FXUS63 KLSX 091703 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1203 PM CDT Thu Jul 9 2026 .KEY MESSAGES... - Severe thunderstorms capable of producing damaging winds, large hail and a few brief tornadoes are possible this afternoon and evening. Locally heavy rainfall is also possible with these storms through tonight. - Another round of severe thunderstorms is possible on Friday afternoon and evening, and locally heavy rainfall will also be possible with these storms. - Generally dry weather is expected Sunday into next week with warmer than average temperatures. && .SHORT TERM...THROUGH FRIDAY/... Issued at 1036 AM CDT Thu Jul 9 2026 Severe thunderstorms and locally heavy rain remain the primary forecast concerns today and Friday. An MCS over northeast Kansas looks like it's spinning up an MCV at this time. Short range guidance moves this MCV east through the day at varying speeds. The RAP may be the slowest, only pushing the vortmax as far as the Mississippi River by 00Z. Various CAMs, the GFS and short range ensembles such as the HREF and REFS move the wave more quickly eastward, crossing the Mississippi river into Illinois around 18Z today. The timing and track of the MCV will be part of what determines convective trends this afternoon. The faster solution would limit afternoon convection mainly to areas along and east of I- 44 in Missouri and along and south of I-70 in Illinois. However the slower solution puts the entire area under threat of thunderstorms this afternoon. Since the model consensus is faster, am leaning in that direction with highest PoPs over east central and southeast Missouri into southern Illinois. Kept low chance PoPs over most of the remainder of the area keeping the slower RAP in mind. Models are showing 3000+ J/Kg CAPE with 30-35kts of deep layer shear which is good enough for storms to become organized into severe multicell clusters or even a severe MCS. Primary threat remains damaging winds in excess of 50kts. Boundary interactions along with a bit of extra directional shear associated with the MCV may be enough for a brief tornado as well. Additionally, there's enough CAPE in the hail-growth zone to support hail up to around 1 inch in diameter. The severe threat should diminish with weakening instability during the evening. The low level jet increases to 30-40kts tonight which will produce moderate to strong moisture convergence along any low level boundary laid down by earlier convection. This should result in more thunderstorms tonight, though precisely where the storms will develop will depend on where the low level boundary settles. Some guidance keeps it farther north closer to the I-70 corridor, while others are more across the Ozarks. Regardless, the low level jet veers overnight which should make the storms progressive along the boundary, but there will probably be more than one wave of thunderstorms that track along it. There is therefore a locally heavy rain threat, though widespread flooding doesn't look likely at this time as flash flood guidance is pretty universally well in excess of 2"/hr. Convective trends for Friday and Friday night are very dependent upon where the effective boundary ends up Friday morning, and on another convectively enhanced wave moving across Missouri into Illinois. There's a question of how unstable the atmosphere will get Friday afternoon into Friday evening depending on where the effective surface boundary is. However, severe wind gusts and hail will be possible again. && .LONG TERM...FRIDAY NIGHT THROUGH WEDNESDAY/... Issued at 1036 AM CDT Thu Jul 9 2026 Models vary in how strong the aforementioned convectively enhanced short wave is as it moves from Missouri into Illinois late Friday night into Saturday morning. Some solutions show a strong wave which develops a closed low at the surface that drives a composite weak cold front and lingering convective boundary through Missouri Saturday morning. This would dry out the low levels and likely end the chance for thunderstorms. Others are weaker and allow the boundary to linger over the area through Saturday which would keep a chance for precip going longer. There's no particular reason to favor either solution at this point as it will depend on convective trends upstream over the Great Plains. Guidance continues to show a strong upper level ridge developing over the western U.S. late Saturday into Sunday which spreads east across the Plains and Midwest by early next week. Models continue to show some differences in the speed with which it builds east, and the ultimate strength of the ridge. However, the consensus continues to put the thermal ridge north of our area across the Upper Midwest. The NBM temperature forecast therefore looks pretty good with dry and warm, but not excessively warm weather next week. && .AVIATION... (For the 18z TAFs through 18z Friday Afternoon) Issued at 1200 PM CDT Thu Jul 9 2026 Thunderstorms are likely to develop by mid-late afternoon, mainly in southeast Missouri and south-central Illinois. Additional storms are forecast this evening into the early overnight hours from central to southeast Missouri. That leaves the central Missouri and the metro St. Louis terminals on the northern edge, so maintained PROB30 groups. Any storm that does manage to directly impact a terminal could produce some gusty winds and lower visibility below 3SM in a downpour. Gosselin && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Flood Watch from 4 PM CDT this afternoon through Saturday morning for Crawford MO-Iron MO-Madison MO-Reynolds MO- Saint Francois MO-Sainte Genevieve MO-Washington MO. IL...Flood Watch from 4 PM CDT this afternoon through Saturday morning for Randolph IL. && $$ WFO LSX