861
FXUS63 KLSX 150845
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
345 AM CDT Wed May 15 2024

.KEY MESSAGES...

- A brief period of dry weather is expected today before unsettled
  weather returns late tonight through Friday. A few thunderstorms
  could be strong Thursday afternoon and evening.

- Locally heavy rain is possible Friday over southeast Missouri
  and southwest Illinois. Recent rainfall has left many areas
  saturated. Locations within low lying terrain and areas with
  poor drainage could see ponding/localized flooding.

- Warmth returns over the weekend into next week with temperatures
  roughly 5-10 degrees above normal.

&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 344 AM CDT Wed May 15 2024

Water vapor imagery shows the broad upper trough beginning to pull
east early this morning with multiple smaller vorts rotating around
the trough. One such feature is evident over southern Illinois,
which could lead to an isolated shower or two over far southeast
sections of the CWA before 12z. Otherwise, the trend will be toward
dry conditions today.

The main concern today will be cloud cover with low level saturation
lingering behind the departing low. GOES-16 nighttime microphysics
products show low clouds pivoting through the region with the
western extent just east of Columbia and Jefferson City. Soundings
show the extent of the cloud cover will be greatest over the
southern half of the CWA with low level saturation gradually thin
from north to south. HREF grand ensembles provide some indication
thats clouds will at least break over northeast Missouri and west-
central Illinois this morning, gradually working south late this
morning into the afternoon. Broken skies will allow peeks of
sunshine with a slight bump in high temperatures (low/mid-70s).

Upper level vorticity ejects east out of the Plains tonight as a
warm front lifts northeast into Missouri. Thunderstorms will be more
numerous along the Kansas-Oklahoma border, where activity will be
ongoing from initial develop that takes shape over the Texas and
Oklahoma Panhandles. This activity will have a long way to go before
reaching Missouri and there is fairly high confidence that it will
be in a weakening phase as it arrives along and behind the leading
edge of a wing of mid-level WAA. WAA reaches the central sections of
Missouri between 06z-09z tonight at which time the HREF paintball
plots (>40dBZ) show a decrease in coverage as it outruns the core of
instability to the southwest. Showers and weakening thunderstorms
reach the Mississippi River in the 12z-15z window and lasts into at
least late morning, which poses questions to afternoon potential.
HREF grand ensemble shows mean cloud remaining at or above 60% until
after 3 p.m., leaving very little time for instability to recover. A
more pronounced lobe of vorticity traverses the southern half of
Missouri through the morning and is situated just south of STL
between 18z-20z. Mean SBCAPE values of 1000-1500 J/kg cover southern
sections of Missouri and Illinois, reaching as far north as St.
Louis. This coincides with 0-6km bulk shear of 25-30 kts and mid-
level lapse rates of 6-6.5C.

Given the questions and marginal environment, if thunderstorms are
capable of becoming organized, it would be over southeast Missouri
and southwest Illinois. Confidence is quite low with severe
potential likely being isolated with marginally severe hail and
damaging wind. At this time, confidence is not high enough to
message the threat outside this discussion. Should trends show
otherwise, later updates will have opportunity to address changes.


Maples

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 344 AM CDT Wed May 15 2024

The axis to an upper ridge is expected to extend northward into the
Ohio Valley as an upper trough moves over the southern Plains
Friday. A stacked surface/mid-level low strengthens over Oklahoma
during the morning before moving east-northeast into southern
Missouri Friday afternoon into Friday evening. A surface boundary is
likely be lingering over the central or southern sections of the
forecast area as deep layer ascent at the left exit region of the
upper jet gives rise to widespread showers and thunderstorms.
Westerly flow nearly parallels the west-east oriented boundary with
PWAT values once again approaching 1.5 inches, if not slightly
higher over southern Missouri and Illinois. Widespread rainfall with
waves of embedded thunderstorm activity is likely to impact southern
sections of Missouri and Illinois with locally heavy rainfall. Given
how wet it has been in recent weeks and how efficient rainfall could
be within thunderstorms, there may be concerns for ponding and
localized flooding in low lying area or areas with poor drainage.

There continue to be differences in timing and evolution of the
pattern heading into the weekend. If there is going to be any real
deviations from the dry weekend, it would be Saturday, when some
guidance closes the upper low over southern Illinois and slows its
eastward progression. This would keep low end probabilities (30%)
over east/southeast sections of the forecast area with dry
conditions more favored over west/northwest portions of the forecast
area. How this plays out could also hold temperatures down a few
degrees from prior forecasts. Sunday has the better bet on being dry
as the upper low departs east and a surface front clears the area
and upper ridging builds in from the west.

Above normal temperature trends continue into next week. It has the
potential to be yet another active period as multiple upper level
shortwaves track across the mid section of the Lower 48.


Maples

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1234 AM CDT Wed May 15 2024

An upper level system is departing to the east early this morning
with low level moisture linger behind. Nighttime microphysics
products show stratus beginning to thicken over much of the area.
The northern fringe of the cloud cover lies just north of KUIN,
which may result in bouncing observations. Initial IFR conditions
should give way to higher ceilings, but MVFR visibilities develop
through sunrise.

The remainder of the TAF sites are well under the stratus deck
with the primary issue being IFR ceilings and minor visibility
restrictions. IFR potential is greatest between 08z and 12z with
minor time differences between terminals. MVFR stratus hangs on
through at least morning with gradual lifting in the afternoon.
It`s possible that bases straddle MVFR/VFR until 18-21z, when
ceiling should rise above 3.5k+ ft.

Maples

&&

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

$$

WFO LSX