Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
955 PM CDT Wednesday may 6 2015
Update...the threat of showers and thunderstorms for the western County Warning Area has
mostly ended as the cape is less than 100 j/kg with much higher
cin values. A weak vorticity maximum over eastern Iowa will move north-northeastward
to northwest WI by late tonight. The dense fog over the eastern portions of the
lake counties is the bigger challenge for the night. The pressure
gradient is expected to gradual increase throughout the night. The
925 mb winds will turn south-southwesterly at 25-30 kts by 06z with the boundary
layer winds to turn south-southeasterly by or after 09z. This may limit the
dense fog to the current locations so no dense fog advisory at
Aviation/06z tafs/...1/8-1/2 mile fog and 200 foot ceilings to
continue over kmke and kenw for early Thursday am but southerly winds will
develop toward sunrise and expect a quick dissipation of fog and
stratus after sunrise. Areas of 1-3sm visibilities expected inland over
southeast WI but visibilities will also quickly dissipate after sunrise Thursday.
Over south central WI...only a few areas of MVFR visibilities are
expected. Otherwise VFR conditions will prevail for late Thursday am
into the evening. There will be small chances of thunderstorms for Thursday
afternoon and night as southern WI is on the eastern periphery of an upper
Previous discussion... /issued 358 PM CDT Wednesday may 6 2015/
Tonight and Thursday... forecast confidence medium.
A surface warm front currently in northern Illinois will continue to trek
north late this afternoon. The lake is causing it to stall... so it
is now taking on a northwest-southeast orientation. The front is apparent on
visible satellite with relatively clear skies and east winds on the
north side and cumulus clouds on the south side. The air is well-
mixed and drier behind this warm front.
North of the warm front... across southern WI... dewpoint
temperatures are in the lower 60s inland from the lake. Elevated
cape is around 400 j/kg. I cannot rule out a stray thunderstorm
along the warm front as it gets into south central WI late this
afternoon... but chances are low.
Chances for surface-based thunderstorms diminish after dark this
evening. There is really not much cape or moisture available in the
upper levels for the low level jet to trigger thunderstorms across
southern WI late tonight... so kept the forecast dry.
Expect dense marine fog to spread inland once again tonight. A dense
fog advisory may be needed for the Lakeshore counties. Winds will
become southerly by middle Thursday morning and dissipate any lingering
The surface warm front will be well into northern Wisconsin on
Thursday. This means southern WI will be very warm with breezy south
winds. A weak cold front and a shortwave will drift into western
Wisconsin Thursday afternoon and likely trigger thunderstorms. Storm
motion will be west-southwest so there is a chance for showers and
thunderstorms over mainly the western mkx forecast area Thursday
afternoon. Any well-organized storms could bring brief damaging
winds... so Storm Prediction Center highlighted the western forecast area in a marginal
Short term....short term...Thursday night through Saturday
This period marked by multiple short waves crossing the region in SW
flow ahead of broad positively tilted trough...with a cold front
slowly dropping northwest to southeast through the County Warning Area...exiting to the southeast
Thursday night...forecast confidence high
Showers and thunderstorms should be ongoing at start of period with
warm advection on 35-40 knots low-level jet ahead of weak lead short
wave that moves through Thursday night into Friday morning. Low-
level winds weaken after 06z as wave lifts off to the northeast...so
have tapered off probability of precipitation from south to north between 06z and 12z. Being
in the warm sector...lows will only cool into the lower 60s.
Friday and Friday night...forecast confidence medium
Thunderstorm chances remain highest along and southeast of the
surface cold front and 850 mb trough that bisect the state from
northeast to southwest...with the front sagging into the northwest at 12z
Friday. Models differing on whether or not the front...parallel to
the middle-upper level flow...passes through the County Warning Area by 12z Saturday.
The baroclinic zone hangs up over southern Wisconsin as a secondary
surface wave...associated with the next short wave in a series...
moves through late Friday afternoon and evening.
Will bring likely probability of precipitation into the eastern half of the County Warning Area Friday
afternoon and all but the far west Friday evening with the approach
of the short wave. Higher cape values on slower NAM versus GFS but
enough instability to warrant thunderstorms. Lingering chance probability of precipitation
in the east after 06z Saturday to account for differing
timing/configuration of departing surface wave.
High temperatures in the middle to upper 70s predicated on being in the
warm sector and seeing some sunshine. With the slow moving front and
no real strong push of colder air...lows in the 50s expected.
Saturday and Saturday night...forecast confidence medium
Have a collaborative slight chance for the entire day Saturday...but
looking more like the morning and early afternoon may be dry...with
the cold front becoming stationary to the south over Illinois before
returning as a warm front Saturday night. Will keep Saturday night
probability of precipitation in the chance category...with the highest in the south near the
Illinois border. A cooler day Saturday with north winds and highs in
the 60s to around 70...and lows in the 40s.
Long term...Sunday through Wednesday...forecast confidence medium
The extended dominated by closed SW U.S. 500 mb low opening up as it
lifts northeastward and crosses the upper Midwest...merging with a Canadian
trough that closes off Tuesday north of the upper Great Lakes.
Expect thunderstorms to become likely by Sunday night into Monday
as warm front and then an occluded front passes through the region
as the surface low tracks from the Central Plains across Wisconsin
reaching SW Quebec by Tuesday morning. Wisconsin will remain under
the southern flank of cyclonic flow through mid-week...with periodic
small chances for showers near to slightly below normal temperatures
through the period.
Expect marine fog to spread inland once again this evening. Look for
LIFR ceilings/visibility to redevelop along the Lakeshore. Fog will dissipate
by late Thursday morning as gusty southerly winds begin.
Otherwise...there is a small chance for showers and thunderstorms
over far southern WI later today into this evening along a surface
dense fog should linger over the nearshore waters through Thursday
morning due to warm and moist air over the relatively cold water.
Increasing southwest winds in the boundary layer later Thursday
morning should thin any lingering fog. Southwest wind gusts will
approach Small Craft Advisory levels on Thursday as well.
Lm...dense fog advisory until 10 am CDT Thursday for lmz643>646.
tonight/Thursday and aviation/marine...mrc
Thursday night through Wednesday...rem