Sunrise 6:16 AM   Sunset 7:43 PM Moonrise 1:49 PM   Moonset 3:11 AM

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FXUS63 KLSX 180829 AFDLSX

Area Forecast Discussion National Weather Service Saint Louis MO 329 AM CDT Thu Apr 18 2024

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 06z TAFs through 06z Thursday Night)
Issued at 1048 PM CDT Wed Apr 17 2024

VFR flight conditions are expected overnight with surface winds becoming light easterly.

Showers and storms now over NE KS could impact KUIN as early as 12z, and KCOU/KJEF as early as 15z if they hold together and become more organized as they move east. We are still expecting the main thrust of showers and storms at KUIN and KCOU/KJEF in the 17-20z time frame with flight conditions deteriorating to MVFR.
Flight conditions could lower to IFR for a brief period with a stronger storm/heavy rain and could have gusty winds as well.
Given the uncertainty in timing, these later conditions have not been included in this TAF issuance. A cold front will bring gusty northwest winds mid-late afternoon.

For the St. Louis metro terminals, showers and thunderstorms are expected in the 20-23z time frame with potential for strong storms. Flight conditions could lower to IFR for a brief period with stronger storms/heavy rain and could have gusty winds as well. Given the uncertainty in timing, these later conditions have not been included in this TAF issuance. A cold front will bring gusty northwest winds by early evening.

Glass

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

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