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000 
FXUS63 KMKX 202247
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
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...Beach Hazards Statement until 9 PM CDT this evening for WIZ052-
     060.

LM...Small Craft Advisory until 9 PM CDT this evening for LMZ643.

&&

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