Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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013 FXUS63 KLSX 182238 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 538 PM CDT Tue Jun 18 2024 .KEY MESSAGES... - A few showers and possibly weak thunderstorms are possible across the Ozarks and southern Illinois late this afternoon, and again tomorrow afternoon across northeast Missouri. Storms tomorrow afternoon have a slightly higher chance of being strong, but chances are low overall. - Chances for showers and thunderstorms are higher (30-50%) between late Saturday and Sunday, but confidence in timing and coverage remain low. - Otherwise, temperatures will gradually rise through the week and peak over the weekend when heat indices will likely reach between 90 and 100 degrees, with the potential to climb above 100 in the St. Louis metro area. && .SHORT TERM... (Through Late Wednesday Night) Issued at 323 PM CDT Tue Jun 18 2024 The primary forecast item of interest in the short term continues to be the potential for isolated showers and thunderstorms, both later this afternoon across the Ozarks and southern Illinois and again across northeast Missouri tomorrow afternoon. As of 2 PM, deep south to southwesterly flow is in place across the mid-Mississippi Valley region, with a large upper ridge across the eastern CONUS, and a corresponding trough across the west. Embedded within this southwesterly flow is a subtle trough just to our south, which is slowly advancing northward this afternoon and spreading ample mid and upper cloud cover into southeast Missouri and southern Illinois, along with some very light stratiform rain. Elsewhere, fairweather afternoon cumulus has developed, but is being suppressed by warmer temperatures aloft. As the afternoon progresses, the advancing impulse may support isolated convective development within the broader cloud/precipitation shield across the Ozarks and southern Illinois, although meager instability and weak wind shear will prevent this activity from becoming particularly threatening aside from a strike of lightning or two. Any showers than manage to diminish through the evening. Otherwise, temperatures will peak in the mid 80s to near 90 degrees today, which is several degrees cooler than yesterday. Early in the day tomorrow, a weakening cold front will slowly sink into northeast Missouri, perhaps augmented by outflow from overnight showers and thunderstorms. While this boundary is expected to stall as it approaches, it may bring a round of scattered showers and thunderstorms into northeast Missouri in the morning, with possible redevelopment during the afternoon as the atmosphere destabilizes. In spite of strong surface heating (temperatures in the low to mid 90s), instability is not expected to be particularly robust thanks to poor mid-level lapse rates, with SBCAPE potentially reaching 1000- 1500 J/kg. Meanwhile, 0-6km shear values will also be marginal (20- 30kt), as most of the stronger upper level winds will remain near or above the equilibrium level per model soundings. Still, just enough instability and shear will be present to support a few stronger clusters capable of producing isolated damaging wind gusts and small hail. Most of this stronger activity is likely to remain to our north, but a strong storm or two can`t be completely ruled out across northeast Missouri. Elsewhere, warm and dry conditions will prevail tomorrow. BRC && .LONG TERM... (Thursday through Next Tuesday) Issued at 323 PM CDT Tue Jun 18 2024 Over the course of Thursday and Friday, the previously mentioned upper ridge of high pressure across the eastern CONUS will gradually expand westward, causing temperatures to slowly increase and largely suppressing convection as it does so. While the upper ridge is expected to reach its peak heights on Friday, temperatures will continue to increase heading into Saturday as a cold front approaches from the northwest. While confidence is very high that temperatures will climb to well above average, likely at least into the mid-90s, ensemble mean 850mb temperature percentiles are not particularly anomalous in either the NAEFS or GEFS ensembles, which my limit the upper ceiling and mitigate potential heat impacts. Further complicating this picture is the arrival of a cold front sometime late Saturday, which remains likely to bring a round of showers and thunderstorms into the area. The timing of this feature may impact temperatures to some degree, but thus far this has not been well reflected model forecasts. In fact, ensemble temperature forecast spreads remain narrow Sunday and similar to Saturday, suggesting that this front may not bring a meaningful cooling trend. This would be especially true if convection arrives overnight Saturday and diminishes before heating commences during the day. Meanwhile, there have been some minor changes in the potential for showers and thunderstorms between late Saturday and Sunday afternoon. This continues to be the most likely timeframe for showers and storms over the next week (30-50%), and this time range has narrowed slightly from the previous forecast. There will also be at least some potential for strong or severe thunderstorms, although much will depend on the timing of storms and available instability. Still, the passage of a shortwave trough will likely spread at least modest mid-level flow into the area, perhaps enough to produce sufficient shear for updraft organization. Early deterministic model forecasts suggest upwards of 30 to 40 kt of deep layer shear and 1000-2000 J/kg of SBCAPE, although these numbers should be taken with a grain of salt. BRC && .AVIATION... (For the 00z TAFs through 00z Wednesday Evening) Issued at 536 PM CDT Tue Jun 18 2024 Clear, dry VFR flight conditions will continue through the end of the TAF period. Winds will continue to gust periodically through 02-04Z, afterwhich sustained winds will remain elevated near 10 kts expect at some of the more sheltered sites (KJEF, KSUS, KCPS). Southerly winds will pick up again tomorrow during the day, sustained near 10 kts, but without significant gusts thanks to weaker winds aloft. MRM && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX