863 FXUS63 KLSX 170828 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 328 AM CDT Thu Apr 17 2025 .KEY MESSAGES... -There is a low threat for severe thunderstorms across the area Friday afternoon and evening, with large hail, and to a lesser extent damaging winds, the main hazards. -The heavy rainfall and flooding threat continues to increase near and along I-44 (MO) and I-70 (IL), with 3-5" of rain possible through Monday morning. -There remains a conditional chance for a strong to severe thunderstorm or two across southeastern missouri and southern Illinois Sunday. && .SHORT TERM... (Through Late Friday Afternoon) Issued at 327 AM CDT Thu Apr 17 2025 Showers and a few thunderstorms will exit the forecast area this morning as the upper level jet entrance and low-level jet forcing this precipitation shift east/northeastward out of the CWA. The remainder of the day will be warm and dry with low-level southerly flow helping temperatures warm an additional few degrees from yesterday, with some portions of central Missouri touching 80 for their high. Southwesterly flow will dominate the mid-level pattern by this evening. Overnight a mid-level disturbance will combine with the ramp up of the low-level jet in the vicinity of a surface low/triple point to spawn showers and thunderstorms northwest of the forecast area over Iowa. Convection is expected to form and track northeast to east with the deep layer shear, missing our forecast area all together, so we have pulled the threat of strong to severe thunderstorms from our messaging. The surface low will be positioned across the intersection of NE-IA-MO-KS overnight, but will gradually track to the northeast through Iowa during the day, helping to advance a cold front into the forecast area by the afternoon. Southwesterly flow ahead of the approaching cold front will pull warm, moist air into the region, pushing dewpoints into the 60s and high temperatures into the 80s areawide. Convection will form ahead of and along the front during the late afternoon and evening, driven by surface convergence and mid-level disturbances. Instability will form ahead of the front, with the HREF and deterministic models indicating 1000-2000 J/kg of SBCAPE available across the forecast area. Strong 0-6 km shear (50+ kts) will also be available ahead of and along the front to organize any severe thunderstorms that form. Most guidance also continues to show a strong low-level warm nose inhibiting surface based convection ahead of the cold front during the afternoon, although a few sources do show CIN eroding during the afternoon. If surface based convection is able to form, large hail and damaging winds would be favored threats while the tornado risk would remain very low due to veered (and therefore unfavorable) 0-1 km shear. Elevated convection, which is more favored at the moment, would result in large hail as the main threat. Delia && .LONG TERM... (Friday Night through Wednesday) Issued at 327 AM CDT Thu Apr 17 2025 The front will stall out and remain quasi-stationary across southern Missouri/Illinois over the weekend, focusing numerous rounds of showers and thunderstorms that will move through the region. These rounds will be driven by the almost continuous stream of mid-level disturbances along the southwesterly flow. Guidance continues to hone in on I-44 (MO) and I-70 (IL) as the location for the axis of heaviest rainfall as the surface and elevated fronts waggle north and south slightly with each passing wave. A few thunderstorms will mix in during Saturday morning as SREF guidance shows a 50-70% chance of at least 250 J/kg of MUCAPE along the axis of heavy rainfall. These thunderstorms will locally enhance rainfall amounts. Currently, a swath of 40-50% chance for at least 3" of rain through 7 AM Monday exists across the axis, with a 10-20% chance for exceeding 4.5" of rain. In areas where thunderstorms linger Friday night into Saturday morning, rainfall amounts closer to 4.5" are expected. The bulk of the rainfall (2-3") will fall from early Saturday morning through Saturday night, and is expected to fall on soil primed from convection Friday afternoon and evening. Flooding of low lying areas is possible, particularly where thunderstorms linger. Additionally, at least minor flooding is expected along the Meramec, Big, Bourbeuse, and Black rivers and more significant rises along the creeks and streams in those basins. Through the weekend the mid-level flow will remain southwesterly as a trough swings from the Four Corners region into the mid- Mississippi Valley. At the surface a low will move northeast from Texas into the forecast area late Sunday into early Monday, bringing one more shot of rainfall to the entire forecast area. Despite the strong forcing, instability will be the limiting factor for strong to severe thunderstorm development with only a 50% chance for MUCAPE greater than 500 J/kg, 0-6 km shear greater than 30 kts, and CIN less than -25 J/kg. I cannot rule out a strong to severe thunderstorm, but confidence in this developing remains low, and we are not publicly messaging this potential. The system will exit the area Monday, and an advancing surface high will help to clear out any remaining light precipitation. The surface high will also help keep the region dry through a portion of Tuesday before chances for precipitation return to the forecast, driven by multiple disturbances passing through the quasi-zonal mid-level flow. Delia && .AVIATION... (For the 06z TAFs through 06z Thursday Night) Issued at 1035 PM CDT Wed Apr 16 2025 Only minor changes were made to the 06Z iteration of the TAFs. VFR conditions are expected to begin the TAF period. Near the start of the period, waves of showers will likely begin to affect UIN and St. Louis area terminals, with showers beginning a few hours later at COU/JEF. A few thunderstorms will also be possible at all terminals, but confidence is low in terminal impacts due to sparse coverage. Ceilings are also likely to lower overnight through tomorrow morning, perhaps to MVFR although this may not be common. Category reductions due to ceilings are most likely at UIN. Meanwhile, breezy south winds are expected from late this evening through the end of the period. Low clouds are expected to gradually diminish through the day tomorrow, and most sites should end the period with VFR conditions. Some marginal low level wind shear is possible at all terminals near the end of the period, but winds will be mostly unidirectional in the low levels, and confidence is low that surface winds will weaken enough to meet wind shear thresholds. BRC && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX