Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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490 FXUS63 KLSX 232343 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 643 PM CDT Mon Sep 23 2024 .KEY MESSAGES... - There in a marginal risk for strong to severe thunderstorms this afternoon and evening mainly across southeast Missouri and southern Illinois. Damaging winds are the main threat. - Conditions will clear Tuesday, leaving dry conditions for the mid- week period. - Uncertainty remains in the return of rain Thursday through the weekend. && .SHORT TERM... (Through Late Tuesday Night) Issued at 207 PM CDT Mon Sep 23 2024 The region is socked in with heavy cloud cover once again today, stunting temperatures significantly. As of 2 PM temperatures have struggled to top 70 degrees in most areas. The mass of showers and thunderstorms that moved into the area this morning have been steadily moving northeast through the forecast area while another area of showers is moving into central Missouri. A weak surface low has developed across southwest Missouri in response to the approaching mid-level shortwave, with its attendant stationary front stretched across the Missouri-Arkansas border. As the mid-level shortwave approaches additional showers and thunderstorms will spawn across southern Missouri and Illinois this afternoon and evening. How many develop and how strong they become depends on how much instability is able to build, which has been limited by the aforementioned heavy cloud cover. Current RAP analysis shows 500- 1000 J/kg of MUCAPE across the southern forecast area with higher values across the warm sector south of the boundary. A few thunderstorms could organize in the 30-40 kts of 0-6 km shear to become strong to severe this afternoon. Damaging winds would be the main concern in areas that are able to clear out and mix a little, but an isolated case of 1" hail also cannot be ruled out. The best environment for severe thunderstorm development will be south of our forecast area where greater instability, as well as enhanced lift and shear from the front will give thunderstorms an extra boost. In this area, proximity to the front will locally enhance the tornado potential (2%), despite ambient 10 kt 0-1 km shear. If the front were to push northward over the next few hours, these conditions could move into the southern CWA. Showers and thunderstorms will linger into the overnight hours, largely across eastern Missouri and western Illinois. With precipitable water remaining near the climatological maximum for this time of year (1.50-1.75") and the potential for convective elements, heavy rain is once again a possibility with individual thunderstorms. The 3 hourly PMM indicates up to 2 inches of additional rainfall are possible within the areas of heaviest precipitation. The progressive nature of the thunderstorms and 3 hour flash flood guidance still near 2-3" will keep flooding at bay, though ponding of water in low lying areas is expected. Showers and thunderstorms will linger into tomorrow across the area as the mid-level trough associated with the previously mentioned shortwave slides into the region. These are not expected to be strong as moisture begins to erode with the approach of a surface high into the mid-Mississippi Valley. Temperatures will begin to rebound tomorrow from their unseasonably cool values today thanks to increase insolation and less precipitation, highs will peak in the low 70s. Delia && .LONG TERM... (Wednesday through Next Monday) Issued at 207 PM CDT Mon Sep 23 2024 The mid-level trough will inch southward during the mid-week period, with mid-level height rises and the advancing surface high keeping conditions dry. At the same time many ensemble systems point to a tropical system (Potential Tropical Cyclone Nine) pushing northward into the Gulf of Mexico. By the start of the weekend the tropical system is expected to get caught in the mid-level trough`s flow, increasing moisture advection into the lower and mid-Mississippi Valleys. The two systems will continue to interact while a Rex Block sets up over the CONUS. Recent runs have shown a northward shift in the system, the result is increasing precipitation chances for the second half of the work week and weekend. As previously mentioned, many of the ensemble systems depict this scenario, which is resulting in very high PoPs (70-80%) in the extended period. These PoPs seem too high given the uncertainty with the development, strength, and movement of this tropical system, but confidence is not high enough to make any significant changes. Temperatures through the week will remain largely near normal. Although more sunshine and less clouds and precipitation would prompted more effective warming, particularly later in the period. Delia && .AVIATION... (For the 00z TAFs through 00z Tuesday Evening) Issued at 643 PM CDT Mon Sep 23 2024 IFR flight conditions are expected to persist through mid-day Tuesday. Showers and a few embedded rumbles of thunder are currently impacting the St. Louis Metropolitan terminals and these will continue to push east/northeast over the next few hours. Otherwise, isolated showers are possible overnight at all terminals. Winds will also weaken overnight and favor the development of fog and reduced visibilities. Winds will increase and become northeasterly by Tuesday morning. VFR flight conditions are forecast to return late Tuesday afternoon. MMG/Carney && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX