Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 212246

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
546 PM CDT Fri Jul 21 2017

The best CAPE gradient remains right along our southern tier of
counties, south into Illinois. Farther to the north we have decent
elevated action going on, but the severe threat should stay close
to where the watch is. The atmosphere is very tropical with PWs
in the 2-2.2inch range. Very efficient rainfall process. Urban and
low lying areas could see flooding tonight, even north of the
watch area by 1 or 2 tier of counties.



Fairly steady thunderstorms moving through southern Wisconsin this
evening. A mix of MVFR/IFR conditions in the heavier rain. Once
the rain moves off toward midnight, we get into a very light flow
regime that will likely lead to IFR CIGS/VSBYS due to very high
moisture in the low levels. Severe storms should remain along the
WI/IL line with heavy rain expected a few tier of counties north
of there. Improving conditions, with respect to cigs/vsbys will
occur later Saturday morning.


.PREV DISCUSSION... (Issued 341 PM CDT Fri Jul 21 2017)

Rest of Today and Tonight...Forecast Confidence Medium.

Removed Marquette Co from the Flash Flood Watch since the axis
of heavy rainfall has shifted south. Heavy rain is the main
concern with the greatest threat across far southern WI. Rainfall
with isolated thunderstorms are expected to continue across
southern WI. The low level jet has strengthened to around 35 knots
pointing into southwest WI where the 850 baroclinic zone is
located. Models show the LLJ strengthening to 40 knots this
evening when moisture transport is maximized. See Hydrology
section for more info.

The surface boundary and outflow is across Iowa and northern
Illinois. Activity has fired up north of this boundary but this
activity will likely end up south of WI. Cape and shear will
increase this evening so we`ll have to see if anything can fire
upstream in Minnesota this evening. If it does, we can still
expect some severe weather.

After midnight the low level jet pivots to the east and slides south
of the state. PWs drop as this forcing leaves the region and precip
should wind down during this time.

Saturday...Forecast confidence Medium.

PWs decrease slightly early Saturday morning but remain high, around
1.5, during the day. Low pressure will move along the WI/IL border
on Saturday. Not too excited and thunderstorm chances tomorrow. Cold
air advection will be in place and the low level thermal boundary
will have shifted south of the state. There will be some surface
convergence after it moves through. Cape looks to be over 1000-2000J
with 35-40 knots of shear so if something get going we could see
some isolated severe storms.

Sunday through Thursday...Forecast Confidence High

Small chances for thunderstorms linger into Sunday with a shortwave
moving through. Most areas will be dry. High pressure will sink into
the region bringing dry weather Monday and Tuesday. Temperatures
will also be cooler with less humidity on northwest winds. Tuesday
night surface low pressure will move across southern Canada
dragging a cold front and some precip with thunderstorm possible.
Behind this system high pressure will briefly nudge in. Low
pressure will approach from the southwest later in the week and a
baroclinic zone will be draped southwest of the state. This
system has potential to bring more rain to southwest wi later in
the week. Beyond this it looks like high pressure will set
bringing quiet weather.

Showers with a few lightning strikes will continue to move across
southeast WI. Another batch of showers with heavy rain at times
will move in this evening. Thunderstorms are possible during this
time. Mainly VFR conditions are expected through tonight, except
for with thunderstorms. Fog is likely again tonight with weak
winds and plenty of moisture in place. MVFR or lower ceilings will
develop over central WI Saturday morning and sag toward southern

Showers are expected through the night. Fog is possible after
midnight. Thunderstorm chances increase this evening with strong
winds the main threat. North winds will increase Sunday night and
Monday but are expected to remain below small craft levels.

12Z DVN sounding showed PW of 1.9 with a freezing level of 14kft.
This is a favorable environment for efficient rainfall. The GFS and
NAM show values up to 2.5 this evening. This is when moisture
transport is maximized on a 40 knot jet. A 850 baroclinic zones lies
across southwest WI with mean layer from out of the west. Forward
propagating corfidi vectors of 20 knots are parallel to this
boundary and flow allowing for training storms. WPC precip
discussion notes the info is nearly double the mean wind which may
lead to backbuilding. The question is where this will set up,
closer to the surface boundary, the 850 boundary, or somewhere in


WI...Flash Flood Watch through Saturday morning for WIZ056>058-



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