Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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334 FXUS63 KLSX 241741 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1241 PM CDT Tue Sep 24 2024 .KEY MESSAGES... - Other than a few afternoon showers and storms today, largely dry conditions are expected through Wednesday. - The next opportunity for rain will come Thursday through the upcoming weekend, but there is still large uncertainty in its timing and rainfall amounts. && .SHORT TERM... (Through Late Wednesday Afternoon) Issued at 412 AM CDT Tue Sep 24 2024 An upper-level shortwave trough is traversing the Mid-Mississippi River Valley this morning, while becoming increasingly negatively tilted. In response, increasing large-scale ascent and weak low to mid-level frontogenesis to the northwest of a developing surface low in southern IL will continue to support showers across northeast MO and west-central and south-central IL. With MUCAPE around 500 to 750 J/kg an isolated thunderstorm or two is also possible. Most showers will depart to the northeast as the trough begins to depart, but confluent low-level flow, steepening low-level lapse rates, and large-scale ascent associated with the arrival of another, more potent upper-level trough will support isolated to scattered showers at times across a broad portion of the CWA through the day. A few stray lightning strikes could accompany these showers. Some showers could even linger past sunset in central and northeastern MO as a weakening cold front arrives. Widespread cloud cover and weak low- level CAA will keep high temperatures today around 70 to the mid-70s F. Clouds will gradually decrease tonight from west to east, potentially leading to patchy fog by Wednesday morning as winds become light/calm with relaxation of the SLP gradient. The coverage of fog will be dependent on how much of the CWA sees clearing overnight, which varies among short-term model guidance. Much drier conditions are expected on Wednesday as mid-level heights rise in association with an increasingly positively tilted ridge "breaking" and promoting formation of an upper-level cutoff low from the aforementioned trough across the Mid-Mississippi River Valley. Cloud cover on Wednesday will be much less than today but there will likely still be scattered to broken diurnal cumulus and mid-level clouds. Marginally greater insolation will permit slightly warmer high temperatures, although again in the 70s F and seasonable. Forcing for any precip on Wednesday will be rather nebulous, but there could be enough residual moisture and insolation to produce an isolated shower or two across southeastern MO and southwestern IL. Pfahler && .LONG TERM... (Wednesday Night through Monday) Issued at 412 AM CDT Tue Sep 24 2024 Thursday through the weekend, the forecast remains highly uncertain due to the expected complex interaction of the upper-level closed low near the Mid-Mississippi River Valley and Potential Tropical Cyclone (PTC) Nine likely making landfall along the Gulf Coast as it tracks northward. This evolution includes a non-linear, Fujiwhara- like interaction (both features simultaneously influencing the movement of one another) followed by absorption of PTC Nine into the cutoff low featured in varying capacities across model guidance. Well in advance of PTC Nine, the cutoff low is expected to interact with a plume of anomalous, tropical moisture to produce showers and a few thunderstorms and even steadier rain at times around its eastern and northeastern flank somewhere across the Mid-Mississippi River Valley and Ozarks by Thursday. Ensemble model guidance differ on the exact placement of these showers/rain as well as their residence time at a given location through the weekend while PTC Nine itself possibly tracks closer to the region. To contextualize the uncertainty, there are still members that have little to no rainfall across much of the CWA, but the number of members that have over 1" of total rainfall in the CWA has increased from 24 hours ago. Regardless of precip, widespread clouds are expected through most the period. These clouds will at least keep temperatures from warming much and in the 70s F; however, NBM interquartile temperature ranges span 5 to 8 F, mainly due to differences in the influence of showers/rain across members. Pfahler && .AVIATION... (For the 18z TAFs through 18z Wednesday Afternoon) Issued at 1238 PM CDT Tue Sep 24 2024 Conditions are gradually trending toward VFR through the afternoon, although occasional drizzle showers continue in the St Louis metro. Brief visibility restrictions are possible for the next few hours. Drier air moving in from the northwest will end this activity this evening with a trend toward clearing conditions. If conditions do clear out this evening, then fog becomes more likely overnight as conditions cool and low level moisture hangs on. This would be most likely in valley locations like SUS, but could occur at any location. Kimble && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX