Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
331 PM CDT Tuesday Jul 29 2014
Tonight...forecast confidence medium.
A middle level shortwave rotating around a stationary closed upper low
centered over James Bay is over northern WI at 3 PM this afternoon
and will continue to make its way southward through southern WI
early this evening. Showers and thunderstorms are developing in
response to this shortwave. Latest rap soundings are now showing
only around 200 j/kg of cape to work with...along with weak shear
and moderate Omega around 7 PM at Madison.
The hrrr and WRF-arw models agree that is the peak time of
convection for our area. Otherwise...there will be some scattered
showers and a few thunderstorms from now through late evening. There
could be some small hail out of the stronger cells and we will have
to watch for the one or two cells that might produce 1 inch hail
this evening. Gusty winds up to around 35 miles per hour will be possible with
these showers as well.
Shower activity will dwindle after midnight in the wake of the
shortwave. Lows are expected in the middle 50s. Light winds and
rainfall could lead to some patchy light fog tonight.
Wednesday...forecast confidence medium.
The upper low will remain stationary and another shortwave is
expected to drop down through the state once again in the afternoon.
This one looks a little drier with less instability...so I am
expecting any pop-up showers to be more scattered in nature.
Thus...kept lower probability of precipitation instead of raising them to likely.
Maximum temperatures should be similar to those today...in the upper 70s inland
from the lake. The overnight shortwave will usher in more northerly
winds Wednesday morning so a lake breeze is a sure bet. Inland areas
will keep the northwesterly winds through the day.
Short term...Wednesday night through Friday night.
Wednesday night...forecast confidence high
Region caught between short wave riding 250 mb speed maximum that passes
to the northeast and east as it rotates around nearly-stationary
upper low wobbling near James Bay ... sheared vorticity on the
western periphery of middle-upper level cyclonic flow through the
Dakotas and Iowa. Best forcing and instability stay close to eastern
short wave...but some Omega moves across northeast half of County Warning Area Wednesday
evening. Also some brief forcing for ascent with weak upper level
divergence with right entrance region of the jet maximum passing
through....though NAM is earlier than GFS with this feature.
Will keep slight chance probability of precipitation in the early evening in all but the far
SW...and linger between 03z and 06z in the NE half of County Warning Area.
Decoupling with diminishing cloud cover after 06z will allow lows to
cool into the middle-upper 50s.
Thursday and Thursday night...forecast confidence medium
Yet another short wave drops through mainly the northeast portion of
the state...but some weak 850 mb warm air advection/convergence
crosses southern WI ahead of the wave Thursday afternoon...under cooling
500 mb temperatures. Resulting forcing and instability during peak
heating brings another round of showers and possible
thunderstorms...which will be more likely if more moist surface dew
points on NAM pan out with surface based cape rising to near 900 j/kg.
Timing of clouds and precipitation will affect high temperatures but given
925 mb temperatures of 20c to 21c west and 19c to 20c east and westerly
gradient wind...even though under 10 knots...strong enough to hold
off lake breeze until middle-late afternoon should see highs climb into
the middle to upper 70s east and upper 70s to lower 80s west.
Will carry some chance probability of precipitation into the evening due to model
differences for position of weak surface trough with the GFS and
European model (ecmwf) laying it over the northern County Warning Area with some surface convergence then
slowly wash it out overnight.
Friday and Friday night...forecast confidence medium
More potent short wave trough moves through WI Friday
afternoon/evening...giving the best chance for widespread showers
and thunderstorms for the week. While all of southern Wisconsin should
see some precipitation...the models are focusing the strongest
forcing over the eastern half of the County Warning Area so will keep likely probability of precipitation
going there. Secondary short wave...of varying strength among the
models...moves through around/after midnight so will go with slight
chance/chance probability of precipitation for the entire night.
Long term...Saturday through Tuesday...forecast confidence medium
Upper trough axis slowly pushes east from the vicinity of Lake
Michigan...but short wave energy still affecting southern WI
Saturday...either in the form of a lingering broad circulation as
depicted on the GFS and Gem...or a trailing weaker short wave in the
European model (ecmwf). Will keep slight chance/chance probability of precipitation on Saturday with the
higher probability of precipitation over the southeast. Still under cyclonic flow into
Saturday evening but drier air moving in with high pressure short
wave ridging building in for overnight into Sunday. Precipitation
chances begin to rise with trough digging into the eastern Great
Lakes as upper flow buckles in response to re-developing western
A VFR period is expected. Look for isolated to scattered showers and
thunderstorms over southern WI from late afternoon through middle to
late evening. Gusty winds up to 35 miles per hour and briefly reduced visibilities and
ceilings would accompany any convection. Small hail is a possibility as
The southeast winds in the nearshore will veer back to the west
tonight. The winds will once again become onshore on Wednesday as a
weak front sags south across the lake.
Tonight/Wednesday and aviation/marine...mrc
Wednesday night through Tuesday...rem