750
FXUS63 KLSX 121104
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
604 AM CDT Wed Jun 12 2024

.KEY MESSAGES...

- Temperatures will warm well above normal to end this week and
  start next week. The warmest conditions are expected Thursday, Sunday
  and Monday with heat index values reaching 100-105 in some
  locations.

- There is a threat for severe thunderstorms Thursday evening. The
  primary threats will be large hail and damaging winds.


&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 325 AM CDT Wed Jun 12 2024

Warming trend continues today as southwesterly low level flow
ushers in 850mb temperatures into the mid to upper teens, then
surge to between 20-25C on Thursday. So expect widespread highs in
the 90s by Thursday. In the meantime, as dewpoints rise through
the period, expect hot and humid conditions with heat indices
around 100 degrees by Thursday for the first time this summer.

Otherwise, the main forecast issue in the short term is the
increasing chances of showers and storms, especially by Thursday
evening. With the upper level ridge over the western CONUS slowly
sliding to the east, flow aloft becomes northwesterly overhead.
So as systems slide west to east along the U.S./Canadian border,
each successive system will ever so slightly push the cold front
southward towards the forecast area. A few of the latest CAMs
indicate that far northern portions of the forecast area could see
showers and storms move in towards daybreak on Thursday. However,
confidence is low as a majority of the latest forecast soundings
depict a decent cap, limited moisture, and no upper support, so
kept forecast for tonight/early Thursday morning dry at this time.

During the day on Thursday, a majority of the deterministic models,
even the latest NAM, as well as ensembles now have similar timing
and placement of the cold front, just north of the IA/MO border by
18z Thursday. South of the front, surface dewpoints in the upper 60s
to the mid 70s will pool along and ahead of the cold front. This
will likely result in moderate instability with ML CAPEs 3000+ J/kg.
However, capping is forecast to be in place and there is some
uncertainty of when it will weaken enough to allow convection to
initiate. At this time, the latest forecast soundings indicate that
the cap will break by early evening, so have increasing POPs after
21z Thursday. Now as far as convective mode, could initially see
supercell development, especially because of the high ML CAPEs, 0-
6km bulk shear 30-40kts, and ML LR 7.5-8C/km. So the main threat
will be large hail initially then as activity transitions to line
segments as it moves east across the region, damaging winds become
the main threat. However, as the cold front and associated activity
pushes south through the evening, the shear lessens as well as the
instability. Thus there is still some uncertainty on exactly how far
south strong to severe storms will make it before waning. For now
best chances will be along and north of I-70.

Byrd
&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 325 AM CDT Wed Jun 12 2024

For the remainder of the forecast period, the main issue will be
the well above normal temperatures as well as heat indices meeting
and/or exceeding 100 degrees.

By Friday morning, the cold front will be south of the forecast area
and stall out, then it will move back north as a warm front on
Saturday. So temperatures on Friday will be a bit cooler, but still
above normal in the upper 80s to low 90s. However, there is some
uncertainty on exactly where the cold front will stall out. If it
remains over southern portions of the forecast area, dewpoints in
the 70s along and south of the front could make it rather
uncomfortable in this area as winds will also be rather weak and not
provide any relief.

Beyond that, the upper level ridge will build into the region with
increasing southerly surface flow as the front moves back north as a
warm front on Saturday. So a resurgence in hot and humid weather
returns for the rest of the weekend and into next week.

As for any precipitation chances, some guidance suggests a few
showers and thunderstorms along the warm front as a weakening upper
shortwave trough over the central Plains lifts northeast into the
Great Lakes region Saturday. A majority of the latest deterministic
and ensembles keeps the best chances of showers and storms just off
to the west and northwest of the forecast area at this time. Beyond
Saturday, there are considerable timing, strength and location
differences among the deterministic models on how the pattern
evolves and additional chances of showers and storms. For now we
remain dry through early next week with the next chance for rain by
Tuesday.

Byrd
&&

.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 603 AM CDT Wed Jun 12 2024

Dry and VFR flight conditions through the forecast period. Winds
will out of the south to southwest at 7 knots or less.

Byrd
&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$
WFO LSX