Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 241744
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1144 AM CST Fri Feb 24 2017

.UPDATE...
Issued at 1144 AM CST Fri Feb 24 2017

Well...the forecast worked out pretty much as expected, with the
exception of just a small 30-50 mile shift in the heaviest
expected precipitation band. Areas near and either side of a
Charles City, IA - Rochester, MN - Eau Claire, WI line did the
"best" in this setup, with a solid 6-12+ inches falling overnight,
while amounts on either side of this axis tapered rapidly.
Additionally, the snow that fell was very wet/heavy in nature,
making it difficult to blow around. As such, true blizzard
conditions have been hard to come by, though during the heaviest
snow, plenty of 1/4SM visibilities were noted due simply to the
rate of snowfall.

So, where do we head from here? Have already downgraded the
southern tier of the Blizzard/Winter Storm warning to an advisory
as just not much snow fell there. Current radar trends continue to
support the notion of a broad deformation-driven snow axis
returning to the region from the west this afternoon and evening,
with a return to more ice in the clouds ending some ongoing
freezing drizzle/rain (or even just rain in a few spots where
temps are above freezing). Additional accumulation remains
somewhat tricky as the overall forcing is waning with eastward
extent, though there are plenty of sub-1SM visibilities to the
west, suggesting at least some decent accumulation rates of
1/2-3/4 inches per hour. Still envision another 1-4" of
accumulation, highest western areas but not sure that`s enough to
warrant a continued warning. For now, per coordination with
surrounding offices, will hang onto current headlines, but may
well need some additional tweaks or downgrades into the afternoon
pending trends. We are definitely seeing improvement on roads per
reports and plow cams so will continue to monitor.

&&

.SHORT TERM...(Today through Saturday)
Issued at 259 AM CST Fri Feb 24 2017

Early this morning, water vapor satellite showed the upper level
low was over central Nebraska. The surface low was displaced from
the upper level system and was over northern Missouri.
Precipitation was falling across much of the area from this system
with snow along and north of Interstate 90 and rain/drizzle or a
mix to the south.

The upper level low still has a positive tilt to it, but the
24.00Z models along with the 24.06Z RAP all continue to show this
taking on more a neutral tilt as it works east today into this
evening. This will cause the pv advection in the 500-300 mb layer
to increase but the models all take the stronger advection south
of the forecast area. Some jet dynamics will help to increase the
forcing on the poleward side of the upper level low as the
divergence aloft should be amplified by coupled jet structure.
Good low level thermodynamics will produce 5 to 10 ubar/s of up
glide on the 290K isentropic surface this morning and then 3 to 6
ubar/s from late this afternoon through much of the afternoon.
This will be come lighter, but continue well into this evening
with a weak trowal that is expected to form on the back side of
the surface low.

With plenty of forcing, no doubt the precipitation will continue,
but the concern going forward is how much additional
precipitation, what type will it be and will blizzard conditions
be reached. A bit of a lull in the precipitation at this time, but
all the meso scale models suggest the deformation zone should
spread back across most of the area by the start of the morning
commute. The heaviest precipitation should then be this morning
before diminishing some but continuing through the day with the
forcing from the trowal. With the decrease in intensity for this
afternoon and evening, the qpf amounts from the latest guidance
look to be not quite as high as before which will then cut into
the overall snow accumulations. Thinking another 4 to 8 inches
today in the warning areas and with what has already fallen, some
locations still likely to get a foot or more, but not sure about
the 16 inch totals we were forecasting on the high end.

As for the precipitation types, the dry slot on the east side of
the upper level low was working north across southern Iowa. As
this spreads into the area, especially the southeast sections, it
will take the ice out of the clouds and set up the potential for
more rain or freezing rain depending on air and road temperatures.
The dry slot looks to remain into the afternoon before moving east
and allowing ice clouds to spread back over the area. However,
this could result in some ice accumulations of a 1 to maybe 2
tenths of an inch across northeast Iowa into southwest and central
Wisconsin.

Observations across southeast Minnesota indicate that blizzard
conditions have yet to be reached with the visibility only briefly
dropping below a mile for short periods of time. The strongest
winds will be through this morning before these start to diminish
as the surface low moves from northern Illinois into lower
Michigan. The wet, heavy nature of the snow will also have some
impact on just how much it can be picked up and moved around.
Concerned that there may not be prolonged periods of whiteouts and
that the blizzard warning may have to be downgraded. Do not plan
to make any changes to this headline this morning but the dayshift
may have to do so once the winds start to diminish.

So for now, no significant changes to the forecast. All headlines
will continue as is, but maybe a bit less snow than earlier
expected. Storm total amounts of 6 to 14 inches in the warnings
with up to a tenth of an inch of ice accumulations possible south
of a Austin to Black River Falls line. In the advisory area, snow
totals of 2 to 5 inches with 1 to 2 tenths of an inch of ice
accumulation.

.LONG TERM...(Saturday night through Thursday)
Issued at 259 AM CST Fri Feb 24 2017

A weak system will zip across the Upper Midwest Saturday night and
Sunday morning and could drop a little bit of light snow across
portions of central and northern Wisconsin. The next chance for
some precipitation will come Monday night and Tuesday as a system
comes out of the central Rockies and lifts northeast toward the
Great Lakes. With highs Tuesday expected to be in the 40s, the
precipitation should start out as light snow and then go over to
rain. Another chance for mainly light snow will come Wednesday
night into Thursday as a system dives southeast out of Canada.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1144 AM CST Fri Feb 24 2017

Messy wintry storm continues across the area with one more batch
of snow expected to work through the region this afternoon and
into the evening hours before tapering off. Overall IFR conditions
will prevail, with a few hour period of more enhanced snowfall
possible, especially at RST, though it appears the risk for
widespread blowing snow is quickly waning. Still, an additional
few inches of accumulation remains possible before precipitation
ends near or before midnight. Ceilings are then expected to
gradually lift overnight, scattering out on Saturday as high
pressure works in our direction. Winds will remain northerly at 10
to 20 knots with higher gusts at RST overnight, shifting
northwesterly and slowly diminishing Saturday morning.

&&

.HYDROLOGY...
Issued at 259 AM CST Fri Feb 24 2017

A few forecast points remain above flood stage or are forecast to
reach flood stage in the near future as a result of the recent
warm conditions and precipitation. For information on specific
forecast points see the latest river flood statements.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory until 6 AM CST Saturday for WIZ041>044-
     053>055-061.

     Winter Storm Warning until 6 AM CST Saturday for WIZ017-029-
     032>034.

MN...Winter Weather Advisory until 6 AM CST Saturday for MNZ096.

     Blizzard Warning until 6 AM CST Saturday for MNZ079-086>088-094-
     095.

IA...Winter Weather Advisory until 6 AM CST Saturday for IAZ010-011-
     019-029-030.

     Blizzard Warning until 6 AM CST Saturday for IAZ008-009-018.

&&

$$

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


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