Scientific Forecaster Discussion
fxus63 karx 261136
Area forecast discussion
National Weather Service La Crosse WI
636 am CDT Mon Jun 26 2017
Short term...(today and tonight)
issued at 356 am CDT Mon Jun 26 2017
At 3 am, the water vapor showed a short wave trough moving
southeast across Iowa. With the best lift and moisture transport
remaining southwest of our area, expect that northeast Iowa will
stay dry this morning.
Another trough located over the Minnesota arrowhead will move
southeast across Wisconsin late this morning and afternoon. The
latest rap has it 0-1 km mean layer convective available potential energy generally less than
250 j/kg with weak shear. Soundings suggest that the moisture is
shallow between 750 and 650 mb, so thinking that the showers and
thunderstorms should remain isolated across western Wisconsin.
With the soundings continuing to show an inverted v in the sub
cloud layer and very steep 900 to 700 mb lapse rates, the stronger
cells will produce gusty winds up to 35 mph.
In the wake of this trough, another one will be moving through
southeast through Wisconsin tonight. With the loss of diurnal
heating, there will be even less instability than its predecessor,
thus, quickly drop off the precipitation chances this evening. At
this time only north-central Wisconsin has any mention of
isolated showers this evening.
Long term...(tuesday through sunday)
issued at 356 am CDT Mon Jun 26 2017
On Tuesday, high pressure will be overhead. This will high will
produce sunny skies and allow US to mix up to 800 mb. This would
result in high temperatures ranging from the mid 70s to lower 80s.
Due to this, bumped up the the temperatures up a couple of degrees
over forecast guidance.
On Tuesday night, the best moisture transport and instability
remains west of the forecast area, so lowered the rain chances
between 1 am and 7 am Wednesday. In addition with the dew points
coming in later, lowered the minimum temperatures in central and
On Wednesday morning, the strong moisture transport moves into the
upper Mississippi River valley. As a result, expect a rapid
increase in the areal coverage of showers and storms across the
area. While the surface convective available potential energy remains low, the most unstable convective available potential energy
will climb up to 1000 j/kg ahead of the warm front. The deep shear
is by far the greatest in the NAM (up to 60 knots). However, if
you eliminate the winds in the lowest 1 km, this shear drops off
to less than 30 knots, so not anticipating any severe weather
during this time frame.
On Wednesday afternoon and evening, the Storm Prediction Center
has much of the area under a slight risk of severe storms.
Northeast Iowa is even highlighted for a potential of significant
severe weather. The NAM has by far the greatest instability and
shear across the area. With dew points climbing into the mid 70s
ahead of an approaching cold front, it has the 0-1 km mean layer
convective available potential energy climbing up to 2 to 4k j/kg. In addition, its 0-6 km shear
climbs into the 45 to 50 knot range. In this scenario, there
would be potential supercell development which would likely
develop into a line as their cold pools coalesce.
Meanwhile, the European model (ecmwf) has similar deep shear, but its instability
remains across western Iowa, southeast Nebraska, and Kansas. In
this scenario, the severe weather would remain to our southwest
and south. The GFS solution is between these two and it would have
its threat of severe weather across eastern Iowa, southwest
Wisconsin, and northern Illinois.
On Thursday, it looks like the precipitation will be south of
the area and there should be more sunshine than previously
thought. Due to this, bumped up the high temperatures a few
From Friday into the weekend, a series of troughs will move
through the region. While there is good agreement for Friday and
Saturday that there will be precipitation, Sunday looks more
questionable and may end up being dry. Confidence was not high
enough to pull the precipitation chances, so did not make any any
changes at this time.
Aviation...(for the 12z tafs through 12z Tuesday morning)
issued at 636 am CDT Mon Jun 26 2017
Scattered showers/isolated thunderstorms are possible this
afternoon, but bulk of activity is expected to be north and east
of taf airfields, so will maintain a dry forecast. Could see some
afternoon thermal cumulus in the 4000 to 5000 ft agl layer.
Otherwise, mostly sunny/clear skies will be the rule. Light
northwest wind this morning and afternoon will become variable or
light from the west this evening and overnight as high pressure
moves into the region. Still a small chance for some valley fog at
klse Tuesday morning, but latest forecast guidance does not
saturate the near surface layer, so will keep it out for now and
monitor trends through the period.
Hydrology...Wednesday and Wednesday evening
issued at 357 am CDT Mon Jun 26 2017
On Wednesday and Wednesday evening, the precipitable water values
will climb into the 1.5 to 1.8 inch range and warm cloud depths
climb into the 3.5 to 4 km range. This could potentially result
in the showers and storms becoming very efficient rain producers.
There remains a potential for 1 to locally 3 inches of rain
falling during this time period. If the heavier rain would fall in
a short period of time or on one of the wetter parts of the
landscape, some flooding or flash flooding could occur.