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000
FXUS63 KLSX 241105
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
605 AM CDT Wed Apr 24 2024

.UPDATE...
Issued at 400 AM CDT Wed Apr 24 2024

The earlier issued Dense Fog Advisory has been canceled.
Visibility has improved across the region. Some patchy areas of
fog remain, but they are localized and improving.

Kimble

&&

.KEY MESSAGES...

- Multiple rounds of thunderstorms, some severe, are expected late
  this week and this weekend. While we cannot rule out severe
  weather just about anytime from early Friday morning through
  Sunday, we are focused on two primary periods:

- Thunderstorms Friday afternoon and evening will pose a
  conditional threat for all severe weather hazards. The
  greatest uncertainty is whether these thunderstorms will form in
  our area after activity earlier in the day.

- Thunderstorms Sunday afternoon and evening will pose a threat
  for all severe weather hazards including tornadoes. The greatest
  uncertainty with this is related to the timing of a cold front.
  The threat could be focused in our area or to our west. With a
  slower front, the threat could continue into Monday.


&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 315 AM CDT Wed Apr 24 2024

Early this morning we find ourselves within an area of surface high
pressure centered near the western Great Lakes in the wake of
yesterday`s cold front. Areas of dense fog early this morning, due
to leftover moisture after yesterday`s rain, is eroding from north
to south as dry advection eventually wins out. This fog gives way to
a mostly clear sky today with perhaps some afternoon cumulus.
Temperatures top out near or just below seasonal averages, in the
60s to low 70s. Tonight should be our coolest night of this air mass
as surface high pressure begins to moves east through the Great
Lakes with our area on the southwest periphery. Dewpoints in the 40s
suggest that lows should similarly be in the 40s, but parts of
central Illinois could see some 30s closer to the surface high.

On Thursday, the initial rumblings (no pun intended) of an upcoming
active weather period will show up to our west. Moisture surging
back northward through the Plains will set off showers and
thunderstorms across Kansas during the day on Thursday. With mid
level flow out of the WNW, clouds and remnant showers will have a
tendency to drift downstream across Missouri and potentially into
our area. We`ll still be in the cooler, drier air mass with little
to no instability, so we expect any strong to severe storms on
Thursday to remain well to our west. With the potential for more
cloud cover, especially over central Missouri, temperatures may
actually be a few degrees cooler than Wednesday, mainly in the
60s.

Kimble

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 315 AM CDT Wed Apr 24 2024

Big Picture Overview:
As upper ridging moves east a trough will move into the
Southwestern US with multiple shortwave troughs exiting through
the Plains late this week and through the weekend. Gulf of Mexico
moisture spreads north and east across the Plains and into the
Mississippi Valley setting up an unstable environment capable of
producing thunderstorms each day. With a predominantly southerly
flow at the surface and moderate to occasionally strong
southwesterly flow aloft, enough shear will also exist to produced
organized strong to severe thunderstorms. As each shortwave trough
moves through the Plains it and its related fronts will serve as
focusing areas for thunderstorm activity. The timing of these
waves and the recovery of the air mass in between waves are the
primary sources of uncertainty for the threats each day. While we
cannot rule out thunderstorms just about any time as we remain in
the unstable air mass, the focus for our area is on two primary
periods: Friday afternoon and Sunday evening.

Thursday Night into Friday Morning:
The better low level moisture makes its initial surge northeast
into and through our area along an ill-defined warm front as a
shortwave trough moves into the Northern Plains. We will see areas
of showers and thunderstorms in this moist advection particularly
along and ahead of the warm front. In this sort of scenario,
thunderstorms would be elevated in nature and pose primarily a
hail threat depending on the available instability. At the moment,
models are not very aggressive with the instability with this
initial moisture push, perhaps in part due to a potential
thunderstorm complex over southwest Missouri late Thursday into
Thursday evening weakening the moisture gradient as it moves
through our area late Thursday night into Friday morning. While it
doesn`t look great right now, we don`t place too much faith on
lower resolution long range models to resolve this sort of
elevated instability and so we remain alert for at least a
conditional threat of hail with this opening round of
thunderstorms.

Friday Afternoon and Evening:
We will likely see at least some daytime heating on Friday to
help destabilize the newly arrived moist air mass and generate
surface based instability Friday afternoon and evening. How much
destabilization occurs and where it occurs depends in part on the
convection earlier in the day on Friday. The better forcing with
the shortwave trough will be exiting to the northeast through the
day, and there are no other obvious low level features to trigger
convection in our area. Thunderstorm initiation in our area Friday
afternoon would depend on other more subtle boundaries that may
locally develop. Convective initiation is more likely further to
the west along the dryline in Kansas and Oklahoma with these
potentially tracking into our area Friday evening. There should be
enough shear available for supercells, although they may begin
merging into a broader convective complex before they arrive. The
primary threat with storms during this period would be large hail
and damaging winds, although an isolated tornado could not be
ruled out given the available directional shear.

Saturday Night:
Shortwave ridging in the wake of Friday`s shortwave trough will
promote subsidence and a capping inversion across our area during
the day on Saturday. This will make it difficult for thunderstorms
to form despite good moisture and likely the strongest heating of
the period. We may well see our greatest surface based CAPE
values during this period, but without a focus or a weakened cap
it will be difficult to trigger convection in our area on
Saturday. However, convection is expected to form along the
dryline to our west as well as along a warm front in the vicinity
of northwest Missouri and southern Iowa during the evening. Our
best chance for thunderstorms Saturday night will be with a
developing convective complex associated with the activity to our
west which could then track into the northern portion of our
forecast area overnight Saturday night. This would primarily pose
a threat for damaging winds, although a QLCS tornado would also be
possible.

Sunday Afternoon and Evening:
As the next shortwave trough moves into the Northern Plains on
Sunday it will finally send a clearing cold front southeastward.
Thunderstorm initiation is expected in the warm sector ahead of
this front on Sunday, with the available shear and instability
favoring supercells congealing into a broader convective complex.
All severe weather hazards would be possible with these including
large hail, tornadoes, and damaging winds. There could also be a
threat for locally heavy rainfall with training thunderstorms. The
biggest source of uncertainty for this period is where the cold
front will be and by extension where thunderstorms will initiate.
There remains considerable uncertainty in the guidance on how
quickly the front moves through, with a slight trend emerging
favoring holding the front off to our west. If the front is
slower, then the main severe weather threats on Sunday may be to
our west and there could be a renewed threat in our area lingering
into Monday. If the front is closer to or in our area then the
primary severe weather threats would be in our area as well with
the clearing front eliminating the threat for Monday.

Once the front moves through we will see the moisture pushed off to
our south and end our daily threats for thunderstorms. The air mass
behind this front has origins in the Pacific (not cold) with a
trajectory through the western mountains (dry) so we won`t see a
noticeable temperature drop behind it. In fact, ridging builds back
in quickly behind the departed trough opening us back up to even
warmer temperatures for the middle of next week.

Kimble

&&

.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 605 AM CDT Wed Apr 24 2024

Pockets of fog early this morning at valley terminals will burn
off quickly this morning, by around 12Z. Winds become light out of
the north under VFR conditions. A back door cold front is moving
southwest through northern Illinois this morning with some MVFR
ceilings behind it. This is expected to lose its push southwest
with time and clouds dissipate before reaching any of the TAF
terminals. If it does make it in, it will likely be for a brief
window of MVFR ceilings. As winds go light again tonight we could
see fog form at valley locations once again.

Kimble

&&

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

$$

WFO LSX