC at 06Z Friday
shows a gradual trend to the north and west. IF this trend
continues, it would favor a warmer air mass across the forecast
area and more of a wintry mix than pure snow for some areas. This
is especially true if the 17.00Z ECMWF verifies which deepens the
surface low near the MN/IA/WI border by Friday morning, keeping
the primary deformation snow band across southern MN into northern
WI. 00Z GEFS plumes show La Crosse snow totals ranging from 0 to
10 inches, so still A LOT of spread. Given this uncertainty, will
continue to follow a model consensus, resulting in high end POPs
for much of Thursday, then tapering off from west to east on
Friday. To minimize potential flip-flopping of the forecast, will
maintain a mainly snow p-type, but may need to eventually
introduce more of a mix if recent model trends continue.

Next weekend still looks dry, but cold as temperatures plummet
behind the departing wave. Still a question on how cold, but highs
back into the teens to low 20s seem reasonable at this time.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1206 PM CST Sun Dec 17 2017

Overcast skies with IFR to LIFR ceilings are expected to persist
into the overnight. Visibilities should remain near or above 2
miles through the afternoon at KRST, and above 5 miles at KLSE.
Expecting another lowering of visibility this evening until winds
begin to increase out of the southwest early Monday morning. Could
see fog redevelop at KRST this evening, but confidence is not
high enough to include in the TAF at this time. The persistent
low-level cloud cover should begin to scatter out late Monday
morning as southwesterly winds increase. Could see some gusts
approaching 20 kt at KRST heading into Monday afternoon.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.

&&

$$

UPDATE...Hollan
SHORT TERM...Rogers
LONG TERM...Rogers
AVIATION...Hollan



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