Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 260514 AAC
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
1214 AM CDT Wed Jul 26 2017

.UPDATE...


&&

.AVIATION(06Z TAFS)...

Middle to high clouds will continue to push through the area
overnight into Wednesday. Ceilings will lower to around 4000 feet
or so with the showers and thunderstorms moving through the area
later Wednesday afternoon into Wednesday night. Ceilings down to
2500 feet are possible later Wednesday night, along with light
fog.

South winds overnight will become west southwest on Wednesday,
gradually weakening during the day. Winds will shift to the north
behind the cold front later Wednesday night.

Scattered showers and thunderstorms will push through the area
later Wednesday afternoon into Wednesday evening. They should exit
later Wednesday night. Best chance for severe storms and locally
heavy rainfall will be southwest of Madison. Gusty winds and
hail remain possible elsewhere during this time.

Wood

&&

.PREV DISCUSSION... (Issued 951 PM CDT Tue Jul 25 2017)

UPDATE...

Elevated convection, driven by low level jet pointing into
northeast and east central Wisconsin, clipped northeastern
Sheboygan County. This activity will continue to shift away from
the area late this evening. There are a few weak showers in
northwestern portions of the area, but these are not expected to
last very long.

Kept low PoPs going for later tonight across the far northern
portions of the area, in case some showers or storms from the
north drift into those areas. Most of the night should remain
dry. Mesoscale models have any approaching elevated convection
later tonight into Wednesday morning weakening and staying just
north of the area, where the better upward vertical motion fields
are expected to be located.

00Z NAM run continues to focus the main area of convection for
later Wednesday afternoon into Wednesday night to the southwest of
the area. The low level jet focuses mostly in this area. It does
have some convection affecting the forecast area, as the cold
front moves through the area Wednesday night.

Deep layer bulk shear is at 25 to 35 knots, with varying amounts
of mean layer CAPE, depending on the model. Think current Slight
Risk area from SPC looks good for now, and may need a further
nudge to the southwest if other models are similar to the NAM.
Locally heavy rainfall is possible with high precipitable water
values and other favorable heavy rainfall parameters.

Wood

MARINE...

South winds will linger at 10 to 20 knots across the nearshore
waters of Lake Michigan overnight. This will bring waves of 2 to 4
feet, highest toward the Sheboygan area. Winds will gradually
weaken and become more west southwest on Wednesday, with lower
waves.

Scattered thunderstorms should move through the area later
Wednesday afternoon and night. Gusty winds and hail are possible
in any stronger storms.

Gusty northeast winds Thursday into perhaps Friday will bring
building waves of 3 to 6 feet to the nearshore waters of Lake
Michigan. These waves may linger as late as Friday night. A Small
Craft Advisory may be needed for this period.

Wood

PREV DISCUSSION... (Issued 648 PM CDT Tue Jul 25 2017)

UPDATE...

AVIATION(00Z TAFS)...

Winds will become south and decrease somewhat tonight, then become
west southwest on Wednesday. Middle to high clouds will continue
to push through the area early this evening, before more extensive
clouds move in later this evening and overnight. Middle to high
clouds will linger on Wednesday, with some diurnal cumulus
possible.

Lower ceilings to 3500 feet with scattered thunderstorms should
move east through the area late Wednesday afternoon and evening.
Visibilities down to around or below alternate minimums are
possible with any storms, along with gusty winds and hail. Best
shot for strong to severe storms, and heavy rainfall, will be
south and west of Madison during this time.

Wood

PREV DISCUSSION... (Issued 226 PM CDT Tue Jul 25 2017)

DISCUSSION...

Tonight - Confidence...Medium
Precip shield associated with morning MCS has shown a considerable
diminishing trend and this will continue. Plenty of debris
cloudiness will remain into this evening. With the surface high
shifting off to the east the return flow will pick up. Already
seeing a boost in the dew points back to near or above 60 here
with higher numbers upstream poised to move in our direction. At
this time it appears the better focusing will be to our north
where 500 millibar vort action/lower level frontal convergence
and axis of LLJ meet up. Cannot rule out some parts of our north
being grazed so have some POPS in place there a bit later on.

Wednesday - Confidence...Medium
Main focus will be on convective trends with approaching frontal
boundary from the northwest. While the primary front stays to our
northwest during the day, there is a 500 millibar shortwave that
is proggd to arrive during the afternoon hours. There is also some
divergence noted with the west/east moving jet stream across the
upper Lakes region. the progs suggest the western and northern CWA
would have the better chances for TSRA development so have a NW-SE
gradient in POPS. Some model differences noted on degree of
instability during the afternoon with wind fields not exactly
overly favorable. 925 temps rise into the low/mid 20s celsius so
expect plenty of temps reaching the mid 80s.

Wednesday night...Forecast confidence is high.

There`s very good consensus among the models that the cold front
will push south through the area, likely exiting southern
Wisconsin shortly after midnight. The arrival of a mid level short
wave tracking in from NE/IA will help to energize the activity in
the afternoon and evening across southern WI...especially the
southwest. Shear is decent enough, but not impressive. CAPE
values are quite varied among the models with the NAM most
aggressive. But again, nothing too extreme. In fact, the NAM is
most aggressive with many of the fields and is likely suffering
from it`s usual convective feedback issues. The models do tend to
focus the heaviest rainfall amounts across our western forecast
area, with lighter amounts in the east. However, as we`ve seen
with all of the events this month, we need to remain open to
shifts in these placements once the activity gets going. There
will be ongoing convection prior to it arriving in southern WI and
how that evolves will certainly have an influence on subsequent
development.

The SPC has backed off on the Slight Risk across southern
Wisconsin. Keeping it mainly south and west of a Baraboo to
Watertown to Paddock Lake line. Our far southeast and northeast
are now just in a marginal risk. I like this adjustment, focusing
more on our southwest, closer to the nose of the low level jet and
expected cold pooling behavior.

Thursday through Saturday...Forecast confidence is high.

High pressure builds across the area during this period. There is
a potent short wave that drops northeast Wisconsin later Thursday
afternoon through Thursday night. There may be some diurnal
showers across the north that could sneak into our northern CWA
before they die at sunset. Temps and humidity will be comfortable.

Sunday and Monday...Forecast confidence is low.

The GFS is suggesting a cold front could be the focus for showers
and storms during this period, while the ECMWF is much drier
looking. We`ll carry small chances for this potential.

AVIATION(18Z TAFS)...MCS has decayed with just debris cloudiness
remaining. Progs hinting at some MVFR cloudiness forming later
tonight into SC WI. Main concern will be with potential for
strong to severe storms later Wednesday/Wednesday evening though
some development possible prior to 00z esp in SC WI.

MARINE...Winds and waves expected to remain below small craft
levels into Wednesday. However, a southerly flow will be in place
ahead of a frontal boundary. Scattered storms, a few strong, are
most likely Wednesday afternoon into the evening hours.

BEACHES...

Brisk northeast winds by Thursday afternoon will likely result in
high swim risk conditions along the Lake Michigan shore.

&&

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

$$

Update...Wood
Tonight/Thursday and Aviation/Marine...Collar
Thursday Night through Wednesday...Davis


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