Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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114
FXUS63 KMKX 241437
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
937 AM CDT Mon Jul 24 2017

.UPDATE...
Scattered to overcast clouds will persist through the morning.
Drier air will move in this afternoon as high pressure moves
overhead helping to break up the clouds. Quiet weather is
expected.

&&

.MARINE...
There will be a few northeast wind gusts to 25 knots today. Winds
will start to weaken this afternoon as high pressure moves
overhead. Waves around 4 feet will linger through the day or early
evening. A Small Craft Advisory is in effect.

&&

.PREV DISCUSSION... (Issued 654 AM CDT Mon Jul 24 2017)

UPDATE...

Low clouds are persisting across much of central and southeast WI
this morning. There is a back edge to them and it`s working its
way down into east central WI. South central WI continues to
remain cloud free. I would expect some cumulus clouds to develop
there later this morning, but overall we should see a clearing
trend everywhere through early afternoon.

AVIATION(12Z TAFS)...

The large area of low clouds with bases of 1500 to 2500 ft caused
by cool, moist air trapped beneath the inversion is not advecting
any farther south this morning. South central WI continues to
remain cloud free. I would expect some cumulus clouds to develop
there later this morning. Overall, expect the clouds to gradually
mix out and dissipate through early afternoon.

Clear skies and calm winds tonight could allow for some patchy fog
in the usual places. Another quiet and mostly sunny day is
expected Tuesday.

PREV DISCUSSION... (Issued 339 AM CDT Mon Jul 24 2017)

TUESDAY AND TUESDAY NIGHT...Forecast confidence is high.

Surface high pressure will be over the area Tuesday morning, before
progressing into the lower Great Lakes by evening. Light southeast
surface winds will veer southerly by afternoon, with a gradual
increase in temperatures and moisture ahead of a surface trough over
the Great Plains. It will feel a bit muggy by evening, as dewpoints
climb into the 60s area-wide, and perhaps even to around 70 west of
Madison. We may see a few thunderstorms drop down from the
northwest late Tuesday night before weakening, but there is low
confidence in this occurring so only brought slight chances for
precipitation into our northwest during this time.

WEDNESDAY THROUGH THURSDAY...Forecast confidence is medium.

A deep layer cyclone will progress from northern Ontario into
Quebec, with a trailing surface trough/cold front extending
southwest from the surface low. The front will slide southeast
across Wisconsin during the day, likely reaching our area during the
late afternoon or early evening hours. Surface convergence along the
front will spark showers and thunderstorms, with impacts potentially
including severe weather and heavy rainfall.

Several parameters favor efficient rainfall production Wednesday
night. These include very deep warm cloud depths nearing 5 km,
modest forward propagating corfidi vectors around 15 knots, and
precipitable water (PW) values around 2 inches. Climatology suggests
that these values are around 175% of normal for this time of year.
It`s also worth noting that both the 24.00z GFS and NAM
deterministic solutions place a reservoir of 2.5 inch PWs just to
our south over northern Illinois. So, what`s clear is that we`ll
be in an environment that`s quite supportive of heavy rainfall
Wednesday night. What`s less clear is exactly where the axis of
heavier rainfall will be. The 24.00z deterministic guidance falls
into two camps. The GFS/NAM keep the low-level baroclinic zone
just south of the state line, with a convective complex diving
southeast along the instability gradient from central Iowa into
northern Illinois during the evening. This solution would place a
swath of 2-3 inch rainfall across these areas, with the heavier
precipitation gradient quickly tapering off across far southern
Wisconsin. The ECMWF/Canadian solution is a bit slower, and brings
the heavier rain amounts right across southern Wisconsin late
Wednesday night. The QPF from these models is quite a bit less
however, averaging 0.75 to 1.5 inches. Current indications are
that the steering flow will be progressive enough to keep things
moving along and prevent much training of convection, but we`ll
need to keep an eye on this in case the front should slow up. For
now, be aware of the possibility for heavy rainfall on Wednesday
night.

Severe thunderstorms are also possible from late afternoon into
evening, given the overlap of favorable kinematic and instability
parameter spaces. Sufficient speed and directional shear will be
provided via a mid-level westerly speed max juxtaposed with a
developing southwesterly low-level jet. This will bring a veering
wind profile with height, yielding effective bulk shear values of 35
to 40 knots in the 0 to 6 km layer. Instability varies by model
owing to different convective evolutions, but in general we`re
expecting around 1 to 2 kJ/kg of mixed layer CAPE. At this time,
shear within the lowest kilometer appears fairly weak, so the
greatest severe risks would be damaging wind and large hail. This
will likely be another case where the mesoscale details could make
or break a potential event, so we`ll continue to evaluate the
risk going forward.

Showers and thunderstorms could persist into Thursday morning if
the less progressive solutions depicted by the ECMWF/Canadian
models verifies. Otherwise, expect dry conditions by afternoon,
with high pressure building in from the northwest.

FRIDAY THROUGH MONDAY...Forecast confidence is medium.

High pressure will slowly progress from the Upper Midwest into the
lower Great Lakes, bringing quiet weather and near normal
temperatures over the weekend. The deterministic GFS persists in
bringing a few showers or storms Saturday night, but this solution
remains unsupported among the other guidance. Another chance for
a shower or storm may be late Sunday night or Monday, but there is
model disagreement regarding this at the moment.

AVIATION(09Z TAFS)...

The large area of low clouds caused by cool, moist air trapped
beneath the inversion is having trouble advecting farther south
early this morning. Expect the clouds to gradually mix out and
dissipate through late morning. In addition, there is a back side
to these clouds that is timed to reach Fond du Lac around 11:30
this morning.

Clear skies and calm winds tonight could allow for some patchy fog
in the usual places. Another quiet and mostly sunny day is
expected Tuesday.

MARINE...

North to northeast winds are actually calming down after the main
push of cooler air tracked into Illinois overnight. The persistent
north winds with a few gusts to 25 knots will lead to 3 to 5 foot
waves along the nearshore today, so a Small Craft Advisory is in
effect.

BEACHES...

A Beach Hazards Statement is in effect for a High Swim Risk along
the Lake Michigan Shoreline today through this evening. This means
Dangerous pounding waves and life-threatening currents are
expected at the beaches. People visiting the beaches should stay
out of the water.

Brisk north to northeast winds will build high waves along the
shoreline. Waves will taper off from north to south this
afternoon and evening, so the counties in southeast WI have a
later end time than the northern counties.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...Beach Hazards Statement through this evening for WIZ066-071-072.

     Beach Hazards Statement until 7 PM CDT this evening for WIZ052-
     060.

LM...Small Craft Advisory until 10 PM CDT this evening for LMZ645-646.

     Small Craft Advisory until 7 PM CDT this evening for LMZ643-644.

&&

$$

Update...Marquardt
Today/Tonight and Aviation/Marine...Collar
Tuesday THROUGH Sunday...SPM



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