Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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256 FXUS63 KLSX 240839 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 339 AM CDT Fri May 24 2024 .KEY MESSAGES... -Thunderstorms are expected this afternoon and evening along the approaching cold front, several will be strong to severe and capable of all hazards. -Severe thunderstorms are expected again on Sunday during the morning and again during the afternoon/evening, with large hail, damaging winds, and a tornado possible. -Next week will usher in a pattern change toward drier, more seasonal weather. && .SHORT TERM... (Through Late Saturday Afternoon) Issued at 332 AM CDT Fri May 24 2024 An MCS has developed across portions of eastern Nebraska just ahead of a surface cold front. The cold front will march eastward into the forecast area this evening as a mid-level trough slides to the northeast through the day. Ahead of this system, showers and isolated thunderstorms have blossomed in the warm sector across northern Missouri and Iowa. Through the early morning hours it has become apparent that the MCS is most likely going to move east/northeast in step with the mid-level trough sliding through Iowa and leaving the CWA largely free from convection this morning. This will leave the 1500-2000 J/kg of SBCAPE (albeit well capped) already present across our area untapped, and in the absence of widespread convective debris, will only increase through the day. A mixture of discrete and multi-cell thunderstorms would be able to develop along the cold front as it marches through the area this afternoon and into the evening. In an environment with 2000+ J/kg of MLCAPE and 25-40 kts of 0-6 km bulk shear, several will be able to become strong to severe. Hail and damaging winds will be the main hazards given the 7-8 degree/km mid-level lapse rates and robust instability, though I cannot rule out a tornado as well. The best areas for severe thunderstorm development will be across central, northeast, and east-central Missouri into west-central Illinois where convergence along the front will be strongest. Convection will diminish as the cold front pushes through the area and weakens with time, largely during the evening hours. The cold front will exit the forecast area overnight and into the early hours Saturday, leaving a surface high and rising mid-level heights in its wake. Saturday continues to be the best day of the holiday weekend in terms of lack of precipitation chances. Late Saturday afternoon and into the evening a surface low will form across the Central Plains in response to an approaching mid-level trough. The warm front associated with this system will surge northward Saturday evening into early Sunday, increasing moisture and instability across the mid-Mississippi Valley. A LLJ will develop at the same time, both enhancing the northward surge of moist, unstable air, and also initiating and organizing convection across the forecast area with 35-50 kts of 0-6km bulk shear. MRM && .LONG TERM... (Saturday Night through Thursday) Issued at 332 AM CDT Fri May 24 2024 Deterministic guidance quickly grows this convection upscale into an MCS that tracks eastward along the Missouri-Iowa border during the morning hours. Thunderstorms along the southern edge of this system will be capable of becoming strong to severe as they will have access to an environment with dewpoints in the mid 60s, MUCAPE 1000- 2000 J/kg, and 0-6km shear vectors between 40-50kts. All hazards will be possible with any storms that become strong to severe, particularly if they are able to become rooted in the boundary layer. How far south this system extends into the forecast area remains uncertain, and this detail will impact severe thunderstorm chances later in the afternoon. The further south this system extends will use up more instability across the forecast area and stunt recovery. But it will also push the outflow boundary further south into the forecast area, which will be an area of potential development during the afternoon and evening. Another round of thunderstorms is expected along the cold front associated with the surface low as it moves through the region during the afternoon and evening Sunday. Overall coverage will be scattered along the cold front as convergence is much weaker than during the morning. However, where the cold front intersects with the outflow boundary from the morning convection will be an area of enhanced development both in coverage and intensity of thunderstorms. Continued moisture advection through the day combined with daytime heating and a strong shear profile will provide a robust environment for severe thunderstorms, particularly in areas untouched by morning convection. Where thunderstorms are able to form during the afternoon, they will quickly become severe, capable of large hail, damaging winds, and tornadoes. Given the likely track of the MCS and the intersection with the cold front, the Enhanced Risk from SPC looks reasonable. The mid-level trough axis and associated surface cold front will push east of the forecast area Sunday into Monday, ushering in a significant pattern change. Ensemble guidance depicts a mid-level trough becoming established over the eastern CONUS and a mid-level ridge rooting over the Rockies. This will leave the mid-Mississippi Valley in northwesterly flow for much of next week, resulting in near normal temperatures. While ensemble guidance does indicate that disturbances will slide through the northwesterly flow to impact the region, these will be weaker with less available moisture. This will result in lower rain chances overall and smaller amounts of rain when it does occur. This pattern will persist almost to the end of the forecast period, when the mid-level trough-ridge pattern begins to shift east. MRM && .AVIATION... (For the 06z TAFs through 06z Friday Night) Issued at 1038 PM CDT Thu May 23 2024 Dry and VFR conditions exist at the terminals tonight, and other than a few low clouds that may not be widespread enough for impacts, there are few aviation concerns until convection returns to the forecast. The threat for thunderstorms will return to the region from west to east Friday, but exactly when convection will impact any one area is still mildly uncertain and depends on convection to our north and west tonight. Two scenarios still look plausible: - One where an undisturbed line of thunderstorms tracks through the region during the late morning and early afternoon, ending any threat for later. - Another where the line deteriorates and leaves instability untouched for later in the day. Either way, conditions improve by 03Z when a cold front advances through. More benign conditions prevail from then on through the end of the TAF period. MRB && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX