915
FXUS63 KLSX 181640
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1140 AM CDT Tue Jun 18 2024

.KEY MESSAGES...

- Today will be the coolest day of the week, however temperatures will
  remain above average with peak heat index values ranging from
  90-101. There is a chance for afternoon isolated to scattered
  showers and thunderstorms mainly across southeast and eastern
  Missouri and southwest Illinois (20-30%).

- Above normal temperatures are expected to continue through the
  weekend with highs in the 90s and Friday and Saturday being the
  hottest days.


&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 429 AM CDT Tue Jun 18 2024

Current water vapor imagery is showing a mid-level ridge centered
over the Mid-Atlantic area which is dominating the eastern CONUS.
The associated deep high pressure is resulting in pronounced deep-
layered southerly flow across the Mississippi River Valley. This
southerly flow continues to transport warm air and Gulf moisture
into the region.

Today will be slightly less warm, compared to yesterday, with highs
in the mid 80s to low 90s area-wide. However, with dew points in the
70s, there will be no issue for heat index values to reach 90-101
degrees. The main catalyst for highs being slightly lower is due a
large fetch of increased cloud cover moving into the region. In
addition to increased cloud cover a vorticity maximum rounding the
mid-level high, currently evident in water vapor imagery over
Louisiana, will bring a chance for afternoon scattered showers and
thunderstorms. Deterministic guidance is in agreement that the
vorticity maxima will traverse across southeast Missouri and
southern Illinois. This will help focus afternoon convection and
with weak low-level support and SBCAPE values between 500-1500 J/kg
where the vort max traverses. Locations that do see a
shower/thunderstorm can expect downpours and some gusty winds in the
strongest of storms. Should the vorticity maximum traverse further
to the north, the footprint of isolated to scattered afternoon
convection will be larger. Shower and thunderstorm chances will
subside after sundown due to a loss of instability and daytime
heating.

By Wednesday, the mid-level ridge/high will begin to expand
westwards. As it does so, another weak vorticity maxima will round
its northwestern periphery and traverse across northwest and north-
central Missouri Wednesday afternoon. This will help focus another
chance (20-30%) of afternoon showers and thunderstorms.
With closer proximity to the center of the high, Wednesday will be
another hot day with temperatures a hair warmer than today area-wide
with highs in the upper 80s to mid 90s. Despite the slightly warmer
temperatures tomorrow, heat index values will not be as high as
dewpoints in the 60s are forecast.

MMG/Glass

&&

.LONG TERM...  (Wednesday Night through Monday)
Issued at 429 AM CDT Tue Jun 18 2024

The mid-level high will continue its westward shift through Saturday
with surface high pressure dominating the region. The attendant
height rises aloft will favor increasing high, temperatures back up
into the mid to upper 90s by Saturday. Confidence in this warm-up is
further increased as the NBM interquartile ranges only have a 5
degree spread with the lower quartile being in the mid-90s. Due to
the dominance of deep high pressure, rain chances will be
suppressed Friday through Saturday afternoon and evening.

Ensemble guidance is in consensus that a mid-level trough will
traverse across the Upper Mississippi River Valley late Saturday
into Sunday. As it does so, a shortwave and vorticity maxima will
help provide support for a cold front to move across the region.
This will provide the next chance for shower and thunderstorm
activity. There are differences among the deterministic guidance on
the timing of the front, so the exact timing of the shower and
thunderstorms are uncertain at this time. However, confidence is
increasing that precipitation chances will ramp-up overnight
Saturday into Sunday morning with over 70% of ensemble guidance
indicating rain during this time frame.

Temperatures will remain above normal despite the passage of a cold
front on Sunday and continue into early next week as slight ridging
builds back in over the central CONUS. Confidence in these above
normal temperatures is increasing as the lower quartile of the NBM
max temperature distribution is above the average high
temperature for late June.

MMG/Glass

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1135 AM CDT Tue Jun 18 2024

VFR conditions are expected to prevail throughout the 18Z TAF
period. VFR cumulus will impact local terminals this afternoon,
and may briefly dip to MVFR levels. A few showers and a weak
thunderstorm or two may develop in southeast Missouri this
afternoon, and there is a low chance (20%) that a storm could
develop near STL. Chances remain too low to include in the TAF,
but an amendment may be needed later this afternoon if storms
develop nearby. Gusty southerly winds will persist this afternoon,
and diminish by sunset. A low chance of valley fog exists at a
few terminals overnight, but confidence remains too low to include
in the TAF at this time.

BRC


&&

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

$$

WFO LSX