Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
1104 am CDT Sat Sep 20 2014
Still a lot of uncertainty on convective development today. The
earlier line of showers has diminished...and the second area is
approaching. Models tend to increase this as upper divergence
increases due to the entrance region of the departing pre trough
axis upper jet. Then most models have an evening lull before the
second stronger middle level trough approaches late this evening.
The NAM forecast soundings continue to show parameters supporting
a slight risk of severe. However 850 mb winds of 30 knots weaken
this afternoon with little if any convergence. As a result will
continue a small to medium chance of thunderstorms this
afternoon...with a early evening lull.
The fist line of showers have diminished. VFR conditions with
mainly high level clouds late this morning. Then cumulus should
develop. Still a chance of thunderstorms this afternoon. Then a
lull early this evening before scattered showers and possible
thunderstorms returns late this evening ahead of a rather strong
middle level short wave trough sweeps in from the northwest.
Previous discussion... /issued 329 am CDT Sat Sep 20 2014/
Today...forecast confidence is low.
Line of convection continues to weaken but has advanced into
northwest County Warning Area early this morning. With upper level jet weakening and
shifting off to the east...expect this trend to continue. However
can not rule out more scattered convection spreading eastward across
remainder of County Warning Area through middle-morning as possible mesoscale eddy/middle
level wave spreads eastward across southern WI/northern Illinois area.
Also...upper level divergence associated with right entrance region
strengthens over southern WI this morning. Moisture in place with precipitable water
values between 1 and 1.5 inches. However not enough confidence to
go with higher than chance/scattered probability of precipitation at this point after 12z. Most of
the mesoscale guidance shows this line of convection continuing to
diminish early this morning with little to no activity the rest of
the morning. Also...better low level forcing refocuses off to the
Quite a bit of clouds this morning limiting instability. Will
continue chance probability of precipitation this afternoon with likely in the far southeast as
stronger middle-level short wave slides across southern WI. Moderate to
strong bulk shear of 35 to 45 knots. Clouds should thin by the
early afternoon so middle 60 dewpoints may result in MUCAPES exceeding 1500
j/kg this afternoon.
Tonight...forecast confidence is medium.
Convection threat diminishes by early evening as upper level wave
and weak surface front slides off to the east. Focus shifts northwest
where a vigorous middle-level short wave with strong middle-level
potential vorticity slides across southern WI from late evening through
the early morning. Middle-level lapse rates increase to 6-7c over
northern County Warning Area with this feature.
In addition...a moderate low to middle-level frontogenesis band of
enhanced forcing will sweep across southern WI. Hence will bump up probability of precipitation to
likely in the far north trailing off to schc in the far southwest.
Short term...Sunday through Monday night...forecast confidence is
Models continue to show potent 500 mb vorticity maximum sliding
southeast through the area Sunday morning...then exits to the
south Sunday afternoon. The GFS was a bit slower than the
NAM/ECMWF/Canadian models with the passage of this feature.
Area forecast soundings from NAM/GFS not showing a lot of
moisture Sunday morning...but enough to support chances for
showers. Despite some uncertainty here with areal coverage of the
showers...raised probability of precipitation a bit Sunday morning.
Held onto low probability of precipitation Sunday afternoon...as daytime heating allows
for weak instability despite exiting 500 mb vorticity maximum.
Highs Sunday look to be in the lower to middle 60s.
A dry period then is expected Sunday night into Monday night
across the area. Models are showing surface high pressure sliding
east southeast through the region. This will continue to bring
cool temperatures to the area as well...as 925 mb temperatures are
slow to warm during this time.
Long term...Tuesday through Friday...forecast confidence is
GFS/European model (ecmwf) are in decent agreement with the high slowly moving off
to the east of the region Tuesday into Wednesday. The influence of
the high should keep the area dry. Temperatures should warm to
near seasonal normals.
GFS/European model (ecmwf) then have 500 mb shortwave trough stalling and
dissipating over Minnesota Wednesday night into Thursday...as
downstream ridge over the western Great Lakes region lingers.
Kept probability of precipitation going for Wednesday night...though latest models keep
the area dry. Later forecasts may consider removing probability of precipitation for
Wednesday night if later runs show 500 mb shortwave not reaching
The high well east of the region may continue the quiet weather
conditions into the end of the week. Temperatures should remain
near seasonal normals.
May be some MVFR ceilings associated with any morning convection that
spreads in from the west. Otherwise VFR conditions will return later
today with a chance for afternoon convection diminishing from west to
east as main forcing shifts off to the east. Scattered -shra and
possible T returns this evening ahead of vigorous short wave
sweeping in from the northwest.
Gusty south to southwest winds will continue today with potential
gusts to 25 knots. Hence will continue Small Craft Advisory and extend
to 00z. Gusty west to north winds are expected to develop late
tonight and Sunday behind a strong cold front and low pressure
that sweeps across the western gtlakes in the morning.
Lm...Small Craft Advisory until 7 PM CDT this evening for lmz643>646.
today/tonight and aviation/marine...mbk
Sunday through Friday...wood