197
FXUS63 KLSX 050349
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1049 PM CDT Tue Jun 4 2024

.KEY MESSAGES...

- There is an chance for scattered showers and isolated thunderstorms
  through early evening. There is a marginal severe threat should
  a thunderstorm become severe with the primary hazards being
  gusty winds up to 60 mph and quarter sized hail.

- There is another chance (60-70%) of thunderstorms overnight. If
  a thunderstorm does become severe, the primary hazards will be
  gusty wind up to 60 mph and heavy downpours that could cause
  locally heavy rainfall.

- Dry weather will follow for the rest of the week with around
  average high temperatures for early June. The next chance (30%)
  of rain will be this weekend.


&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 250 PM CDT Tue Jun 4 2024

Most recent water vapor imagery shows a cut off upper level low and
associated vorticity maximums spinning over southern Iowa and
northern Missouri. As this cut off upper low continues to the
northeast through the afternoon, it will provide support for
scattered showers and thunderstorms through the early evening.

Persistent morning cloud cover across the Mid-Mississippi River
Valley has so far limited instability area wide. Fields of cumulus
clouds have developed where there has been breaks in the clouds.
These are the areas that will be the prime spots to watch for
scattered shower and thunderstorm development. SPC Mesoanalysis shows
that there will be anywhere from 1000-2000 J/kg of SBCAPE and 20-25
kts of 0-6km bulk shear during peak heating. Should a thunderstorm
become severe the threats would be marginal with wind gusts up to 60
mph and quarter sized hail. The most likely scenario for areas that
do see development will be heavy downpours, lightning, and brief sub-
severe (less than 60 mph) wind gusts. The severe threat will
diminish around sundown as instability will continue to wane and
the upper level support will be to the northeast.

As the cut off upper low moves to the northeast, a separate
shortwave trough will enter the area and provide support for a cold
front to trek across the region tonight through Wednesday morning
and be the focus for shower and thunderstorms. The severe threat
with any thunderstorms along the cold front is minimal but not
impossible, primarily across central and northeast Missouri where
ensemble SBCAPE values max out around 1000-1500 J/kg by the time the
front makes it to those areas. Drier air aloft will also be moving
in and allow for dry air entrainment into thunderstorms. Thus, the
primary threat with any thunderstorm that does become severe along
the front will be gusty winds up to 60 mph. With any showers or
thunderstorms there is also a threat for heavy downpours that could
cause locally heavy rainfall.

The cold front, and its associated shower and thunderstorm activity,
will exit the area by Wednesday mid-morning. Temperatures behind the
front will not feel drastically different from today under abundant
sunshine. Wednesday will however feel less humid as a drier airmass
moves in. A dry secondary cold front will move across the Mid-
Mississippi River Valley late Wednesday night. The combination of
this dry cold front and clear skies causing sufficient radiational
cooling will allow Wednesday night low temperatures to drop into the
low- to mid- 60s.

MMG/Britt
&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 250 PM CDT Tue Jun 4 2024

Ensemble guidance is in consensus that a large scale upper level low
will shift into the Great Lakes region by Thursday and halt there
through the weekend and keep the Mid-Mississippi River Valley in
deep northwesterly flow. This combination of northwesterly flow and
drier air moving into the region will allow for a dry end to the
work week. Confidence in a rain-free end to the week is further
enhanced by over 90% of ensemble guidance members being dry. By the
weekend, there is a chance (30%) for rain and potentially
thunderstorms as shortwaves move across the area. However, we will
still be in northwesterly flow and there are differences in the
timing and magnitude of the shortwaves amongst deterministic
guidance there is little confidence in how any rain and
thunderstorms will materialize on Saturday and Sunday.

Temperatures through the next week are forecast to be around to
slightly below normal for early June with high temperatures in the
low- to mid-80s. Uncertainty in weekend temperatures is evident in
the NBM interquartile ranges with a 5+ degree temperature
difference.

MMG/Britt
&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1048 PM CDT Tue Jun 4 2024

As of 0345z, the cold front has moved into northwestern portions
of Missouri with showers and storms forming along and ahead of it.
So still expect the activity will move from west to east
overnight, moving through KCOU, KJEF and KUIN between 06z and 10z
Wednesday, then through the St. Louis metro area TAF sites between
08z and 13z Wednesday. Will see brief periods of MVFR ceilings and
visibilities with the activity. Then the ceilings will lift and
scatter out between 13z and 16z Wednesday.

Otherwise, south winds will veer to the west to northwest behind
the cold front. Winds will become light and variable after 00z
Thursday.

Byrd
&&

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

$$
WFO LSX