Scientific Forecaster Discussion
fxus63 karx 211115
Area forecast discussion
National Weather Service La Crosse WI
615 am CDT Sat Oct 21 2017
Short term...(today through sunday)
issued at 230 am CDT Sat Oct 21 2017
Ripple in the upper level flow spinning out of the Southern Plains
early this morning, and should lift (with a branch of the low level
jet) northeast across Iowa/Minnesota this morning. Expect an expanding area
of showers and thunderstorms as the shortwave feeds on about 500
j/kg of MUCAPE. Most of this activity should skirt just west of the
local area, exiting over northwest WI by noon.
Models still in good agreement on sweeping a north-south running
cold front west-east across the region this evening, roughly
bisecting the local area at 06z. The front leads its parent trough
in, with this feature clearing east Sunday afternoon. The GFS and
NAM still wanting to build about ~500 j/kg or so of MUCAPE. The cape
profile in latest BUFKIT soundings isn't quite as skinny as previous
model runs - perhaps a tad more supportive of updrafts. Still,
overall instability is pretty meager. Shear on the other hand is
ample - especially in the 0-3 km layer (35kts). The deeper 0-6 km
shear is well displaced into the cold air Post the front, but around
20 kts from 0-1km in the instability axis. Warming ahead of the
boundary looks to keep the area capped off from much of the
convective potential until the front moves in this evening - and
starts to interact with the low level jet. Meso models paint this
scenario pretty well. Thanks to the shear, a few storms this evening
could be strong - potentially severe. Wind the main threat.
Lingering showers gradually exit east Sunday, with clearing from
west to east. Cooler air, more typical for the season, also returns.
Long term...(monday through friday)
issued at 230 am CDT Sat Oct 21 2017
"much colder" will be the key phrase for the upcoming week as the
upper level pattern takes on a pronounced northwest/troughing
A couple troughs of note...with the first dropping southwest out of
Canada and through the upper Mississippi River valley Mon/Tue. Cold
air advection, favorable low level lapse rates and the upper level
energy with the wave look to have enough saturation for some shower
production. Even some hints of elevated instability to play with Mon
afternoon. Not enough confidence to add thunder to the forecast at
this time, but can't rule out completely.
The next trough of note takes a similar path but a bit more west,
dropping through the northern/Central Plains before shifting
east/northeast across the Great Lakes this weekend. A sfc cold front
leads the shortwave across the region. The GFS and ec have some
differences in timing/positioning with the main players, but both
suggest at least scattered showers in the Thu-Fri time frame. With
850 mb temps progged to tumble to around -8c by Sat, some of those
showers could/would come in the form of snow. We might finally be
putting the lid on Summer and opening the door wide open to fall.
Break out the Coats, and retire the shorts.
As for those temps, both the ec and GFS keep highs at or below the
seasonable normals for the middle part of the new week, with even
colder air for the weekend.
Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 615 am CDT Sat Oct 21 2017
Mild/moist southerly airflow will push scattered shra/ts across
the area this morning with a break in the action anticipated
after 16z. Ceilings/vis expected to be slipping into MVFR at krst
today with mainly VFR conditions at klse. A cold front will then
push a more organized line of shra/ts across krst in the
22/00-22/03z time frame and across klse in the 22/03-22/06z time
frame. MVFR conditions expected in this line of shra/ts.
Otherwise, breezy south winds today ahead of the cold front,
swinging to the west becoming 10-15 mph with the cold frontal
passage this evening.