Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 212340

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
640 PM CDT Mon Aug 21 2017

.UPDATE...Expect the strongest convection to pass south of WI for
the evening hours but will nonetheless have showers and sct
tstorms move from west to east across the region this evening.
Warm, moist advection will increase ahead of the approaching
storms on a swly LLJ through the evening hours. The rain will
linger into the overnight hours before ending. Cold and dry air
advection and subsidence will then clear the skies during the
morning daylight hours.


.AVIATION(00Z TAFS)...Other than greatly reduced Cigs and Vsbys
within storms, MVFR Cigs will develop during the early morning
hours on Tue. Dry air on breezy nwly winds will then clear the
skies during the morning daylight hours. A few fair wx cumulus may
occur Tue aft.


.PREV DISCUSSION... (Issued 236 PM CDT Mon Aug 21 2017)


Tonight...Forecast Confidence is medium...

A mid-level trough currently over the high plains will move
through the region overnight as low pressure at the surface moves
through Wisconsin. Ahead of this system, quite a bit of moisture
will be drawn into the region, with PWs exceeding 1.5 inches,
reaching perhaps as high as 2 inches. Today`s extensive cloud
coverage will really hamper the ability for any surface based
convection to get going. However, models are still depicting
MUCAPE values reaching around 1000 j/kg ahead of the trough.
Moreover, deep layer shear will increase to around 40 kts as the
mid level trough moves through the area.

With this combination of moisture, lift, and instability, the
stage is set of convection this evening and into tomorrow
morning. All of the synoptic scale models in good agreement in
bringing a large swath of precip through the region. Some of the
meso models are a less bullish on widespread precip overnight, and
the HRRR continues this trend, but given the previously mentioned
ingredients, will continue to likely precipitation chances
tonight. There is less confidence regarding the strength of the
storms. The instability and shear parameters mentioned earlier are
sufficient to support severe storms, but the likely elevated
nature of the storms will limit the potential for damaging winds
to reach the surface. Moreover, the seasonally high PWs and low
values of CAPE above -20C will limit the large hail potential.
Therefore we cannot rule out the chances for severe storms this
evening, but locally heavy rain appears to be the greatest threat
with these storms, and even this is questionable given the latest
trends on the HRRR meso model.

Tuesday...Forecast Confidence is high...

The upper level trough will moves out of the area in the morning,
leaving us in NW flow into the afternoon hours. NW flow at in the
lower levels will help scour out the remaining moisture, helping
skies clear in the afternoon.

Tuesday night and Wednesday...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 cool north wind should diminish Wednesday


Wednesday through Saturday...Forecast confidence is High.

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 weakens.
A shortwave drops down from the northwest Thursday. The GFS is
strongest, with showers developing, while the ECMWF is dry.

The upper flow over Wisconsin then becomes more zonal Friday. A
weak shortwave moves into the Upper Mississippi Valley Saturday.

High pressure and cooler air remains into Friday. The high is
expected to be over the eastern Great Lakes Saturday, with weak
warm advection on the back side of the high.

Sunday and Monday...Forecast confidence is High.

The upper shortwave drops into Wisconsin Sunday, but is fairly
weak. The shortwave remains through Monday on the ECMWF.

The ECMWF is a bit more aggressive on rain amounts, with the GFS
more spotty and very light.


VFR conditions will persist through the day before cigs begin to
gradually fall ahead of an approaching trough. Models generally in
good agreement showing thunderstorms arriving around the MSN area
by mid-evening and the eastern sites a few hours later. Cigs will
continue to fall into the morning hours, perhaps dropping into
IFR if a storms moves over a TAF site. Any storms that do move
through could potentially produce very heavy rainfall. Severe
weather is a lesser threat, but we can`t rule out hail and gusty
winds as well. Cigs will reach their minimum (near the VFR/MVFR
threshold) right around sunrise and then begin to increase well
into VFR late tomorrow morning.

As the trough moves through the region in the late evening/early
morning hours, 2kft winds will increase to around 35kts. This will
put us right on the threshold of LLWS at all TAF sites.


Winds will eventually turn to the southwest and west later
tonight as a trough of low pressure moves through. West to
northwest winds will approach Small Craft Advisory levels late
tonight into Tuesday morning. Showers and thunderstorms are
expected tonight into Tuesday morning, with quiet weather
returning for the remainder of the week.




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