Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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300 FXUS63 KLSX 220344 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1044 PM CDT Sat Sep 21 2024 .KEY MESSAGES... - Our best opportunity to receive widespread beneficial rainfall will arrive tonight and last through Sunday morning. Additional chances for showers and thunderstorms exist until Tuesday. - There is a chance of a few strong to severe thunderstorms on Sunday afternoon ahead of a cold front. The primary threat with the strongest cells will be damaging winds. - Temperatures next week will start out well below average behind Sunday`s cold front and will moderate through the week. && .SHORT TERM... (Through Late Sunday Night) Issued at 255 PM CDT Sat Sep 21 2024 Surface analysis shows that yesterday`s cold front has regressed back to the north, clearing the area as a warm front. Most of Missouri remains fairly cloudy from ongoing showers and thunderstorms, limiting temperatures. Across the Mississippi River, temperatures have had more freedom to climb with current observations near 90 degrees in some locations. There is a 20 - 30% chance of showers and thunderstorms areawide this afternoon and evening, but plenty of dry time is expected. The greatest opportunity for stronger thunderstorms will be in southern Illinois where sunshine has been more abundant today. 3000+ J/kg of SBCAPE is present here, and bulk shear values are 20 - 25 kts. Ongoing convection that has moved into this environment has strengthened, becoming capable of small hail and gusty winds. Additional development here will likely be capable of the same. The better chance for showers and thunderstorms will arrive very early Sunday morning. This will also serve as our best opportunity to see widespread beneficial rainfall within the next week. In the mid-levels, multiple series of vorticity maxima will eject into the Mid-Mississippi Valley from a cutoff low entering the central Plains. In the lower levels, a southwesterly 20 - 30 kt low-level jet will develop overnight as a cold front pushes southeast into northern Missouri. Both the LLJ and cold front will bring additional moisture into the CWA, with PWAT values forecast to reach nearly 2". This is corroborated by the HREF grand ensemble which shows a swath of at least a 50 - 70% chance of this occurring Sunday morning. For reference, the maximum PWAT value at SGF for this date is 1.79". Although ample moisture and forcing will be in place tonight, the concern for flooding is slim to none. Six hour flash flood guidance values are nearly 5 inches since antecedent conditions are so dry. To achieve these values, we will need very slow moving thunderstorms, which deterministic guidance is quite confident won`t happen. We`ll then need training thunderstorms, which is possible given the orientation of the low-level jet. However, with the mix of showers and thunderstorms expected and the progressive nature of the system, it seems unlikely that any given location will see a series of thunderstorms that would produce 2" - 3" of rain per hour or 5" in 6 hours. Because of this, the largest impact of this system will be a drought-improving rain. Sunday morning`s showers and thunderstorms are forecast to exit the forecast area late in the morning, with some clearing expected in its wake. By this point the cold front will be well within the CWA, and afternoon highs will reflect that. A near 20 degree temperature range is forecast, with temperatures in northeast Missouri pushing 70 degrees and temperatures in southeast Missouri nearing 90 degrees. Southeast Missouri is also where the most instability will reside. Mid-level forcing and low-level convergence will be much more pronounced than they are today, promoting the potential for thunderstorm development in the afternoon. Most guidance is in consensus that this will happen, but confidence decreases concerning the potential for severe thunderstorms. How much instability we get will be tied to cloud cover, which most models have been bulking up on in recent runs. Southwest Missouri has the greatest chance for late morning clearing, so stronger thunderstorms may develop there and shift east into our area. There is still the possibility that convection that develops in our CWA becomes strong to severe given the amount of forcing and bulk shear (30 - 40 kts) we have in store already. We may not need an excess amount of CAPE when we have so much else in place. If a thunderstorm does become severe, damaging winds will be the primary threat. Jaja && .LONG TERM... (Monday through Next Saturday) Issued at 255 PM CDT Sat Sep 21 2024 The potential for showers and thunderstorms will continue Monday and into Tuesday as vorticity maxima keep ejecting out of a mid-level shortwave in the central Plains. Instability will be at a minimum, however, as low-level cold air advection tanks our temperatures behind the front. Highs on Monday are forecast to remain in the 60s for nearly half of the CWA, with the warmest locations only reaching the upper 70s. These values are below average for late September, and do not promote the development of strong thunderstorms. The shortwave axis is forecast to pass the region sometime on Tuesday, ending our rain chances until at least late in the work week. Low- level moisture will be scoured out, leaving dewpoints in the 50s and low 60s. Those dewpoints alongside highs in the 70s will create a seasonable feel to accompany the quiet weather. The slight chance for showers and thunderstorms returns late in the week as a cutoff low spins somewhere over the central or southern CONUS, but guidance varies wildly concerning this feature so there`s not much to discuss. Jaja && .AVIATION... (For the 06z TAFs through 06z Sunday Night) Issued at 1044 PM CDT Sat Sep 21 2024 VFR conditions are expected for a few more hours with isolated showers moving through eastern sections of Missouri prior to 06z. Conditions deteriorate around 06z with showers and thunderstorms moving in from west to east. Just prior to 04z this evening, showers and thunderstorms spread from near the MO/IA border through west-central MO and into the southern Plains. This broad mass of activity will translate eastward overnight through into Sunday morning. MVFR ceiling/visibilities will be the primary impact with localize pockets of IFR possible at any terminals directly impacted by thunderstorms. This potential is greatest at KCOU/KJEF. While these conditions cannot be ruled out at KUIN and metro terminals, thunderstorms will likely weaken somewhat as they track east early Sunday morning. Though a bulk of the precipitation was originally expected to shift east through mid-morning Sunday, some of the latest trends in data show a slightly southward shift in the axis of heavier rainfall. If this trend continues, impact may extend slightly later into the morning. There is a brief lull behind this initial wave and the arrival of the cold front Sunday evening, when activity flares up again along and ahead of the front. The position of the front will determine exact placement of higher potential Sunday evening, but the general consensus if for rainfall to shift southeast of KUIN/KCOU/KJEF around 00z and metro terminal after 02z-03z. A few isolated showers may linger with MVFR ceilings along and behind the front. VFR could make a return at KUIN before the end of the TAF period. Maples && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX