Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 211108
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
608 AM CDT Wed Jun 21 2017

.UPDATE...

IFR fog in river valleys will dissipate quickly with the morning
sun.

&&

.AVIATION(12Z TAFS)...

IFR fog in river valleys will dissipate quickly with the morning
sun. otherwise VFR.

Some ACCAS or showers expected later this afternoon ahead of a
developing low level jet pushing toward southern Wisconsin.

Thunderstorms are expected to spread into south central WI during
the early evening hours, then across much of the area ahead of a
warm front which will move across southern Wisconsin late tonight.
MVFR cigs and IFR vsbys in showers/thunderstorms.

&&

.PREV DISCUSSION... (Issued 230 AM CDT WED JUN 21 2017)

TODAY AND TONIGHT - Forecast Confidence...Medium.

Surface high centered across the Great Lakes today, will bring
pleasant weather to start the day with low dew points.

The upper level trough over the Great Lakes moves east, as a zonal
jet sets up from Montana to lake Superior. Upper level divergence
increases this evening and continues tonight. 700 mb upward motion
is a little quicker and increases this afternoon.

700 mb dew points increase this afternoon as the 700 mb winds
increase from the west, 850 mb dew points increase mainly tonight,
as a 40 knot 850 mb southwest jet moves across southern Wisconsin,
increasing to 50 knots late.

GFS forecast soundings at Madison bring elevated CAPE of 725
Joules/kg this evening, that rises to near 1500 Joules/kg by
midnight.

With the elevated moisture beginning this afternoon, expect some
ACCAS or showers to move in this afternoon, then with the
approaching 850 mb low level jet, and increasing elevated CAPE,
expect thunderstorms this evening and overnight. Some hail expected,
with SPC outlook expanding the slight risk across south central and
into portions of southeast Wisconsin.

THURSDAY THROUGH FRIDAY - Forecast Confidence...Medium.

A few thunderstorms may linger into Thursday morning as strong low
level jet and moisture advection slides off to the east.  Will lower
pops to chance category as somewhat drier and warmer air builds into
southern WI as surface and elevated warm front lift to the
northeast.  Building Capping inversion should keep convection threat
low into Thursday afternoon.  Low level winds back to the SSW
Thursday afternoon and evening ahead of approaching mid-level short
wave.  Thinking that surface warm front will be carried northward
into central WI late Wednesday night or Thursday morning by strong
low level winds. Morning Cap expected to erode in the afternoon
resulting in surface based Cape of 2-3k joules.  Better chance for
convection refiring in northern CWA in the afternoon closer to
surface front. Otherwise, triggering mechanism holds off until the
evening when right entrance region of upper jet passes across
southern WI. Lift also enhanced by passing cold front and mid-level
trof. Impressive sloping frontogenetical forcing passes across
southern WI between 00 and 06Z Friday.  0-6km bulk shear in the 35
to 45 knot range with an unstable atmosphere and low pressure
possibly strengthening and moving northeast across central WI during
this period.  CWASP values in the .6-.7 range in the evening. Energy
Helicity Index from the NAM increases to 3.5 in NW CWA during the
evening.  At this point, this area would be the focus for tornado
formation in the late afternoon and early evening.  Only concern for
Thursday would be lingering cloud cover limiting instability,
especially in the north.

Convection winds down later Thursday night into early Friday as
surface cold front slides off to the southeast, and more stable
conditions settle into southern WI.  Drier air continues to feed
into southern WI on Friday on breezy WNW winds.

EXTENDED PERIOD...

FRIDAY NIGHT THROUGH TUESDAY - Forecast Confidence...Medium.

Medium range guidance is in reasonable agreement on unsettled
weather continuing across the western Great Lakes region for this
period.  Spokes of short wave energy will track southeast in
broadscale long wave troffing over central Canada into the upper
Midwest region, carried by WNW steering flow.  These pieces of
energy will bring occasional showers and a few rumbles of thunder to
southern Wisconsin over the weekend into early next week.  In
addition, cooler air will be settling over the region resulting in
temperatures averaging below normal for late June.  The GFS 5-day
500H means does hint at this cooler pattern finally breaking down
later next week around the first of July.

AVIATION (09Z TAFS)...

Clear skies and light winds will allow for some patchy mainly MVFR
fog, but some IFR in river valleys. This will dissipate quickly with
the morning sun.

Some ACCAS or showers expected later this afternoon ahead of a
developing low level jet pushing toward southern Wisconsin.

Thunderstorms are expected to spread into south central WI during
the early Wed evening hours.

MARINE...

Light winds and sunshine will allow for a lake breeze to develop by
late Wed morning. Showers and thunderstorms are likely Wednesday
evening and Wednesday night. Then again late Thursday when some
severe storms possible. Breezy southwest winds on Thursday.

&&

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

$$

Update...Hentz
Today/Tonight and Aviation/Marine...Hentz
Thursday THROUGH Tuesday...MBK



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