Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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909 FXUS63 KLSX 311039 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 539 AM CDT Fri May 31 2024 .KEY MESSAGES... - Rain returns to the region overnight tonight, and after a largely-dry Saturday night through Sunday, additional thunderstorm chances linger in the region through Wednesday. - Temperatures will warm from the roughly-seasonable values we`ve recently enjoyed into the low/mid-80s starting Sunday, approaching 90 degrees by mid-week. Exactly how warm we get will depend on currently-uncertain rainfall timing and coverage. && .SHORT TERM... (Through Late Saturday Afternoon) Issued at 230 AM CDT Fri May 31 2024 Regional radar mosaic depicts a nearly stationary area of showers, with a few thunderstorms, stretching across the Kansas/Missouri border and north into the Missouri River Valley. This activity, forced by a weak and sluggish mid-level shortwave, weak low-level jet, and nebulous surface convergence, is causing nothing more than increased high cloud cover over the forecast area. Recent high- resolution model guidance now suggests that this mid-level wave will be slower to translate east than originally forecast, and that dry air will be more prominent in the mid-levels. As such precipitation chances today were slowed from west to east. Cloud cover and weak easterly winds from a departing surface high to our northeast will keep temperatures in the upper 70s to low 80s today, near to slightly-below normal once again. By the late afternoon into the early evening, a weak surface low will develop in the Lower Mississippi Valley as the mid-level wave amplifies slightly. As the low tracks north and east, weak but widespread moist isentropic ascent will allow for a shield of stratiform rain to advance into the region from southwest to northeast. It is worth noting that one or two CAMs do not show this feature, which would lead to far more localized rain (under thunderstorms) if it were to occur. That said, the synoptic pattern seems more conducive to light, stratiform rain. While most will see amounts around 0.50-0.75 inches or less, there are hints in high- resolution model soundings that a few embedded thunderstorms are possible in eastern and central Missouri into west-central Illinois along an axis of deep moist convergence. If more convective elements can occur, and are slow-moving or training, amidst precipitable water values very likely (near 100% chance) exceeding the 90th climatological percentile, locally-heavy rain would bring a threat for nuisance flooding. That said, this threat is limited spatially and nearly all will see unimpactful rain through Saturday morning. Rain is slow to exit Saturday thanks to a secondary mid-level wave of PVA forcing additional showers and thunderstorms. Chances decrease through the day from west to east as the wave passes, though it doesn`t look like a washout in any one spot whatsoever. The rain and cloud cover will work to keep temperatures in check once again, with most of the region struggling to warm out of the upper 70s. That said, if we clear out sooner than forecast, central Missouri has the potential (15-20% via high-resolution ensembles) to reach 80 degrees. MRB && .LONG TERM... (Saturday Night through Thursday) Issued at 230 AM CDT Fri May 31 2024 By late Saturday, quasi-zonal flow becomes more established aloft, putting the region`s rain chances at the mercy of subtle shortwaves that introduce timing and location uncertainty to the forecast. Saturday night, and most of Sunday, will remain largely dry with mid- level shortwaves staying to our south and west. There is a low chance (15-20%) for a few thunderstorms Sunday afternoon in central and southeast Missouri where better instability will exist, but a lack of upper-level ascent will keep any activity widely-scattered at best. Waves of mid-level forcing will continue to march through the Midwest from Monday through Wednesday, bringing the threat for showers and thunderstorms each day. That said, ensemble cluster analysis varies on exactly when and where rain will fall. This results in broad-brushed rain chances in the forecast, though the first half of the week does not look like a washout. There is also very little threat for stronger thunderstorms at this point, with these shortwaves not looking very strong. What will be more noticeable are our warming temperatures: stronger 850mb warm-air advection amidst low-level southwest flow will bring temperatures into at least the low to mid-80s on Sunday. We warm further as the work week begins, with 850mb temperatures likely (60- 80% chance) reaching the 75th climatological percentile by then. Tuesday is forecast to be the warmest day of the week, when the low- level thermal ridge is strongest over the region (19-22 degrees Celsius). That said, exactly how warm we get will be much more closely tied to where, when, and how widespread precipitation will be on each day. While global-scale ensembles have very low probabilities (5-10%) of reaching 90 degrees on Tuesday, I wouldn`t rule it out if precipitation holds off and strong southerly low- level flow prevails. MOS guidance does nod to this potential, and is likely closer to the high-end possibility. Any temperatures this warm would be relegated to the metropolitan areas, most likely, though most of the region would sit in the mid/upper 80s. While global-scale ensembles vary on forecast details through mid- week, they do converge on a pattern change by late Wednesday. A rather strong upper-level ridge (near the 99th climatological percentile in model guidance) establishes over the western CONUS, with a well-defined wave tracking through the northern Plains into the Great Lakes. The resultant surface low would draw a cold front through the bi-state area and usher in a more concrete chance for widespread showers and thunderstorms. That said, exactly when the front passes and how strong it will be remains uncertain, leading to lower precipitation chances overall. Following this frontal passage, the Mississippi Valley will be squarely in deep northwest flow. This will bring our low, but persistent rain chances to an end from Thursday into the weekend, and cool temperatures back to near/slightly-below normal (upper 70s to low 80s). How long we stay in this pattern is uncertain, with some guidance bringing a ridge into the Plains by late Saturday and others holding onto the northwest flow through next weekend. MRB && .AVIATION... (For the 12z TAFs through 12z Saturday Morning) Issued at 531 AM CDT Fri May 31 2024 Dry and VFR conditions currently prevail at the terminals, with high cirrus streaming into the region from convection to the south and west. East winds will gradually shift out of the south through the day as a surface low and widespread showers approach the TAF sites. There`s some question as to how much instability we`ll see, which will limit thunder chances through the event. The best chances will be overnight tonight, which is delineated in the VCTS lines for the St. Louis metropolitan and central Missouri TAFs. KUIN still has no thunder mentioned as it is unlikely to see instability that far north. MVFR CIGs are still expected to plague the region for much of the TAF period. Guidance is still pulling CIGs down far lower to IFR and LIFR in some instances, though given how rare that is in June and the current setup, I am hesitant to believe that output and remain on the optimistic side of guidance. MRB && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX