Scientific Forecaster Discussion
fxus63 karx 191851
Area forecast discussion
National Weather Service La Crosse WI
150 PM CDT Mon Aug 19 2019
Short term...(tonight through tuesday)
issued at 150 PM CDT Mon Aug 19 2019
Upper level shortwave trough slides east across the northern plains
this evening with the low level jet/850 mb moisture transport nosing
into the shortwave by 09z as it shifts across northern Iowa.
Convection should take off in earnest as this occurs, and meso
models show just that with rapid development between 06-09z over
northwest IA, dropping southeast into western Illinois toward 12z. Despite
overnight timing, there looks to be some elevated instability to
work on along with favorable shear, moreso in the 0-3 km layer.
Expect some strong to severe storms across Iowa overnight. Current
location would keep the bulk of this threat just southwest of the
local forecast area. Certainly a scenario that bears a close watch,
especially for northeast Iowa.
As the mesoscale convective system exits southeast, some models (esp rap) hang an inverted
trough/piece of upper level energy northwest of mesoscale convective vortex. In those
scenarios, enough low level saturation also hangs back, providing
ingredients for cont shower/isold storm chances across the south
into the afternoon. Don't anticipate any strong/severe threat with
any of this activity. Will hold small pops for now.
Another bit of upper level energy looks to slip east/southeast
across northern Minnesota Tue morning, into northern WI by the evening.
Differences in timing between the variety of models...with latest
hrrr about 12 hours quicker on bringing in convection north of I-94
compared to the 12z nam12. That said, many of the other models only
produce a smattering of showers/storms. Going against the convection
is no/little tap to Gulf moisture (focused south with that exiting
mcs), cloud cover across the south from the mesoscale convective system (how extensive,
thick), and BUFKIT soundings suggest capping that could/will need to
be overcome (either warming or by the front/shortwave). What does
get going will have some instability to work with - rap suggests
upwards of 2500 j/kg of SBCAPE over northern WI by 00z Wed and
potentially +30 kts of effective shear. If that manifests, enough
support for some stronger/isold severe storms. A lot of "ifs" though
for Tue afternoon/evening...and confidence is not high at this time
with how it will playout.
Long term...(wednesday through monday)
issued at 150 PM CDT Mon Aug 19 2019
Models in good agreement with the overall, big picture for the last
half of the week.
Upper level shortwave trough slated to drop southeast across the
Great Lakes Wed/Wed night. Little saturation to work on though -
most of that will be cleared south/southeast with the Tue system. A
sfc high then progged to build in from the west for Thu/Fri while
the axis of an shortwave ridge a loft treks east across the northern
plains. Should see a dry end to the work week, with temps likely a
few degrees below the late Aug normals.
The ridge quickly shifts east for the weekend, allowing for a
shortwave trough to nudge in from the west. Showers/storm chances
will accompany the trough and its associated sfc front. Should also
see a bump up in temps as winds turn more southerly.
Aviation...(for the 18z tafs through 18z Tuesday afternoon)
issued at 1231 PM CDT Mon Aug 19 2019
VFR conditions expected into the overnight hours with clear skies
and light winds. Fog is possible in the river valleys, but
confidence is somewhat low as debris cirrus from ongoing storms to
the south could hinder its development, along with increasing winds
just above the surface toward the early morning hours. Looks like
storms should stay well south of the taf sites, but there are some
hints of an associated MVFR cloud deck sneaking into rst by mid-
morning. For now, keeping it at bkn035, but will need to monitor
trends in future issuances.