430 FXUS63 KLSX 181710 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1210 PM CDT Wed Jun 18 2025 .KEY MESSAGES... - Severe thunderstorms remain possible this afternoon and evening, particularly over southeast Missouri and southwest Illinois. - Heat index values may climb above 100 degrees starting Friday and last into early next week. && .SHORT TERM... (Through Late Thursday Afternoon) Issued at 258 AM CDT Wed Jun 18 2025 Latest radar mosaic is showing scattered thunderstorms developing over central Missouri with an organized MCS moving east across southern Missouri and northern Arkansas. While the CAMS have the MCS staying south of the CWA, they do show a MCV moving northeast that will help generate additional shower and thunderstorms, mainly across central and northeast Missouri into west Illinois early this morning between 5 and 9 am. The outflow from this activity may very well either push into the eastern and southern parts of the CWA this morning, or perhaps out of the CWA altogether depending on the strength of the outflow boundary. If the boundary does stay in the the CWA, it will act as a focus for additional thunderstorm activity as latest runs of the HRRR is showing some additional development over the eastern CWA late this morning in the form of discrete cells. Forecast soundings from the RAP at SLO and FAM this afternoon are showing MLCAPES of 1000-2000 J/kg with deep layer shear 30-40 knots which will be enough to support organized multi-cells and perhaps supercells, with the main threat being damaging winds with large hail and tornadoes also possible. The chance for showers and thunderstorms will come to an end from northwest to southeast as front currently extending from Iowa into Nebraska will move southeast across the area tonight. Dry weather is expected on Thursday as an upper ridge begins to build in from the Plains. Britt && .LONG TERM... (Thursday Night through Tuesday) Issued at 258 AM CDT Wed Jun 18 2025 The LREF has the aforementioned ridge building over the Midwest bringing dry and warm weather late week into the weekend. The NBM is showing IQR of only 2-4 degrees at STL from Friday through Monday with highs in the lower to middle 90s. With dewpoints forecast in the 70s, heat indices climbing above 100 degrees seem reasonable with the LREF probabilities AOA 50% over parts of the area on Saturday and Sunday afternoons. Britt && .AVIATION... (For the 18z TAFs through 18z Thursday Afternoon) Issued at 1205 PM CDT Wed Jun 18 2025 The threat for thunderstorms during the first few hours of the TAF period has shifted east of the terminals. Scattered showers and thunderstorms are possible this afternoon into evening, though chances are low (20-30%) so have included a mention only in a PROB30 group. Confidence in if and where thunderstorms develop is low at this point. Thunderstorms will diminish tonight, leaving VFR conditions for the remainder of the forecast period. Delia && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX