Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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894 FXUS63 KLSX 200328 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1028 PM CDT Wed Jun 19 2024 .KEY MESSAGES... - A few showers and thunderstorms are possible this afternoon. While most of these storms will be weak, a few strong wind gusts can`t be ruled out, particularly across northeast Missouri. - Heat will steadily build tomorrow through the weekend, with afternoon heat index values likely reaching between 95 and 105 and morning temperatures in the low to mid 70s Friday and Saturday. - Confidence is increasing that a round of showers and thunderstorms will move through the area between Saturday night and Sunday morning, with precipitation chances between 30 and 70% in most areas. A few strong/severe thunderstorms will be possible, but this potential is less certain. && .SHORT TERM... (Through Late Tonight) Issued at 159 PM CDT Wed Jun 19 2024 Weather thus far today has been generally quiet as the mid- Mississippi Valley region remains underneath the western periphery of a slowly expanding upper ridge of high pressure. However, a nearly stationary front is draped across northwest/north central Missouri as of 1 PM, with numerous post frontal showers and a few thunderstorms ongoing behind it. Just ahead of this front across the warm sector, strong surface heating within a relatively clear area has quickly eroded what little CIN remained, and this has allowed for rapid cumulus development and even a few weak showers. This deepening convection has developed in two general areas as of 1 PM: across north central Missouri, and farther southeast along a loosely defined boundary and/or thermal ridge aligned with the I-44(MO)/I-70 (IL) corridors. As the afternoon progresses, increasing instability (1000-1500 MLCAPE) will support slowly deepening convection, but a lack of appreciable deep layer wind shear is likely to prevent storms from amounting to much more than weak pulse-type convection. Meanwhile, significant dry air aloft is likely to impede updraft growth in most areas, and storms are likely to struggle to develop in this environment...particularly along the southern line of convection described above. However, areas in the vicinity of the front may have slightly more shear to work with (near 20 kt of effective bulk shear), and less dry air aloft, which may support a few stronger cores capable of producing occasionally strong downburst winds. Otherwise, temperatures have already reached the upper 80s to near 90 degrees early this afternoon, and will likely rise another few degrees by 3 to 5 PM. While a few showers may linger in northeast Missouri into the evening, most showers will diminish prior to sunset. Clearing skies should allow for efficient cooling into the upper 60s to low 70s, with a few patches of very shallow fog possible in wind-sheltered valleys. BRC && .LONG TERM... (Thursday through Next Wednesday) Issued at 159 PM CDT Wed Jun 19 2024 Between tomorrow and Saturday, the broad upper level ridge of high pressure across the eastern CONUS will slowly expand westward, gradually increasing both temperatures and subsidence aloft. This will lead to both increasing surface temperatures and even lower precipitation chances, although a few stray afternoon showers can`t be completely ruled out across far northern Missouri and west- central Illinois tomorrow afternoon. Within this timeframe, gradually building heat is expected to be the most noteworthy and potentially impactful weather, although current forecast temperatures and humidity remain below levels that typically result in significant impacts locally. Peak afternoon temperatures remain most likely to occur on Saturday by a slim margin, with ensemble mean 850 temperatures increasing slightly to between 19 and 21 C, corresponding to actual surface temperatures generally in the mid to upper 90s and heat index values near the century mark. Temperature trends have also increased slightly on Friday as well, and both days are now expected to see very similar temperatures. Confidence in these temperatures is relatively high considering the expected lack of precipitation and cloud cover that would complicate the forecast, and narrow NBM/LREF temperature ensemble spreads support this as well (although the NBM remains several degrees warmer than most other available guidance). Overnight Saturday, confidence is increasing regarding the timing of an approaching cold front and associated round of showers and thunderstorms. Ensemble members continue to trend toward an overnight frontal passage, which does limit the upper end of available instability, but in spite of this models continue to produce just enough to support vigorous updrafts. Meanwhile, a passing shortwave along the northern periphery of the ridge may also provide a boost in mid-level winds, and just enough to produce sufficient deep layer shear for updraft organization. That is the long way of saying that a few strong or severe storms will be possible overnight Saturday through early Sunday, with the greatest chances across northern Missouri and west-central Illinois, although the currently forecast parameters remain marginal. While this cold front is expected to move into the area by Sunday, it likely will not provide significant relief from the heat. This is because this front is not expected to be particularly strong, and will likely stall across the region during the day Sunday. Meanwhile, increasing humidity may actually cause heat index values to remain steady or even increase slightly from the previous afternoon, unless lingering morning convection and cloud cover can provide just enough cooling to keep us from reaching the currently forecast values. Ensemble mean temperatures do dip a few degrees Sunday and spreads increase slightly, so temperatures Sunday are a bit less certain than on Saturday. Overall though, the trend for above average temperatures will continue into early/middle next week, as the upper ridge is expected to retrograde into the western CONUS and amplify. Ensemble mean temperatures in both the GEFS and NAEFS ensemble suites also increase to between the 90th and 98th percentile by Tuesday, which is notably higher than projections this weekend. While this may not necessarily translate to hotter surface temperatures, the potential exists for impactful heat early next week. BRC && .AVIATION... (For the 06z TAFs through 06z Thursday Night) Issued at 1025 PM CDT Wed Jun 19 2024 Mainly dry and VFR conditions are expected through the period. Only exception will be the potential for some brief fog between 08-12Z at SUS with MVFR or low VFR visibilities. Otherwise, just light winds are expected. Britt && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX