Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 150342
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
942 PM CST Sun Jan 14 2018

.UPDATE...
Snow continues to fall across southern Wisconsin as the cyclone
pushes southeast out of MN. Latest model guidance shows very
little difference from previous forecasts, so little change
needed. Did adjust this evening`s snow ratios up toward 25:1 given
upstream observations. This increased evening totals up about 1",
and should give us 1-2" totals across the area before midnight.

&&

.MARINE...
SCA remains in effect through tonight. Winds currently gusting
between 25 and 30 kts will begin to diminish after midnight as
the approaching low pressure center draws near. However, waves in
ice free areas will remain in excess of 4 ft through 12z south of
Port Washington and through 15z between Port Washington and
Sheboygan.

As the low moves out of the area on Monday night and Tuesday,
winds will turn to the NNE and will approach SCA criteria on
Tuesday morning. Given the fetch over the lake, waves in ice free
waters will likely reach SCA criteria as well and remain there
until Tuesday evening.

Winds still expected to become gusty for the end of the week,
remaining at or above SCA criteria for much of the time from
Wednesday through Friday.


&&

.PREV DISCUSSION... (Issued 550 PM CST Sun Jan 14 2018)

UPDATE...
Radar echoes have overtaken all of southern Wisconsin and it has
not taken long for the low levels to saturate. Snow is reaching
the ground nearly everywhere, save for the eastern 2 or so tiers
of counties, and it won`t be long there. Precip will gradually
increase in intensity as WAA currently overhead is supplemented
with lift from the upper level wave. Effects of this wave can been
seen upstream on the ARX radar, where fairly widespread 20-30 dBZ
echoes can be found along with visibilities down to 1SM.

Between ample lift from the developing cyclone and a nearly
saturated column below 600mb through tomorrow evening, all the
ingredients appear to be in place for the previously advertised
long duration light snow event. Models continue to show an area of
enhanced low level convergence just behind and to the north of the
surface cyclone on Monday afternoon, impacting the lakeshore
areas. This feature will likely tap some energy from the lake
given the easterly flow at that time, so this could cause a
traffic headache Monday afternoon. However, it is very difficult
to pin down exactly where these mesoscale bands will end up, so
please stay tuned for updates as the vent progresses.

AVIATION(00Z TAFS)...
Snow has pushed into nearly all of southern Wisconsin, with just
the lakeshore areas yet to saturate. High confidence that the
entire area will be seeing snow within the next hour or two, and
models in good agreement keeping that snow in place for the next
24 hours or so.

Despite the good agreement on the overall pattern for this system,
there are a few wrinkles in this forecast that need attention.
First of all, much of the model guidance is picking up on a narrow
band of moderate to heavy snow tomorrow afternoon along the
lakeshore counties. Given the good model consensus and knowledge
of previous events, it seems like a good bet that this band will
develop. However, as is always the case with these narrow lake-
enhanced bands, it is very difficult to pin point exactly when and
where this band will set up, so please continue to monitor for
updates.

The second issue is that meso models seem to be having a hard time
with the cigs across the area this evening. Given the current
observations, much of the guidance is either far to low or far too
high. I ended up blending the guidance which gave me cigs in the
1.5kft range, which seems reasonable given the RH profile of the
larger scale models. However, given the large spread in the model
guidance, confidence in cig forecast is not particularly high.

PREV DISCUSSION... (Issued 233 PM CST Sun Jan 14 2018)

DISCUSSION...

Due to higher snowfall amounts expected with lake effect snow
Monday night into Tuesday, have issued a Winter Storm Warning for
the lakeshore counties tonight into Tuesday morning. The Winter
Weather Advisory continues elsewhere tonight into Monday night.

Tonight And Monday...Forecast Confidence is High.

Snow will spread into the area from both the west and the south
this evening, as steady warm air advection continues and low
levels saturate. The snow will continue with differential cyclonic
vorticity advection moving in later this evening and overnight,
as low pressure at the surface slides southeast into northern
Illinois.

The inverted surface/850 mb trough on Monday then pivots counter
clockwise across the area, and actually takes the north to south
lake effect/enhanced snow band from central Lake Michigan with
it. There are hints of a TROWAL developing, though it may remain
just to the south. Still, the trough/lake enhanced band will
result in enhanced upward vertical motion and snowfall rates on
Monday. This is in addition to the light snow with the 500 mb low
moving through.

At this time, it appears that the northeastern counties would
have the best shot at these enhanced snowfall rates, though it
could rotate through a good part of the rest of the area. Thus,
increased snowfall amounts in the northeast counties Monday.

Deep dendrite zone in the low levels combined with the good
upward vertical motion with the trough/lake enhanced band tonight
into Monday should combine with relatively high snow to liquid
ratios to bring 4 to 6 inches of powdery accumulation. The
highest amounts will be in the northeast counties, and lowest
amounts southwest of Madison. This will impact the morning and
evening commutes on Monday.

Monday night and Tuesday...Forecast confidence is medium.

The sharp low level trough axis will shift south across the area
overnight. In the wake of the trough, winds will back to
northeast along the shore and with delta T`s in the 15-16C range
coupled with steep low level lapse rates and deep(ish) omega under
the impressive upper low, it appears we have a good chance for a
period of moderate lake effect snow. The lake effect should
transition from multi-band in the easterly flow early, to a
focused band after midnight with the more northeast flow. The
higher resolution models are depicting a snow band better than the
GFS/ECMWF.

The problem, if this band stays just off shore, the amounts for
the lake counties will be quite a bit less. However, the evidence
seems strong enough to support posting a warning at this time.
CIPS analogs for this event have a mean snow amount that is
rather high. But, looking at the various members doesn`t bring
lots of confidence. Some are very high in snow, others quite low
and it`s hard to find an ideal match.

The snow will taper off quickly on Tuesday, but could linger
through the morning across the far southeast. This will be a long
duration snow...on the order of 36-40 hours in some places.

Wednesday through Friday...Forecast confidence is high.

This looks like a dry period with high pressure generally in
control. Temps remain cold through Wednesday, but we get into a
return flow by Thursday with progressively warmer temps into
Friday.

Saturday and Sunday...Forecast confidence is low.

Both the GFS and the ECMWF show a decent low pressure system
developing to the south and moving through the region. The EC
continues to look warmer than the GFS. One says more rain, the
other suggests snow. Plenty of time to work that out, but for now,
it will suffice to say it looks unsettled later next weekend.

AVIATION(21Z TAFS)...

Some flurries may move into western portions of the area into
early this evening. No accumulations are expected with this.

The snow is expected to spread eastward and northward across the
area between 00Z and 03Z Monday. A long duration powdery snow
event is expected tonight into Monday night/Tuesday morning.
Ceilings should remain in the 2000 to 2500 foot range tonight into
Monday night, possibly a bit lower at times. Visibility values
should be in the 1 to 2 mile range during this time, again
possibly a bit lower at times.

A lake enhanced snow band should pivot southward across the
northern and eastern portions of the area Monday into Monday
night. There may be moderate to heavy snow rates of 1 inch per
hour or greater associated with this band. Depending on how
quickly it moves through the area will determine how much more
snow it will drop.

For now, expecting a general 4 to 9 inches of snowfall
accumulations tonight into Monday night/Tuesday morning. The
lowest amounts will be southwest of Madison, and highest toward
the Sheboygan area and near Lake Michigan. General snowfall rates
should be up to 1/4 inch per hour, with 1/2 inch per hour rates
possible at times.

South winds tonight will veer west Monday morning and northwest
in the afternoon. They should become more northerly Monday night
into Tuesday. There may be gusts in the 15 to 20 knot range.

MARINE...

Small Craft Advisory remains in effect from 6 PM CST this evening
until 6 AM CST Monday morning south of Port Washington, and until
9 AM CST Monday morning north of there. The pressure gradient is
expected to tighten during this period, as low pressure approaches
the area from the west.

This will bring increasing south winds, with gusts of 25 to 30
knots at times. This will also bring building waves of 3 to 6 feet
tonight, before gradually subsiding to 2 to 4 feet on Monday.

North to northwest winds will approach Small Craft Advisory
levels at times Monday night into Tuesday, as the low gradually
moves away from the region. South to southwest winds Wednesday
into Friday will approach Small Craft Advisory levels as well.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory until midnight CST Monday night for
     WIZ046-047-051-056>058-062>064-067>070.

     Winter Storm Warning until noon CST Tuesday for WIZ052-060-066-
     071-072.

     Winter Weather Advisory until 6 AM CST Tuesday for WIZ059-065.

LM...Small Craft Advisory until 6 AM CST Monday for LMZ644>646.

     Small Craft Advisory until 9 AM CST Monday for LMZ643.

&&

$$

Update...BSH
Tonight/Monday and Aviation/Marine...Wood
Monday Night through Sunday...Davis


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