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FXUS63 KLSX 191943 AFDLSX

Area Forecast Discussion National Weather Service Saint Louis MO 243 PM CDT Sun May 19 2024

KEY MESSAGES

- Abnormal warmth persists through Tuesday, when another run at 90 degrees is likely (60-70% chance) for the St. Louis and Columbia metro areas.

- Thunderstorm chances exist through a large portion of the forecast, though the threat is not continuous nor uniform areawide.

- The most tangible, though conditional, threats for severe weather exist Tuesday (primarily in central/northeast Missouri and west- central Illinois) and Wednesday (along and south of the I-44 and I- 64 corridors in Missouri and Illinois, respectively).

SHORT TERM
(Through Late Monday Night)
Issued at 241 PM CDT Sun May 19 2024

A diffuse surface boundary is currently draped across northeast Missouri and west-central Illinois, and its movement is largely at the mercy of differential heating as its upper-level support is far- removed from the region. Air on the warm/humid side of the boundary is very unstable, with SBCAPE values exceeding 2500 J/kg according to SPC Mesoscale Analysis. Heights aloft continue to gradually rise with a narrow ridge axis extending from the Ark-La- Tex region through into the Ohio River Valley. The combination of weak surface convergence and subsidence aloft are counteracting the positive buoyancy in place to keep convection in check. The very strong instability may be able to overcome this subsidence, so isolated to widely-scattered thunderstorms remain possible through the early evening in northern Missouri and west-central Illinois. While low, the potential for near-severe hail will exist with any convection going into the evening.

The boundary lifts further north overnight with the low-level jet strengthening a bit. Severe convection is poised to eject from the central Great Plains on the nose of this LLJ and track along a MUCAPE gradient into Iowa. As such, most of the night tonight will be dry and warm with southerly winds gradually strengthening. At the same time, mid-level troughing in the western CONUS becomes more pronounced, and by Monday morning we are squarely in deep southwest flow aloft. Temperatures once more jump well into the 80s, but may be muted ever-so-slightly compared to today because of more abundant cloud cover. Weak upper-level diffluence will exist as opposed to strong subsidence, but even still the forcing mechanisms are not very strong. There is also a question as to how much we destabilize, with some guidance keeping the MLCAPE well-capped, and others allowing for a more buoyant mixed layer. Without much low-level lift to focus convection on Monday, I'm not confident we see anything robust or strong quite yet. If convective debris lasts longer into the morning and early afternoon, redevelopment will be inhibited. If not, or if we have a well-defined MCV, we could see marginally- severe thunderstorms across the forecast area. We are not currently messaging the threat given the wide range of possibilities and low overall threat.

Most short-range guidance relegates convection Monday night far to our north across Iowa along the warm front/MUCAPE gradient. This situation would leave us warm and dry once more in the open warm sector on Monday night into Tuesday morning, though low PoPs (15- 20%) persist in northern Missouri and west-central Illinois to account for subtle uncertainty in the exact frontal position.
Otherwise, most of the resident instability will remain untouched in advance of the more tangible severe weather threat on Tuesday.

MRB

LONG TERM
(Tuesday through Next Sunday)
Issued at 241 PM CDT Sun May 19 2024

Little has changed in the overall thinking for our severe weather potential Tuesday, marking a return to an active weather pattern for us. Amidst the broad upper-level trough in the western CONUS, a shortwave will eject into the central Plains and into the Mississippi Valley. There is still some variance in exactly where that wave tracks, which would limit the southern extent of the severe weather threat somewhat. That said, the mid-level jet streak is strong (near the 90th climatological percentile) and will lend lift and shear to the resident instability to present a severe weather threat. It is worth noting that if we continue a northward trend in the wave, it's entirely possible a very small area of the region sees severe weather. The area most likely to see severe weather remains across the northern and western forecast area, where the best forcing will exist during diurnal peak heating. To nod that peak heating for a moment, Tuesday looks like another warm day with a few spots reaching 90 degrees and widespread mid/upper 80s otherwise.

Assuming the current presentation of the mid-level wave is sufficient, convection still looks to initiate in a discrete mode along a pre-frontal boundary in the open warm sector. There is a trend to slow down the synoptic system overall compared to previous forecasts, meaning the severe weather threat may not manifest until close to sunset in central and northeast Missouri. By this point, the storm mode would be in question as deep-layer shear begins to back and become more parallel with the low-level lifting mechanism.
That said, the shear orientation is not certain and discrete convection may persist by the time thunderstorms reach us. If the former occurs, the highest threats would be for damaging winds and tornadoes. If the latter occurs, all hazards will threaten the region. Another question is how long severe convection will last and how far south it will progress. As thunderstorms continue south and east Tuesday evening, instability will wane and they will be further detached from mid-level support. As such, a gradient will exist in the threat for severe weather (illustrated well by the SPC Day 3 Convective Outlook).

The cold front attending the synoptic system will slowly march through the region overnight Tuesday into Wednesday, but there are no indications it will exit completely by Wednesday afternoon. As such, a renewed threat for severe weather will exist along and ahead of this boundary. That said, a few uncertain factors are still in the air. The first is the effect of convection overnight Tuesday on instability redevelopment for Wednesday. It doesn't appear that would be an issue, with most convection ending during the late evening Tuesday. The second is the mid-level lift being displaced north of the best instability and low-level forcing. While indications are the mid-levels may exhibit some weak confluence near the instability, there is ample upper-level jet support for deeper convection. The area generally highlighted by the SPC Day 4 Convective Outlook is of greatest concern (essentially along and south of the I-44/I-64 corridor). Discrete convection that does develop will likely grow upscale quickly, posing mostly a damaging wind and tornado threat.

The front finally passes late Wednesday into early Thursday, leading to a brief but largely-tranquil period from Thursday into early Friday. That said, the front doesn't move far to our south, and its ultimate position is unclear. Because of this uncertainty, 30-50% PoPs linger through into the weekend. I doubt we will see a persistent thunderstorm threat that entire time, but with fairly large uncertainty surrounding the front position, the NBM PoPs were used through the extended forecast. The next more appreciable rain chance returns Friday into the weekend, at which point the CIPS/CSU severe weather probabilities begin to inch back into the region.
Ensemble guidance hints at another amplified wave later this weekend, but considerable timing and amplitude differences exist.

MRB

AVIATION
(For the 18z TAFs through 18z Monday Afternoon)
Issued at 1219 PM CDT Sun May 19 2024

VFR conditions are currently forecast through the valid TAF period. Weak southerly winds exist at all regional terminals, with a warm front now north of KUIN. Strong instability exists across the region, and though the front is weak and mid-level forcing is non-existent, we may see an errant thunderstorm near KUIN this afternoon. Confidence in even vicinity impacts at the terminal is low, and as such was kept out of the TAFs at this time.

Remnant thunderstorms from the central Great Plains will enter central Missouri tonight into tomorrow morning. A few thunderstorms may occur, but by that stage there is low confidence in much thunder. VCSH was added, but may need to be adjusted to include thunder if confidence increases in that. The convection will decay further as it approaches St. Louis, and may not make it there at all. If it does, it would be in the 17-20z timeframe.

MRB

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

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