Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 230945

Area Forecast Discussion
National Weather Service La Crosse WI
345 AM CST Mon Jan 23 2017

.SHORT TERM...(Today and tonight)
Issued at 345 AM CST Mon Jan 23 2017

Main fcst concerns this period: persistent low clouds, patchy fog,
overall dampness and continued mild late Jan temperatures.

06z data analysis had at 987mb sfc low over the southeast Conus with
troughing northward to Lk Superior. A weak sfc ridge axis extended N-
S thru the plains, producing weak gradient sfc-850mb NW flow over
much of MN/IA/WI early this morning. This gradient flow helping to
keep the dense fog at bay across much of the region this morning,
however sfc obs and fog-product imagery showed widespread low
stratus remained across the Upper Midwest, keeping early morning
temps mostly in the 30s, some 20-25F above the normals.

No issues noted with 23.00z model initializations. Solutions as one
mid level trough axis moves east across the region today, with hgts
falling and the flow turning SW tonight as strong western troughing
progresses into the plains. Overall trend favors faster/stronger of
earlier runs with the plains troughing at 12z Tue. Short-term fcst
confidence (for much of the same as the weekend weather) is good
this cycle.

For the short term, the weekend weather is going to remain thru
today/tonight, less the widespread dense fog. Lower levels remain
quite saturated, with plenty of moisture stuck in/under an
isothermal layer/inversion near 800mb. Plenty of low stratus all
the way upstream to southern Can and well west to the sfc-850mb
ridge axis. Going to play the persistence card today/tonight,
holding onto generally low overcast skies and patchy fog. Bit
deeper moisture over the area this morning east of the mid level
trough axis, and -DZ signal on area radars NW of the fcst area
into north-central MN. Will carry a patchy -DZ mention across the
area thru 15z. Day crew may need to carry it further thru today
until the mid level trough axis passes and moisture shallows. Low
cloud will continue to mute the diurnal temp ranges. A diurnal
rise of only a handful of degrees today, with lows tonight mostly
a couple degrees either side of 30F quite reasonable under the
continued cloudy skies. Should clouds actually break for a time
today, some sunshine and much reduced snow cover from the past few
days would allow some highs to pop into the 40s. Again, not
optimistic for this.

.LONG TERM...(Tuesday thru Sunday)
Issued at 345 AM CST Mon Jan 23 2017

For Tuesday thru Wed night: main fcst concerns this period are the
impending winter storm and snow amounts mainly later Tue afternoon
into Wed morning.

Model runs of 23.00z show gradually improving agreement on the
troughing moving into the plains at 12z Tue and its evolution as
lifts toward/thru the mid/upper MS valley this period. ECMWF has
offered a rather steady solution the past 3 days, with other models
shifting toward its solution. Thus will continue to lead toward an
ECMWF weighted model consensus this period. Fcst confidence in the
Tue-Wed night period and the details improving this cycle.

Impacts of the strong mid level trough/shortwave and attendant lower
level features/thermodynamic forcing set to spread into the west
end of the fcst area around mid-day Tue, then east across the area
Tue afternoon thru Wed afternoon. Following the ECMWF lead,
strongest/deepest of the forcing/lift looking to spread W to E
across the fcst area Tue afternoon into Wed morning. Rather
impressive 850-500mb FN convergence signal to move across the area
Tue afternoon/evening, with rather impressive lifting thru the DGZ
spreading across the area, especially between 24.21z-25.06z. In
fact, 23.00z Nam would produce 6+ inches of snow at both KRST/KLSE
between 22-02z Tue evening!  Not buying that at the moment though.

Column over the bulk of the fcst area progged cold enough for precip
to form/fall as snow (no elevated warm layers for ice this time),
with precip with this system looking to be controlled by the
depth/warmth of the sfc/boundary layer. Temps Tue at precip onset
looking to be in the mid/upper 30s at many locations, quickly
falling into the lower 30s once any heavier precip would move in,
quickly changing an -RA at onset to -SN. Bulk of precip across most
of the fcst area with this system expected as snow, with several
inches expected most area. Exception will be the far south end of
the fcst area where there may be a bit of a warm layer aloft
intrusion and the boundary layer may remain warm enough for a more
of a -SN/-RA mix and possibly some sleet. Appearing as though the
south edge of the snow band is going to have a rather tight snowfall
gradient, with minimal snowfall expected in the far south end of
Grant Co. WI, to 5 to 6 inches possible in the northern parts of
Fayette Co. IA to Richland Co. WI. Continued to trend toward 80-90%
precip chances spreading across the area Tue afternoon into Wed
morning, tapering off thru Wed afternoon/night as the low/trough
axis passes.

The biggest concern with this system is the snow-liquid ratio.  For
now, stayed with the model/ensemble of ratios more in the 6:1 to
10:1 range Tue afternoon/evening, which blends well with neighboring
grids and produce a smooth snowfall band across the fcst area
boundaries. However, fear once the changeover from any -RA to snow
is complete and it`s late Tue afternoon/Tue night, the ratios may
end up in the more traditional 10:1/12:1 range, adding 2 to 3 inches
to the snow totals in the well blended grids.  Time will tell, but
in the mean time, expanded the winter storm watch into Fayette/
Clayton ctys in IA and much of western into central WI for this

With the now warmer ground surfaces many areas, the initial snowfall
on most roads likely to partly melt, becoming slushy/slippery very
quickly with onset of the snow.

For Thursday thru Sunday (days 4 to 7)...

Main fcst concerns this period: return to more near normal late Jan
temps thru the period.

Medium range model runs of 23.00z continue with a strong signal of a
change in the mid level flow pattern to west coast ridging/eastern
NOAM troughing once the Tue/Wed system passes, setting the region up
under NW flow aloft. Fcst confidence for temps returning to near
normal for the Thu-Sun period is good this cycle.

Trough passing Wed establishes NW flow aloft over the region. This
then set to persist thru the weekend as hgts rise along the west
coast of NOAM. Initial airmass behind the departing Wed low/trough
not that cold, with highs Thu still looking to be a category or so
above normal. Some detail differences among the models, but Thu-Sun
shapes up to be a period of gradually cooling 925-850mb temps, with
850mb temps by Sun around 1 std deviation below normal. Thus,
consensus highs by Sat/Sun mostly in the lower-mid 20s (over what
should be fresh snow cover) look well trended but may well end up a
category or two too warm. Small lingering -SN chance Thu as a
secondary shortwave dropping into the trough would keep deeper/
tighter cyclonic flow over the area quite reasonable. Beyond that,
model detail differences with timing/strength of shortwaves to
ripple down thru the NW flow. May yet need some small -SN/flurry
chances any of the days Fri-Sun. Plenty of time to detail that


.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1135 PM CST Sun Jan 22 2017

Yet another night of IFR/LIFR ceilings, although dense fog may not
be as problematic given slightly stronger northwest surface winds.
Generally kept minimum visibility restrictions in the 1SM (KRST) to
3SM (KLSE) range overnight, with TEMPO groups late tonight/ early
Monday morning for slightly lower thresholds. Conditions will
improve slowly late Monday morning/early afternoon, potentially
rising into the MVFR range later in the period. Winds will be light
through the period, generally from the west-northwest.


WI...Winter Storm Watch from Tuesday afternoon through Wednesday
     afternoon for WIZ032>034-041>044-053>055.

MN...Winter Storm Watch from Tuesday afternoon through Wednesday
     afternoon for MNZ079-086>088-094>096.

IA...Winter Storm Watch from Tuesday afternoon through Wednesday
     afternoon for IAZ008>011-018-019-029-030.



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