Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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985 FXUS63 KLSX 111111 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 611 AM CDT Thu Jul 11 2024 .KEY MESSAGES... - Scattered thunderstorms will continue to threaten parts of the region each day through Saturday morning, with plenty of dry time in between rain chances. - Temperatures rise above normal through the weekend, eventually dragging heat index values into the triple-digits by Sunday, before increased rain chances and a cold front offer some relief. && .SHORT TERM... (Through Late Friday Afternoon) Issued at 327 AM CDT Thu Jul 11 2024 Weak, isolated thunderstorms continue to percolate at this hour across parts of Missouri along a lobe of mid-level positive vorticity advection. Aloft, a sluggish shortwave is oozing east across eastern Iowa and northwest Illinois. Several other areas of PVA rotate around the cyclonic flow, stimulating additional convection. It is this feature that will be the focus for activity today across the region. As daytime heating sends temperatures into the mid/upper 80s again and surface-based instability values into the 1500-2000J/kg range, the threat for isolated to widely-scattered thunderstorms will increase. Unlike yesterday, the mid-level forcing will gradually weaken with time as most guidance begins to shunt the shortwave east of the bi-state area. Despite this, the copious instability that will develop and a weak surface boundary traversing the region will be sufficient for some degree of convection that threatens gusty winds and localized heavy downpours. The best chances (30-45%) are east of the Mississippi River, where the stronger forcing will exist during peak diurnal heating. With the shortwave departing to our north and east overnight and upper-level ridging building in from the west, weak high pressure across the Upper Mississippi Valley will help to weaken winds out of the northeast. Convective activity should wane overnight for most of the region save for areas near the Ozarks, allowing for clearing skies. All of this lends increased confidence in areas of fog impacting parts of the St. Louis metropolitan area and areas along and east of the Mississippi River. While dense fog is not forecast at this time, localized low-lying areas could very well see visibilities approach one-quarter mile overnight. The strong upper-level ridge over the western CONUS begins to expand east on Friday, which will slowly draw the surface boundary north again as a warm front later in the day. Before this, weak and isolated convection may fire in the vicinity of the front. That said, the synoptic-scale subsidence aloft will overcome most updrafts outside of the front itself. As the front begins to lift Friday evening, persistent warm air aloft will cap any convection and keep the region largely dry. MRB && .LONG TERM... (Friday Night through Wednesday) Issued at 327 AM CDT Thu Jul 11 2024 As the western ridge broadens across the central CONUS late Friday into Saturday, pushing a warm front into the region, there is potential for convection Saturday morning along the front on the nose of a veering low-level jet in northern Missouri. At this point, most guidance has most surface-based/mixed layer instability well- capped amidst rising heights aloft. However, if the subsidence is weaker than currently expected, a cluster of stronger thunderstorms would persist along the Missouri/Iowa border into central Illinois. Given the uncertainty in thunderstorm intensity for the reasons described above, we are not currently messaging the SPC Day 3 Marginal Risk for severe weather. Behind the front, temperatures jump into the 90s areawide sending heat index values into the low 100s. Ensemble guidance continues to paint 850mb temperatures in the 90th climatological percentile across the Missouri/Iowa border. This anomalous 850mb warmth expands south by Sunday amidst persistent warm-air advection and southwest boundary layer winds. The strong subsidence will suppress convection through late Monday at the earliest, but the strong 850mb thermal ridge and southwest surface winds will result in temperatures rising further still into the next work week. All available guidance agrees that Monday and Tuesday will be the warmest days of the forecast period, with near-certainty (90%+ chance) of low to mid-90s each day. The St. Louis and Columbia metropolitan areas will make a run at 100 degrees, but it may be a bit too humid to get that hot. This combination of heat and humidity will threaten vulnerable populations and those outdoors, and if the current 105+ degree heat index values persist, heat products will be needed in the next few days. The largest source of uncertainty surrounds how humid we`ll be. A southwest wind can sometimes help advect drier air from the Ozark Plateau, which isn`t represented in this forecast. If that occurs, it would act to keep heat index values in check. By late Monday, a pronounced shortwave trough tracks along the periphery of the ridge (now as far north as the U.S./Canada border) and erodes its western extent. While this has little impact on us in and of itself, additional shortwaves will work to deepen a longwave trough across the Great Lakes region. This will draw a front slowly but surely into the Mid-Mississippi Valley, and both increase our rain chances and bring relief from the abnormal heat. While a relative cooldown is all but certain according to available ensemble guidance, exactly when the relief arrives and how far temperatures drop is still not clear. Most reasonable scenarios support a slower frontal passage, which would promote multiple rain chances going into the end of next week but keep temperatures in check. Regardless, by Thursday temperatures fall back to near-normal at the warmest. MRB && .AVIATION... (For the 12z TAFs through 12z Friday Morning) Issued at 601 AM CDT Thu Jul 11 2024 Patchy fog is impacting the region this morning, however only KSUS has fallen below VFR conditions among the TAF sites. Weak convection continues to percolate over central Missouri, which will briefly impact KCOU and KJEF but will most likely not have lightning with it. Most of the TAF period will be dry and VFR, but there is a general threat for thunderstorms. Considerable differences exist in exactly which terminals (if any) would be affected today, and timing uncertainty also exists. As such, no thunderstorms are mentioned in the TAFs but remain an outside possibility. Tonight, winds weaken out of the northeast and fog will become a more distinct possibility at a few terminals. The highest chance for fog is at KUIN, though the river valley TAF sites (KSUS and KJEF) may also be impacted. MRB && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX