Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 150432
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
1132 PM CDT Sat Apr 14 2018

.UPDATE...

No major changes to the forecast from earlier this evening. Still
looks like mostly a mixed bag of wintry precip overnight tonight,
though snow should become more common across the northwestern CWA.
The overall message remains the same--expect to see considerable
travel impacts through tonight and much of tomorrow.

&&

.AVIATION(06Z TAFS)...

IFR conditions will continue overnight tonight, as mixed winter
precipitation impacts the TAF sites. The exact timing of the
various types of precipitation is quite challenging, but overall
a mix of sleet and snow is expected, with occasional periods of
freezing rain mixed in. Snow will become more common with time,
with much of the area seeing mostly snow by tomorrow afternoon and
evening. Winds will remain quite gusty from the northeast tonight
and tomorrow, eventually becoming northwesterly late tomorrow
evening/night.

&&

.PREV DISCUSSION... (Issued 945 PM CDT Sat Apr 14 2018)

UPDATE...
Have added Milwaukee, Racine, and Kenosha counties to the winer
weather advisory. Getting some reports of slippery conditions due
to sleet and freezing rain.

Sleet is the main precip type due to a warm layer around 700 mb and
it looks like it will continue through at least the evening before
cooler air works its way in. As a result, have trimmed snowfall
totals a little toward central WI. The counties under the winter
storm warning still have the best chance of seeing several inches of
snow. Rap soundings seem pretty representative now. They show the
column colder just to the north of our forecast area where precip
type is all snow. As temps cool on the back side of this system
there is general model consensus of a change over to snow for
areas north and west of Madison. The Rap sounding shows the
Madison area hovering right at freezing aloft, indicating snow or
sleet. The evening model guidance shows mainly sleet and/or
freezing rain for southeast Wisconsin, and uncertainty with snow.
Regardless, slippery conditions will result across all of
southern Wisconsin.

Snow will likely continue through Sunday south-central areas but
precip type is uncertain in the southeast.

&&

MARINE...
Gale force gusts will continue into Sunday morning. Winds will
remain at Small Craft Advisory levels through Monday but will shift
to the north Sunday night.

&&

PREV DISCUSSION... (Issued 722 PM CDT Sat Apr 14 2018)

UPDATE...
Moisture is surging into the region this evening associated with
850 warm air advection. There is strong lift from the left exit
region of an upper jet and waves of vorticity from the approaching
upper closed low. Precipitation type is mainly sleet and snow with
occasional freezing drizzle. Surface air temperatures are at or
below freezing except for the Kenosha area and MKE pavement
temperatures have dropped to 34 degrees. Icing is likely,
especially on elevated surfaces, while the warmer ground is
helping mitigate icing on pavement. There is a warm layer around
800mb causing the sleet. The surface low is moving across Illinois
and temps at the surface and aloft should decrease a little as it
continues eastward. So freezing rain and glazing of surface
continues to be a concern as these temperatures drop. Precip type
remains a tricky forecast due to model uncertainty with low level
temperatures and amount of saturation in the snow growth region.
This make it tricky to determine when the change to snow will
happen, which of course affects snowfall amounts. In the meantime
expect the wintry mix to continue.

Allowed the wind advisory to expire but wind gusts around 30 knots
will continue through the night.


PREV DISCUSSION... (Issued 350 PM CDT SAT APR 14 2018)

TONIGHT AND SUNDAY...Forecast Confidence - Medium to High.

A late season winter storm will bring mixed precipitation across
southern Wisconsin this evening through Sunday. Latest forecast
models came in a little colder overall, with better saturation in
the ice growth region. This suggests a better chance for snowfall,
particularly for areas north of a line from Lone Rock to Madison to
Port Washington. Decided to upgrade the Winter Weather Advisory to a
Winter Storm Warning for these northern areas due to the higher snow
totals. Additionally, strong winds, freezing rain, and sleet will
add to the mess made by the higher snow amounts. Expanded the Winter
Weather Advisory in the south to add Walworth County due to the
higher confidence in some icing. Kept the advisory out of the far
southeast due to more of a rain threat there and less ice/snow
totals.

The biggest forecast challenge with this system continues to be how
the models are handling the warm layer aloft and saturation in the
ice crystal growth zone. There are significant differences among
solutions, from the colder and more saturated GFS which has higher
snow totals than the forecast, to the warmer and drier aloft NAM
which would suggest more freezing rain and sleet. Even this
afternoon, model soundings would suggest no chance of snow (either
freezing rain, rain, or sleet), yet snow has been reported at times
in many locations. All this leads to a fair amount of uncertainty in
precip types overnight. That being said, surface temperatures will
fall below freezing most places tonight, so still confident in the
potential for impactful wintry mix despite the uncertainty in exact
precip types. The biggest slug of precip will come through this
evening into Sunday morning, with light precip lingering into Sunday
afternoon.

Another concern is how much icing will be on roads vs elevated
surfaces. There were some slick spots this morning in northern
portions of the forecast area. Limited the icing amounts some due to
the potential for melting on the ground, especially during the day
Sunday.

Will keep the Wind Advisory going until 7 pm, with gusts to 45 mph
still being reported. Winds will ease a little this evening into
tonight, but will still remain on the gusty side into tomorrow.

Strong onshore winds and pounding waves in excess of 10 feet will
continue tonight into early Sunday.  Have not had any reports of
lakeshore flooding or erosion, but threat will continue.  Hence will
extend Lakeshore Flood Advisory until 15Z Sunday.

SUNDAY NIGHT AND MONDAY...Forecast Confidence - Medium.

700H low pressure exiting the area a bit slower Sunday night due to
increasing negative tilt and broadening of low pressure over the
Great Lakes.  Hence will be bumping up pops for Sunday night, with
lower chance hanging on for Monday.  Forecast soundings from GFS and
NAM showing more moisture lingering in the mid-levels and better
chance for ice crystal initiation falling into the cloud bearing
layer.  So a better chance for more light snow and light
accumulations Sunday night into early Monday.  With lingering
cyclonic curvature and thermal trof over the area, some snow and
rain showers may develop on Monday.  Low level lapse rates exceed 8
degrees with some weakly elevated instability.  Temperature recovery
on Monday will be minimal.

MONDAY NIGHT AND TUESDAY...Forecast Confidence - Medium to High.

A quiet period with some minimal recovery in temperature expected as
short wave ridging affects the area.  May finally get rid of the
persistent stratus to result in a partly to mostly sunny day on
Tuesday.

EXTENDED PERIOD...

TUESDAY NIGHT THROUGH SATURDAY - Forecast Confidence...Low to
Medium.

The period of quiet weather will be brief as all Medium range
guidance continues to show another deepening low pressure system
affecting southern WI Tuesday night into possibly Thursday.  The 00Z
ECMWF slowed this system down so that it affects the area Wednesday
into Wednesday night.  This would result in a better threat of
liquid precipitation on top of warmer ground.  Meanwhile, the latest
GFS and Canadian show this system to be more progressive and begins
to affect the area later Tuesday night.  GFS forecast soundings
favor precipitation beginning as liquid but surface temps may have
fallen below freezing Tuesday evening.  May be a time of light
glazing later Tuesday night before warming to all liquid for a time.
 ECMWF and GFS show surge of warm air aloft into the area ahead of
850H low pressure, while CMC a little bit colder.  Although timing
differs, colder air will be located just to the north and may result
in a wintry mix across northern CWA on Wednesday.  Colder air
funnels back into southern WI later Wednesday or Wednesday night
which would result in changing the mix back to all snow.  SLR will
start out low but should increase during the event.  Mixed precip
changing to light snow may result in a Winter Weather Advisory for
northern/western CWA depending on timing of the heaviest precip.

Another central Plains low pressure area looks to swing our way over
the weekend, but large discrepancies in medium range guidance.
ECMWF now slower and not as warm as earlier 00Z ECMWF, while GFS
looks like another round of mixed precip. No prolonged warming
expected.



&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory until midnight CDT Sunday night for
     WIZ058>060-062>072.

     Lakeshore Flood Advisory until 10 AM CDT Sunday for WIZ052-060-
     066-071-072.

     Winter Storm Warning until midnight CDT Sunday night for WIZ046-
     047-051-052-056-057.

LM...Gale Warning until 10 AM CDT Sunday for LMZ643>646.

&&

$$

Update...Boxell
Tonight/Sunday and Aviation/Marine...Boxell
Sunday Night through Saturday...99



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