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000
FXUS63 KLSX 181718
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1218 PM CDT Thu Apr 18 2024

.MESOSCALE DISCUSSION...
Issued at 1206 PM CDT Thu Apr 18 2024

Surface analysis at 1700 UTC shows a west-east warm front from
near KCOU>>KCPS>>KSLO. This boundary has been pushing northward at
a good clip, and should continue to push another ~50 miles to the
north over the next couple of hours. The western extent (into
central Missouri) of the warm sector may get impinged upon over
the next few hours as surface observations, satellite imagery, and
radar reflectivity all show a well-defined outflow boundary
gradually pushing southeast with time. A couple of storms have
developed along the southwestern periphery of this outflow, and
with instability increasing ahead of it, would expect additional
development over the next few hours. Behind this boundary, storms
should be elevated. However, with MUCAPE values of 500-1000 J/kg
and stronger deep-layer shear (~50-55 knots), cannot rule out a
couple of marginally severe hail reports in northeast Missouri and
west- central Illinois. However, the main focus will be further
southeast in the warm sector. In addition to storms firing along
the outflow boundary, the actual synoptic cold front is further
west back into southeast Kansas and western Missouri. The speed of
the cold front is expected to be a bit faster than the outflow
boundary, so it should tend to catchup to it by late afternoon.
Instability within the warm sector will be strong (SBCAPE
2000-3000 J/kg). However, deep-layer shear further southeast is
not as robust. Latest RAP forecasts for late this afternoon
indicate about 35 knots of deep- layer shear. While certainly not
weak by any nature, it is a bit on the weaker end of the spectrum
for very organized convection. This weaker shear also puts
convective mode a bit into question. I would expect some supercell
structures this afternoon, though probably more transient in
nature due to the aforementioned marginal effective shear.
Multicellular clusters will also likely exist across the area this
afternoon, which would also bring a damaging wind threat. Large
hail (up to golf ball sized) is the main threat in any supercells.
There should be a transition to more line/bowing segments by late
afternoon/early evening, but not sure if everything will come
together in the form of a well-organized QLCS. If that does
occur, it would be most likely in parts of southeast Missouri and
southwest Illinois early this evening. Regardless, the main threat
should transition more to damaging winds by early this evening
given the expectation for more linear convection. In terms of
tornadoes, the low-level shear/helicity forecast is not expected
to be particularly strong, with LCL values increasing through the
afternoon. The best chance for isolated tornadoes likely will be
along/near the warm front and close to the track of the surface
low. This is where very low- level shear/helicity should be
enhanced the most. The threat should end from west/northwest to
east/southeast this evening, exiting by about 0200 UTC out of
parts of southeast Missouri and southwest Illinois.

Gosselin

&&

.KEY MESSAGES...

- Strong, but sub-severe, thunderstorms will impact northern
  Missouri and west-central Illinois this morning. The potential
  for these cells to grow into an organized complex, a key factor
  into the timing and evolution for severe weather today, now
  looks low (less than 10% chance).

- Discrete strong to severe thunderstorms will develop this
  afternoon in central Missouri and move northeast along and ahead
  of a cold front, threatening large hail and damaging wind. They
  will grow into a broken line of thunderstorms and dive southeast
  through the St. Louis region, southeast Missouri, and southwest
  Illinois. All hazards can be expected with the broken line,
  though damaging wind is the most likely and widespread of the
  three.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 319 AM CDT Thu Apr 18 2024

Regional surface analysis shows a deepening surface low beginning to
eject from the Oklahoma/Texas Panhandles to the southern Great
Plains. A warm front extends from the low from west to east, and is
draped across the Ozark Plateau. Within the warm sector, while
temperatures are uniformly in the mid-60s to low 70s, quality low-
level moisture is pooling closer to the low and a dryline in the
Plains. At the same time, a subtle shortwave impulse aloft is
drawing a separate cold front down into the central Plains and
increasing convergence along the warm front. Convection developed
overnight north of the warm front on the nose of an intensifying low-
level jet in eastern Kansas. From this, one linear structure is
persisting tracking east near the warm front in the northern
Kansas City metro area and posing a large hail and damaging wind
threat at this hour.

These thunderstorms will gradually lose their low-level jet
forcing with sunrise, and while they may amalgamate as they move
east, there doesn`t appear to be enough organization to lay a
strong cold pool to its south. As the morning goes on, instability
will attempt to recover across the region as the low begins to
track northeast and the cold front enters the state. I`m confident
that instability will recover sufficiently (1000-2000 J/kg of
surface-based CAPE) for severe convection, but strong mixing will
mitigate stronger destabilization by limiting moisture in certain
areas. Now that the airmass will be largely untouched under
abundant some degree of sunshine, forecast soundings mix the open
warm sector notably. This is particularly true lee of the Ozarks
in central Missouri, where orographic influence and a southwest
wind will further limit surface moisture. That said, the cold
front itself will serve as a synoptic feature to draw the moisture
north from the southwest, and dewpoints will certainly be on the
rise on the eastern side of the Ozarks up through east-central
Missouri.

Without a strong morning MCS cold pool to spark new convection in
the late morning, thunderstorms will be forced to initiate ahead
of the approaching cold front during the early afternoon in
central and northeast Missouri. This solution is leading to a
slightly later onset time for the severe convection threat, now
closer to noon. There is consistent signal that discrete
supercells will develop first, probably due to the subtle weakened
instability where mixing is greatest. They will be high-based
(1.00-1.25km) given the deep mixing. Because of this, a tornado
threat will be nearly non- existent at first unless mixing is
woefully overestimated. The initial severe weather threat will be
for isolated to scattered large hail and damaging wind. Deep-layer
shear vectors do hint that the storm motions will favor cell
interaction and upscale growth through the early afternoon from
the western edge of the St. Louis metro northeast to central
Illinois. At the same time, the convection as a whole moves into
more intense, uncapped surface- based instability (upwards of 2000
J/kg during the mid-afternoon, waning from then on). From this
point on, large hail becomes a more limited threat and damaging
winds come to the forefront. Despite its linear structure, CAMs
still depict the remnant supercell mesovortices embedded in the
line. Despite 0-3km shear being marginal at best for QLCS
tornadogenesis, the presence of an embedded mesovortex would
invalidate that limitation. As such, the threat for tornadoes
along the line is very real.

Once the broken line organizes, it will pick up speed and dive
southeast. The southern extent of this line is still unclear: it
will extend to at least Franklin County, Missouri but persistent dry
air off the Ozarks may delay further southward development of the
line until it finds more appreciable moisture. By the time the
line reaches roughly the I-44 corridor, sufficient instability
will allow for that southward intensification. The St. Louis
region will see its greatest impacts from roughly 5-8pm, with
areas further south and east in the crosshairs through about 10pm.
The cold front will quickly vaporize any instability, ending the
threat for the region. Cooler and drier air will filter in
overnight, as well as some degree of low clouds, and usher in a
relatively tranquil period from Friday through the weekend.

MRB

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 319 AM CDT Thu Apr 18 2024

Temperatures this weekend will remain below normal for mid-April,
featuring highs in the mid-50s to mid-60s through Sunday under
the influence of the deep longwave trough in the north-central
CONUS. Low (10-20%) precipitation chances will glance southern
Missouri along the stalled frontal boundary, but the overwhelming
majority of the region will be dry. Lows overnight Saturday into
Sunday morning still threaten patchy frost for a very small
portion of northeast and north-central Missouri, however a weak
northerly wind and RH values below 75% currently preclude much
threat for frost outside of sheltered areas and valleys.

A shortwave passes through the region aloft Sunday night into Monday
morning, but poor moisture return will keep the feature devoid of
precipitation. Alongside this, the upper-level flow pattern will
amplify to some degree. Exactly how much is unclear, with varied
solutions in the global ensemble guidance, and will influence our
modest warmup that starts early next week. Some flavor of broad
upper-level ridging will form over the south-central CONUS by
Monday, expanding further north and east through the week. The early
work week will almost certainly (80%+ chance) be at most near-
normal, so highly-abnormal warmth won`t return quite yet. Beyond
today, the next threat for any showers and thunderstorms returns at
some point between early Tuesday and early Wednesday along a cold
front. At this very early stage, instability looks to be at a
premium, with the absolute worst-case SBCAPE values in the global
ensemble members reaching a meager 400 J/kg. While this can
certainly change, and bears watching amidst a strong shear
parameter space, the threat for any impactful weather here looks
slim to none right now.

MRB

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1206 PM CDT Thu Apr 18 2024

Chances of showers and thunderstorms will gradually increase over
the next several hours along/ahead of a cold front. The best
chance for storms at KUIN/KCOU/KJEF is mid-late afternoon, and
closer to 0000 UTC in/around metro St. Louis. Hail, strong winds,
and very low visibilities are all possible in thunderstorms. Some
trailing shower activity may linger for a few hours behind the
cold front, with ceilings lowering to low MVFR along with a wind
shift to the northwest. A brief period of IFR ceilings is
possible this evening/early overnight, but was not confident
enough to add in at this time.

Gosselin

&&

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

$$

WFO LSX