Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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530 FXUS63 KLSX 221134 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 634 AM CDT Wed May 22 2024 .KEY MESSAGES... - Much cooler temperatures across the region today behind the cold front. Highs will be 10 to 15 degrees cooler than yesterday. Some showers and thunderstorms are possible south of I-70, but the severe weather risk is low. - The rest of the week and weekend will feature multiple opportunities for showers and thunderstorms with daytime temperatures in the 70s and 80s. It won`t be a total washout, but periods of rain are possible just about any day. The driest day is looking to be Saturday, while Sunday has the best chance of stronger storms near our area. && .SHORT TERM... (Through Late Thursday Afternoon) Issued at 313 AM CDT Wed May 22 2024 As of 3AM the cold front has just about cleared our forecast area to the southwest, with winds shifting to the west behind it and dewpoints falling into the 50s. The front will settle near the Ohio River this afternoon with our area solidly on the cool side of this boundary with high pressure building in from the west. A mid level shortwave trough tracks east across the region this afternoon, likely sparking some elevated showers and thunderstorms as it does so. Instability on the cool side of the boundary is pretty weak, though, so we are not expecting much potential for severe weather. The better severe weather threat will be along the surface cold front where greater instability and shear exists. The Day1 Outlook from SPC has continued to shift the greatest threat southeast as the frontal position becomes more clear, although we remain in the lowest level marginal risk. If a storm does get strong to severe today the threat would most likely be hail. This threat is low enough that we are not messaging it aggressively. Surface high pressure pushes to the east tonight into Thursday, allowing a light southerly return flow to develop. The remnant front will have lost its definition at this point especially after convection along it Wednesday afternoon. Even so, as moisture oozes back north it will create another opportunity for showers and thunderstorms tonight into the day Thursday, mainly for the southern half of our forecast area. We will be developing shortwave ridging in the wake of Wednesday`s trough, though, so forcing to focus lift to trigger showers and storms will get harder to come by despite increasing instability on Thursday. Kimble && .LONG TERM... (Thursday Night through Tuesday) Issued at 313 AM CDT Wed May 22 2024 The general pattern remains fairly well agreed upon with a west to southwest flow aloft and multiple shortwave troughs through the weekend bringing multiple chances for showers and thunderstorms most days. The strongest of these waves comes through around Sunday which represents our clearest threat for showers and thunderstorms. On Friday the best shortwave energy passes by to the north with a wave moving through the Northern Plains. The pull of warm, moist air into this system will bring an unstable air mass through our area and provide a chance of showers and thunderstorms. While instability is maximizing, shear remains pretty weak with the best flow aloft well to our north closer to the core of the trough. In fact, overall lift is rather weak in our area ahead of the cold front and current forecast POP from NBM may be too high. If storms do develop then they will likely be weakly organized in a very unstable environment and weak to moderate shear. The marginal risk from the SPC Day3 outlook seems appropriate at this stage with locally damaging winds or large hail the likely threat if stronger storms develop. The uncertainty on where the focus for storms will be provides us enough uncertainty to delay messaging this just yet. Friday`s trough to our north slings a cold front through our area Friday night bringing a slightly cooler and drier air mass in behind it. This sets Saturday up as likely the best day of the weekend with the lowest rain chances and very comfortable temperatures. The next trough tracks further south than the prior one, closer to our forecast area on Sunday and deepening as it moves through. Moisture return ahead of it Saturday night into Sunday morning represents our best chance of showers and thunderstorms of the weekend. How this evolves is still uncertain and could come with a threat for strong to severe storms depending on how it plays out. The greatest uncertainty is on the timing of the wave as well as how far north the surface warm sector lifts ahead of it. North of the surface warm front storms will be elevated and primarily a threat for large hail depending on the amount of instability available. In the warm sector where surface-based instability is present a broader severe weather threat is possible especially if the timing of the wave aligns well with daytime heating. At the moment, that threat looks greater to our south. Behind this trough we get at least a couple of days of cooler weather again with a drying trend as well. Lingering PoPs beyond Sunday are primarily due to the potential for a backside wave moving through the northwest flow behind the departed trough. If this occurs it will likely be a period of light showers Monday or Tuesday within an otherwise largely dry pattern. There is considerable spread in the guidance on that backside trough so confidence is fairly low at this point. Kimble && .AVIATION... (For the 12z TAFs through 12z Thursday Morning) Issued at 633 AM CDT Wed May 22 2024 Much lighter winds expected today out of the west with mostly VFR conditions expected. A few elevated thunderstorms are possible in the St Louis metro this afternoon. Winds gradually become southerly by tomorrow morning with a chance of a few showers or maybe a thunderstorm as well. Kimble && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX