Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 270723

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
223 AM CDT Tue Jun 27 2017

Today and tonight...Forecast confidence is high.

High pressure will provide quiet weather across the area today. A
few afternoon clouds will allow temps to warm into the mid 70s
with light winds and low dewpoints. The high will slide off to
the east tonight. Strong warm and moist air advection arrives
toward sunrise on Wednesday. We could see some initial showers or
storms clip our far western counties toward 12z Wednesday, a
prelude to a more active day on Wednesday.

.WEDNESDAY AND THURSDAY...Forecast confidence is medium.

A low amplitude wave will progress east-northeast from the western
Dakotas Wednesday morning into the western Great Lakes Wednesday
night. Surface low pressure attendant to the upper wave will track
from the northern High Plains to the Michigan U.P., with the
associated cold front pushing slowly southeast through southern
Wisconsin Wednesday night. The primary forecast concern for this
period is the chance for severe thunderstorms and heavy rainfall
late Wednesday afternoon into Wednesday night.

The latest forecast model guidance continues with good support for
large-scale ascent on Wednesday into Wednesday night. Look for a
shortwave perturbation to propagate east-northeast from the western
Dakotas in the morning into northern Wisconsin by evening. Forcing
for ascent now arrives a bit earlier than previously suggested, with
increasing mid-level moisture, differential cyclonic vorticity
advection, and 500 hPa wind fields by Wednesday morning. This, in
tandem with strong low-level warm advection should be enough to fire
off a round of showers and thunderstorms during the morning to early
afternoon hours. Forecast models suggest these storms would be
elevated, with effective inflow layers above 800 mb. Severe weather
is not likely from this round as the best instability and shear
remains to our west during this time.

Showers and thunderstorms should re-develop during the late
afternoon to evening ahead of the slowly southeastward advancing
cold front. Some of these thunderstorms could be severe, but the
risk is conditional given the evolution of the pre-storm
environment. So let`s talk a bit about what we`re seeing at the
moment. From a kinematic perspective, things look pretty good, given
the strong 50+ knot low-level jet impinging on southern Wisconsin
from the southwest. This would promote a backing low-level wind
profile, with around 35 knots of shear within the lowest 3
kilometers. Things get a little trickier when it comes to
instability. By evening it should be feeling pretty muggy as 65+
degree dewpoints advect northward via strong southerly winds.
Forecast models project enough surface heating (temperatures into
the mid to upper 70s) along with this moisture to build around 1500
J/kg of MLCAPE ahead of the cold front. The fly in the ointment
remains the moderate to strong low-level warm advection that the
model guidance continues to generate well into Thursday evening.
This raises the possibility that we see debris clouds and weaker
showers/storms lasting into the evening, potentially keeping us
from reaching those higher CAPE numbers. Of course, given the
strength of the shear involved, we may still see a few severe
storms even with the more limited instability. If severe storms do
develop, the best time frame is from around 6 pm to midnight. All
modes of severe weather, including tornadoes, would be possible.
Heavy rainfall and perhaps localized flooding is a concern during
the evening and night given strong southwesterly moisture
transport along along the frontal zone and precipitable water
values around 1.75 inches.

Any lingering precipitation should end Thursday morning as the
frontal zone pushes south Into central Iowa and northern Illinois.

.FRIDAY THROUGH MONDAY...Forecast confidence is low.

A mid-level shortwave will escort a surface wave along the stalled
frontal zone on Friday, with showers and thunderstorms expected. It
appears that the front should remain just to our south, keeping
the greatest instability over northern Illinois.

The upper flow pattern remains progressive into early next week.
Showers and thunderstorms are again possible on Saturday/Saturday
night with a shortwave and surface trough. The next chance for rain
appears to be on Monday. Temperatures will be seasonal.



Look for VFR conditions through the TAF period. Scattered clouds
are expected today with high pressure keeping winds light. Mid
and high level clouds will increase tonight as the high slips off
to the east. Showers and thunderstorms will gradually spread west
to east after 12z Wednesday.



Low pressure approaching from the west will bring breezy/gusty
southerly winds to the nearshore waters Wednesday through
Wednesday night. A small craft advisory will likely be needed for
that time.




Today/Tonight and Aviation/Marine...Davis
Wednesday through Monday...SPM is the U.S. government's official web portal to all federal, state and local government web resources and services.