Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
342 PM CDT Tuesday may 5 2015
Tonight and Wednesday...forecast confidence is medium.
Area of rain showers over southern Wisconsin back into southern
Minnesota and northern Iowa was being driven by 925mb to 850 mb
moisture convergence/frontogenesis response...as well as an
approaching weak 500 mb vorticity maximum. Mesoscale models has this
area of rain showers gradually shifting east northeast through the
area most of this evening. Thus...continued scattered to numerous
rain showers into the evening hours.
925 mb/850 mb convergence/frontogenesis response pushes northward
through the area later tonight and exits Wednesday morning...with
modest warm air advection later in the day. Surface warm front also
pushes northward later tonight into Wednesday into and through the
area. Upward vertical motion fields are not very strong later
tonight into Wednesday. Area forecast soundings do show some mean
layer cape developing during the afternoon...more on NAM than GFS.
Kept lower end probability of precipitation going for this period.
Light winds and moist low levels under shallow inversion should
allow for some fog to develop across the area later tonight into
Wednesday morning. Patchy dense fog is possible...especially near
Lake Michigan with onshore winds and cool moist airmass below
inversion. Some uncertainty here...so limited dense fog mention to
nearshore waters for now...but could come onshore. Will need to be
watched this evening.
Cool temperatures will linger tonight...with clouds and scattered
precipitation. Upper 40s to around 50 for lows seem reasonable.
Highs Wednesday will warm into the middle to upper 70s well
inland...with 50s to near 60 close to Lake Michigan with continued
Wednesday night and Thursday...forecast confidence is high.
The middle level shortwave trough lifts northeast and weakens as it
reaches the upper Mississippi Valley and western Wisconsin area
Thursday. Upper level winds are weak...but increase a little
across Iowa Thursday afternoon. Very weak upper divergence...mainly
south central Wisconsin around noon. Weak 700 mb upward motion
Thursday. A band of higher 700 mb relative humidity pushes into mainly south
central Wisconsin Thursday. The 850/700 mb winds do
increase...especially at 850 mb around sunrise Thursday west of
Madison...but the main low level jet remains over Iowa and
western Wisconsin. The zero to 1 km mixed layer cape vales
increase to around 800 joules/kg mainly from Madison west. The
local severe weather parameter rises to around 75 percent late
Thursday afternoon and early evening.
The models are mainly precipitation free Wednesday night but the 12z GFS
and 00z European model (ecmwf) brings moderate precipitation into south central
Wisconsin Thursday afternoon. The NAM forecast soundings show
the cap eroding during the afternoon over south central Wisconsin
with the forecast area in a warm and increasingly moist airmass.
However a lake breeze may keep the shoreline areas cooler.
Thursday night and Friday...forecast confidence is high.
A second upper level trough moves northeast into Wisconsin
Friday...along with a 70 knot speed maximum along with weak upper
level divergence. The stronger middle level shortwave is
quicker...moving across the Lake Superior area...with a weak
trough axis into southeast Wisconsin around noon.
Weak 700 mb upward motion occurs Friday morning with an area of
higher 700 mb relative humidity...with some drying during the afternoon...
especially north areas. Southern Wisconsin is in the 850/700 mb
entrance region of the speed may lifting across west central and
northern Wisconsin. 850 mb relative humidity increases Friday.
As low pressure pushes to north of Lake Superior...a trailing
cold front enters southern Wisconsin Friday. The highest zero
to 1 km mixed layer cape rises to around 1300 joules/kg along the
surface cold front around noon over southern portions of south
central Wisconsin before model convection lowers it when it moves
into southeast Wisconsin. NAM nsharp soundings show marginal
severe storms potential.
Saturday and Sunday...forecast confidence is medium.
The GFS and European model (ecmwf) both have a large upper low near Colorado
Saturday that pushes into the Central Plains Sunday. The 00z European model (ecmwf)
is a little faster.
The cold front exits Southern Lake Michigan by Friday evening. A
large area of high pressure moves across southern Canada...keeping
the surface boundary across central Illinois and the Ohio River valley.
The 00z European model (ecmwf) keeps the boundary much closer...and even lifts it
north into southern Wisconsin. As a result the GFS keeps southern
Wisconsin dry...while the European model (ecmwf) has periods of moderate
precipitation with a strong low reaching northwest Iowa Sunday
afternoon while the GFS still has it back over the Central High
Monday and Tuesday...forecast confidence is medium.
The rather strong middle level shortwave pushes into the upper
Mississippi Valley Monday and into the western Great Lakes
The surface low then moves east across Wisconsin Monday on the
European model (ecmwf) and across northern Illinois Monday night on the GFS. Either
way shower and thunderstorm chances will be rather high with a
period of moderate to heavy precipitation values on the models.
Look for the area of rain showers to shift east northeast across taf
sites through this evening. East northeast winds are expected to
weaken tonight into Wednesday...as they become southeast to south.
May see a few showers or thunderstorms on Wednesday...as warm front
pushes north through the area. However...too much uncertainty with
areal coverage to include in tafs at this time.
Ceilings should remain MVFR/IFR category into this evening. Ceilings
and visibilities should lower to around or below alternate minimums
later tonight into Wednesday morning...mainly for fog. Patchy dense
fog is possible...but will not mention in tafs. Lowest values would
be at Milwaukee and Kenosha. VFR conditions should return by
Light winds and a cool moist airmass should allow for areas of fog
to form tonight and linger into Wednesday or Wednesday night. There
could be some dense fog development during this time...and added
patchy dense wording in forecast. Dense fog advisory may be needed
for this period...and will need to be monitored into this evening.
Tonight/Wednesday and aviation/marine...wood
Wednesday night through Tuesday...hentz