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Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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250 FXUS63 KLSX 151121 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 621 AM CDT Thu Aug 15 2024 .KEY MESSAGES... - After some morning thunderstorms, hot and humid conditions overspread the region today with heat index values above 100 degrees. - Additional thunderstorms, some severe, are expected to develop late this afternoon and this evening ahead of an approaching cold front. Damaging winds and large hail are the primary threats although locally heavy rain is also possible. - Our stretch of wet weather ends with the cold front tonight as drier conditions are expected this weekend and into next week. Temperatures will be near normal for this time of year. && .SHORT TERM... (Through Late Friday Afternoon) Issued at 239 AM CDT Thu Aug 15 2024 An area of thunderstorms continues early this morning mainly along an axis from southeast Kansas into eastern Iowa and northwest Illinois. This is primarily due to moist advection in a low level jet centered around 850MB. This jet weakens as it shifts east this morning and convective activity likely decreases in coverage as well. With sun today we`ll warm into the upper 80s to low 90s with dewpoints climbing into the low to mid 70s. Heat index values push above 100 degrees but likely stay near or below 105, keeping us just below Heat Advisory. With the heat and humidity today, we`ll see much more instability than we`ve seen in recent weeks. SBCape of 3000 or more is likely by late this afternoon just as mid level flow increases around the base of a trough to our north. While the instability and shear parameters will be in place for strong to severe thunderstorms, primarily in the form of supercells or convective clusters, the uncertainty primarily centers on the trigger for thunderstorm initiation. There`s not a clear forcing mechanism to trigger storms in the warm sector this afternoon, however outflow boundaries from this morning`s convection could do the trick. Latest CAMs are more pessimistic on the afternoon convection, but I wouldn`t rule it out at this stage. If storms don`t get going in the warm sector by late this afternoon, better forcing will be able to focus along the advancing cold front this evening. More recent CAMs have hit this possibility a little harder, stretching the threat window a bit later in the evening, possibly until a little after midnight. The primary threats with thunderstorms this afternoon and evening will be damaging winds and large hail, although a tornado cannot be ruled out in areas where low level shear can maximize. The cold front makes good progress through the area overnight tonight, exiting our forecast area during the morning on Friday. This will bring drier air into the region and shift the focus for thunderstorms to the southeast for Friday, likely keeping our area dry. Temperatures behind the front will actually be fairly similar to today, in the low 90s, as plenty of sun will act on the drier air mass to warm things up. Kimble && .LONG TERM... (Friday Night through Wednesday) Issued at 239 AM CDT Thu Aug 15 2024 Ridging develops over the Rockies this weekend while troughing winding up over the Great Lakes is slow to progress eastward. The combination will put us in a northwesterly flow aloft which is generally a dry and somewhat cool pattern. As the trough does finally begin to push east early next week it will allow a flow of cooler and drier air out of the north behind it. This will lead to some pleasant late summer weather with temperatures only warming into the low 80s with low humidity. Nighttime temperatures drop well into the 60s, and perhaps the 50s. Kimble && .AVIATION... (For the 12z TAFs through 12z Friday Morning) Issued at 621 AM CDT Thu Aug 15 2024 Early morning thunderstorms are most likely in the St Louis metro over the next few hours. We should see a break in the activity lasting through much of the day with light to moderate SSW winds. Additional thunderstorms are expected this afternoon, although there`s considerable uncertainty on where and when. Current thinking is that afternoon storms develop south of all terminals, but thunderstorms developing over northern Missouri may impact all terminals in the evening or into the early overnight. Strong winds are possible with thunderstorms this afternoon and evening. Kimble && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX