Scientific Forecaster Discussion
fxus63 karx 221908
Area forecast discussion
National Weather Service La Crosse WI
200 PM CDT sun Sep 22 2019
Short term...(tonight through tuesday)
issued at 205 PM CDT sun Sep 22 2019
Showers with a isolated storms will continue into the evening hours.
Meso models still suggesting the potential for moderate to heavy
rainfall through 00-02z across northeast Iowa/southwest WI. While
amounts up to now have not been excessive, any heavy rain in these
areas (grounds still saturated from previous rains) have the
potential to result in flooding. Will let Flash Flood Watch continue
for now, but it might be cancelled early after anticipated pcpn
exits this evening.
Shortwave ridging aloft with a sfc high to the south promises a dry
start to the new work week, with these conditions continuing into
the better part of Tue. Relatively mild temps will also make a
Tue evening looks to flip the Switch Back to shower/storm chances as
upper level shortwave trough drives east from the northern plains to
across the upper Mississippi River valley. Sfc cold front leads the
trough in, tracking into the local forecast area between 00-06z Wed.
Relatively mild/moist airmass will move in ahead of the front -
highs in the mid 70s with mid 60s dewpoints expected. Low level
warming points to a cap around 850 mb that may never erode, but
ample instability above that with MUCAPES above 1 km of 1500 j/kg
progged. If storms could become sfc based, add a grand onto those
values. Bulk shear from 1-7km upwards of 40kts. Some potential for
supercells at outset. Overall severe threat has taken a couple ticks
up and Tue evening bears a close watch. The storms/line segments
look to be fairly quick moving, helping to lower a heavy
Long term...(wednesday through sunday)
issued at 205 PM CDT sun Sep 22 2019
Cold air aloft coupled with cyclonic flow should lead to some
showers Wed, Post the departing upper level level trough. The bulk
of these rain chances should hold across northern reaches of the
region - moreso north of I-90 (perhaps even I-94). Shortwave ridging
brings the Promise of a dry and sunnier day for Thu.
Showers and storms then become likely later Thu night through Friday
as an upper level shortwave trough lifts out of the Desert
Southwest, spinning across the Great Lakes by 00z Sat. Slug of low
level warm air advection and strong push of 850 moisture transport,
all ahead of northward moving sfc warm front, will be responsible
for the first slug of pcpn. The upper level trough and associated
cold front will continue to fire showers/storms into Fri evening.
In addition to what looks like a wet Friday, the models are now
trending toward bringing a juicier airmass back into the area. Warm
cloud depths from 3.5-4 kft in the ec and GFS along with pws near 2
inches set the stage for efficient rain producers. Orientation of the
anticipated shower/storm bands could favor training storms at times.
This scenario supports heavy rain and the potential for yet more
flooding. Going to have to keep a close eye on the evolution of this
system as we near the end of the new work week.
Could see more rain chances for the weekend, with better chances
currently from Sat night into sun.
Aviation...(for the 18z tafs through 18z Monday afternoon)
issued at 1223 PM CDT sun Sep 22 2019
VFR conditions expected to prevail through the taf period. Ongoing
showers across Iowa have been steadily dissipating as they work
northeast, so not including mention at either taf site for now.
However, guidance is in good agreement of redevelopment later this
afternoon, so continued -shra mention into early evening. Some obs
sites under ongoing showers have reported MVFR ceilings/visibility,
but trends in guidance have been toward less shower coverage, so
elected to not tempo in MVFR mention at this time, but it may be
needed in updates. Otherwise, showers will come to an end this
evening, but there are some hints at some lingering clouds into
Monday. Winds will generally remain westerly and under 10 kts.
WI...Flash Flood Watch through late tonight for wiz061.
Iowa...Flash Flood Watch through late tonight for iaz029-030.