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Area forecast discussion 
National Weather Service Milwaukee/Sullivan WI
938 PM CDT Monday may 4 2015

Update...vorticity maximum over western Iowa and associated middle level
thetae advection...frontogenesis...and resultant showers/thunderstorms
will gradually move into southern WI late tonight and through Tuesday am. The
vorticity maximum will move eastward by early afternoon with a gradually
dissipation of showers during the afternoon.


Aviation/06z tafs/...mainly VFR light to MDT showers during the
early Tuesday am hours but ceilings will lower to 1.0-2.5 kft by the
middle to late morning along with areas of MVFR visibilities. The showers
will be slow to dissipate during Tuesday after but low ceilings and visibilities
will remain.


Previous discussion... /issued 247 PM CDT Monday may 4 2015/

Short term...

Tonight and Tuesday...forecast confidence...high

A shortwave moving through a stalled frontal zone will bring more
rounds of showers and a few storms tonight into Tuesday. Models are
overall coming into better agreement...suggesting the rain will hold
off in the southwest until as late as midnight...then spread across
the southern two-thirds of the forecast area overnight. There is a
good chance it will remain mostly dry up towards Fond Du Lac and

There has been a trend in the latest models to hold onto some light
rain chances into Tuesday afternoon across much of the forecast have leaned forecast in that direction.

With southern Wisconsin stuck on the cooler and more stable side of
the frontal zone...not much in the way of instability through
tomorrow. Thus went no higher than just a chance for thunder.

Clouds will help to keep temperatures a few degrees above normal values
tonight. The east winds...rain chances...and clouds will result in
below normal highs Tuesday...most noticeably near Lake Michigan
where it will struggle to around 50.

Tuesday night and Wednesday...forecast confidence is high.

An upper ridge builds into Wisconsin from the Ohio River valley
as a shortwave trough pushes into the Central Plains. Very weak
upper level divergence. The 850/700 mb layer is saturated north
areas and lifts north late Tuesday night along and ahead of a
weak 850 mb warm front. The 850/700 mb levels dry a little
Wednesday. The 925 mb relative humidity follows the same trend...but lags around
6 hours behind.

Low pressure expands across the plains with an increasing south
flow across Wisconsin...mainly west of the Mississippi River. This
lifts the surface boundary north...but with high pressure over the
Great Lakes...and southeast winds...the warm front will be
weakening and hard to follow at the surface. Zero to 1 km mixed
layer NAM cape rises to around 1000 joules/kg from Madison
west...and only around 70 joules/kg towards Sheboygan.

The NAM and 00z European model (ecmwf) still generate moderate precipitation
values Tuesday night...but the GFS is dry. Then with the lack of
low level wind convergence and weak upper forcing...the models
only have areas of scattered mainly light amounts of quantitative precipitation forecast. The NAM
forecast soundings have only a weak cap around 700 mb.

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 southern Minnesota Thursday afternoon. Only weak 700 mb
upward motion Thursday. A band of higher 700 mb relative humidity pushes into
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
Minnesota. The zero to 1 km mixed layer cape vales increase to
around 1000 joules/kg mainly from Madison west.

The models are mainly precipitation free Wednesday night and Thursday.
Southern Wisconsin is in a warm and increasingly moist airmass.
However a lake breeze may keep the shoreline areas cooler.

Long term...

Thursday night and Friday...forecast confidence is high.

The weakening shortwave near Western Lake Superior Thursday night
pushes off into southern Canada Friday. The GFS has another very
weak shortwave pushing into southern Wisconsin Friday.

As low pressure exiting northern Minnesota pushes to north of
Lake Superior...a trailing cold front enters southern Wisconsin
Friday. Both the GFS and European model (ecmwf) bring light to moderate
precipitation into the area.

Saturday and Sunday...forecast confidence is high.

The GFS and European model (ecmwf) both bring a strong middle level trough to the
central rockies area Saturday...that moves into the northern
and Central Plains Sunday.

The cold front exits southeast Wisconsin Friday night. A large
area of high pressure moves across southern Canada...keeping the
surface boundary across central Illinois and the upper Ohio

This will keep most of the shower and thunderstorm chances just
south of the Illinois border Saturday and Sunday and west of the
area on Sunday. However there will be small chances of
precipitation due to the proximity of the surface boundary to the
south...and the developing low over the plains to the west.

Monday...forecast confidence is medium.

The rather strong middle level shortwave pushes into the upper
Mississippi Valley.

A rather strong low then exits the plains toward southern
Minnesota. The 12z European model (ecmwf) puts much of southern Wisconsin in the
warm sector with the surface low moving into northwest Wisconsin.
However the GFS puts the low over southern Wisconsin with most of
southern Wisconsin missing the warm sector. Either way shower and
thunderstorm chances will be rather high with moderate to heavy
precipitation values on the models.

Aviation/00z tafs/...

A shortwave moving through a stalled frontal zone will bring more
rounds of showers and a few storms tonight into Tuesday. Best chance
will be after midnight through Tuesday morning...mainly across the
southern two-thirds of the forecast area.

Will likely see lower ceilings redevelop later tonight into Tuesday with
southern Wisconsin stuck on the cooler side of the frontal boundary.
Best chance for lower ceilings will be across the southern half of the
forecast area. Visibilities will probably drop to MVFR for a time in any
moderate rainfall.


Mkx watches/warnings/advisories...


tonight/Tuesday and aviation/marine...ddv
Tuesday night through Monday...hentz

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