Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 100513
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1113 PM CST Mon Jan 9 2017

.UPDATE...
Issued at 847 PM CST Mon Jan 9 2017

A very challenging evening forecast. Light precipitation continues
to blossom over southeast Minnesota and northeast Iowa where the
10.01Z RAP shows a weak short wave trough moving across with some
weak mid level frontogenesis. Further complicating matters, the
precipitation is falling out of high cloud bases, on the order of
6 to 8 thousand feet, but is being reported as freezing drizzle or
sleet. The forecast has been adjusted to agree with ground truth,
but with none of the short range models handling this, lots of
uncertainty on how long this mixed precipitation will continue.

Looking ahead to later tonight and into Tuesday, the 10.00Z NAM
has come in and continued the trend of strengthening the system as
it moves across the forecast area. This could have some
substantial impacts to the forecast in regards to the
precipitation types. By being stronger with the system, the
forcing will be stronger and deeper which should allow for more
ice to be introduced into the system, especially in the
deformation zone on the northwest side of the surface low. Ahead
of the surface low, there little to no agreement between the NAM
and the RAP this evening. By late tonight, the NAM would suggest
little to no ice in the clouds ahead of the surface low over
northeast Iowa into southwest Wisconsin south of Interstate 90,
which the RAP agrees with. By 12Z though, the NAM has essentially
filled this all back in with ice while the RAP continues to show
no ice ahead of the surface low. The stronger system may also
allow warmer temperatures aloft to get drawn into the southern
parts of the area. The NAM would indicate temperatures warm
enough for complete melting across northeast Iowa at 12Z and warm
enough for partial meting across southwest Wisconsin at 15Z. The
RAP is not as warm at 12Z and would have partial melting possible
south of Interstate 90 but then by 15Z would have complete melting
occurring over northeast Iowa into southwest Wisconsin.

The 09.18Z GFS was more in line with the NAM, so would expect the
00Z to trend that way as well. With all these uncertainties, tough
to start making too many adjustments to the forecast and may have
to wait until the new GFS comes in to have a good feeling on
which way to go. One thing that does seem likely, is for the
possibility of more snow across southeast Minnesota where is seems
more probable that ice will now be in the clouds with temperatures
aloft cold enough to primarily keep it all snow. It may be
necessary to expand the winter weather advisory into this area to
account for this.

&&

.SHORT TERM...(Tonight through Tuesday)
Issued at 300 PM CST Mon Jan 9 2017

Snow to a wintry mix is the main forecast concern tonight into
Tuesday. Ice in cloud-low level warming playing a big role in pcpn
type, and adds some uncertainty to the forecast.

1) This evening: low level warm air advection and favorable
isentropic upglide on the 280:295 K sfcs, coupled with a northwest-
southeast running frontogenetic band, will continue to fuel the
current band of snow across MN and western WI. Time/height x-
sections and bufkit soundings favor mostly snow as the pcpn type,
although some threat for freezing pcpn across the far south.

2) Overnight: focus for pcpn turns more to a west-east running
frontogenetic boundary, which meso models and nam/gfs place right
around the I-90 corridor.

Persistent dry low/mid layer across the south per bufkit soundings
(roughly 700-850 mb) continues, and likely too large to overcome
with a seeder-feeder process. Thus, freezing pcpn the higher threat.
Temperatures/dew points warming through the night, and likely at
or above 32 F by 12z for far northeast IA/southwest WI. That said,
road temps are cool and could support ice accumulation. Tough,
tough scenario - and will have to be monitored closely. Obvious
impacts to the morning commute and school buses if icing is
realized.

To the far north, temperature profiles and relative humidity still
say ice will be in the cloud and it will stay cold, so snow
remains the pcpn type.

In between (around I-90) it looks more like a mixed bag, with
partial to complete melting of any hydrometeors possible, bringing
sleet/freezing pcpn into question...along with snow.

3) Tuesday: Upper level shortwave spins across the Northern Plains
and then across the local area. Models suggesting a bit stronger
compared to some previous runs, and maybe a bit farther south. This
trough will work on that west-east running boundary, and also help
with lift along its associated cold front (which it pushes into
western WI by 18z Tue). Expect some enhancement in the pcpn areas by
18z as this all comes together, favoring the the I-94 corridor and
northeast IA/southwest WI. QPF also higher.

4) End result? Snow accumulations from 3 to 5 inches from I-94
north. Winter weather advisory still looks good for Clark/Taylor
counties in north-central WI. See some potential for expansion
southward a county, but will leave current setup as is.

I-90 corridor around an inch (or so) for snow, with some light
icing possible.

South of I-90, icing the main concern for a few hours, closer to 12z
Tue. Challenging forecast here with increasing temps, ice in cloud,
and road temps all coming into play.

5)Caveat: 18z GFS/NAM coming in more aggressive with generating
qpf along the west-east boundary region Tue morning as the
shortwave trough approaching - and lingering it a bit longer.
Potential snow/icing amounts may be on the rise if this trend
continues.

Keep up on latest forecast for adjustments and potential expansion
to headlines.

.LONG TERM...(Wednesday through Monday)
Issued at 300 PM CST Mon Jan 9 2017

GFS/EC/NAM favoring more of a quick moving, zonal flow for the
middle/later part of the week. All favor bringing a shortwave across
northern parts of the region Wed, sliding a sfc cold front through
the area as it does. Band of pcpn would result, mostly as snow. Some
ice in cloud/warming concerns in the south though that make mixed
pcpn a possibility. Amounts look relatively small at this time, with
a few inches of snow possible in the north, less south - but that
threat for icing. Something to watch after we get through the
tonight/Tue weather system.

Quick shot of colder air post the Wed system, but temps already
looking to bounce back to the Jan normals by the end of the weekend.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1113 PM CST Mon Jan 9 2017

As the weak short wave trough and mid level frontogenesis continue
to move east, the band of precipitation has as well. This looks
like it will past both airports by or shortly after 06Z. The break
in the precipitation is not expected to last long as the forcing
starts to increase again late tonight ahead of the next short wave
trough and surface low. Based on the latest model guidance, expect
this next band to be all snow for KRST but enough warm air aloft
could get brought in over KLSE for a period of a wintry mix late
tonight into Tuesday morning. This warm air aloft should cool
enough to then support all snow from about late morning through
the afternoon. The heaviest snow in the deformation zone looks to
have a chance to impact both airport and will show the visibility
going down to less than a mile in this. The VFR conditions will
quickly go down to MVFR and then IFR late tonight as the
precipitation starts. Improvement back to VFR is expected to occur
fairly quickly once the snow ends Tuesday afternoon. Some concern
for additional visibility restrictions in blowing snow at KRST
Tuesday afternoon and evening as the winds switch to the west and
gust to around 25 knots on the back side of the surface low.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory until 3 PM CST Tuesday for WIZ017-029.

MN...None.
IA...None.
&&

$$

UPDATE...04
SHORT TERM...Rieck
LONG TERM....Rieck
AVIATION...04



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