849
FXUS63 KLSX 041952
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
252 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 18z TAFs through 18z Wednesday Afternoon)
Issued at 1230 PM CDT Tue Jun 4 2024

Scattered showers and thunderstorms will be in the vicinity of
the airports early-mid afternoon. There may be the potential for
additional development later this afternoon into the early this
evening, but confidence in this trend is too low to include in the
TAFs at this time. There is higher confidence with thunderstorms
chances late this evening and overnight ahead of a cold front that
will pass southeast through the area. Any thunderstorms that
occur through tonight have the potential to reduce ceilings and
visibilities to MVFR (possible IFR) conditions in brief downpours
with gusty winds. Otherwise, drier conditions are expected behind
the front on Wednesday with VFR conditions.

Britt

&&

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

$$

WFO LSX