Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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966
FXUS63 KMKX 142047
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
347 PM CDT Sat Oct 14 2017

.SHORT TERM...Tonight and Sunday

Forecast Confidence is Medium.

Dynamic storm system continues to take shape over the region. The
past few hours worth of observations and convection allowing
model (CAM) runs have not changed the going forecast all that
much. As a result, there remains some questions as to the exact
evolution of our mesoscale pattern this evening.

As anticipated, surface low pressure is taking shape over Iowa,
while the mid level shortwave enters the Upper Midwest. The
surface warm front/inverted trough extending from the low
pressure in Iowa has moved very little since this morning. As a
result, our region remains entrench in easterly flow, while
southerly flow is still over central Illinois. As the mid level
shortwave continues its eastward trek, the surface low will move
to the northeast.

The biggest question is where exactly the low track will occur.
After looking at all available guidance and blurring ones eyes,
the low should track over southern WI this evening, exiting to the
northeast overnight. This could put the southeastern third (or
so) of the area in the warm sector for a few hours. If this is the
case, there is a concern for severe storms.

Given such a dynamic system, shear is quite good. Deep layer shear
is well in excess of 60kt, 0-3km shear will be 40-45kt from the
west southwest and 0-1km bulk shear will be 40kt out of the
southwest. While the atmosphere is unstable, CAPE values are
rather modest. Generally speaking, mixed layer CAPE values in the
500 J/kg (give or take) along and southeast of the warm front,
which is certainly enough given strong dynamics/forcing/shear.

As a result, the key to tonight`s forecast is the behavior of the
warm front. Any location that is along/southeast of the warm front
will be prone to severe wind gusts and a few tornadoes. While hail
can be expected with any strong storms, severe hail may be tough
to come by given warm cloud depths and marginal mid level lapse
rates. Heavy rain and localized flash flooding continue to be a
concern as well. Precipitable water values of 1.75" are quite
unusual for this time of year and we`ve already observed stout
rain rates in northern Illinois. The going watch is well placed,
with the great risk for flash flooding associated with the
heaviest rates and in urban/slow drainage area.

This system will waste no time accelerating to the northeast, with
gusty west winds spreading over the region overnight. Light
showers will continue to be possible in the cyclonic flow of the
departing low through Sunday morning. Northwest winds will remain
quite gusty on Sunday, with slow clearing expected from west to
east during the afternoon.

.LONG TERM...

Sunday night through Saturday...Forecast confidence is high:

High pressure is expected to bring plenty of sunshine and dry
weather to southern Wisconsin for most of next week. High temps
will likely warm to a few degrees above normal by mid-week...then
possibly be 10-15 degrees above normal by the end of the week.
South to southwest winds will be breezy at times. On the lighter
wind days, wouldn`t be surprised by a lake breeze developing in
the afternoon given the relatively cooler lake temperatures near
the shore.

&&

.AVIATION(18Z TAFS)...

Rather messy aviation situation across the region. Stratus deck
has remain IFR and will continue to do so well into tonight.
Visibility will be a bit more dependent on rainfall, wind
direction and eventually where a surface warm front ends up moving
to. The forecast remains pessimistic with a few rounds of
rainfall, mainly IFR conditions (primarily ceilings) and mainly
MVFR visibility (some bouts of IFR with drizzle or heavier
downpours of rain). The storm system responsible for these
conditions will accelerate to the northeast this evening. This
will result in gradually improving ceiling and visibility
overnight from west to east. However, there will be southwesterly
low level wind shear to contend with. Heading into Sunday, MVFR
ceilings will persist through the morning and winds will shift to
the northwest and become gusty.

&&

.MARINE...

Southerly gusts and waves will likely be a little higher this
evening into tonight, building some higher waves. A Small Craft
Advisory remains in effect north of Port Washington and goes into
effect south of there at 7 PM.

Winds will veer west to northwest later tonight into Sunday and
increase behind departing low pressure. Gust to 35 knots are
likely, so a Gale Warning remains in effect from later tonight
into Sunday afternoon.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...Flash Flood Watch until 1 AM CDT Sunday for WIZ064>072.

LM...Small Craft Advisory from 7 PM this evening to 4 AM CDT Sunday
     for LMZ644>646.

     Gale Warning from 4 AM to 4 PM CDT Sunday for LMZ643>646.

     Small Craft Advisory until 4 AM CDT Sunday for LMZ643.

&&

$$

Tonight/Sunday and Aviation/Marine...Gagan and DDV(Marine)

Sunday Night through Saturday...DDV



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