Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 230556 AAC
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
1156 PM CST Mon Jan 22 2018

.UPDATE...

Rain has transitioned to snow over all except the far southeast
portions of the area, as the air column cools with wet bulb
saturation and some cold air advection. Getting some moderate
snowfall across the higher reflectivity areas, and this should
gradually pivot eastward across the area overnight. The snow
should exit to the east by 12Z or so Tuesday morning. Expecting a
2 to 3 inch snow accumulation in the Winter Weather Advisory area,
with 1 to 2 inches on the periphery of this area.

Main issue will be the snow and freezing standing water on
untreated roads overnight into Tuesday morning in the advisory
area, as well as other areas that got decent rainfall from earlier
on Monday.

Some light freezing rain or drizzle may occur overnight into early
Tuesday morning over mainly south central Wisconsin, where ice
crystals may be lost. Not sure if there is enough of a risk for an
advisory for this area as of now.

Wood

&&

.AVIATION(06Z TAFS)...

Rain will transition to snow across the far southeast counties by
07Z, with the snow lingering overnight. The snow should end from
west to east by around 12Z Tuesday or so. Look for 2 to 3 inches
of accumulation in far southeast Wisconsin, with an inch or so
near Madison, and lower amounts to the north. Rates of 1/4 to 1/2
inch per hour are possible, mainly in southeast Wisconsin.

There may be light freezing rain or drizzle overnight into early
Tuesday morning over south central Wisconsin. Not confident to
mention in TAFs at this time with some uncertainty.

Ceilings below alternate minimums are expected with the snow,
then gradually improve to MVFR category on Tuesday, then VFR later
in the day. Visibilities will be around 1 mile or so with the
snow, then gradually improve to VFR category by middle morning
Tuesday. Winds will become north to northwest and gusty overnight,
lingering into Tuesday, before weakening Tuesday night.

Wood

&&

.PREV DISCUSSION... (Issued 930 PM CST Mon Jan 22 2018)

UPDATE...

Most of the thunder has ended or will be ending shortly in the
eastern counties. Rain is changing to snow in the Madison area
already, and the transition is expected to shift eastward into the
overnight hours. This transition will occur a little sooner than
currently forecast. Will adjust snowfall amounts upward a bit in
Madison to account for the earlier changeover.

Still looking at the southeast counties seeing the best shot at
the highest accumulations, where the best low level frontogenesis
response will be located. A general 1 to 3 inches is expected in
this area, perhaps an inch or two, or more, higher if models are
correct with higher QPF in that general area. Kept Winter Weather
Advisory for the southeast counties going as is for now.

May see light freezing drizzle north and west of Madison later
tonight into early Tuesday morning. Not sure if it will be
impactful enough for an advisory. Will let the night shift take
another look at it.

Wood

MARINE...

Allowed the Dense Fog Advisory to expire for the nearshore waters
of Lake Michigan at 03Z. Webcams and observations suggest that the
dense fog has ended, with the rain moving through and into the
area also helping improve visibilities.

Small Craft Advisory continues into Tuesday night across the
nearshore waters of Lake Michigan. Gusty north to northwest winds
are expected to develop overnight, with gusts up to 30 knots on
Tuesday. These winds should bring building waves of 4 to 8 feet
later tonight into Tuesday, with the highest waves toward the open
waters.

Wood

PREV DISCUSSION... (Issued 754 PM CST Mon Jan 22 2018)

UPDATE...

Area of showers and scattered thunderstorms are rotating through
the eastern portions of the area early this evening, with some
showers lingering elsewhere. Robust low level frontogenesis
response helping to drive this area of precipitation. The
convection has produced pea size hail covering the ground in
several spots.

This convection should gradually dissipate by later this evening,
as any remaining elevated CAPE is lost. Still may see more pea
size hail covering the ground in spots until this occurs.

Will also need to watch for runoff from rainfall affecting area
rivers and low spots on roads where water may collect.

Dense fog may end with the precipitation moving through the
southeast counties. May drop the advisory before the ending time
of 03Z if values continue to climb across this area.

Rain is expected to change to snow from northwest to southeast
across the area later this evening and into the overnight hours.
The best area for accumulating snow will be over the southeast
counties, where the best low to middle level frontogenesis
response will remain.

Models have trended higher with QPF amounts in this area, and
this makes sense with the saturated dendrite zone being tapped.
Adjusted snowfall amounts up a bit in this area, and may need
further adjusting upwards, depending on how the 00Z model runs
handle it. There is some uncertainty here. For now, the Winter
Weather Advisory looks on track.

The other issue would be the potential for some light freezing
rain or drizzle to affect the western counties later tonight into
early Tuesday morning. The ice crystals dry out in these areas,
which may lead to some light freezing rain or drizzle occurring.
Still evaluating this, and will see what the 00Z model runs do
with the ice crystals aloft.

Wood

AVIATION(00Z TAFS)...

Look for showers with scattered thunderstorms to affect most of
the eastern part of the area this evening. The storms may produce
pea size hail that covers the ground in spots. The storms should
gradually weaken and dissipate by later this evening.

The rain should change to light snow from northwest to southeast
across the area later this evening into the overnight hours. It
should change to snow at Madison around 05Z Tuesday, and
Waukesha/Milwaukee by 08Z, with Kenosha by 09Z. The best area for
accumulations will be in the southeast counties, where a period of
moderate snow is expected.

1 to 3 inches of snowfall is forecast at this time in that area,
including Milwaukee, Waukesha and Kenosha. Amounts may be 1 to 2
inches higher in localized areas, depending on where any enhanced
snow bands develop. Some uncertainty here at this time with
amounts. Rates of 1/10 to 1/4 inch per hour are expected, with 1/2
inch per hour possible.

Snowfall amounts elsewhere, including Madison, should be around
1/2 inch or so. The snow should end from west to east between 12Z
and 15Z Tuesday.

Ceilings below alternate minimums are expected tonight into
Tuesday morning, before increasing to MVFR by the afternoon.
Visibility values of 1/4 to 1 mile may linger until later this
evening in southeast Wisconsin, before becoming 1 to 2 miles later
tonight with the snow. VFR values should return once the snow
ends Tuesday morning. Winds will be shifting to the north
northwest by later tonight, and become gusty into Tuesday.

Wood

PREV DISCUSSION... (Issued 322 PM CST Mon Jan 22 2018)

SHORT TERM...

Tonight and Tuesday...Forecast Confidence is Medium...

An occluding low pressure system will progress across the lower
Great Lakes through Tuesday. The associated warm frontal zone
should hold steady along the Wisconsin/Illinois border, with some
warmer temperatures sneaking into the far south. With the frontal
zone in this vicinity, we`ll need to keep an eye on our far south
into this evening as thunderstorms now developing over central
Illinois push north into southern Wisconsin this evening. While
convective allowing mesoscale models suggest these storms will
weaken over time (the most likely scenario as instability is
quite meager), there is a large amount of (largely unidirectional)
shear present. We`ll need to watch for a very small severe
potential along our southern border, but this appears to be a
rather small risk.

Our attention then turns towards tonight, as the low pressure
system pulls away to our east. Precipitation will transition from
rain to snow as temperatures cool aloft, with a TROWAL and
frontogenesis band contributing to a burst of wet snow generally
after midnight tonight. Current model solutions place the highest
QPF across our southeast, where a Winter Weather Advisory for
impacts was issued. It`s important to note that the placement of
mesoscale banding features such as this are tough for the synoptic
forecast models to nail down. As such, it`s possible that the
location of this band may change. The key message is that
travelers late tonight into tomorrow morning should prepare for
the possibility of slippery/slushy roads resulting from the snow.
Reduced visibility is also possible late tonight/early tomorrow
morning in bursts of moderate snow.

Temperatures will fall below freezing late tonight, and will
remain there tomorrow. Lows tomorrow night will drop into the
teens.

LONG TERM...

Tuesday night through Thursday...Forecast confidence is high:

High pressure will bring quiet weather Tuesday night through
Thursday. Temperatures will be near normal Wednesday, but will
quickly return to above normal conditions Thursday as southerly
winds develop on the back side of the departing high.

Friday through Monday...Forecast confidence is medium:

Low pressure is expected to pass by well to the north on Friday.
Southerly winds ahead of the surface trough will continue to pull
milder air into the region. Temperatures Friday will likely hit at
least the mid-40s, with 50 possible per some of the milder model
solutions.

Could see a little rain and a few snowflakes Friday night as the
trough passes through, though models overall look fairly dry with
the passage. The latest ECMWF is slower with the trough and
develops a surface low in the area, which would keep rain chances
going into Saturday. This is a new solution and is currently an
outlier, so kept the forecast dry for Saturday.

Milder temps will probably linger Saturday, with highs back to
near normal Sunday into Monday.

AVIATION(21Z TAFS)...

Rain showers and drizzle will continue into this evening with low
ceilings mostly persisting. The exception is in the far south and
southeast, where ceilings may rise for a time into early evening,
before falling once again tonight. Visibility will bounce around
some, with lower periods occurring in between rain showers. There
remains a small chance for thunder into this evening, but
confidence is too low to include it in the TAFs.

A period of wet snow is expected late tonight into early Tuesday
morning. Accumulations of 1 to 3 inches are possible, with the
higher amounts possible in the Milwaukee, Kenosha, and Waukesha
areas.

MARINE...

Extended the small craft advisory into Tuesday night, with high
winds and waves likely to persist until this time. While
visibility has improved some in the nearshore waters this
afternoon, will keep the marine dense fog advisory in effect until
this evening, as poorer visibility may return. Mariners should
exercise caution.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory until 10 AM CST Tuesday for WIZ065-066-
     069>072.

LM...Small Craft Advisory until midnight CST Tuesday night for
     LMZ643>646.

&&

$$

Update...Wood
Tonight/Tuesday and Aviation/Marine...SPM
Tuesday Night through Monday...DDV


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