Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 211807 AAA
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
107 PM CDT Wed Jun 21 2017

.UPDATE...

Weakening area of showers should clip the southwest counties early
this afternoon before moving to the southeast. Rest of the
afternoon looks relatively dry, with a dry airmass in place. Left
small PoPs for thunder in grids later this afternoon in the
western counties, but the best moisture and upward vertical motion
look to hold off until early evening. Middle to high clouds may
limit highs somewhat if they linger over the area this afternoon.

Mesoscale models suggest that the best chances for elevated
convection with the low level jet nose this evening will be across
the far northern and northeastern portions of the forecast area.
Kept this area in the highest PoPs later this evening into the
overnight hours, before it slides off to the northeast.

Area forecast soundings are showing enough elevated CAPE with
fairly strong effective shear to bring a large hail risk to the
area. SPC Slight Risk looks good for now, given this combination.
The rest of the area may not see much in the way of elevated
convection tonight, with the cap increasing and the best upward
vertical motion to the north and northeast parts of the area.

There is some potential for convection to move southeast through
the area later tonight, though confidence is shaky as the best low
level jet forcing moves out of the area. Kept PoPs going for now
across the area.

Mesoscale models suggest another round of severe storms is
possible later Thursday afternoon into Thursday evening. Strong
upward vertical motion with frontogenesis with the cold front is
expected to initiate storms just to the northwest of the area by
the middle to late afternoon hours. Adjusted PoPs upward in the
northwestern portions of the area later in the afternoon, as
storms may be developing or pushing into that area.

High PoPs remain Thursday evening/night, as the front and storms
push southeastward through the area. Given the expected mean layer
CAPE and deep layer/low level shear, all hazards are possible.
Heavy rainfall is also a concern, given the favorable moisture
parameters expected. SPC Slight Risk looks good for now.

Wood

&&

.AVIATION(18Z TAFS)...

Southeast winds are expected near Lake Michigan this afternoon
with lake breeze. Area of showers may clip Madison and areas to
the southwest of there for a few hours early this afternoon.
Middle to high clouds will linger at times into this evening. Some
diurnal cumulus clouds may develop as well.

Best chances for storms mainly from middle to late evening will
be toward the Montello, Fond du Lac and Sheboygan areas. They may
be severe with large hail possible. These may linger for a time
later this evening, before moving to the northeast by later
tonight.

Low level wind shear is forecast mainly later tonight into early
Thursday morning, with 2000 foot southwesterly winds around 40 to
45 knots.

Chances for storms linger into Thursday, with the best chances for
a round of severe storms later Thursday afternoon and Thursday
night. A strong cold front will slide southeast across the area
during this time. Severe storms with all hazards are possible
during this time, with heavy rainfall possible as well. Ceilings
and visibilities will be greatly reduced in any storms.

Gusty southwest winds are expected Thursday, with gusts to around
20 knots possible at times.

Wood

&&

.MARINE...

Storms may develop across the area this evening, with the best
chances north or North Point Lighthouse. Severe storms with large
hail are possible in this area. Chances for storms are lower to
the south. These storms should linger for a time later this
evening and overnight, before moving to the northeast.

Chances for storms linger later tonight into Thursday. The best
chances for another round of severe storms will be Thursday night,
as a strong cold front moves southeast through the area. All
hazards of severe weather are possible Thursday night as the
storms move through.

Gusty southwest winds may reach Small Craft Advisory levels at
times Thursday, with a tight pressure gradient and good low level
mixing.

Wood

&&

.PREV DISCUSSION... (Issued 608 AM CDT Wed Jun 21 2017)

UPDATE...

IFR fog in river valleys will dissipate quickly with the morning
sun.

AVIATION(12Z TAFS)...

IFR fog in river valleys will dissipate quickly with the morning
sun. otherwise VFR.

Some ACCAS or showers expected later this afternoon ahead of a
developing low level jet pushing toward southern Wisconsin.

Thunderstorms are expected to spread into south central WI during
the early evening hours, then across much of the area ahead of a
warm front which will move across southern Wisconsin late tonight.
MVFR cigs and IFR vsbys in showers/thunderstorms.

PREV DISCUSSION... (Issued 230 AM CDT WED JUN 21 2017)

TODAY AND TONIGHT - Forecast Confidence...Medium.

Surface high centered across the Great Lakes today, will bring
pleasant weather to start the day with low dew points.

The upper level trough over the Great Lakes moves east, as a zonal
jet sets up from Montana to lake Superior. Upper level divergence
increases this evening and continues tonight. 700 mb upward motion
is a little quicker and increases this afternoon.

700 mb dew points increase this afternoon as the 700 mb winds
increase from the west, 850 mb dew points increase mainly tonight,
as a 40 knot 850 mb southwest jet moves across southern Wisconsin,
increasing to 50 knots late.

GFS forecast soundings at Madison bring elevated CAPE of 725
Joules/kg this evening, that rises to near 1500 Joules/kg by
midnight.

With the elevated moisture beginning this afternoon, expect some
ACCAS or showers to move in this afternoon, then with the
approaching 850 mb low level jet, and increasing elevated CAPE,
expect thunderstorms this evening and overnight. Some hail expected,
with SPC outlook expanding the slight risk across south central and
into portions of southeast Wisconsin.

THURSDAY THROUGH FRIDAY - Forecast Confidence...Medium.

A few thunderstorms may linger into Thursday morning as strong low
level jet and moisture advection slides off to the east.  Will lower
pops to chance category as somewhat drier and warmer air builds into
southern WI as surface and elevated warm front lift to the
northeast.  Building Capping inversion should keep convection threat
low into Thursday afternoon.  Low level winds back to the SSW
Thursday afternoon and evening ahead of approaching mid-level short
wave.  Thinking that surface warm front will be carried northward
into central WI late Wednesday night or Thursday morning by strong
low level winds. Morning Cap expected to erode in the afternoon
resulting in surface based Cape of 2-3k joules.  Better chance for
convection refiring in northern CWA in the afternoon closer to
surface front. Otherwise, triggering mechanism holds off until the
evening when right entrance region of upper jet passes across
southern WI. Lift also enhanced by passing cold front and mid-level
trof. Impressive sloping frontogenetical forcing passes across
southern WI between 00 and 06Z Friday.  0-6km bulk shear in the 35
to 45 knot range with an unstable atmosphere and low pressure
possibly strengthening and moving northeast across central WI during
this period.  CWASP values in the .6-.7 range in the evening. Energy
Helicity Index from the NAM increases to 3.5 in NW CWA during the
evening.  At this point, this area would be the focus for tornado
formation in the late afternoon and early evening.  Only concern for
Thursday would be lingering cloud cover limiting instability,
especially in the north.

Convection winds down later Thursday night into early Friday as
surface cold front slides off to the southeast, and more stable
conditions settle into southern WI.  Drier air continues to feed
into southern WI on Friday on breezy WNW winds.

EXTENDED PERIOD...

FRIDAY NIGHT THROUGH TUESDAY - Forecast Confidence...Medium.

Medium range guidance is in reasonable agreement on unsettled
weather continuing across the western Great Lakes region for this
period.  Spokes of short wave energy will track southeast in
broadscale long wave troffing over central Canada into the upper
Midwest region, carried by WNW steering flow.  These pieces of
energy will bring occasional showers and a few rumbles of thunder to
southern Wisconsin over the weekend into early next week.  In
addition, cooler air will be settling over the region resulting in
temperatures averaging below normal for late June.  The GFS 5-day
500H means does hint at this cooler pattern finally breaking down
later next week around the first of July.

AVIATION (09Z TAFS)...

Clear skies and light winds will allow for some patchy mainly MVFR
fog, but some IFR in river valleys. This will dissipate quickly with
the morning sun.

Some ACCAS or showers expected later this afternoon ahead of a
developing low level jet pushing toward southern Wisconsin.

Thunderstorms are expected to spread into south central WI during
the early Wed evening hours.

MARINE...

Light winds and sunshine will allow for a lake breeze to develop by
late Wed morning. Showers and thunderstorms are likely Wednesday
evening and Wednesday night. Then again late Thursday when some
severe storms possible. Breezy southwest winds on Thursday.

&&

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

$$

Update...Wood
Today/Tonight and Aviation/Marine...Hentz
Thursday THROUGH Tuesday...MBK



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