Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
350 am CDT Sat may 23 2015
Today and tonight...forecast confidence is high.
An upper level ridge builds across the upper Mississippi Valley as a
jet maximum increases in the northwest flow over Wisconsin...lifting to
the northern Great Lakes late tonight. Generally weak upper level
divergence across southern Wisconsin. An narrow elongated area of
cyclonic vorticity lifts north trough southern Wisconsin today middle
level relative humidity increases today and tonight with bands of weak upward
motion moving in from the southwest. The 850/700 mb winds are
fairly weak as the lower and middle level ridge extends across the
forecast area. The developing south 850 mb flow begins to push
toward south central Wisconsin by sunrise Sunday but the 850/700 mb
moisture convergence stays southwest. The NAM/GFS and European model (ecmwf) are all
dry through tonight. Low chance of showers late tonight with the
approaching increase in 850/700 mb wind maxima...and some mainly middle
level virga possible with the increasing middle level moisture today.
With the weak surface ridge...a lake breeze will cool things off
near Lake Michigan in the afternoon.
Sunday... forecast confidence medium.
The low level jet strengthens over the Midwest and points into
southern WI Sunday through Monday while several rounds of middle level
shortwaves move across WI. There is a slowing trend with the precipitation
arriving into southern WI... so reduced probability of precipitation for the first part of
Sunday morning. As is the case with any shortwave and warm air
advection- induced precipitation... timing of The Rounds of precipitation is
The first wave of showers and a few embedded thunderstorms should
lift through southern WI late Sunday morning with a 500mb shortwave
and 850mb-700mb frontogenesis on the nose of a low level jet. The
moist profile with high precipitable water values will lead to
limited instability. The NAM is driest with this feature and keeps
southeast WI mostly dry.
Another shortwave is expected to lift through the area either late
Sunday afternoon or Sunday evening. The lowest chance for precipitation on
Sunday will be in southeast WI. Overall... the highest chances will
be west and north of Madison.
Clouds and precipitation will keep temperatures moderated in the lower 70s
inland. Increasing southeast winds will lead to cooler Lakeshore
By late Sunday night... the strongest axis of the low level jet will point
into northeast Iowa and southeast Minnesota. This should cause a break in
precipitation for southeast WI... so I reduced probability of precipitation down to chance there.
Meanwhile... a low level trough axis will track from northeast Iowa
and southwest WI into south central WI late Sunday night. Kept
likely probability of precipitation for this area.
Memorial Day... forecast confidence medium.
That trough axis will continue to track through southeast WI Monday
morning. Instability is still weak with this feature... thus
kept the likely rain showers with a chance for thunderstorms wording.
Models are coming into great agreement that the dry slot will wrap
into southern WI Monday afternoon. With the warm sector over the
area... instability increases significantly... especially with the
NAM model. Bulk shear becomes high as well with another push of the
low level jet into the region. The cwasp values on the NAM are
around 85 and lower 70s for the GFS. Storm Prediction Center also noted the strong
instability and added most of Wisconsin to the marginal... 5
percent... risk for severe weather. The ingredient lacking Monday
afternoon in southern WI will be a trigger mechanism to get
thunderstorms. We will be capped around 750-800mb during the daytime
hours... but a surface trough tracking into south central WI early
Monday evening may be enough to induce a few thunderstorms that have
the potential to be strong. We will continue to keep an eye on this
The dry warm sector over southern WI Monday afternoon will allow for
some sunshine and highs in the upper 70s to around 80 inland. It
should turn out to be a very nice Memorial Day afternoon.
Tuesday through Friday... forecast confidence medium.
The main surface trough from that closed upper low over the upper
Midwest will finally slide through southeast WI Tuesday. The trough
may stall and lead to lingering shower chances Tuesday night into
Wednesday... although Wednesday is looking drier with the latest
Models differ with timing of short wave energy in a southwesterly
flow regime...leading to a chance for showers and slight chance for
thunderstorms Thursday into the weekend. Next weekend may end up
being a wet one again.
Temperatures look to remain at or above seasonal norms through the
VFR conditions expected with a ridge of high pressure over the Great
Lakes. Increasing middle level clouds expected. Some mainly virga
Increasing south winds are expected to occur on Memorial Day...as a
tight pressure gradient develops with low pressure over Minnesota. A
Small Craft Advisory may be needed for later Sunday night
and Memorial Day.
Today/tonight and aviation/marine...mrc
Sunday through Friday...hentz