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Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
1151 am CDT Tuesday may 21 2013
Aviation/18z tafs/...limited convective potential with lack of
forcing and dry air advection in low levels as winds start to
shift SW. Isolated development north of the County Warning Area will continue to
pull away to the NE. This was in agreement with latest hrrr run.
Everything pretty much shuts down with cumulus field filling in from
Iowa and northwest Illinois. Some ceilings are MVFR within this cloud field. More
rain showers and lower ceilings on Wednesday as low develops and moves to
our south and east.
Previous discussion... /issued 537 am CDT Tuesday may 21 2013/
Today and tonight...forecast confidence medium
Areas of stratus will prevail early this morning with some clearing
by middle to late morning. This will aid daytime heating and
instability for late this morning and afternoon. Various model
soundings vary with expected cape values from very little up to 1000
j/kg. This has to do with differences in middle level lapse rates and
dewpoints. Will count on marginal cape perhaps up to 500 j/kg.
Triggers for convection today are weak with a couple weak vorticity
maximums to swing northward in the southerly flow aloft of the upper low. A weak
surface trough may also become present as southerly winds turn more westerly. Will
go with chances of thunderstorms from late am through the afternoon. Low
probabilities of severe weather will be possible given MDT deep layer
shear and middle level dry air.
The upper low will finally begin to move southeastward into western Iowa by 12z
Wednesday. The forward tilt upper trough will become more negative tilt
with saturation and rain possible in far southeast WI by 12z.
Wednesday and Wednesday night...forecast confidence medium.
The stacked low pressure system finally slides into the area on
Wednesday and passes overhead Wednesday night. There is impressive
and rather sustained q-vector convergence through the period with
deep moisture available. This looks like our best chance for a
widespread rainfall. Instability is marginal so will just carry a
chance for storms. No severe weather is expected.
Winds also turn onshore across the southeast...so areas close to
the lake will cool off quite a bit.
Thursday and Thursday night...forecast confidence is high.
Will linger a chance for rain across the southeast Thursday
morning as the remnants of the upper low pull away. Then much
cooler and drier weather overspreads the area on increasing
northeast winds as high pressure builds in. We could see lows
Thursday night dip into the upper 30s in many areas as winds fall
off with the ridge axis moving overhead.
Friday and Friday night...
Cool and dry for Friday with high pressure holding strong.
As the ridge slides off to the east Friday night...the return flow
warm air advection could bring in some late night thunderstorms.
This is a pretty small chance at this point.
Saturday and beyond...forcast confidence is low to medium.
The GFS and the European model (ecmwf) are in disagreement with the pattern this
weekend into early next week. The European model (ecmwf) maintains the ridge over
our area and the Great Lakes through next Tuesday...resulting in
dry weather. Meanwhile the GFS pushes a surface boundary farther
east...acting as a focus for showers and storms through the same
period. A blend would keep any risk for storms mainly over our far
western County Warning Area...and those chances look small.
Aviation/12z tafs/...IFR to MVFR stratus has formed over southern WI and
southwestward into eastern Iowa. This stratus will likely not dissipate until middle
to late morning for kmsn. A much lesser time period of stratus is
expected over the southeast WI taf sites. The stratus should lift into
scattered-broken cumulus congestus of 4-5 kft for the afternoon. A weak wind
shift line will gradually shift winds from southerly to westerly today. There
is a chance of thunderstorms mainly this afternoon as the winds shift.
Drier air will arrive on the westerly winds for tonight but low pressure over
Iowa will bring good chances of rain to far southern WI including kenw
toward sunrise Wednesday am. Areas of light fog could develop late tonight
before the clouds and rain chances move back into the area.
Widespread rain with low ceilings and visibilities is then expected on Wednesday.
Today/tonight and aviation/marine...gehring
Wednesday through Monday...Davis
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