Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI
FXUS63 KMKX 310830
Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
330 AM CDT TUE MAY 31 2016
.TODAY AND TONIGHT...Forecast Confidence Is Medium.
A subtle weak upper level shortwave will move across southern
Wisconsin today, with the main strong shortwave moving across the
northern plains through tonight. A 75 knot 250 mb jet max
increases to 85 knots by midnight, as it noses into northwest
Weak to moderate upper level divergence is expected today and
tonight, along with 700 mb upward motion. A 850/700 mb southwest
speed max of 40 knots is lifting across west central and northwest
Wisconsin, with the entrance region across southern Wisconsin
later this afternoon. 850/700 mb dew points and RH increases today
GFS surface dew points increase into the lower to mid 60s, but
will use the drier MOS dewpoints that remain in the upper 50s for
the most part. Lower values off Lake Michigan will be used,
especially in northeast areas.
GFS precipitation increases through the day, and especially
tonight as the surface cold front approaches. Zero to 6 km bulk
shear is rather weak today, but does increase to 33 knots west and
northwest of Madison late tonight.
GFS zero to 1 km CAPE does increase to around 925 Joules/kg at
Madison this afternoon, although likely a little overdone due to
too high dew points. Even so, elevated CAPE is around 2000
Joules/kg. therefore, expect the chance of thunderstorms to spread
east today, and across the forecast area tonight.
The Storm Prediction Center has south central Wisconsin in a
marginal severe risk today and tonight, with the best chance late
afternoon and tonight, as the instability and shear increases.
.WEDNESDAY THROUGH THURSDAY NIGHT...Forecast Confidence Is Medium.
Models are in decent agreement with bringing the surface cold
front eastward into the area Wednesday, then exiting the eastern
portions of the area by 06Z Thursday. NAM/GFS are showing fairly
robust low level frontogenesis with the frontal passage.
The NAM also brings an area of 500 mb vorticity maxima northeast
through the far eastern and southeastern counties Wednesday
afternoon, with the GFS/Canadian clipping the far southeast
Wednesday morning. The main vorticity maximum within the rather
flat 500 mb trough crosses northern portions of the area later
Wednesday into Wednesday night. There are hints of upper
divergence Wednesday morning from the right entrance region of a
250 mb jet streak.
So, all of this leads to continuing the likely pops for Wednesday,
tapering off Wednesday evening. May need higher pops in later
forecasts if models remain consistent with forcing for upward
Area forecast soundings are showing tall skinny CAPE of several
hundred J/kg on NAM before the front moves through, with less CAPE
on GFS. Deep layer shear is decent, in the 30 to 35 knot range.
Marginal risk for severe storms seems reasonable from Storm
Prediction Center for Wednesday, though most storms will remain
below severe levels.
Cold air advection behind the frontal passage Wednesday night into
Thursday will bring a somewhat cooler but drier airmass into the
region. Mild temperatures in the middle to upper 70s Wednesday
ahead of the cold front will remain in the lower to mid 70s for
Thursday. Lows Wednesday night and Thursday night should drop into
the lower to mid 50s in most areas. Dew points will lower behind
the front, which will bring less humid conditions.
.FRIDAY THROUGH MONDAY...Forecast Confidence Is Medium.
ECMWF/GFS are in decent agreement with bringing next system
southeastward into the region later Friday into Saturday night.
They slide a 500 mb shortwave trough southeast through the region
during this time, with an associated surface low/cold front moving
through the area later on Saturday.
There should be enough upward vertical motion to continue chance
pops for most of this period. GFS forecast soundings were too high
with dew points during this time, so went with only slight chances
for thunder. Temperatures should remain near seasonal normals.
Kept pops going Sunday into Monday, as 500 mb vorticity maxima
continue to pass through or remain close enough to the area.
Expecting VFR conditions through the TAF period except for brief
MVFR/IFR in scattered showers and thunderstorms. The coverage of
the showers and thunderstorms will increase as a cold front
approaches from the west tonight.
Light winds and low wave heights are expected to continue into
mid-week. There will be a slight chance of showers and
thunderstorms today, with increasing chances tonight and Wednesday
with passage of a cold front. Look for southeast to east winds
today and tonight ahead of the cold front. Wind speeds will
increase tonight, but are expected to remain below small craft
TODAY/TONIGHT AND AVIATION/MARINE...Hentz
WEDNESDAY THROUGH MONDAY...Wood