Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
315 PM CDT Sat Sep 20 2014
Tonight and Sunday...forecast confidence - high.
Models increase the upper divergence this afternoon due to the
entrance region of the departing pre-trough axis upper jet. Then
most models have an evening lull before a strong middle level
shortwave 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 are
weakening this afternoon with little if any convergence. The high
res arw model shows mainly lighter showers. Cape is not as strong
as the NAM so prefer the high-res arw's weaker convection.
Thunderstorm chances end by early evening as the upper level trough
axis slides off to the east. Focus shifts northwest where a strong
middle-level short wave moves across southern Wisconsin from late
evening through the early morning. Middle-level lapse rates increase
to 6-8c/km with this feature along with an increase in 700 mb upward
Therefore expect another band of showers to move through late
evening and early morning.
The second upper level trough axis moves across southern Wisconsin
late morning to just after noon along with another rather strong middle
level shortwave. By this time 850/700 mb cold air advection is
occurring...but there is some weak upward motion and NAM has around
240 joules/kg of cape. Still a lot of low level moisture...and some
showers are possible. Still enough lingering moisture around 4 thousand
feet for stratocumulus into the afternoon before drying occurs from
the north late.
Short term...Sunday night through Tuesday.
Sunday night...forecast confidence high.
Dry for the start of the period as forcing is south of County Warning Area with
potent short wave...and broad 1025 mb surface high builds into the
region. Skies clear sufficiently and winds calm for patchy...to
areas of fog over south central WI...with little to no fog in the
east with winds holding up a bit higher. Lows dip into the upper 30s
to around 40 in the east away from the lake...with low 40s west.
Monday/Monday night...forecast confidence high.
Coolest highs will be in the east...in spite of westerly winds...as
models brush the far east with the western edge of the 925 mb cold
pool Monday morning...though the coolest 850 mb temperatures remain well
east. 925 mb temperatures yield low to middle 60s eastern third...with upper
60s elsewhere. Light winds and mostly clear skies bring patchy fog
back into the picture Monday night.
Tuesday/Tuesday night...forecast confidence high.
Temperatures begin to warm in developing southerly flow between the
expansive high to the east and a Lee trough over the plains. 925 mb
temperatures rise to around 15c...bringing low 70 highs...though onshore
winds in the afternoon hold highs to the middle 60s.
Long term...Wednesday through Saturday. Forecast confidence
Models having trouble resolving evolution of weakening plains trough
that brings a small chance for rain showers to the northwest County Warning Area as it
lifts through Minnesota Wednesday night. GFS continues to drop a
compact short wave into the remnants of the original
trough...closing it off as it retrogrades back toward the Central
Plains. However...the Canadian Gem and European model (ecmwf) are now showing some
similar variation of the same theme. In spite of the Gem and GFS
spinning various vorticity maxima over the region through the day
Thursday...they develop no quantitative precipitation forecast with the dry southeasterly flow
around the high. Will keep rest of extended dry...with above normal
highs...and at or above normal lows for the end of the week.
Cumulus/stratocumulus around 35 hundred feet ahead of a weak trough.
Showers and still a chance of a thunderstorm 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 that sweeps in from the northwest.
MVFR ceilings expected behind this trough late tonight and into Sunday
morning. Then yet another middle level shortwave could bring some
showers through around noon. The MVFR ceilings will gradually raise to
VFR as drier air moves in from the north later Sunday afternoon.
Gusty southwest winds will continue to diminish late this
afternoon as a trough approaches.
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 Great Lakes in the morning. Small craft
advisories may be needed
Tonight/Sunday and aviation/marine...hentz
Sunday night through Saturday...rem