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Area forecast discussion 
National Weather Service La Crosse WI
625 am CDT Wednesday Jun 3 2015

Short term...(today and tonight)
issued at 321 am CDT Wednesday Jun 3 2015

Latest water vapor satellite imagery showing upper level trough over
the Pacific northwest and several shortwave troughs embedded in
the southwesterly flow aloft from the Rocky Mountains into the
northern plains. The first shortwave trough located over central
South Dakota and northern Nebraska was producing a line of showers
and thunderstorms across southern South Dakota and eastern Nebraska
per latest mosaic radar.

Main concern will be the convection with the shortwave trough/mesoscale convective vortex
over southern South Dakota and northern Nebraska holding together
and moving into the forecast area today. The 03.00z GFS/NAM/ECMWF
are in decent agreement weakening the shortwave trough and
associated lift/moisture transport and convergence as shortwave
trough pushes east into weak shortwave ridge over the Great Lakes
region today. The 02.00z GFS/NAM/ECMWF indicate focus of moisture
transport/convergence over northern Iowa with shortwave
trough/MCV...and over northern Wisconsin/Lake Superior with
surface low today and tonight. The 02.00z GFS/NAM/ECMWF and latest
hi-resolution models...hrrr/arw/nmm...show convection to weaken
this morning. However...the 03.00z GFS/NAM/ECMWF suggest
redevelopment with mesoscale convective vortex/along weak surface cold front by 18z time
frame and focus over western part of the forecast area. This is
evident from the latest 03.06z hrrr. As the weak cold front/mesoscale convective vortex
tracks across the forecast area...the 03.00z GFS/NAM/ECMWF show
weak moisture convergence/lift to support scattered showers and
thunderstorm development across the forecast area tonight...with
the better lift/nose of low level moisture transport just south
and north of the forecast area. Have concentrated the higher
chances of showers/thunderstorms across the far north and southern
parts of the forecast area.

Severe risk remains nearly non existent across the western forecast
area today...due to limited instability and weak shear...less than
20 knots in the 0-3km layer...over the region.

Long term...(thursday through tuesday)
issued at 321 am CDT Wednesday Jun 3 2015

Thursday into Friday night...the 03.00z GFS/NAM/ECMWF are in
slightly better agreement moving surface front south of forecast
area. However...the models differ on placement of surface front and
timing of shortwave troughs overtopping upper level ridge into the
Great Lakes region. Also...the 03.00z GFS/NAM/ECMWF differ on the
strength of the frontogenetical band of precipitation across
northern Wisconsin Thursday night into Friday morning...as the
03.00z GFS is more robust than the NAM/ECMWF. At this time...the
focus for higher chances of showers and thunderstorms Thursday
into Friday will occur over the southern part of the forecast
area/northeast Iowa and southwest Wisconsin...where the nose of
moisture transport/convergence interacts with surface front.
Showers and thunderstorms will be possible across the northern
sections of the forecast area...depending on the strength of
frontogenetical forcing/lift.

Main forecast concerns Saturday into Tuesday are precipitation
chances through much of the period. The 03.00z GFS/ECMWF/Gem are in
good agreement in digging upper level trough over the Great Lakes
region through the period. Biggest differences between the models is
the strength of the upper level trough and timing of shortwave
troughs into the Great Lakes region and upper Midwest. Strongest
shortwave trough pushes across the Great Lakes region Saturday
night and Sunday and provide the higher chances of showers and
thunderstorms. Weaker shortwave trough moves over the forecast
area and warrants a small chance of showers and
thunderstorms...mainly across the eastern parts of the forecast
area Sunday night into Monday.

&&

Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 625 am CDT Wednesday Jun 3 2015

Challenging forecast as various surface boundaries and upper level
disturbances will have a hand in rain showers/ts production today and
tonight. Its not clear cut when the greatest threats will lie.

That said...latest mesoscale models and radar trends favor pushing the
current area of rain showers/ts from southern Minnesota/Iowa eastward. As it does
though...it will start to run out of a favorable airmass and should
weaken. Could impact krst for a few hours...but likely stay west of
klse.

In the afternoon...instability axis re-establishes itself across the
area...and a surface low and upper level trough should tap into
this...firing another area of shra/ts. Some mesoscale models suggest the
main mass would stay along/north of I-90...and could impact
krst/klse.

Could be a period of MVFR ceilings/visibility with any thunderstorm...but with
confidence lower on when this would be more likely...will not add to
forecast. In addition...some hints that lower/MVFR ceilings will move in
overnight as the surface low moves in. This matches with latest
fog/stratus imagery and surface observation. Some MVFR visibility too. Uncertain that
it would reach krst before 12z so will not add...but the potential
will be monitored.

&&

Arx watches/warnings/advisories...
WI...none.
Minnesota...none.
Iowa...none.
&&

$$

Short term...dtj
long term...dtj
aviation...rieck

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