Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 190213
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
913 PM CDT Tue Apr 18 2017

.UPDATE...MUCape continues to decrease along approaching cold
front per SPC Meso analysis. With diminishing convergence along
front, wl remove thunder threat for the rest of the night, but
still outside chance isolated convection may develop. Cold front
knocking on door of western CWA and will sweep across southern WI
during the overnight, generating isolated to scattered light
showers.

&&

.MARINE...Cancelled Small Craft Advisory 1 hour early across
northern zones as winds and waves have fallen below criteria.
Approaching cold front will sweep across near shore waters between
05Z and 12Z resulting in south winds turning to the west to
northwest. Wind speeds will increase a bit but should remain
below 20 knots.

&&

.PREV DISCUSSION... (Issued 611 PM CDT Tue Apr 18 2017)

AVIATION(00Z TAFS)...Initial surge of warm air and moisture
advection triggering scattered light showers across southern WI.
As deeper moisture continues to stream northward, expect isolated
to scattered showers to redevelop as surface cold front moves
through overnight. Can not rule out an isolated thunderstorm as
weakly elevated instability is expected ahead of passing cold
front and southward extension of passing mid-level wave.

Biggest question is extent of low cloudiness expected late tonight
into Wednesday morning across the area. Large area of low clouds
resulting in MVFR cigs over MN and northwest WI wl rotate
eastward. Surge of colder air loses some of its punch as it pushes
into the southern half of WI later tonight. Trimmed back amount of
time MVFR cigs are forecast but can not yet totally rule out a
period of lower cigs.

PREV DISCUSSION... (Issued 304 PM CDT Tue Apr 18 2017)

TONIGHT AND WEDNESDAY...Forecast Confidence Is High.

A fast moving low pressure system takes a track across the Lake
Superior region this evening. A cold front trailing southwest from
the low will drop into our western forecast area this evening,
reaching the WI/IL border by sunrise Wednesday. We should see some
showers along and ahead of the front as it moves through, and
there is just enough CAPE on the soundings to support an isolated
rumble of thunder.

Winds will become northeast Wednesday morning, as high pressure
moves across south central Canada. High temperatures will occur
early in the day, then fall through the afternoon east as brisk,
cool east winds, thickening cloud cover, and rain take over.

The entrance region of the exiting northern stream upper jet
across the upper Great lakes, along with an approaching southern
stream jet from the central Plains, will result in increasing
upper divergence. Also, the nose of the 45 knot low level jet,
moving into northeast Iowa and southern Wisconsin Wednesday
afternoon, along a tight 850 mb baroclinic zone, will increase
the low level moisture convergence.

This forcing should bring rain and scattered, elevated
thunderstorms to the area late Wednesday afternoon, as the surface
warm front over northern Illinois begins to lift north. Some
storms will have hail potential due to 450 Joules/kg of elevated
CAPE far south toward evening.

WEDNESDAY NIGHT THROUGH FRIDAY...Forecast Confidence Is Medium.

Models are still in some disagreement with where the surface low
will track Wednesday night. The GFS and to some extent the ECMWF
take the low northeast into southwest Wisconsin, with southern
portions of the area getting into the warm sector by 12Z Thursday.

The NAM/Canadian are taking the surface low across far southeast
Wisconsin later Wednesday night, keeping most of the warm sector
airmass just to the south. The low track will also affect where
the warm front will move through. Took a blend of these models,
which brings the warm front and warm sector into the far southern
counties later Wednesday night.

The best upward vertical motion fields should be Wednesday
evening, with the 850 mb jet pointing northeast into or just north
of the forecast area. A 500 mb vorticity maximum may cross the
area during the evening as well. This should be the period of
highest PoPs across the area, especially in the northern 2/3 of
the area.

Area forecast soundings suggest elevated convection will be
possible, with much higher CAPE on the NAM than the GFS. Deep
layer shear increases as well in the evening. SPC has southern
portions of the area in a Marginal Risk for severe storms for late
Wednesday afternoon into Wednesday night. It seems the evening
will have the best chances for anything strong developing, with
isolated large hail the main hazard.

By the time the surface low and warm front arrive later Wednesday
night, the best upward vertical motion fields exit the area to
the east. So, not sure if there will be much in the way of
convection after 09Z Thursday. Some uncertainty here, given the
lack of agreement with the model low track during this time.

Some showers should linger into Thursday, mainly in the northern
counties, as the 500 mb shortwave trough passes by to the north.
Cold air advection Thursday into Thursday night should bring
cooler and drier air into the region. High pressure moving into
the Lake Superior area Friday will continue the dry forecast.
Cooler 50s on Thursday are anticipated, with upper 40s to lower
50s near the lake on Friday. Milder values are forecast well
inland on Friday.

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

High pressure should slide slowly southward across the region
Friday night into Saturday, continuing the dry conditions. A cold
front will then slide southward through the area Sunday, quicker
near Lake Michigan. Things look dry during this time across the
area. Onshore winds each day should keep highs near Lake Michigan
in the upper 40s to middle 50s, with milder values well inland.

Models then show fairly well developed low pressure, and an
associated well defined 500 mb shortwave trough, sliding eastward
from the Dakotas to northern Lower Michigan Monday into Tuesday.
It appears that enough warm air advection develops to include PoPs
for most of this period in the forecast. May need higher PoPs
later on if the models remain consistent with the timing and
placement of features with this system. Temperatures will
continue to be cooler near Lake Michigan, with slightly warmer
values inland.

AVIATION(18Z TAFS)...

VFR conditions are expected through today. Look for brisk south
winds, reaching gusts to 25 to 30 knots inland, especially away
from Lake Michigan. The winds near the lake are not as strong due
to the inversion. These winds will gradually veer to the south
during the afternoon and should increase. Winds will then diminish
in speed this evening.

Low pressure tracking across northern Wisconsin will drag a cold
front into southern Wisconsin this evening. The front should reach
KMSN by around 03-05z and KMKE about 07-09z Wed. A band of
showers will be ahead of the front. CIGS will likely drop to MVFR
levels in the wake of the front.

The cold front will become stationary across northern Illinois
Wednesday morning, before lifting north during the evening.

MARINE...

A tight pressure gradient, between exiting high pressure and low
pressure tracking across Lake Superior this evening, will bring a
rather short period of elevated south winds into this evening.
The strongest winds and largest waves will be found up toward
Sheboygan. The lake breeze and resultant inversion have kept lake
shore winds a bit lower. A Small Craft Advisory has been issued
into this evening.

East winds and waves should reach Small Craft Advisory conditions
again Wednesday evening, ahead of strong low pressure that will
track across central Wisconsin Thursday morning.

&&

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

$$

Update/Marine/Aviation...MBK
Tonight/Wednesday...Hentz
Wednesday Night through Tuesday...Wood



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