FXUS63 KLSX 031120 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 620 AM CDT Sat May 3 2025 .KEY MESSAGES... - Showers and a few weak thunderstorms are forecast today, particularly within and east of the St. Louis metropolitan area. Rain chances gradually shift east through Monday morning, with thunderstorm chances becoming very low (less than 10%). - Cool temperatures this weekend will give way to a return to more seasonable warmth by Tuesday, lasting through the rest of the week. && .SHORT TERM... (Through Late Sunday Afternoon) Issued at 339 AM CDT Sat May 3 2025 Mid-level water vapor satellite imagery shows the axis of the amplified trough pivoting through the Mid-Mississippi Valley at this hour. Along and ahead of the trough axis, scattered showers are percolating and areas of low stratus are gradually expanding. Meanwhile, weak but persistent northerly winds are promoting cool and gradually-lowering surface dewpoints. The upper-level wave is still forecast to stall and cut off from the main flow. Between the steepening low-level lapse rates from the cooling aloft and cyclonic flow promoting some ambient lift, showers will become more widespread during the day. The highest chances, up to 80%, will be present in east-central Missouri and southwest Illinois where forcing will be most concentrated. Breaks in the clouds may allow for sufficient instability to allow for a few weak, brief thunderstorms in eastern Missouri and western Illinois. That said, instability and wind shear are so marginal that no strong to severe convection will develop. Temperatures today will be up to 10 degrees cooler than yesterday, with nearly all available guidance showing weak but persistent cold air advection persisting. The very low threat for thunderstorms ends with sunset abating what little instability can develop. Mid-level PVA will continue to support showers along and east of the Mississippi River overnight as the trough cuts off and a high-amplitude Omega block develops over much of the CONUS. Sunday will see the greatest influence of the cutoff low across the region. It will have the most effect across southern Illinois, where temperatures will struggle to warm above 60 degrees and widespread showers will develop. Rain chances decrease with westward extent, with nearly all guidance currently keeping even low (15%) rain chances east of the Mississippi River during the day. Temperatures will generally be warmer further west due to more scattered cloud cover and warmer air aloft. There's some uncertainty as to how far east the cutoff low will be, which plays into the westward extent of the rain chances. Thunder chances are very low by this point, with the abundant clouds inhibiting much surface heating where precipitation is most likely. MRB && .LONG TERM... (Sunday Night through Friday) Issued at 339 AM CDT Sat May 3 2025 Monday's sensible weather will be very similar to Sunday's owing to the Omega block keeping the synoptic features in the CONUS stagnant. However, the block appears short-lived as the easternmost cutoff low becomes dislodged by a northern-stream wave in southern Canada. As a result, heights gradually rise Monday night into Tuesday and the region enters a warm air advection regime. A dry, warmer, and mostly- sunny day will result on Tuesday, with the NBM IQR for high temperatures jumping 10 degrees into the 70s and remaining there all week. The expanding longwave trough in the southwest (possibly developing into a brief Rex block depending on the guidance one looks at) will bring new rain chances behind the warmer air sometime mid-week. Exactly when those rain chances return is uncertain and depends on the evolution of this longwave, but may begin as early as Tuesday evening according to global ensemble guidance. This round of precipitation, forced by a brief uptick in mid-level diffluence, features the highest rain chances (60% for some) of the next work week and is followed by persistent low (20-30%) chances. That said, there is very little to suggest that any rain would be impactful. NAEFS and EPS guidance unilaterally show relatively normal PWATs compared to climatology, and while exact depictions of the trough vary, they all show the wave being fairly weak. The weak forcing, leading to broad and weak cyclogenesis, coupled with innocuous moisture, leads to fairly unimpactful chances for rain through the end of the week. To further support this, even the NBM and global ensemble 95th percentile for QPF struggles to reach 1.50 inches by Friday. While it is fairly early, there is general consensus that the weekend stays largely dry with the longwave trough becoming less influential under a larger-scale ridge. MRB && .AVIATION... (For the 12z TAFs through 12z Sunday Morning) Issued at 610 AM CDT Sat May 3 2025 MVFR to IFR stratus and scattered showers exist at the St. Louis metropolitan terminals this morning under a stalling upper-level low. Elsewhere, stratus is either largely eroded or VFR. CIGs will rise through the morning, with guidance now much less pessimistic with degrading conditions. Showers will blossom at the St. Louis metropolitan terminals this afternoon associated with the upper-level feature, which may briefly lead to MVFR restrictions. While a rogue thunderstorm cannot be ruled out this afternoon in St. Louis, it's very likely that the terminals stay devoid of lightning. MVFR CIGs redevelop over southern Illinois tonight where the center of the upper low will exist. While the central Missouri terminals and KUIN look very safe from these restrictions, the impactful CIGs are knocking on the eastern doorstep of the St. Louis terminals. The site with the highest threat is KCPS, but confidence is not high enough in where the MVFR conditions will materialize to include in the TAFs just yet. MRB && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX