Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 241109

Area Forecast Discussion
National Weather Service La Crosse WI
509 AM CST Tue Jan 24 2017

.SHORT TERM...(Today through Wednesday)
Issued at 356 AM CST Tue Jan 24 2017

Main fcst concerns this period include impending winter storm/snow/
wintry mix to spread into/across the area later this afternoon and

Data analysis at 06z had a weak ridge of high pressure from ND to
eastern IA, with 996mb low moving east out of eastern CO. Weak sfc-
850mb ridge axis doing little to help us with the persistent low
stratus and patchy fog across the Upper Midwest. Sfc obs/fog-
product imagery showing a persistent cloud deck across the region.
Early morning temps under this cloud deck on the mild side once
again, mostly in the upper 20s to mid 30s region-wide.

No issues noted with 24.00z model initializations. Models have
come into a rather tight consensus (even with sensible weather
outcomes) as strong shortwave energy lifts out of the southern
Rockies this morning and closes off a mid level low over western IA
by 12z Wed, then moves this to southern Wi by 00z Thu. Overall trend
favors slower/stronger of the earlier models with evolution later
today thru Wed. Given the tight model consensus, even among the hi-
res/WRF models, short-term fcst confidence ahead of a strong winter
storm is good this cycle.

In the short-term, staring down the barrel of a rather strong winter
storm to impact the fcst area starting late this afternoon and
continuing into Wed. Initial surge of lift, from lower level warm
advection/isentropic lift, thru 850-500mb FN convergence and 300mb
ageostrophic flow/divergence in the left exit region of a 120-150kt
jet max spreads NE across the area this afternoon. A few detail
differences, but additional rounds of lift to rotate NE/N across the
area late tonight and thru Wed as the sfc-mid level low approach.

Per model soundings and X-section, much of the initial strong lift
goes into saturating the rather dry 900-500mb portion of the column.
This works to delay the onset of precip across the area to mainly
late this afternoon across the SW end of the fcst area to as late as
midnight over the far N end of the fcst area. Initially a warm nose
of air around 850mb over at least the S 1/2 of the fcst area this
afternoon, but all models showing evaporative cooling as the
column saturates to rather quickly erode this, leaving the boundary
layer temps to control precip type. With the slower evolution, more
of the precip is pushed into late this afternoon/tonight, when the
boundary layer is diurnally cooling anyway. Thus a trend toward
precip more as snow, and higher snow-water ratios. Slower evolution
also looking to allow the forcing/lift to produce more precip across
the area with PW values in the inflow airmass around 6/10 of an inch,
some 200+% of normal. All this points to more snow/higher snow
amounts as this system moves across the area. Snow amounts in the
main band, mainly along/south of I-90 now looking to be in the 6-9
inches range, with locally higher amounts possible. Realigned the
winter headlines to better match the timing of the precip spreading
NE across the area. Replaced the winter storm watch with a winter
storm warning and adding a winter weather advisory (all thru 00z Thu)
to the rest of the counties across the far N and S ends of the fcst
area for lesser snow totals (north and south) and more of wintry mix

Signal among models now for potential loss of ice in the clouds over
the south 1/3 of the fcst area late tonight into Wed morning.
Depending on boundary layer temps, this could result in a wintry
mix or a period of -FZRA/-FZDZ across parts of NE IA/SW WI where sfc
temps would be 32F or less. Added mention of patchy freezing precip
to these areas late tonight/Wed morning.

Models continuing to show some near neutral stability/-EPV in/just
above the strong forcing/lift centered on 00z this evening. Perhaps
even some weak elevated CAPE in the inflow airmass also centered on
the same time. previous shift added a thunder mention across roughly
the south 1/3 of the fcst area late this afternoon/evening. Left this
for the most part but did trim it from chance mention to slight
chance as elevated CAPE signal has trended downward with the 24.00z

Stayed with a blend of guidance highs/lows for today-Wed, once again
showing little diurnal movement.

.LONG TERM...(Wednesday thru Monday)
Issued at 356 AM CST Tue Jan 24 2017

For Wed night thru Thu night: main fcst concerns are lingering -SN
in to Wed evening, cooling temps thru the period.

24.00z models in good agreement for the main wave/trough axis to
exit Wed night, with a rather strong northern stream secondary trough
axis/shortwave to drop into the region for Thu/Thu night. With
models maintaining a decent consensus thru this period, fcst
confidence for Wed night thru Thu night is good this cycle.

Deep cyclonic flow and slower exit of the mid level trough Wed night
keep some deeper moisture and deformation band lift over much of the
fcst area into Wed evening. Continued with some 30-60% snow chances
Wed evening, highest E/S of KLSE with some additional minor
accumulations over the SE 1/2 of the fcst area. May need to extend
some of the winter storm warning/advisories in central/SW WI into
Wed evening, but can detail that as the storm evolves over the
24 hours. Mid level portion of the column dries behind the
departing lead trough for later Wed night thru Thu night. Any
lingering precip chances after midnight Wed night looking to be
small and mainly very light snow or flurries as the column slowly
cools. Some deeper moisture progged over the area Thu night as the
northern stream shortwave/ trough axis drop in, and may yet need
more of a 20-40% -SN chance Thu night into Fri morning. Left the
consensus dry fcst Thu night as is for now. After the past week,
the slow cooling under the developing NW flow aloft has highs/lows
closer to normal by Thu/Thu night.

For Friday thru Monday (days 4 to 7): main fcst concerns this period
are the return to more near normal late Jan temps and on/off small
-SN/flurry chances.

Medium range model runs of 24.00z in good agreement for a pattern
transition to western NOAM ridging and eastern NOAM troughing for
our day 4-7 period, putting the region under colder NW flow aloft.
Given the decent between model and run-to-run consistency (except on
the details of shortwaves rippling thru said NW flow), fcst
confidence for Fri-Mon above average this cycle.

Day 4-7 period to be dominated by colder NW flow aloft and a return
to more near normal late Jan weather/temps. 850mb temps cool to
near/a bit below normal for Fri-Sun, with highs/lows these periods
perhaps even a few degrees below normal. Some warmup by Mon as low
level warm advection ahead of a stronger shortwave dropping out of
central Can would spread across MN/WI/IA. Before that though, models
are at odds on the details of at least a couple shortwave to ripple
SE thru the flow and across the area Fri-Sun. Given the cooler
column and deeper cyclonic flow on the poleward side of the jet
axis, each of these looking capable of a period of -SN/-SHSN/
flurries as they would pass across the region. Consensus hit and
miss small -SN chance in the Fri-Sun period OK until the shortwave
detail differences get sorted out.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 509 AM CST Tue Jan 24 2017

The clouds remain over the area again this morning with IFR
ceilings. These may briefly bump up to MFR by late morning or
early afternoon with a bit of diurnal warming, but they will not
stay there very long. The winter storm taking shape over Kansas
this morning will advance northeast into Iowa overnight. This will
push a band of snow over both airports this evening that will then
continue through the overnight hours. Both the ceiling and
visibility will go back down to IFR once the moderate to heavy
snow begins and then stay there through the night.


WI...Winter Storm Warning from 6 PM this evening to 6 PM CST
     Wednesday for WIZ032>034-041>044-053>055.

     Winter Weather Advisory from 3 PM this afternoon to 6 PM CST
     Wednesday for WIZ061.

     Winter Weather Advisory from 9 PM this evening to 6 PM CST
     Wednesday for WIZ017-029.

MN...Winter Storm Warning from 3 PM this afternoon to 6 PM CST
     Wednesday for MNZ094.

     Winter Storm Warning from 6 PM this evening to 6 PM CST
     Wednesday for MNZ079-086>088-095-096.

IA...Winter Storm Warning from 3 PM this afternoon to 6 PM CST
     Wednesday for IAZ008>011-018-019.

     Winter Weather Advisory from 3 PM this afternoon to 6 PM CST
     Wednesday for IAZ029-030.



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