Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 172248

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
548 PM CDT Tue Apr 17 2018

The 18z intermediate runs of the NAM/GFS holding steady and
supporting the current forecast. Quiet conditions tonight will
continue through about sunrise. Precip will then spread rapidly
from southwest to northeast across the area. See the previous
short term discussion below for the technical details on the
forecast. No changes at this time.



VFR conditions will continue through 12z Wednesday, but then look
for rapidly deteriorating CIGS/VSBYS as precipitation spreads
southwest to northeast across southern Wisconsin Wednesday
morning. Lower MVFR CIGS will arrive from the northeast ahead of
the precip as winds turn north northeast and pull in a lower cloud
deck off Lake Michigan. IFR conditions push in by early afternoon
and begin to clear out by late Wednesday evening. KMSN could see a
very quick 3-5 inches of snow between noon and 8 pm, peaking
between 1-5 pm. KMKE/KUES/KENW will be in the 2-4 inch range
between 1-10pm, peaking about 2-6 pm.


.PREV DISCUSSION... (Issued 345 PM CDT Tue Apr 17 2018)

Tonight through Thursday morning...Forecast confidence is medium

Focus is on a deepening, closed low propagating east across the
plains tonight on into southern WI/northern IL tomorrow. This mid
level low closes off a bit further to the west of the region, but
still takes a favorable path across southern WI to provide some
deep lift on the PVA side of the trough. The 850mb closed low
takes a track a bit further south to where the warm nose stays
into northern IL and doesn`t cause as much of a changeover as
previously had been forecast. Thermal profiles shows the wet bulb
temperature staying below 0C but the ambient air temperature
starting out 1-3C above freezing. So, at the onset there could be
a very brief period of rain or sleet, but it will very quickly
transition over to all snow as the p-type. Deep surface to 600mb
frontogenesis creates a strong upward circulation as the trough
approaches, which is why there is such concern for heavy snowfall
rates. Some EPV noted above this layer which also gives some extra
concern for some instability aloft with this system.

The two main `hold ups` at this point are that the system appears
to peak just a bit further to the west (where warnings are in
place) and then slowly weakens as it moves into the Great Lakes
and it also is a quick hitter with not much residence time. Also,
being able to nail down exactly where the Fgen band sets up a day
ahead of time is extremely difficult. So, there is potential for
an upgrade to a warning overnight into early tomorrow morning, I
just didn`t want to have to overwarn for a large area when the
band will likely be the width of 1-2 counties.

Overall, main concern is for the timing of this coming right at
the evening commute. While road temps warmed today, when you get
snowfall rates of 1-2" per hour it will easily accumulate on the
roads. There is some increase in the winds overnight, but not
strong enough to cause blowing and drifting snow.


Thursday through Tuesday...Forecast confidence is high.
The good news is that beyond this system we appear to be in a
gradual warm up through the rest of April. Not much to speak of
precipitation- wise with the focus being on tomorrow.

Borderline MVFR conditions are possible along with a few flurries
this afternoon as an inverted trough swings south across eastern
WI. The main concern goes to tomorrow when IFR to LIFR conditions
are possible as heavy snow develops. Could see a brief period of
1/4sm to 1/2sm heavy snow in the afternoon/everning as the band
moves through. Still trying to nail down the location of where
this heavy snow band sets up though, with central WI the most
likely area.

Strong winds and waves are expected Wednesday night into Thursday
as a potent storm system moves through the southern Great Lakes.
Conditions will improve going into Friday as high pressure moves
in from the west. A small craft advisory has been issued for
Wednesday afternoon into Thursday afternoon timeframe.


WI...Winter Weather Advisory from 10 AM Wednesday to 4 AM CDT
     Thursday for WIZ046-047-051-052-056>060-062>072.

LM...Small Craft Advisory from 1 PM Wednesday to 7 PM CDT Thursday
     for LMZ643>646.



Tonight/Wednesday and Aviation/Marine...Halbach
Wednesday Night through Tuesday...Halbach is the U.S. government's official web portal to all federal, state and local government web resources and services.