Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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983 FXUS63 KLSX 210344 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1044 PM CDT Thu Jun 20 2024 .KEY MESSAGES... - Steadily building heat is expected over the next several days, with heat likely peaking early next week. While heat may not be extreme on any individual day, the prolonged nature of this heat has the potential to amplify impacts. - While a stray shower or weak thunderstorm will be possible across northeast MO and west-central IL this afternoon, better chances for showers and storms (30 to 70%) exist overnight Saturday and Sunday morning. A few strong or severe storms will be possible, but chances of this are low. && .SHORT TERM... (Through Sunday) Issued at 235 PM CDT Thu Jun 20 2024 Mostly benign weather conditions remain in place this afternoon as a ridge of high pressure centered across the eastern CONUS continues to slowly expand further into the Mississippi river valley. Meanwhile, a stationary front remains draped near the MO/IA state line, with widespread deepening cumulus development along and south of the boundary across the warm sector. While warm mid-levels have kept the environment quite stable across much of the region, some modest MLCAPE has managed to develop across primarily northeast Missouri and west-central Illinois as of noon, and this is also where the deepest convective development has occurred thus far. As the afternoon progresses, a few showers and even a weak thunderstorm or two may emerge from this cu field, but most of this activity is likely to remain near or just north of our northernmost counties (Knox, Lewis, Adams, Brown). If showers do manage to form in these areas, short bursts of heavy rain and a strike of lightning or two will be the most likely hazards. As we head into the weekend, the previously mentioned upper ridge will continue to expand westward, and temperatures will slowly increase. So far today afternoon temperatures are very similar to yesterday, and we will likely once again reach the upper 80s to mid 90s by late afternoon, with only moderate humidity. However, this may be the "coolest" day of the next week, and by Saturday we`ll likely see temperatures reaching well into the mid 90s to perhaps near the century mark. This forecast has remained rather consistent, with ensemble mean 850 mb temperatures increasing to around 19-21 C, along with narrow ensemble temperature spreads. The good news leading up to Saturday is that humidity is likely to remain only moderate, which should help to keep heat index values in check through Saturday, and below our typical thresholds that lead to heat impacts over short durations. However, this may change Sunday as humidity is expected to increase ahead of a weakening frontal boundary, with only a minimal decrease in temperatures expected. In fact, the heat index values Sunday may actually end up slightly higher than Saturday, even if actual temperatures are a few degrees lower. This heat is also very likely to persist, if not worsen, heading into early next week as well. Before we get to that, though, the previously mentioned front is also expected to arrive with a round of showers and thunderstorms overnight Saturday. Confidence in the timing of this activity is higher than average at this time range, owing to very good agreement among ensemble members. What is less certain is just how far south this activity will persist Sunday morning, and whether lingering showers or precipitation will have any impact on temperatures Sunday afternoon. At the present time, it seems unlikely that precipitation will provide meaningful relief Sunday. Meanwhile, the overnight/early morning timing is not climatologically favorable for severe thunderstorms, but models continue to produce just enough instability (500-1500 J/kg of MUCAPE) and 0-6km shear (20-25kt) to at least consider the potential for a few strong or severe thunderstorms in northern areas during this timeframe. During the afternoon Sunday, this front is expected to stall somewhere across the area and weaken. In addition to the potential implications on heat that have already been discussed, this leaves open the possibility of additional thunderstorms Sunday afternoon, particularly along and south of wherever the boundary ends up. Confidence is much lower regarding this potential and ensembles are much less enthusiastic about precipitation during the afternoon, particularly the NBM. However, there are a non-trivial number of LREF members that maintain precipitation chances through Sunday afternoon, with ample instability in place along with continued modest deep layer shear. BRC && .LONG TERM... (Monday through Next Thursday) Issued at 235 PM CDT Thu Jun 20 2024 On Monday, guidance consensus is that the shortwave responsible for Sunday`s front will continue eastward and deepen as the western upper-level ridge amplifies and tries to re-expand eastward. However, both ensembles and deterministic guidance differ on how quickly both of these features phase, with a slower solution keeping northwesterly flow aloft over the Middle Mississippi Valley and temperatures at or cooler than values expected Sunday. A quicker solution, where the upper-level ridge builds more aggressively into the region, favors temperatures warming above what was experienced Sunday. Confidence is increasing that Tuesday will likely be the hottest day of the period. Then, guidance consensus is that 850 mb temperatures will be around 23 degrees C, climatologically correlating with temperatures in the upper 90s assuming deep mixing. What will give a boost to this heat is that a majority of guidance also has southwesterly surface flow, which will provide adiabatic warming as air moves downslope off the Ozarks. This will support surface temperatures around 100 degrees for most locations in the CWA, backed by the 50th-75th percentile of global ensembles and the NBM. As early as Tuesday night, guidance consensus is that a shortwave will move through the Midwest, though deterministic guidance and ensemble clusters differ on its amplitude. This impacts not only the timing of the associated cold front, but also the potency of the post-frontal airmass. A shallower wave will have a slower FROPA and only take temperatures down by a few degrees, while a deeper wave will have a quicker FROPA and greater impact on temperatures. While how much we cool exactly is uncertain, confidence is high that we will cool to some degree the middle of next week, as the 75th percentile of ensemble guidance tops out in the low 90s. Elmore && .AVIATION... (For the 06z TAFs through 06z Friday Night) Issued at 1042 PM CDT Thu Jun 20 2024 Dry and VFR conditions are expected through the period with winds at 7 knots or less. Britt && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX