FXUS63 KLSX 220339 AFDLSX Area Forecast Discussion National Weather Service St Louis MO 1039 PM CDT Fri Aug 21 2026 .KEY MESSAGES... - Seasonably cool late summer temperatures are expected this weekend into next week with few opportunities for rain. && .SHORT TERM /THROUGH SUNDAY/... Issued at 210 PM CDT Fri Aug 21 2026 Currently a ridge of high pressure over the Intermountain West has spread hot and dry conditions out into the Plains. A trough centered over Hudson Bay has extended its influence southward into the Great Lakes, with this trough expected to continue pushing into the Great Lakes and New England over the coming days. A leading shortwave trough is pushing through Wisconsin toward Chicago this afternoon, with a cold front sliding southward behind it. There are two primary areas of convection expected in the next 12 to 24 hours along this front: The first is closer to the shortwave trough tracking into northern Illinois. Here some modest moisture convergence aided by lift from the shortwave trough will lead to scattered thunderstorms potentially merging into a broader complex pushing southeast overnight. It is likely that these storms track to the east of our forecast area, but there's an outside chance (20 percent) that they do move through our far eastern counties overnight. While there is some severe weather threat with this, the threat will be greatest in the evening hours closer to the core of the thunderstorm complex. The threat locally will be considerably lower and later in the night. The second is to the west over Nebraska and Kansas. Here a low level jet this evening will send modest moisture northward toward the front and trigger another thunderstorm complex to develop. This tracks southeast potentially into western Missouri overnight, but is unlikely (less than 10 percent) to make it into our forecast area. The most likely scenario for us tonight will be a subtle passage of the cold front overnight, with winds turning out of the north by morning. Expect a mostly sunny day behind the front on Saturday. Despite cold advection, deep post-frontal mixing will ensure it's another warm day in the mid to perhaps upper 80s. That deep mixing, though, is expected to mix in some drier air from aloft. We've deliberately included more of the higher resolution guidance in the forecast to reflect this drying. Some guidance has dewpoints falling well into the 50s with a corresponding increase in temperature. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Issued at 210 PM CDT Fri Aug 21 2026 As we go through the weekend, the trough spins into southeastern Canada while the ridge expands its influence eastward into the Rockies and Great Plains. Surface high pressure moving in behind tonight's front will ensure that our area remains more under the influence of the cooler air mass from the trough despite rising heights aloft. Highs only reach the low 80s with lows in the low 60s or even upper 50s at times. As the ridge nudges eastward our area will become better situated to have some effects from ridge running shortwave troughs, what we sometimes call the "ring of fire" of thunderstorm complexes rotating around a ridge. At first, low level moisture is pretty limited due to northerly low level flow near the Gulf, but over the next few days better moisture does eventually lift northward into the Southern Plains. It has more difficulty moving eastward, though, as the influence of the surface high moving into the western Great Lakes and Ohio Valley remains strong. We've seen a growing consensus in guidance at this point for the focus of convection Sunday into Monday to stay just to our southwest along with the effective warm front. Some cloud cover from this activity to our west may actually help to hold temperatures down even further on Monday, potentially in the 70s. Meanwhile on the other side of the warm front the heat is on with temperatures potentially near 100. The next more substantial shortwave trough tracks through the northwest flow on Wednesday, but it's not clear we ever get into the warm, moist air by then. So despite the passage of another weak cold front, we may not get any rain out of it. NBM POP remains low (less than 20 percent), and most of the low resolution ensemble guidance keeps the focus for precipitation to our south where the better moisture exists or to our north where the core of the upper wave will be. Additional shortwave troughs have some potential (20 percent or less) to bring additional rounds of showers and storms through the end of the week, but confidence gets lower with time on when and where those waves will track. There is a general consensus, though, that the ridge eventually expands far enough east that we eventually get in on the warmer weather again. NBM probability of 90 degree heat increases to about 30 percent by Friday. && .AVIATION /00Z TAFS THROUGH 06Z SUNDAY/... Issued at 1036 PM CDT Fri Aug 21 2026 Mainly VFR flight conditions are expected through the TAF period at all local terminals as a dry cold front drops through the area during the morning hours and causes winds to shift to out of the north-northwest. There is a threat of fog at KSUS, but the front's passage may mitigate this. If fog does develop, dry air and the wind shift with the front will help quickly dissipate it by around sunrise. && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ SHORT TERM...Kimble LONG TERM...Kimble AVIATION...Elmore