Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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561 FXUS63 KLSX 010344 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1044 PM CDT Fri May 31 2024 .KEY MESSAGES... - Rain will overspread the area tonight lasting into Saturday morning with potential for a little embedded thunder. Isolated to scattered shower and thunderstorms will then impact the area Saturday afternoon and early evening. - A short period of largely dry weather will prevail from late Saturday evening through Sunday morning, with shower and thunderstorm chances return Sunday afternoon continuing through the middle of next week. - After a day of cooler weather and below normal temperatures on Saturday, we rebound to above normal temperatures and heat more typical of summer starting Sunday lasting into the middle of next week. && .SHORT TERM... (Through Late Saturday Night) Issued at 154 PM CDT Fri May 31 2024 A broad mid-upper trof with embedded vort maxes and currently centered through eastern KS will move eastward into MO tonight, with the axis moving east of the CWA by early Saturday evening. These vort maxes along with an evolving and strengthening warm conveyor belt featuring a veering S-SE LLJ will modulate precipitation coverage and intensity over the next 36 hours. Precipitation has been slow to evolve initially but as we head into the evening the combination of the increasing LLJ, attendant moisture transport and convergence along with larger scale forcing should lead to widespread showers spreading and developing northward through central and eastern MO, then shifting east/northeast into the overnight hours. There is not much in the way of instability forecast with the precipitation tonight, but the appearance of negative showalter indices late evening and overnight attendant with some mid-level cooling and better low- level forcing would suggest a threat of some embedded elevated thunderstorms overnight into mid-Saturday morning, especially towards the backside of the overall precipitation shield. The moisture transport will lead to PWs increasing to 1.5+ inches during this time frame and given this moisture increase, slow cell motions, and some potential for cell training we could see some localized corridors of heavier rainfall within an SSW-NNE zone extending from between STL-COU into west-central IL. Both the PPM and LPPM guidance from the HREF support this idea with pockets of 1.5-3.0 inches of rainfall. Unless this falls in an hour or so or the amounts are higher, which would probably necessitate more convection, then the overall flood threat appears low at this time. The main thrust of more continuous precipitation/greater coverage of showers should move east of the CWA by 18z or so on Saturday as the mean mid-upper trof axis progresses east. However diurnal destabilization in the wake of the axis passage (afternoon MLCAPE of 1000-1500+), and weak surface convergence attendant with the surface trof that is attendant with the upper system, will fuel the development of additional isolated-scattered showers and thunderstorms across eastern MO and SW IL into early evening along/ahead of the advancing surface trof/convergence axis. The aforementioned storms would pose a threat of heavy downpours and lightning. All indications are any ongoing showers and storms should dissipate rather quickly over SW IL on Saturday evening with loss of heating/instability, and the remainder of Saturday night should be dry and tranquil. Glass && .LONG TERM... (Sunday through Next Friday) Issued at 154 PM CDT Fri May 31 2024 Quasi-zonal mid-upper level flow will develop Saturday night into early next week. This regime will initially feature lower- amplitude disturbances through Sunday night, with more prominent short-waves centered thru the mid-upper MS Valley traversing the region Monday-Tuesday. These disturbances along with plentiful instability will bring prolonged thunderstorm chances to the region, first returning late Sunday afternoon in the western CWA. All day rains don`t appear likely, and despite the prolonged precipitation chances, there will be plenty of dry time. Depending on the timing of the waves and amount of deep layer shear present, this potential could include organized multicell strong storms in the afternoon/evening. Also attendant with the quasi- zonal regime will be the return of largely south-southwest low level flow and WAA, resulting in a warmer weather and above normal temperatures somewhat typical of June/early Summer. Longer range guidance strongly suggests a significant pattern change will take place starting late Tuesday and progressing through the end of next week featuring a deepening northern stream upper trof sliding into the eastern CONUS. This trof is eventually forecast to be the dominant steering feature for the MS Valley eastward. While there is generally a consensus in this change and the development of NW flow aloft, there are substantial differences with the ensemble systems evident in the LREF cluster analysis regarding the details of the depth and position of the trof and the magnitude of heights aloft. The initial short wave trof starting the pattern change will drive a cold front through the area, most likely in the Tuesday night-Wednesday morning time frame. An earlier cold fropa would possibly bring a potential for stronger storms due to greater instability. The differences in the pattern details and heights aloft lead to lower confidence in temperatures beyond Wednesday with the LREF IQR for highs on Thursday on the order of 15 degrees ranging from the low-mid 70s to upper 80s. Glass && .AVIATION... (For the 06z TAFs through 06z Saturday Night) Issued at 1035 PM CDT Fri May 31 2024 A weather system continues to gradually move through the region, impacting all local terminals with rain and increasingly low ceilings. The rain is mostly prevalent over central and eastern portions of Missouri, primarily impacting KCOU and KJEF. KUIN, KSUS, KSTL, and KCPS are on the eastern edge of this rain, and are expected to see intermittent rain through the night. Brief, embedded heavier showers and possibly a thunderstorm may produce lower visibilities at time, but confidence in these directly impacting terminals remains low. Additionally, low stratus will continue to creep into the region from southwest to northeast, eventually bringing IFR ceilings to all local terminals by mid Saturday morning. While the greater rain chances will clear out during the morning, the ceilings will be slow to improve. VFR flight conditions are current forecast by the end of the current forecast period; however, given the timing and hints from some model guidance, I can`t rule out that some local terminals remain MVFR or IFR into tomorrow night. While the better rain chances clear out during the morning, the chance for scattered showers and possibly thunderstorms will carry into the afternoon for most of the local terminals. This convection is expected to remain east of KCOU and KJEF at the moment. Elmore && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX