Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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150 FXUS63 KLSX 260922 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 422 AM CDT Thu Sep 26 2024 .KEY MESSAGES... - Rain associated with tropical moisture and Tropical Cyclone Helene will spread into the region overnight lasting through Friday evening, primarily along/south of I-44 in MO and I-70 in IL. Total rainfall of 1 to 3" is forecast across southwestern IL and southeastern MO with minor flooding possible where locally higher amounts fall. - Breezy conditions are also expected on Friday with occasional gusts of 25 to 40 mph, but higher wind gusts are possible in southwestern IL Friday evening, depending on the track of Helene. - Mainly afternoon and evening showers are expected Saturday through Monday with coverage decreasing each day. && .SHORT TERM... (Through Late Friday Night) Issued at 421 AM CDT Thu Sep 26 2024 Early this morning, an upper-level cutoff low is centered over southern IL, with much of the CWA under the influence of weak subsidence in its wake and a weak inverted SLP ridge. With only a few mid-level clouds present, efficient radiational cooling has allowed river valley fog to develop, especially along the Missouri River and in the Ozarks. High temperatures today are anticipated to be a couple degrees warmer than Wednesday with greater insolation through only scattered diurnal cumulus and ahead of upper and mid- level clouds arriving during the evening. As the upper-level cutoff low drifts southward into the Lower Mississippi River Valley this evening, tropical moisture will begin to wrap around the cutoff low and spread into southwestern IL and southeastern MO. The initial arrival of this moisture could be accompanied by development isolated to scattered showers and thunderstorms during the evening while instability is available, but a more widespread shield of stratiform rain will arrive overnight as moisture deepens and isentropic ascent increases. This shield of rain will persist through most of Friday but its northwestward extent varies across CAMs and other short-term model guidance due to variability in the exact position of the cutoff low while it interacts with a weakening, post-landfall TC Helene tracking northwestward through the Southern Appalachians/Tennessee Valley and Lower Ohio Valley. HREF 6-hour probabilities of measurable rainfall are 60 percent or higher along and south of I-44 in MO and I-70 in IL, leading to some confidence that the rain shield will extend across much of that area through Friday. Under the shield of rain, high temperatures will only reach the upper 60s to around 70 F, with 70s F elsewhere. Strong low-level flow (around 40 to 50+ kt at 850 hPa) and a tightening SLP gradient will lead to breezy conditions throughout the day with occasional gusts of 25 to 40 mph. Late Friday, TC Helene is progged to rotate around the closed low and reach the vicinity of the Mississippi-Ohio River confluence as it becomes absorbed into the cutoff low. Not only will this process potentially augment the rain shield, but it will also strengthen winds across southwestern IL. The latest HREF has 60 to 80 percent probabilities of wind gusts exceeding 50 mph Friday evening; however, these gusts are conditional on the track of TC Helene being nearby the CWA. There will be a gradient in total rainfall through Friday night across the CWA, the interquartile range of QPF in ensemble models are between nothing to 0.25" in central/northeastern MO and west- central IL, 0.25 to 1.50" near I-44 (MO) and I-70 (IL), and 1.00 to 3.00" in southwestern IL and southeastern MO. With PW above the 99th climatological percentile, the heaviest rain is expected across southwestern IL and southeastern MO with rainfall, although little to no instability will limit rain rates. It is worth noting that the 90th percentile of QPF in those areas is around 4", which cannot be ruled out locally with minor flooding possible, but flash flooding is unlikely. Pfahler && .LONG TERM... (Saturday through Wednesday) Issued at 421 AM CDT Thu Sep 26 2024 Saturday through Monday, the upper-level cutoff low is anticipated to have fully absorbed TC Helene and begin progressing slowly eastward, away from the Mid-Mississippi River Valley. In response, large-scale ascent will also depart, but lingering moisture and modest daytime heating will support diurnal showers, most numerous on Saturday afternoon. With clouds and coverage of showers lessening each day, a slight warming of temperatures is expected through the 70s F to near 80 F by Sunday and Monday. Thereafter, global model guidance signal an upper-level trough traversing the northern CONUS, sending a cold front through the Mid-Mississippi River Valley around Tuesday, but there is uncertainty in the strength of the front and whether temperatures fall from above average to near or below average following its passage. Pfahler && .AVIATION... (For the 06z TAFs through 06z Thursday Night) Issued at 1051 PM CDT Wed Sep 25 2024 Light and variable winds, clear skies, and lingering low-level moisture will favor the development of fog overnight at KSUS and KCPS. Fog cannot be ruled out at KJEF but confidence in its development is too low to include in TAF. Expect northerly winds to increase Thursday morning with diurnal cumulus. Cloud cover is forecast to increase in coverage late Thursday evening. MMG/Glass && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX