Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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640 FXUS63 KLSX 211956 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 256 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 18z TAFs through 18z Sunday Afternoon) Issued at 1231 PM CDT Sat Sep 21 2024 This morning`s showers extending from northeast through central Missouri continue to very slowly diminish in coverage. For this reason, took VCSH out of the St. Louis metropolitan terminals this afternoon. There is a slight chance of showers and thunderstorms through this evening, but coverage and chances are too small to include mention in the TAFs. Widespread rain with some thunder is forecast to begin late tonight and continue into tomorrow morning ahead of a cold front. Winds will shift dramatically during this time period, making it difficult to choose a direction at any given terminal. This is exacerbated given the outflow boundaries in the area and the exiting warm front. Winds during the period are not expected to be especially gusty, with prevailing winds generally at 10 kts or less. Jaja && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX