FXUS63 KLSX 081107 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 607 AM CDT Wed May 8 2024 .KEY MESSAGES... - Scattered to widespread severe thunderstorms are expected from the late this morning into this evening. All hazards are expected: large hail, damaging winds, a few strong tornadoes, and flash flooding. - A period of much needed calmer weather and cooler temperatures is expected to end the week and continue through the weekend. && .SHORT TERM... (Through Thursday Night) Issued at 316 AM CDT Wed May 8 2024 Surface observations show a low developing over the Southern Plains beneath southwesterly flow aloft. A cold front stretches through the Ohio Valley and into the Mid-South. As the surface low strengthens this morning, the front will lift northward as a warm front. South of this front, a warm, unstable airmass will advect into the Middle Mississippi Valley. The nose of the low level jet will veer into southwestern Missouri this morning and force convection along the warm front that will then move east/southeast across the front and within a warm sector characterized by nearly 3,000 J/kg of SBCAPE and 60 kts of 0-6 km shear. Thusly, this morning-early afternoon convection will have the potential to be strong to severe, but its mode is uncertain, with two scenarios possible. One is where it congeals and grows upscale into an MCS that passes through southeastern Missouri and into the Ohio Valley. This would pose primarily a damaging with threat, though embedded supercells would also lead to a large hail threat. A tornado threat would also accompany any embedded supercells, as well as mesovortices on any line segment that is able to bow toward the northeast given 0-3 km bulk shear of about 30 kts oriented southwest to northeast. This scenario may dampen the severe potential of any afternoon/evening thunderstorms. The second scenario is that morning- early afternoon convection is able to stay more discrete and supercellular, which would lead to a greater risk of tornadoes and large hail that would carry into the afternoon and evening as multiple rounds of storms develop. Regardless of either scenario, CAMs have been consistent in producing multiple rounds of convection through the afternoon and into the evening within the warm sector (generally along and south of I-70) thanks to a persistent low level jet nosing into the area and upper-level divergence via a jet streak. The warm sector will potentially remain just as conducive for severe thunderstorms through the afternoon into the early evening as it is in the late morning, with any storm capable of all severe hazards. Strong tornadoes are of particular concern during the afternoon given 0-1 km SRH upwards of 250-300 m2/s2. Also of concern is the potential for flash flooding. Mean PWAT from the 00z HREF maxes out at 1.5" over southern portions of the CWA, with the same guidance producing a swath of 2-5" in that portion of the area given training storms. Because of this, we have issued a Flood Watch for portions of southeastern Missouri and southwestern Illinois. Convection will exit the CWA this evening ahead of a cold front sweeping through the region. This front will usher in cooler air for Thursday, with highs running 5-10 degrees below what is expected today. Moisture wrapping around the backside of the departing low will produce low chances (30%) for rain mainly north of I-70 during the afternoon and early evening. Elmore && .LONG TERM... (Friday through Tuesday) Issued at 316 AM CDT Wed May 8 2024 At the start of the extended period on Friday, guidance consensus is that an upper-level trough will be east of the region with a shortwave digging southward over the CWA along the through's western periphery. A minority of ensembles and deterministic guidance have light rain occurring over the CWA with its passage, but given it is the minority, a dry forecast has been maintained for now. The deep northwesterly flow will advect cooler air into the area, with a majority of ensemble guidance suggesting temperatures will struggle to get out of the 60s for most locations. Yet another shortwave will pass through the Midwest late Friday into Saturday and send a cold front through the CWA early Saturday. Despite this cold front, mid- level temperatures will actually warm through the day as low to mid- level ridging builds into the area, leading to Saturday being a few degrees warmer than Friday. Sunday into early next week, guidance consensus is that the trough will continue eastward as upper-level flow over the central U.S. becomes quasi-zonal. Some deterministic guidance has a shortwave passing through the Mid-South and Ohio Valley within this flow late Sunday into Monday, bringing low rain chances to the area. As we get toward the middle of next week, guidance consensus breaks down, with ensemble clusters and deterministic models roughly split between northwesterly flow and southwesterly flow over the Middle Mississippi Valley. If the former comes to reality, then temperatures will stay mild and conditions likely dry. If the latter solution is realized, then temperatures will warm and the area could experience more active weather. Elmore && .AVIATION... (For the 12z TAFs through 12z Thursday Morning) Issued at 557 AM CDT Wed May 8 2024 Multiple rounds of thunderstorms are expected to impact the local terminals from late this morning through this evening. Timing in the TAFs represents when confidence is highest in impacts; however, the threat of thunderstorms could occur in a longer time frame than currently indicated. Thunderstorms will be capable of large hail, gusty winds, and tornadoes, particularly at KCOU, KJEF, KSUS, KSTL, and KCPS. Additionally, heavy rainfall will be capable of reducing visibilities. The storm and rain threat will exit this evening, though I can't rule out some showers around KUIN early tonight. Elmore && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Flood Watch from 10 AM CDT this morning through this evening for Cole MO-Crawford MO-Franklin MO-Gasconade MO-Iron MO- Jefferson MO-Madison MO-Moniteau MO-Osage MO-Reynolds MO- Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO-Washington MO. IL...Flood Watch from 10 AM CDT this morning through this evening for Monroe IL-Randolph IL-Saint Clair IL. && $$ WFO LSX