Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 170516
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1116 PM CST Mon Jan 16 2017

.UPDATE...
Issued at 1028 PM CST Mon Jan 16 2017

Forecast working on quite well, which is unfortunate because that
means it remains a real mess out there. Strong deformation banding
remains in place north of I-90 but continues to very slowly
narrow/shrink, which should continue over the next handful of
hours as the upper low to our south pulls away. However, am a
little worried that a few spots may have picked up in excess of
0.25" of ice accretion, though switching headlines this late in
the game doesn`t seem to make much sense with precip likely
switching over to sleet per trends and some calls to those areas.
That switchover to sleet and perhaps even some snow should
continue into early morning with the deformation band waning but
gradual cooling aloft erasing the remnant warm wedge.

Farther south within the dry slot, periods of more showery
precipitation will continue, turning more drizzly far south with a
lack of any cloud ice available. Could also see some fog into
northeast IA and far southwest WI for a few hours within a zone of
weaker winds near the surface low, so will be watching the
potential to see if we can muster some dense fog (visibility into
central IA much lower currently).

All in all, no changes to going headlines as roads are just a mess
area-wide per non-stop scanner traffic here at the office. The
morning commute will be a challenge - stay safe out there
everyone.

UPDATE Issued at 533 PM CST Mon Jan 16 2017

Well...what a mess. Broad band of warm advection-driven
precipitation continues to overspread the area, just spreading
north of I-94 the past few hours (dry air issues). Warmer air
aloft has been a little more robust than initially thought (no
surprise there...typically happens in these events), with mainly
freezing rain occurring with temps either side of 32F, but even
where temps have risen above 32 given lower wet bulbs and cold
road temps in the 29-33F range. Per scanner reports, roads are an
absolute mess and highly glazed over, with quite a few accidents
reported. Things won`t change much the next few hours, but precip
shield should gradually continue to shift northward as dry
slotting arrives from the south. Things may tighten up within a
stronger deformation axis north/east of La Crosse after 05Z, with
some risk that precip briefly changes back to snow as just enough
colder air arrives aloft to erase the warm nose.

Primary changes at this time are to increase freezing rain/ice
accumulation amounts much farther north per reports/radar CC
trends as well as add Taylor County, WI to the advisory. Precip
hasn`t yet made it that far north but it appears it will, likely
taking the form of sleet, freezing rain, and snow (slightly colder
profiles that far north). It won`t take much ice for any travel
issues to felt an advisory was the most prudent course of action,
especially in light of accidents farther south.

We`ll be here monitoring all evening, with changes as warranted.
If possible, simply don`t travel out there as night fall now means
untreated roads will become even more icy.

&&

.SHORT TERM...(This evening through Thursday)
Issued at 228 PM CST Mon Jan 16 2017

Early this afternoon, water vapor satellite showed the closed
upper level low along the Kansas/Nebraska line with the surface
low over northeast Kansas. Local and regional radars showed the
main band of precipitation was ahead of the system from eastern
Nebraska across northern and eastern Iowa into southern Wisconsin
and Illinois. The predominant precipitation type has been freezing
rain or freezing drizzle. However, how long this is able to
continue, especially across the south is in question. Surface
temperatures have risen to freezing or just above across all of
northeast Iowa into southwest Wisconsin and parts of central
Wisconsin. In addition, dew points have also come up and are the
same or within a degree of the air temperatures which will keep
the surface wet bulb temps at or above freezing as well. Road
temperatures have also been creeping up with most, if not all, of
these now above freezing. Even with the approach of night,
temperatures may not have the opportunity to drop much as the
surface low approaches and this should limit the amount of icing
that will occur. Overall, these are mainly fine tuning adjustments
to the general forecast with the big picture not changing a whole
lot.

As the whole system comes northeast tonight there is still
expected to be a loss of ice aloft from south to north for all but
the far northwest and north sections of the forecast area. The
warm air aloft will continue to be warm enough for melting except
in the areas that keep the ice aloft. This will keep the primary
precipitation types as freezing rain/drizzle except where there is
the combination of air and surface temperatures above freezing.
The most likely locations for mainly non-freezing liquid
precipitation appears to be across our far southern three
counties, but still concerned there could be some localized areas
of additional icing. However, if the roads temperatures start fall
with the loss of daytime heating, the icing problems could come
back. Confidence is not high enough yet to start dropping counties
out of the winter weather advisory.

To the north where is looks like snow may end up the dominant
precipitation type there are some concerns about the snow ratios
and how much accumulation there will be. Snow ratios have been
raised closer to a 6/7 to 1 which gives accumulations of 1 to 2
inches. Concerned that the ratios may still be too low and amounts
could end be a little higher with some 3 inch amounts possible.

The main period of precipitation is still expected to be late this
afternoon into this evening when the forcing is maximized. As the
upper level system approaches, the main short wave trough will
produce a period of moderate pv advection in the 500-300 mb layer
and moderate qg convergence in the 1000-300 mb layer. This forcing
will be aided by some jet dynamics with the divergence aloft
maximized from a coupled jet structure over the region. The warm
air advection ahead of the system will continue to provide 2 to 3
ubar/s of up glide on the 290K surface with the moisture transport
aimed right into the area. All this forcing should then diminish
through the overnight into Tuesday morning with the precipitation
expected to quickly shift east Tuesday morning.

.LONG TERM...(Thursday night through Monday)
Issued at 228 PM CST Mon Jan 16 2017

Southwest flow aloft will remain in place for much of the period.
This will allow the next system coming up out of northwest Mexico
to approach the area but then weaken by the end of the week as
another system crossing the southern Rockies becomes the dominant
system. This system is then expected to also weaken early over the
weekend as yet another one crossing the southern Rockies again
takes over and then tracks across the mid Mississippi River Valley
into the Appalachians. With all these systems coming close to the
region, there will be an extended chance for some precipitation
from Thursday night right into next Monday. It continues to look
warm through this period as well, with highs in the 40s and
overnight lows in the mid to upper 30s. This should allow most of
this precipitation to fall as rain.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1116 PM CST Mon Jan 16 2017

Our messy winter storm continues to slowly wind down across the
area, with the most significant precipitation having come to an
end for LSE/RST. However, periods of lighter wintry precipitation
will continue the remainder of the night and into mid morning
Tuesday, taking the form of mainly rain at LSE and freezing rain
or drizzle mixing over to sleet/snow at RST but with minimal
additional accumulation. IFR to LIFR conditions will remain
through the day, very slowly lifting this afternoon and especially
this evening as high pressure and much drier air arrive.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory until noon CST Tuesday for WIZ017-029-
     032>034-041>044-053>055-061.

MN...Winter Weather Advisory until noon CST Tuesday for MNZ079-
     086>088-094>096.

IA...Winter Weather Advisory until noon CST Tuesday for IAZ008>011-
     018-019-029-030.

&&

$$

UPDATE...Lawrence
SHORT TERM...04
LONG TERM...04
AVIATION...Lawrence


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