Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 210453
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
1153 PM CDT Wed Sep 20 2017

.UPDATE...

The outflow has finally outrun the convection so any redevelopment
will be limited. None the less, thunderstorms continue across the
northeast portion of the CWA and will continue to put down rain
for the next few hours. We may see up to an additional inch in
some areas, which may be problematic across portions of Fund du
Lac county where MRMS estimates show that over 2 inches has
already fallen.

Thunderstorm activity will continue to diminish with storms moving
away from the frontal boundary as the front stalls across the
CWA. Once the storms move out, the focus turns to the potential
for fog toward sunrise. The temperatures have already reached the
dewpoint across much of the CWA, save for the far southeast. Light
winds with these moist conditions set the stage fog, though
broken to overcast skies should keep temperatures from falling
much further. High res guidances is highlighting an area from
roughly Fond du Lac to Lone Rock where winds will be the lightest
and thus visibility will be most limited.


&&

.AVIATION(06Z TAFS)...

Storm and shower activity has all but cleared the TAF sites
so the focus turns to the low stratus/fog potential. Latest
high res guidance has backed off on the IFR conditions in the
southeast. That may work out well given that temperatures are
5-10 degrees higher than the dew point in that area and won`t
fall much more given the cloud cover. For not, I will remove the
IFR conditions from UES, MKE, and RAC, but we will have to watch
this closely through the night.

Guidance is also showing the potential for LLWS tomorrow evening
at MSN. At this point, it looks marginal so I chose to leave it
out. Will have to reevaluate as we get closer to the evening.

&&

.PREV DISCUSSION... (Issued 827 PM CDT Wed Sep 20 2017)

UPDATE...

The storms have more or less behaved themselves across southern
Wisconsin this evening. They are still pulsing up, but are having
a tough time reaching levels of concern. We`re likely getting lots
of pea size hail out there in some of these storms west and north
of Madison, but we haven`t received reports of anything thus far.
This is likely due to the very low mid level lapse rates. The
storms just can`t get the updrafts to support large hail. The rain
has been heavy just north of Marquette/Green Lake/Fond du Lac
counties and it appears that some cold pool dynamics within the
axis of high precipitable water is starting push that heavy rain
south into those three counties. Thankfully, we have been very
dry, so we can take a little water. Urban areas will have to watch
for localized flooding on roads. Overall, things should generally
weaken in intensity and coverage as we head toward the midnight
hour. The atmosphere is stabilizing.

MARINE...

The small craft advisory will be allowed to expire at 9 pm. Winds
and waves have come down. Be alert for possible thunderstorms
north of Milwaukee tonight. Some fog is also possible with this
very warm and humid airmass.

PREV DISCUSSION... (Issued 547 PM CDT Wed Sep 20 2017)

UPDATE...

Not much to add for an update to the previous discussion. We have
an incredible amount of CAPE out there, but the deeper forcing is
pushing off to the north and that`s where the severe storms have
been up to this point. Our main forcing across southern WI will be
with the low level convergence along the front and also a weaker
prefrontal convergence band just west of Madison. Things are
starting to get more organized late this afternoon and nearly all
the guidance gives us a peak of activity between now and about 10
pm. Thereafter, we start to lose the instability, so a diminishing
trend should take over. Severe storms are certainly possible with
the main threat at this point looking like large hail, followed
by strong gusty winds. The more isolated nature of the severe
activity expected over southern WI is what`s keeping a watch from
being issued at this point. But, that could change if things
become more organized.

AVIATION(00Z TAFS)...

Thunderstorms associated with an approaching cold front will
mainly impact KMSN tonight, with a lower threat across the
southeast due to the weakening nature of the front. The storms
will peak between about 00z and about 04z this evening, then
diminish. Look for VFR conditions through much of tonight, except
brief lower conditions in any thunderstorms. We should see an area
of MVFR and IFR conditions develop toward sunrise north of a line
stretching from Dubuque to Sheboygan. This is post frontal and in
an area that will likely see rain this evening. The southeast
should stay VFR, though we`ll have to keep an eye on fog rolling
in off the lake. The lower stuff to the north will push north by
Thursday afternoon.

PREV DISCUSSION... (Issued 245 PM CDT Wed Sep 20 2017)

SHORT TERM...

Tonight - Confidence...Medium
Convection firing from ne IA into SW WI ahead of low level
frontal boundary. Airmass is very unstable with MLCAPE values
reaching 3000 j/kg into sw WI. Dynamics proggd to ride to our
north so lower level forcing to be the primary forcing mechanism.
Mid level lapse rates not favorable though some higher 0-6km
shear sneaking into the cwa, upwards of 30 to 35 knots. Best combo
of forcing and instability looks to be lined up in SC WI where
meso progs show convection at its peak very late in the afternoon
through about 02z or 03z. So have highest pops in that area and
this is also where SPC has the SLight Risk placed and that looks
fine. Coverage of any shra/tsra proggd to diminish with time
overnight with decreasing instability. Depending on degree of
clearing fog may set up once again with the lighter winds
associated with weak winds in vicinity of the boundary.

Thursday - Confidence...Medium
The front is then proggd to lift back to the north. The GFS is
most aggressive with developing some shra/tsra. Mid level flow is
southwest with a broad anticyclonic look and building heights. 850
LLJ is also proggd to remain to our west, so forcing looks again
to be in the lower levels. 925 temps bounce back into the low or
mid 20s celsius so toasty temps and high dew points expected once
again.

Thursday night through Sunday...Forecast confidence high.

An upper ridge will hold from the ern Great Lakes to E TX through
this period with a longwave upper trough over the wrn USA. S WI
will experience sly flow and very warm and humid conditions. Heat
index values will reach the upper 80s to lower 90s through the
weekend with cooler conditions near Lake MI via sely flow.

LONG TERM...Monday through Wednesday...Forecast confidence
medium.

The upper trough over the wrn USA will slowly progress ewd during
this period. Thus the sfc trough/cold front will eventually move
into the region. Chances of showers and tstorms return for Mon-Tue
with temps gradually cooling, but still mild for this time of
year.

AVIATION(18Z TAFS)...Main story will be arriving shra/tsra with
approaching trough/wind shift into srn WI this evening. Still
expect potential for some stronger storms esp into sc WI where
frontal forcing will interact with peak instability. Meso models
decrease precip coverage overnight as instability wanes and better
dynamics ride to our north. Will have to watch for fog
development later tonight with lighter wind regime and lingering
low level moisture with higher dew points still in place.

MARINE...Small Craft Advisory continues into this evening due to
a gusty southeast wind and higher waves which have built highest
towards Sheboygan. No changes planned.

BEACHES...Breezy south to southeast winds have built waves
around 4 feet at beaches in Sheboygan and Ozaukee counties, thus a
Beach Hazard Statement remains in effect. The highest waves and
strongest currents are expected into the evening.

&&

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

$$

Update...BSH
Tonight/Thursday and Aviation/Marine...Collar
Thursday Night through Wednesday...Gehring



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