Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 101721
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
1121 AM CST Sun Dec 10 2017

.UPDATE...

Made some adjustments to precip chcs and wx types for later
tonight and Monday. Backed off a bit on the freezing precip
mention as we should have ice in the dendrite region during the
morning and afternoon events. Will continue to fine tune those
details this afternoon.

&&

.AVIATION(18Z TAFS)...

The lower cloud deck is the main challenge over the next 12 hours.
We should see improving conditions as we approach mid afternoon.
Until then, look for widespread MVFR CIGS, but these should at
least lift to VFR, even if the clouds don`t scatter out. We will
likely then stay VFR until late tonight when we start to see snow
spread in with the approach of the next weather system. CIGS will
drop to MVFR levels at KMSN by about 10z and KMKE and the rest of
the southeast a few hours later. This would also be about the time
light snow will be spreading across. We should see the morning
round last until about mid morning with a second round ramping up
again by early afternoon. It looks like the main precip type will
be snow.

&&

.PREV DISCUSSION... (Issued 1026 AM CST Sun Dec 10 2017)

UPDATE...

The forecast is on track. Low clouds have spread across the area
this morning, but should begin to break up this afternoon under a
region of subsidence. The visible image does suggest these clouds
are rather thin upstream, so for now the potential for mid aftn
sunshine looks reasonable.

MARINE...

Winds are veering to the west then northwest late this morning in
response to a cold front moving through. Modest swly winds will
return for Mon, but become brisk out of the nw for Mon- Mon nt,
following the passage of another strong cold front. A Small Craft
Advisory will likely be needed for Mon- Mon nt, with at least a
small potential for gale force wind gusts.

PREV DISCUSSION... (Issued 535 AM CST Sun Dec 10 2017)

UPDATE...The forecast is on track for today. Stratus clouds are
increasing from the northwest early this morning while a cold
front is expected to pass later this morning. No snow is expected.

AVIATION(12Z TAFS)...
MVFR stratus will increase from the nw early this morning, but
believe some of it will scatter out during the afternoon due to
subsidence. However, cannot rule out areas of MVFR cigs through
the afternoon. Clouds will increase again late tnt, with chances
for light snow early Mon AM. Cigs and vsbys may fall to MVFR
toward 12z, especially north and west of Madison. After sunrise on
Mon, the light snow will overspread srn WI for the day including
areas of freezing rain. Cigs will fall to 600-1500 feet with vsbys
ranging from 1-3SM.

PREV DISCUSSION... (Issued 354 AM CST Sun Dec 10 2017)

SHORT TERM...

Today And Tonight...Forecast Confidence Is Medium.

A wave of low pressure and cold front will move across central
and southern WI this morning, with a weak shortwave trough to
pass across east central WI during the early afternoon. Stratus
will accompany the passage, but cold advection and differential
AVA may provide enough subsidence and drying for partly cloudy
skies for the latter half of the afternoon. If the clearing does
occur then temps should be able to fall into the teens over ern WI
this evening, before the clouds increase again.

For tnt, an amplifying shortwave trough will track from the nrn
high plains to sw MN by 12z Mon with a sfc low just north of MSP
at that time. PVA and warm advection well ahead of the upper wave
will increase clouds and bring chances for light snow early Mon
morning. The best chances for minor snow accumulation before 12z
will be north and west of Madison. Dry air should limit the
eastern extent of the snow until later Mon morning.

Monday Through Tuesday Night...Forecast Confidence Is Medium.

Models are in general agreement with taking focused warm air
advection across the area Monday morning. They also take the
potent 500 mb shortwave trough southeastward across northeast Iowa
into east central Illinois Monday. This track will likely bring
enough differential cyclonic vorticity advection and mid level
frontogenesis response for a decent snow band to develop.

The main question is where it will track. A couple of the
mesoscale models suggest that this band may track east southeast
through parts of southern Wisconsin Monday morning into the
afternoon. Another keeps it south of the area. Given the track,
feel that it is a good idea to go with at least likely PoPs over
the whole area for Monday morning, lingering in the south and east
in the afternoon. Some dry air may limit the eastward extent of
the snow in the morning, so PoPs here may need adjustment in later
forecasts.

May need higher PoPs in the southwestern counties, and lower in
the north, depending on where the snow band sets up. For now, have
a 1 to 2 inch accumulation mainly for Monday, highest in the
southern counties.

Another issue Monday will be the possibility of light freezing
rain or drizzle at times, as forecast soundings are still showing
ice crystals aloft being lost in some areas. Left generally low
PoPs for this to occur, as the trend has been for more saturation
than not in the snow crystal growth region aloft. Some uncertainty
exists here.

The snow, and possibly some light freezing rain/drizzle during
the Monday morning/evening commutes, would bring a fair amount of
impact to the area. A Winter Weather Advisory may be needed in
later forecasts for late Sunday night and Monday, depending on how
things evolve with the snow band track and light freezing
rain/drizzle potential.

Strong cold air advection then moves into the area Monday night,
with gusty northwest winds. Lows should drop into the 10 to 16
degree range northwest to southeast across the area. It may get a
few degrees colder than that, with fresh snow on the ground.

Cold temperatures then continue for Tuesday into Tuesday night,
as a high pressure ridge axis shifts into the region. Again,
temperatures may end up a few degrees colder than currently
forecast, with the snow cover, clearing skies and light winds.

LONG TERM...

Wednesday Through Saturday...Forecast Confidence Is Medium.

The GFS and Canadian models bring low pressure southeastward
through the region Wednesday night, with passing 500 mb vorticity
maxima within northwest flow. There is some placement differences
between the models with these features. Still, there is enough
focused warm air advection and QPF to warrant PoPs for light snow
for Wednesday night.

Another shot of cold air advection for Thursday may bring more
cold temperatures into the area. Some uncertainty here, so did not
adjust the consensus blended model temperatures down much at this
time.

Temperatures may moderate Friday into Saturday, as 500 mb flow
becomes more zonal, and some warm air advection develops in the
low levels. It appears that the best upward vertical motion
remains north of the area during this period. Left consensus
blended model PoPs for now for this period, being so far out.

AVIATION(09Z TAFS)...

MVFR stratus will increase from the nw this morning, but believe
it will scatter out during the afternoon due to subsidence.
However, cannot rule out areas of MVFR cigs through the
afternoon. Clouds will increase again late tnt, with chances for
light snow early Mon AM. Cigs and vsbys may fall to MVFR toward
12z, especially north and west of Madison.

MARINE...

Swly winds of 10-15 kts will shift to the nw for the afternoon,
with the passage of a cold front. Modest swly winds will return
for Mon, but become brisk out of the nw for Mon-Mon nt, following
the passage of a strong cold front. A Small Craft Advisory will
likely be needed for Mon-Mon nt, with at least a small potential
for gale force wind gusts.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...None.
&&

$$

Update...Davis
Today/Tonight and Aviation/Marine...Gehring
Monday THROUGH Saturday...Wood


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