Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 202336

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
636 PM CDT Sun Aug 20 2017

.UPDATE...Meso-scale convective eddy continues to move east across
northern IL. Few light showers may skim far southeast in next hour
or so. Otherwise, the rest of the evening should be dry.
Convergence very weak along boundary located across northwest
Wisconsin and little resultant development despite MUCape around
3k joules. Will keep small chances after midnight due to
increasing upstream warm air advection, but latest mesoscale
models delaying showers and storms until very late tonight into
Monday morning. May be able to decrease more Pops with late
evening update More clearing will likely result in a greater
threat of fog development overnight. Will see how the upstream
cloud cover develops this evening before adding patchy fog.


.AVIATION(00Z TAFS)...Scattered showers and storms may hold off
until very late tonight into Monday morning. Hence less cloud
cover will result in a better chance for fog development
overnight. May add fog to late evening update. Forecast soundings
not showing much low level moisture so not thinking any late night
stratus at this point. Best chance for showers and storms still
Monday evening.


.PREV DISCUSSION... (Issued 208 PM CDT Sun Aug 20 2017)


Tonight and Monday...Forecast Confidence is medium.

The mesoscale convective vortex responsible for showers across far
southern Wisconsin today will continue drifting east across
northern Illinois this evening, taking any remaining showers away
with it. The persistent high clouds over southern Wisconsin should
thin towards evening as this feature departs. Scattered showers
and thunderstorms along a surface trough in northwest Wisconsin
should sink into central Wisconsin this evening. Most guidance
keeps these storms north of our area, but it`s not impossible that
one of these could brush our northern counties.

The convective allowing models are in agreement in developing
thunderstorms after midnight across northwest Iowa in association
with a push of warm/moist advection arriving via the nocturnal
low-level jet. This guidance then steers the resulting
thunderstorm complex east-southeast into far southern Wisconsin
and/or northern Illinois by early Monday morning. A few of these
storms could be strong as the reach western portions of the area,
but the low-level jet will be veering by this time, with the
complex likely weakening by mid morning as it pushes into
southeast Wisconsin and northeast Illinois. This will likely set
up a boundary which will be traversed by additional convection
Monday evening into Monday night.

We should be quiet for a time for early to mid afternoon thanks to
subsidence following the morning storms. The Eclipse viewing
forecast remains rather murky, with odds favoring mostly cloudy
skies during the event. Temperatures during viewing time should be
in the mid to upper 70s, with dewpoints around 70 making it feel a
bit muggy.

Monday night and Tuesday... Forecast confidence is High.

The zonal flow becomes southwest Monday night ahead of a strong
shortwave across south central Canada that will push into the
Upper Great Lakes Tuesday. Southern Wisconsin will be in the
right entrance region of the 125 knot 250 mb jet just ahead of the
trough axis. Moderate upper divergence and 700 mb upward motion
until just after sunrise Tuesday, when they weaken.

A 850 mb thermal ridge will extend across southern Wisconsin
Monday night giving way to strong cold air advection by Tuesday

A surface trough/warm front will be near southern Wisconsin
Monday Night before a strong cold front exits Tuesday morning. Dew
points will be near 70 ahead of the front, with precipitable water
values around 1.75 inches. Main issue is the location of the
boundary. The HRRR experimental has a morning MCS moving across
northern Illinois/southern Wisconsin. This would keep the most
unstable air farther south.

GFS soundings support the SPC slight risk of severe, with zero to
1 km mixed later CAPE of 1700 Joules/kg early Monday evening, with
zero to 1 km shear around 24 knots and zero to 1 km SRH values of
160 m2/s2. While large hail and damaging winds are the primary
threat, there is enough low level veering winds/shear for a small
tornado possibility.

Thunderstorms are expected Monday evening and into much of the
night, with showers ending Tuesday morning. The location of the
initial surface boundary will play the main role in severe
potential. If the HRRR plays out the severe potential would be

Look for brisk northwest surface winds Tuesday.


Wednesday through Sunday...Forecast confidence is High.

A large mid/upper level trough is expected over Southeast canada
and the eastern Great Lakes Wednesday. There is some weak CAPE Wednesday
afternoon, but there is a rather strong cap and very dry air above
the cap.

The upper trough slowly reaches the northeast U.S. Thursday
night. Strong upper level northwest flow is expected into
Thursday, before the eastern U.S./Canadian trough weaken. The
upper flow over Wisconsin then becomes more zonal later Thursday.
A weak shortwave moves into Wisconsin Saturday and Sunday.

The cool north wind should diminish Wednesday, with high pressure
and cooler air Thursday and Friday. The high is expected to be
over the eastern Great Lakes Saturday, with weak warm advection on
the back side of the high.


Prevailing VFR expected through early tonight. Rain showers will
affect areas mainly south of Milwaukee and Madison through the
afternoon. Shower and thunderstorm chances increase late tonight
into early tomorrow morning, followed by additional thunderstorm
chances Monday evening and night. Some of the storms Monday night
may be severe. Some brief ceiling and visibility reductions are
possible later tonight in and near showers and thunderstorms.


Southerly winds of 10 to 20 knots will continue this evening,
with the highest winds and waves generally north of Port
Washington. Winds and waves should remain below Small Craft
Advisory levels for much of next week. Showers and thunderstorms
are possible tonight into Monday morning, and again from Monday
evening into Tuesday morning. Some of the thunderstorms could be




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