Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 180351
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
1051 PM CDT Tue Apr 17 2018

.UPDATE...

The NAM and GFS have come in and are reasonably consistent with
the previous solutions. They are faster than the latest HRRR, so
we`ll have to resolve these differences over the next couple of
hours.  No changes to the current forecast.

&&

.AVIATION(06Z TAFS)...

VFR CIGS cover all but far southwest Wisconsin late this evening.
These clouds look like they will remain in place through the
night. Eventually, lower clouds will stream in from the northeast
and the southwest as low pressure pushes quickly toward the
region. Look for MVFR CIGS over much of the area between 13-15z
Wednesday. Snow or mixed precipitation will also quickly
overspread the area from southwest to northeast between about
17-20z. The snowfall rates could be rather high initially, then
trailing off. Total accumulations at KMSN should be in the 3-5
inch range mainly between noon and 8 pm, peaking between 2-5 pm.
KMKE/KUES/KENW will be in the 2-4 inch range between 2-10pm,
peaking about 3-6 pm. The snow will taper off during the evening.

&&

.PREV DISCUSSION... (Issued 847 PM CDT Tue Apr 17 2018)

UPDATE...
The only change to the forecast on this evening shift has been to
slow things down just a tad...and that trend may have to continue
if the latest solution from the HRRR persists and proves
accurate. The HRRR at 18z tomorrow has the precip still south and
west of Madison, quite a bit slower than our current forecast.
Without any other guidance, it`s too early to make any wholesale
changes.

This is still a very potent low pressure system, but the
surface and upper level features are very much displaced from each
other. The surface low tracks east toward central Indiana by mid
day on Wednesday, while the mid level, compact closed circulation
moves along the WI/IL border. This helps to explain the northern
displacement of the precip from the surface low. However, the
current precip trends off the HRRR also look more southerly. Hmmm.

There continues to be some elevated CAPE in the 600-400mb layer
and given the deep forcing I expanded the thunder mention a bit
farther north. This should be a very narrow window of thunder
threat, mainly near the onset of the precip. The soundings
continue to show a rather muddy solution with regard to precip
type. The warm layer aloft should wet bulb below freezing very
quickly once precip starts. The dendrite zone is at times
saturated and at other times looking dry, though the saturated
layer is right near -10C.

No changes to the advisory at this time.

MARINE...
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.

PREV DISCUSSION... (Issued 548 PM CDT Tue Apr 17 2018)

UPDATE...
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.

AVIATION(00Z TAFS)...

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)

SHORT TERM...
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.

LONG TERM...

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.

AVIATION(18Z TAFS)...
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.

MARINE...
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.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
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.

&&

$$

Update...Davis
Tonight/Wednesday and Aviation/Marine...Halbach
Wednesday Night through Tuesday...Halbach



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