Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 212025
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
325 PM CDT SAT OCT 21 2017

.TONIGHT AND SUNDAY - Forecast Confidence...Medium.

Clouds should gradually move into the area from the west and
southwest into tonight. Gusty south winds should linger into the
early evening hours, until mixing subsides with loss of daytime
heating.

Mesoscale models suggest that the area should remain dry into at
least early this evening. Low level frontogenesis response then
should move east toward and into the area later this evening into
Sunday morning. There is some uncertainty with the timing and areal
coverage of any showers and elevated storms, as the frontogenesis
response weakens quite a bit tonight as the front approaches the
area. There is also a good amount of dry air to overcome across the
area.

The mesoscale models are generally showing a line or broken line of
convection weakening considerably, as it shifts toward the western
counties later this evening, and into the area overnight into early
Sunday morning. Continued to use higher end PoPs across south
central Wisconsin overnight into early Sunday morning, but they may
be too aggressive given the weakening upward vertical motion and dry
air in place.

Mainly elevated CAPE later tonight is in the 200 to 500 J/kg range
on the NAM and HRRR, a bit more on the RAP, and weaker on the GFS
and ECMWF. Kept thunder mention for tonight, with best chances in
the western counties. Not confident in any severe weather occurring,
given the possible weakening trend later tonight.

Better low level saturation and frontogenesis response occurs later
Sunday morning into the afternoon, as the front moves through the
area. Kept high PoPs for Sunday morning, lingering mainly in the
eastern counties in the afternoon.

Mild temperatures should linger with south winds tonight into Sunday
morning, before the front moves through. Temperatures should remain
nearly steady or begin to fall after the frontal passage in the
afternoon.

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

12Z NAM solution has shifted to slower eastward progress of cold
front with widespread -shra lingering over srn WI through Sunday
night.  Meanwhile, GFS and ECMWF showing surge of drier air sweeping
across southern WI during the evening. All short term guidance
however showing lingering southwest steering flow ahead of weak mid-
level trof over western WI, so enough uncertainty and potential for
some -shra to linger into Sunday evening over eastern WI to warrant
adding chance wording into Sunday night.

Sharpening upstream mid-level trof over the northern plains will
settle into the western Great Lakes, which will nudge cutoff low
pressure over the TN valley region northward.  This low will blend
with upstream amplifying troffing to cause rapid cyclogenesis across
the central Great Lakes Monday night and Tuesday.  This deepening
system will likely bring some -shra into southern Wi later Monday
night and Tuesday.  Low level thermal profiles favor all liquid
precipitation, but will be possible for a few snow pellets or flakes
to be mixed in with the rain, especially Tuesday night as the
precipitation begins to end.

Expect temperatures to recover and rise back into the upper 50s to
lower 60s on Monday but colder air will become entrenched over the
area Monday night and Tuesday.  925H temps will fall into the 0 to
3C range.  A raw day can be expected on Tuesday with temperatures in
the 40s.  Period of clearing will allow temperatures to fall into
the mid to upper 30s Tuesday night  but low level mixing and
uncertainty regarding clouds and potential for light mixed
precipitation later in the night should preclude significant frost
development.

.EXTENDED PERIOD...

TUESDAY NIGHT THROUGH SATURDAY - Forecast Confidence...Medium
trending Low.

Medium range guidance in reasonable agreement on long wave trof over
the Great Lakes and Ohio Valley region getting nudged eastward early
in this period as mid-level steering flow increases from the
northwest across the upper midwest into southern Canada.  Mid-level
short wave caught in this flow may bring a period of rain and snow
showers to the area Tuesday night lingering into Wednesday.

Uncertainty increases later next week as 00Z medium range models
diverged in strength, timing and placement of next low pressure
trough moving across the northern Plains into the midwest by the
weekend.  Fortunately, 12Z guidance coming into better agreement
with timing and track.

GFS had been trending with a colder but more progressive solution
with a more open mid-level short wave moving across the northern
Plains, transitioning to cutoff low over the Great Lakes by Friday.
ECMWF meanwhile had slow, warmer solution for the area with long
wave trof developing upstream over the Rockies and high plains,
eventually settling into the southwest CONUS by the weekend.  By 12Z
Saturday, GFS has a 925H temp of -6C over srn WI while ECMWF is 14C!
GFS Ensemble guidance trending toward slower, more open solution
than GFS deterministic run, but ensembles still show considerable
uncertainty.

12Z now in much better agreement on amplifying short wave trof
strengthening in the upper midwest, eventually moving into the
western Great Lakes early in the weekend.  This would carry a cold
front through southern WI Thursday night with gradually colder air
settling in place Friday and through the weekend.

Latest WPC blended model guidance ignored both 00Z GFS and ECMWF and
leaned heavily on EMCWF ensembles and GEFS mean.  WPC trending
toward sharper mid-level trof over the upper midwest with much
weaker surface low pressure trof over the Great Lakes, which is
looking like the right track per new 12Z guidance. Looking more
likely that first widespread frost and/or freezing temperatures may
occur over the weekend.

&&

.AVIATION(21Z TAFS)...Gusty south winds are expected into early this
evening across the area. Southeast lake breeze will continue to
affect areas near the lake until around 00Z Sunday.

Clouds should gradually increase into tonight from west to east. A
period of low level wind shear is expected between 03Z and 09Z
Sunday across western portions of the area, and 03Z to 13Z to the
east.

There may be a delay in the arrival of any showers and some storms
from later tonight to Sunday morning in south central Wisconsin,
with dry air in place. The cold front will move east across the area
Sunday morning, bringing a period of showers. A few storms may
linger Sunday morning.

Mainly MVFR ceilings are expected, with visibility values around 5
miles or so possible. The showers should end in south central
Wisconsin during the afternoon, but should linger to the east. South
winds will veer to the west and northwest with the frontal passage.

&&

.MARINE...Small Craft Advisory continues until 21Z Sunday across the
nearshore waters of Lake Michigan. Gusty south winds with a tight
pressure gradient and good low level mixing are expected during this
period. Gusts of 25 to 30 knots are expected, with the highest gusts
tonight.

Waves of 4 to 7 feet should linger into tonight, highest toward
Sheboygan. These winds and waves should slowly subside Sunday
afternoon, as the pressure gradient weakens.

Other periods of gusty winds and building waves are possible Tuesday
into Tuesday night, and again Thursday night into Friday.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...

WI...None.
LM...Small Craft Advisory LMZ643>646 until 21Z Sunday.

$$

TONIGHT THROUGH SUNDAY/AVIATION/MARINE...Wood
SUNDAY NIGHT THROUGH SATURDAY...MBK


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