Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Milwaukee/Sullivan WI
1013 am CDT Sat Jul 26 2014
Watching area of showers and storms upstream as a weak shortwave
and vorticity maximum approach. Latest rap and hrrr suggest probability of precipitation may need
to be bumped up a bit across the southern half of the forecast
area this afternoon. It does not look like a ton of rain...but
some locations may see at least enough to wet the ground.
There is a nice pocket of clearing across the southeast forecast
area right now. There are also a lot of clouds up stream...so
looks like sun may be limited in the afternoon. If this pans
out...high temperatures will probably need to be lowered a bit.
Watching area of showers and storms upstream as a weak
shortwave and vorticity maximum approach. It does not look like a ton of
rain...but some locations may see at least enough to wet the
ground this afternoon. Will watch upstream conditions and update
tafs as confidence increases.
Latest rap and NAM soundings suggest enough mixing today for VFR
conditions to return most places by afternoon. Should see mainly
VFR conditions then through Sunday. Looks like a decent chance for
showers and storms Sunday as an upper level trough and surface
front move through.
Previous discussion... /issued 322 am CDT Sat Jul 26 2014/
Today and tonight...forecast confidence is high.
Expecting a quiet...warm and humid day today. Warm advection with
a departing shortwave is bringing some showers to northern Illinois
early this morning and some sprinkles across parts of southern WI.
This should mostly be out of the area by or shortly after sunrise.
Models then show a weak middle level ripple coming across the area
this afternoon...on the southern periphery of the impressive
closed system dropping south out of scntrl Canada. However...there
is a formidable dry layer from below 850mb through about 650mb
that will make it hard to precipitate through.
Additionally...there is a rather strong low level cap that will
work to prevent any development. Therefore...will keep it dry
today. Therefore...will keep it dry today. Lower clouds around
this morning...associated with the exiting short wave...should
gradually clear out with sunshine expected. Highs should easily
hit the lower to middle 80s.
The large Canadian system will push into northwest WI by 12z sun
with surface trofing pushing through the area as well. The column
still looks quite dry out ahead of the upper system and we/ll
probably have to wait until Sunday for better saturation and
instability to bring precipitation with the cooling temperatures aloft. Will
intro a small pop across the north after midnight tonight for the
small threat there.
Short term...Sunday through Monday night...forecast confidence is
Models push surface trough/cold front southeast through the rest
of the area during the morning hours...with a secondary trough
moving south into the area by 00z Monday. Strong 500 mb shortwave
trough also slides southeast through the area on Sunday.
Area forecast soundings from the NAM showing a good amount of
mean layer cape and deep layer shear until around 15z Sunday in
the southeast portions of the area...which would bring a hail/wind
risk. GFS soundings show much less instability in the morning in
this area...so think NAM is likely overdone with the instability.
Both models do bring some rather weak tall/skinny cape to the
area during the afternoon. Kept likely probability of precipitation in the afternoon in
most of the area...with rain showers likely and chance of thunder.
Kept lower end probability of precipitation in the evening in the east...until cooler and
drier air moving into the region ends the precipitation. Highs
Sunday should reach the upper 70s to lower 80s...then fall into
the evening with cold air advection.
500 mb trough remains over southeast Canada and into the
northeast and middle Atlantic region. This will keep 500 mb flow
northwesterly over the region...with weak vorticity maxima moving
through at times. High pressure will build into the northern
plains Sunday night into Monday night...with cold air advection
bringing cooler and less humid air into the area.
NAM only model showing quantitative precipitation forecast Monday afternoon/night with showers.
Kept Monday dry as other models keep dry conditions. Temperatures
will reach the lower to middle 70s Monday...with lows mainly in the
50s Sunday night and Monday night.
Long term...Tuesday through Friday...forecast confidence is
European model (ecmwf)/GFS generally show northwest flow at 500 mb remaining over
the region for this period...with weak vorticity maxima moving
through the area at times. Surface high pressure slides slowly
southeast through the Central Plains into the middle Mississippi
River valley area. The models generate quantitative precipitation forecast during each afternoon
into the early evening hours.
GFS forecast soundings show weak instability Tuesday
afternoon...with better mean layer convective available potential energy in the afternoon hours
for later in the week. For now...continued to have lower end probability of precipitation
each day and night for most of this period. Temperatures look to
be slightly below seasonal normals each day.
Look for an area of MVFR clouds this morning...especially from
kmsn on west. These clouds are associated with a weak trough and
warm advection moving through and should begin to improve from
west to east shortly after 12z this morning. Then look for VFR
conditions through the remainder of the taf period.
The chances of shower or storms look very small until we get
closer to the influence of a large upper level closed low dropping
in around 12z Sunday morning. So...generally a very quiet taf
period with light winds less than 10 knots.
Could see Small Craft Advisory conditions develop later Sunday
into Monday...as northwest winds increase in the wake of passing
today/tonight and aviation/marine...Davis
Sunday through Friday...wood