Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 110311
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
911 PM CST Sat Dec 10 2016

.UPDATE...
Issued at 911 PM CST Sat Dec 10 2016

Well...here we are with the event underway and some questions
still remain regarding snow amounts overnight. Overall, am
starting to get a little nervous that our snowfall amounts may be
a touch too high across southern areas and thus a little too low
near the I-90 corridor. Radar trends the past 2 hours are telling,
with a pronounced narrow but intense frontogenetic snow band laid
out across southern MN and WI, and really stretching back into
western SD as well. Recent RAP trends seem to have caught onto
this feature, most pronounced in the 700-500mb layer, which also
just happens to be completely in the DGZ, supporting the growth of
dendrites and thus some pretty high snow-liquid ratios. Per
reports/observations under this band, snow ratios are generally
20-25+ to 1 (not surprising at all with a 300+mb deep DGZ), while
outside of it where there is minimal lift through the DGZ, ratios
are much closer to 12 to 1. That makes for a very tough forecast
heading through the night, not only with regard to snowfall
amounts but also placement of the narrow FGEN band, especially
considering some spots under that band have already picked up 2-4"
of fluffy snow the past 6 hours.

Hard to ignore reality and short term model trends which agree
that the FGEN axis may well end up a little farther north than
earlier thought. This is especially true with recent RAP runs,
showing the elongated FGEN axis pretty much stationary as a subtle
lead 700mb wave passes the next few hours, followed by the more
pronounced upper feature toward morning, with the latter also
helping induce a nice deformation field bisecting the CWA near the
I-90 corridor. That setup does leave some question about how far
south additional precip redevelopment will occur, especially too
far south of the FGEN axis (as well as areas north of I-94 on the
downward branch side...also fighting some dry air). All told, not
ready to bite on the need for any upgrades to the current advisory
area farther north, but it may be needed with little advance
notice pending trends overnight.

Ahhh...the joys of winter weather forecasting!

&&

.SHORT TERM...(This evening through Monday night)
Issued at 220 PM CST Sat Dec 10 2016

It now appears that the southern parts of the forecast area will
have a decent shot of getting some heavier snow and plan to
upgrade to a warning to handle this. The 10.12Z models have come
in with a slightly stronger short wave trough to cross the area as
it deepens once it crosses the northern Rockies tonight. This
allows the frontogenesis zone to become stronger, especially in
the 06Z to 12Z time frame tonight in the 750-600 mb layer. With
this stronger forcing signal, the resulting qpf amounts and snow
amounts will be bumped up and it now appears that 8 inches of snow
could fall in a 24 hour period. In addition, the surface low looks
like it will take a more northeast track Sunday which could linger
some light deformation snows a little longer over southwest
Wisconsin. This may not add all that much snow as the qpf amounts
look to be pretty light in the deformation zone, but again, could
be enough to bump some areas into the warning category. Thus will
take all of our northeast Iowa counties along with the 3 southwest
Wisconsin counties and upgrade to a winter storm warning through
Sunday. Otherwise, very little overall change in the expected
conditions for the remainder of the area. Snowfall amounts of 6 to
8 inches north of the warning up to Interstate 94 still look on
track as does the 4 to 6 inches for north central Wisconsin.

There may also have to be some adjustments made on the back side
of the system to the end times for the Wisconsin part of the
advisory. It looks like the snow could be winding down by late
afternoon or early in the evening, which could necessitate ending
the advisory a little sooner. There is also a signal on the back
side of the system of a potential loss of ice before the
precipitation ends. However, think the models have a bit of a bias
toward taking the ice out too fast as systems are pulling away
from the region and will stay with just snow for now.

Cold, arctic air is still on track to spill into the Upper
Midwest early next week. The arctic cold front will move through
the region Monday afternoon and evening. Another pacific short
wave trough looks like it will zip across the region in
conjunction with the cold front. This wave looks to be strong
enough to generate a short period of weak to moderate pv advection
in the 500-300 mb layer. The QG convergence looks to be weak but
extend through a deep layer from 1000 up to 300 mb. This may be
enough to squeeze out some light snow as the cold air moves in and
will carry a small 20 to 30 percent of snow Monday afternoon and
evening. Temperatures Monday night look to bottom out in the
single digits above and below zero. Wind chill values will become
a concern with values dropping into the single digits and teens
below zero. However, it does not look like an advisory would be
needed at this time.

.LONG TERM...(Tuesday through Saturday)
Issued at 220 PM CST Sat Dec 10 2016

The cold air will remain for much of the week with some slow
moderation expected by the end of the week into the upcoming
weekend. Wind chill advisories may be needed Tuesday
night/Wednesday morning across north central Wisconsin and again
Wednesday night/Thursday morning for much of the area. Overnight
lows during this period generally look to be in the single digits
below zero with west winds of 5 to 15 mph. The next chance for
snow will come for the end of the week into the weekend as the
arctic high pressure moves off to the east. The models are
currently showing a positive tilt long wave trough moving across
the Rockies with southwest flow and embedded short wave troughs
over the Midwest. Decent agreement that a surface low in the
pattern will track between the local area and the Ohio River
Valley into the eastern Great Lakes with the potential to drop
some more snow on the area.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 530 PM CST Sat Dec 10 2016

Periods of snow have begun across the area and will continue
overnight and into Sunday, with the heaviest period of snow still
expected later tonight into Sunday morning. Overall, IFR
conditions will prevail (especially visibility), though a few
periods of LIFR are possible during any bouts of heavier snowfall.
It still appears that RST and LSE will pick up between 5 and 8
inches of snow by the time it tapers off Sunday afternoon, with
the snow initially of a lighter, more fluffy variety gradually
replaced by a slightly wetter snow with time. As the snow departs
(or at least lightens up in intensity) Sunday afternoon and
evening, widespread IFR ceilings appear probable, with even a
small risk for a little freezing drizzle, though confidence in
that is very low.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Storm Warning until 6 PM CST Sunday for WIZ054-055-061.

     Winter Weather Advisory until 9 PM CST Sunday for WIZ032>034-
     041>044-053.

     Winter Weather Advisory until 9 PM CST Sunday for WIZ017-029.

MN...Winter Weather Advisory until 6 PM CST Sunday for MNZ079-086>088-
     094>096.

IA...Winter Storm Warning until 6 PM CST Sunday for IAZ008>011-018-
     019-029-030.

&&

$$

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


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