Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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589 FXUS63 KLSX 210358 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1058 PM CDT Fri Sep 20 2024 .KEY MESSAGES... - An active pattern will continue through early next week. There is chance of showers and thunderstorms present each day through at least Monday. - This period presents our best chance at beneficial rainfall in quite some time. Our highest chances at seeing this will be Saturday night into Sunday morning. && .SHORT TERM... (Through Late Saturday) Issued at 342 PM CDT Fri Sep 20 2024 A cold front is pushing through the forecast area today, though calling it a "cold front" seems like a misnomer given the well above average temperatures on its backside. What this front does distinguish is the boundary between moist and less moist airmasses. On the moist side, surface based instability is maximized with values reaching up to 3500 J/kg. Deep layer shear of 30 - 35 kts is also in place, presenting the possibility of strong to severe thunderstorms later this afternoon into this evening. There are substantial caveats to the potential for severe today. Model soundings show a deep layer of mid-level dry air and low mid-level lapse rates over the forecast area. Short term guidance has remained consistent in keeping portions of southeast Missouri and southern Illinois capped, though how far north this extends into our southern CWA is uncertain. This is important considering the southern pushing cold front and the fact that the only potential for thunderstorms lies in or near this capped air. If we are not impacted by capping, convection will be a characterized by bubbling cu struggling against dry air entrainment, with some updrafts prevailing over others. For this reason, isolated thunderstorm coverage is expected. If a thunderstorm does become strong to severe, the excess dry air aloft will make damaging winds the primary threat. These are expected to diminish late this evening. We`ll see a brief lull during the late night hours. During this period, temperatures will fall into the 60s and 70s - much closer to our average highs for mid-September. Similar dewpoint temperatures will yield an unseasonably sticky night tonight. On the synoptic scale, today`s southeast pushing cold front will buckle back to the north as a warm front overnight, leaving us within a moist warm sector. A 30 kt low-level jet is forecast to develop over Kansas and western Missouri, sparking convection that will move into the forecast area Saturday morning. Showers and thunderstorms will diminish with eastward extent and with time as they stray further from the nose of the low-level jet and as the low- level jet diminishes, respectively. Most locations will remain dry in the afternoon apart from some isolated to widely scattered convection. Widespread 80s are forecast on Saturday with low 80s in northeast and central Missouri where rain and thicker cloud cover will be more persistent. Further east where the opposite is true, highs will reach closer to 90 degrees. Jaja && .LONG TERM... (Saturday Night through Next Friday) Issued at 342 PM CDT Fri Sep 20 2024 The best chance of rain and thunderstorms for our area will be Saturday night into Sunday as a series of vorticity maxima push through northern Missouri. The region will still be quite moist, being in the warm sector between a retreating warm front and advancing cold front to the northwest. At the same time, a west- southwest oriented 40 - 45 kt low-level jet will ramp up, providing a focus for convective initiation overnight. With PWAT values of 1.50" - 2.00", there will be plenty of moisture for the atmosphere to work with. The same can`t be said for instability overnight, but enough will be present for the development of some non-severe thunderstorms. Flooding is not expected to be an issue given very dry antecedent conditions and the progressive nature of the rain. The only impact expected is beneficial rainfall, some of it heavy in the luckiest spots. The current drought monitor has our entire forecast area in at least D0 conditions (abnormally dry), with the majority of our counties in D1 (moderate drought). The period between today and Tuesday will be our best chance at improving those conditions in quite some time. Ensemble probabilities of total rainfall of 1"+ between now and Tuesday evening are between 75 - 90% for much of the CWA. Is this locked in? No, but it does give hope that we`ll see more substantial rain than what we`ve gotten recently. With some locations not having seen more than a third of an inch of rain in several weeks, any rain is beneficial at this point. SPC has our region in a marginal risk again for Sunday afternoon as instability and shear increase. Currently, ensemble guidance shows anywhere from 500 - 1500 J/kg of SBCAPE and 20 - 35 kts of bulk shear at 1pm Sunday. What`s notable is that the instability and shear maxima aren`t collocated well, with instability being further south and better shear to the north, behind the passing cold front. Throw in the possibility of lingering morning convection, and the potential for strong to severe thunderstorms becomes even more uncertain for Sunday afternoon. The potential for showers and thunderstorms will continue into Monday and Tuesday as the aforementioned cold front stalls over the area and vorticity lobes from an approaching shortwave shoot over it. Being generally on the backside of the front at this point, temperatures will be much closer to normal. So although dewpoints may still be in the 60s across much of the region, cooler temperatures will dampen instability and thus our chances for strong thunderstorms during this period. The front will exit the region Tuesday as the shortwave swings through the region, ending our rain chances through at least the end of the week. Jaja && .AVIATION... (For the 06z TAFs through 06z Saturday Night) Issued at 1043 PM CDT Fri Sep 20 2024 VFR conditions will continue through much of the overnight period with the focus shifting westward to eastern Kansas. A decaying complex of showers and thunderstorms continue to move east into western Missouri with broken/overcast mid to high clouds reaching as far east as the Mississippi River. The main question heading into Saturday morning will be how much of this complex survives as it approaches KCOU/KJEF/KUIN. The stalling surface front lifts northeastward with scattered showers and a few thunderstorms expected along the boundary. This northward shift is limited with precipitation generally expected to track northwest to southeast, also leaving in question just how far east activity makes it before total decay. Therefore, much of the prevailing groups maintained VCSH/SHRA mention with VCTS over central Missouri terminal and VCSH near metro terminals. While VFR is favored through much of the period, isolated and intermittent MVFR cigs/vsby cannot be entirely ruled out if a lingering thunderstorm directly impacts a terminal. There could be a lull Saturday late morning into the afternoon with isolated showers/thunderstorms. The better potential holds off until late in the period when a more vigorous shortwave support more widespread showers and thunderstorms late Saturday night into Sunday. Much of this lies in the tail end or beyond the period, which will be addressed in later updates. Maples && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX