Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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292 FXUS63 KLSX 250352 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1052 PM CDT Tue Sep 24 2024 .KEY MESSAGES... - Wednesday and most of Thursday will have the nicest weather of the week with temperatures in the 70s to near 80 and some sun. - Rain returns late this week and this weekend as tropical moisture gets pulled into the region. The most widespread rain comes on Friday but occasional showers and thunderstorms continue through the rest of the weekend. - While there is still uncertainty on where the heaviest rain will fall, there is increasing confidence that an axis of 1 to 3 inches of rain will occur in the region. && .SHORT TERM... (Through Late Wednesday Night) Issued at 321 PM CDT Tue Sep 24 2024 Surface low pressure is tracking northeast of Chicago this afternoon while upper level low pressure is digging southward through western Missouri. Expect a few more showers through the afternoon, with the potential for a few thunderstorms as well. But this activity should decrease around sunset as instability wanes. As the upper low continues to dig southeast toward the Bootheel on Wednesday, we`ll find our area on the relatively more subsident side of the low between the center of the low and a ridge nudging eastward over the Upper Midwest. This should provide some clearing skies and a nicer weather day than we`ve seen in a while. In fact we`re already seeing the clearing approaching from the northwest on satellite this afternoon, so there is hope for more sun tomorrow. While subsidence is the general rule tomorrow, promoting the dry forecast, we will still be near the cold core of the upper trough with strong warming at the surface. Thus I wouldn`t be surprised to see a few afternoon showers or perhaps a thunderstorm on Wednesday, although the chances at any one location are too low to bring mentionable PoP into the forecast. Kimble && .LONG TERM... (Thursday through Next Tuesday) Issued at 321 PM CDT Tue Sep 24 2024 While we finally end our streak of cloudy, damp weather, we`re actually setting up for another one. The upper trough digging across the region today will stall over the Midsouth. Meanwhile an incoming tropical system, recently named Helene, will track northward through the Gulf of Mexico and into the southeast US. These two lows interact with each other, with the end result being a wobbling upper low that lingers near the lower to middle Mississippi Valley right through the weekend. While there are understandably model differences in how these two lows interact (will Helene get fully absorbed in the upper low or will the two circulations remain distinct?) which contributes to general uncertainty in the forecast, there are some things that have become clear. First, after some drying the next few days a surge of tropical moisture will get pulled in from the southeast leading to an initial widespread rainfall. This comes in the late Thursday night into Friday time period. While NBM PoP has risen to 80 percent in the southeast part of the forecast area on Friday, our confidence in this round of rainfall is actually higher than that. Ensemble guidance shows measurable rainfall in nearly 100 percent of members in a 24 hour time period ending Friday night in this area, so some minor remaining timing uncertainty is holding the PoP for each period a bit lower. PWAT values surge to between 1.7 and 2 inches which indicates this synoptically driven rainfall has the potential to be a real soaker. Within the longer range ensemble guidance, the spread in the 25th to 75th percentile is large, ranging from 0.5 to 2.75. It`s the higher end of that where we probably should focus in on a bit more as this shows the more likely higher end potential with the uncertainty primarily on precise location of where this will happen. With this kind of moisture being advected in beneath the upper low there is strong confidence that an axis of 1 to 3 inches of rain will fall regionally. At the moment the most likely target area for this rainfall is southeast Missouri and southern Illinois. Much of this area actually missed out on some of the heavier rain in recent days so this would be a beneficial rain in those areas. If this instead focuses a little further north then we could start to see some concerns about another round of heavy rain onto soils that already saw 3 to 4 inches in recent days. In that case we could see some localized nuisance flooding or tributary creeks and streams rising significantly. The upper low lingers in the region through the rest of the weekend. Just as we`ve seen with the past few cut off lows, as we go forward in time and the low slowly fills, we transition from a more synoptically focused rainfall to scattered showers and thunderstorms driven by diurnal heating beneath the cool pocket of air aloft. So we`ll see in our forecast that PoP gradually lowers through the weekend as knowing where these daily showers will pop up gets harder to determine as forcing becomes more nebulous. With high moisture in place we may see additional significant rainfall from these showers and storms Saturday and Sunday, but it is likely to be more localized in nature each day. There`s a growing consensus in the guidance that the upper low finally pulls away to the northeast on Monday while the next incoming northern stream trough drives a clearing cold front through the region on Tuesday. This tells us that our daily showers and storms have an end date of Monday, with the cold front Tuesday shutting down this activity and bringing in a drier, cooler air mass at least briefly. Kimble && .AVIATION... (For the 06z TAFs through 06z Wednesday Night) Issued at 1052 PM CDT Tue Sep 24 2024 Light winds will persist overnight and as skies continue to clear, fog is forecast at KCOU, KJEF, KSUS, and KCPS. Fog is not forecast at KUIN because of more persistent cloud cover. At KSTL, fog climatologically does not form in these set-ups so confidence in its occurrence is too low to include in TAF. Expect fog to result in MVFR visibilities at KCPS, KJEF, and KCOU and IFR visibilities at times at KSUS. VFR flight conditions will return shortly after sunrise on Wednesday. Winds will be from the north as surface high pressure shifts into the region from the west on Wednesday. MMG/MRB && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX