Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
319 PM CDT Friday Aug 29 2014
Tonight - confidence...medium
very moist flow in place ahead of middle level trough approaching from
the plains. On water vapor seeing some moisture flowing northward into
this system from the remnants of Marie in the Gulf Coast vicinity southeast Texas.
Airmass continues to destabilize across southern WI with ml cape values
around 1500 j/kg. Precipitable water values are 1.8 to 2.0 with some
borderline shear parameters...though best shear is a bit northwest
of the County Warning Area. Middle level lapse rates not supportive of hail though
downdraft cape pushing 1000 j/kg. Expecting convection to increase
into this evening within DCVA zone of advancing trough axis. Will
boost probability of precipitation from west to east as the better dynamics approach and
move into southern WI. Models have highest quantitative precipitation forecast across western WI through 6z
and then weaken convection a bit through 12z as airmass stabilizes
somewhat. Still expecting main threat to be heavy rainfall...with
tall skinny cape...the high precipitable water/S and slow mbe velocities. Appears
urban/small stream flooding though isolated flash flood potential
exists especially in northwest County Warning Area where dynamics and better instability likely to
coincide and closer to proggd quantitative precipitation forecast maximum.
Saturday - confidence...medium
weak surface low shifts northeast of WI. Southern WI will be situated in
low level trough axis. First vorticity passes early with a secondary shortwave
coming in during the afternoon. Will maintain chance of rain showers/thunderstorms and rain in the
afternoon. Any heating with breaks will allow airmass to destabilize
enough to kick off additional convection with shortwave and low level
Saturday night and Sunday...forecast confidence is high
A secondary weaker exits southeast Wisconsin Saturday evening.
Upper divergence across southern Wisconsin is mainly neutral.
The 700 mb relative humidity increases over much of the forecast area Sunday
afternoon as the middle level dew points rise. The NAM/GFS and European model (ecmwf)
are all dry.
Sunday night and Monday...forecast confidence is high
The GFS and European model (ecmwf) takes a rather strong shortwave across
Wisconsin during this period. The GFS/European model (ecmwf) and dgex bring a band
of moderate precipitation amounts into south central Wisconsin
later Sunday night...and across the southeast Sunday morning.
The local severe weather parameter is around 70 to 75
percent...despite the late night/Monday morning timing. However the
nsharp forecast soundings indicate low or only marginal severe
Monday night and Tuesday...forecast confidence is high
The GFS and European model (ecmwf) takes a second weak shortwave across Wisconsin
Monday night and Tuesday. Only the European model (ecmwf) has some scattered light
Wednesday through Friday...forecast confidence is high
The GFS and European model (ecmwf) have a zonal flow across the U.S./Canadian
border area as another trough digs a little across the northwest
U.S. With the upper flow becoming a little more west/southwestthursday.
The models take a weak shortwave or possibly just convective
feedback in the zonal flow across Wisconsin Friday.
Weak high pressure moves across the Great Lakes Wednesday with a
warm front and precipitation moving into southern Wisconsin
Wednesday night and Thursday with moderate amounts of
precipitation...the front then stalls across southern Wisconsin
and sags back south across Illinois on Friday. The European model (ecmwf) has
precipitation lingering into Friday...while the front is farther
south on the GFS with high pressure building into the northern
Expect convection to become more widespread tonight into at least
Saturday morning as middle level trough axis and surface low move
into WI. With enough clearing in the low cloud cover...additional
thunderstorms and rain may develop with surface features and approach of secondary
middle level shortwave. Models suggest stratus potential later
tonight into Saturday with moisture pooling in vicinity of low
level boundaries. Expect some mixing of this as the day wears on.
Tonight/Saturday and aviation/marine...collar
Saturday night through Friday...hentz