Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 201824 AAC
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
124 PM CDT Thu Sep 20 2018

.UPDATE...

Clouds are finally lifting out of the area south to north,
allowing the return of sunshine this afternoon. Temperatures
generally seem on track for the remainder of the afternoon,
though will have to watch conditions where the sun is out, as
temps could shoot up quickly.

&&

.AVIATION(18Z TAFS)...

VFR conditions are returning south to north this afternoon as
clouds clear out.

A line of showers and thunderstorms will move through southern
Wisconsin tonight as a cold front sweeps through the area. The
storms are likely to reach the northwest forecast area in the 11
pm to 1 am range...and eastern areas in the 2 am to 4 am range.
The storms are expected to decrease in intensity as they travel
eastward through southern Wisconsin. Could see some strong to
severe storms roughly from Madison westward as the line first
moves into the area.

Low level wind shear is likely this evening and overnight as a
strong jet moves overhead. Winds will be gusty at the surface
tonight and Friday ahead of and behind the passing front.

Ceilings are likely to go down again later tonight into Friday
morning behind the cold front. Conditions should improve mid to
late afternoon.

&&

.PREV DISCUSSION... (Issued 936 AM CDT Thu Sep 20 2018)

UPDATE...

Showers will continue to decrease in coverage this morning as the
warm front lifts northward. Latest models suggest that the front
may be a tad slower than previously thought. This would delay the
clearing and resultant warming by a couple hours. Bumped high
temps down a degree or so as a result.

MARINE...

Gusty south to southwest winds are expected ahead of an
approaching cold front later today into tonight. The winds will
remain gusty and become west to northwest on Friday behind the
front. Gale force gusts are likely over the north half of Lake
Michigan, with a few gusts possible in the south half.

Highest waves will be in northern and eastern portions of Lake
Michigan through this event.

Kept the timing of the Gale Warning in the north half of Lake
Michigan and the Small Craft Advisory in the nearshore waters.
May consider a Gale Warning in the south half for tonight into
Friday if forecast winds creep up any higher.

PREV DISCUSSION... (Issued 658 AM CDT Thu Sep 20 2018)

UPDATE...

Showers and thunderstorms percolating along an 850-925mb warm
front are showing a diminishing trend this morning. Expect up to a
half inch of additional rain over southeast WI through 9 am. The
front and associated precip will lift northward through the
morning and the afternoon is still on track to be hot and humid
with increasing southerly winds.

AVIATION(12Z TAFS)...

LIFR ceilings around 400 feet developed across most of southern
WI early this morning, although there are patches of VFR. I do not
have much confidence about when the LIFR conditions will move out
because they are hit or miss and not related to rain/no rain.
There may be a period of MVFR later this morning before we scatter
out, but confidence is low.

Expect low level wind shear tonight due to a strong southwesterly
low level jet just off the surface. We will still be gusty at the
surface. A cold front will track through southern WI overnight.
Showers and thunderstorms associated with it will have a
weakening trend as it tracks east.

PREV DISCUSSION... (Issued 413 AM CDT Thu Sep 20 2018)

SHORT TERM...

Today...Forecast Confidence Is Medium.

Showers and a few thunderstorms continue to percolate along the
925-850mb front, supported by the nose of the low level jet, warm
air advection, and frontogenesis. The convection extends all the
way westward through northern Iowa and far southeast South Dakota
early this morning.

Until then, with precipitable water values sitting around 1.8
inches, the rainfall is very efficient early this morning. Brief
heavy showers are bringing visibilities down and dropping a quick
0.75 to 1 inch of rain. The heaviest hit area for the overnight
hours includes the north half of Lafayette county and central
Green County. Precip rates are beginning to diminish, but it will
keep raining over the same area with up to a half inch of
additional rain through mid-morning, so flooding continues to be
a threat.

As time goes on, the frontal boundary will slide northward into
central WI. Southern WI will be in the warm sector this afternoon,
so expect sunshine with hot and very humid conditions. The max
heat index values will approach the mid 90s toward the IL border.

Tonight...Forecast Confidence is Medium.

As low pressure slides from the Twin Cities into western Lake
Superior this evening, winds will increase out of the south in
southern WI. The strong cold front will track through southern WI
late tonight. There is fair agreement between the models that the
leading edge of thunderstorms associated with the front will
arrive in our western counties (Sauk to Lafayette) around 1 am/
06Z. The overall system and resultant storms will be weakening as
they cross southern WI overnight, so the severe threat is quite
low. That said, an organized line that develops to our west may
sustain some of its strength as it tracks into our east, west of
Madison.

Friday Through Saturday Night...Forecast Confidence Is High.

Models are in good agreement with bringing strong cold air
advection into the area Friday into Friday evening. Gusty west
winds will become northwest, within a tight pressure gradient.
Temperatures are expected to gradually fall in the morning, then
hold steady for awhile in the afternoon. Any shower activity
should remain north of the area Friday, where the 500 mb
shortwave trough will be passing through.

High pressure is then expected to weaken as it slides east across
the region Friday night into Saturday night. This will bring
weakening winds, along with colder temperatures. Highs should only
reach the lower to middle 60s Saturday, with lows in the lower to
middle 40s Friday night, and middle 40s Saturday night.

There may be a period of lake effect clouds and perhaps some
light showers Friday night into Saturday morning, as winds shift
northeast in the low levels with favorable delta T values and
moist low level air. The best chances may be south of Milwaukee
for this to occur.

LONG TERM...

Sunday Through Monday...Forecast Confidence Is High.

This period looks to remain dry across the area, as east to
southeast winds keep bringing in dry air. There is some warm air
advection that gradually develops Monday. Temperatures should
slowly warm from the upper 60s to around 70 Sunday, to the lower
70s Monday.

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

Models are generally trying to show a broad 500 mb trough
developing over the Upper Midwest and bringing it slowly to the
east during this period. They have a cold front sliding east
through the region either Monday night or Tuesday, with chances
for showers and a few thunderstorms. There is some uncertainty
with this period being so far out at this point. Strong cold air
advection behind the front may indicate another push of colder air
into the region by Wednesday.

AVIATION(09Z TAFS)...

Ceilings are gradually falling to IFR early this morning where it
continues to rain. The main line of showers and isolated
thunderstorms will lift north through late morning. Expect a hot,
humid, and dry afternoon across southern Wisconsin with increasing
southerly winds.

A strong cold front will sweep across southern WI overnight.
Gusty west winds and lingering low clouds are expected on Friday.

MARINE...

Southerly winds will increase across the lake as low pressure
strengthens over the Plains and tracks toward western Lake
Superior. Southerly winds are expected to reach gales across the
northern portion of Lake Michigan tonight, with gales continuing
into Friday, as winds become westerly behind a strong cold front.
Over the south half, winds will be between 25 and 30 knots.

A Small Craft Advisory is in effect for the nearshore waters
tonight and Friday. Southern portions of the lake will have
lingering high waves through Friday night, due to persistent
northerly winds.

With the increasing winds, waves will build to 6 to 8 feet across
the open waters Thursday evening, reaching their peak across the
northern portions of the lake early Friday morning, with waves
between 10 and 12 feet expected. Waves will gradually diminish
Friday with the wind shift, but southern portions of the lake will
have lingering high waves through Friday night, due to persistent
northerly winds.

High pressure will eventually build into the region over the
weekend, with much less impactful wind and wave conditions by
Saturday morning.

BEACHES...

There is a moderate swim risk for the beaches along Lake Michigan
in Sheboygan and Ozaukee Counties today. Onshore winds will help
bring waves into the 2 to 4 foot range this afternoon in this
area. Lower waves are expected to the south.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...Small Craft Advisory from 7 PM this evening to 7 AM CDT Saturday
     for LMZ645-646.

     Small Craft Advisory from 7 PM this evening to 7 PM CDT Friday
     for LMZ643-644.

&&

$$

Update...DDV
Today/Tonight and Aviation/Marine...Cronce
Friday THROUGH Wednesday...Wood


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