Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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632 FXUS63 KLSX 011740 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1240 PM CDT Sat Jun 1 2024 .KEY MESSAGES... - Rain will gradually taper from west to east today, with a few thunderstorms possible this afternoon along and east of the Mississippi River. After patchy morning fog, Sunday looks largely dry with seasonable temperatures. - Our return to a relatively-unsettled pattern begins Monday, with daily chances for showers and thunderstorms through late Wednesday. This is accompanied by near/slightly-above-normal temperatures, but exactly how warm we get will depend on the uncertain timing and location of that rain. - A pattern change begins late Tuesday that will pull temperatures back closer to normal, if not below normal, and usher in drier weather by late-week. && .SHORT TERM... (Through Late Sunday Afternoon) Issued at 301 AM CDT Sat Jun 1 2024 Widespread light to moderate rain enveloped the bi-state area overnight to mark the start of meteorological summer, forced by a weak surface low, modest moist isentropic ascent, and a meager low- level jet. Nearly all thunderstorms (and heavy rain/associated impacts) were relegated to the Ozarks, where better deep moist convergence near a stationary front interacted with more appreciable MUCAPE. The surface low, and mid-level shortwave associated with it, will gradually lift north and east through the region today. As a result, rain will be slow to exit and cloud cover will remain largely fixed in place. This will keep our temperatures from warming above the upper 70s at best in central Missouri, with cooler temperatures further east where clouds/rain last longer. Along the "cold front" of this weak low, scattered thunderstorms will develop near the St. Louis metropolitan area and south, tracking east through the afternoon. With weak shear and unimpressive upper-level forcing, any convection will be on the weaker side. Low-level (925mb) moisture will linger through the night across part of the forecast area, keeping broken to overcast cloud cover in place for at least those along and east of the Mississippi River. West of that, gradual clearing and weak low-level subsidence will lead to a threat for fog overnight where saturated ground will provide a sufficient moisture source. The best chance for this fog would be in the Ozarks and central Missouri, where cloud cover will be less (though not completely clear). That said, the speed of the clearing is uncertain and will dictate if and and how far east fog develops. If clouds exit earlier than currently forecast, fog could impinge on eastern Missouri and western Illinois as well. A weak surface high and subtle ridging aloft embedded within large- scale quasi-zonal flow will work to keep Sunday largely dry, and with 850mb temperatures steadily rising through Sunday, surface temperatures will rise back closer to normal. There is a low (15- 20%) threat for thunderstorms in central Missouri during the late afternoon and evening as a decaying convective complex ejects from the Plains, but MLCAPE will be somewhat limited and modest subsidence from the ridge aloft will be in place. All of this is to say that any activity later on Sunday in central Missouri looks meager, if it reaches that area at all. MRB && .LONG TERM... (Sunday Night through Friday) Issued at 301 AM CDT Sat Jun 1 2024 The quasi-zonal flow will remain fixed overhead throughout much of the CONUS through late Tuesday, leaving the region open to shortwave impulses that would interact with sufficient moisture and instability to threaten a continuous potential for showers and thunderstorms from late Sunday through Wednesday. That said, this forecast is more a function of the uncertainty for when each wave of rain would arrive - it will not be actively raining this entire time. Along with the persistent rain chances, this flow pattern will draw 850mb temperatures to near the 75th climatological percentile and promote near to slightly-above-normal temperatures (mid/upper 80s) at the surface. There is growing agreement that one of these waves will track into the Mid-Mississippi Valley during the afternoon and evening on Monday, but the exact track and amplitude of this wave varies in the deterministic guidance and ensemble member suite. Regardless, this is our first more tangible threat for showers and thunderstorms. The amplitude/track uncertainties also lead to somewhat varied depictions of shear parameters on Monday afternoon, but even the more bullish guidance sources keep strong to severe thunderstorms a remote possibility at this time. In that scenario, the best threat for any stronger convection would be across the Ozarks where instability will maximize amidst the passing shortwave. The SPC Day 3 Convective Outlook highlights this area in a Marginal Risk, which we are not messaging at this time given the highly-conditional nature of the threat. Early Tuesday morning, guidance continues to depict a strong upper- level trough ejecting out of the Pacific Northwest and tracking east through the northern Great Plains into the Great Lakes region. This feature spawns a surface low that takes a similar track, which will send a cold front south and east through our region. Exactly when this cold front reaches the bi-state is relatively uncertain, but is still reasonably expected during the Tuesday evening to Wednesday morning timeframe. In addition to being another focus for showers and thunderstorms, this upper-level wave and its surface reflection will be the harbinger of a pattern shift that has potential to usher in below-normal temperatures and overall drier conditions. What looks all but certain is that the wave will help establish a ridge-trough pattern over the CONUS, placing the forecast area in northwest flow aloft to promote largely-dry weather. However, model depictions of the flow pattern quickly diverge with the location of a closed upper-level low varying from the Ohio River Valley to the Mid-Mississippi Valley. Others diverge even further from these solutions, but these two are the most prevalent in the guidance. Where the ULL settles will be pivotal in how much we cool late next week. While ensemble guidance overall trends cooler, the ensemble interquartile range spans from near-normal to nearly 10 degrees below normal. The current forecast, which keeps temperatures close to normal, uses the National Blend of Models given the high uncertainty near the end of the period. That said, if the patterns being shown in the ensemble guidance come to fruition, we`ll likely be cooler than currently forecast. MRB && .AVIATION... (For the 18z TAFs through 18z Sunday Afternoon) Issued at 1232 PM CDT Sat Jun 1 2024 Scattered showers will primarily impact KUIN and the St. Louis area terminals this afternoon with the best thunder potential from around 20-22z. Flight conditions will largely be MVFR. Between 22-00z the majority of the showers should move east of the terminals with this activity decaying early this evening across SW IL. Flight conditions should at least temporarily improve to VFR across much of the area this evening, but overnight we will see a mix of fog and lower stratus that lasts into mid Sunday morning. The visibility in the fog could certainly be lower at KCOU and KJEF that the current forecast near 1SM. By mid-late morning on Sunday, fog should have dissipated and ceilings should be improving. Glass && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX