Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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028 FXUS63 KLSX 251046 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 546 AM CDT Sat May 25 2024 .KEY MESSAGES... -Severe thunderstorms are expected late Saturday night into Sunday morning and again during the afternoon/evening, with damaging winds, a tornado, and large hail possible. -Locally heavy rainfall is possible with both rounds of severe thunderstorms on Sunday. -The upcoming work week will be dry with near normal temperatures. && .SHORT TERM... (Through Sunday) Issued at 253 AM CDT Sat May 25 2024 A cold front, associated with a retreating surface low and mid-level trough, will continue to push through the forecast area early this morning. As of 3 AM the front was located from roughly West Plains, MO through the St. Louis metro to Champaign, IL. Behind this front, a surface high and mid- level height rises will result in a dry day with a largely clear sky. By Saturday evening the nose of the upper level jet will advance into the mid-Mississippi Valley at the same time as a negatively tilted mid-level trough. At the surface a low will form across the Central Plains, it`s attendant warm front surging northward through Missouri overnight. The large scale ascent, enhanced by the strengthening north-south oriented LLJ, will spawn convection in the warm sector across portions of Kansas, Nebraska, and western Missouri. Convection will become organized into an MCS and track eastward through the forecast area along the nose of the veering LLJ and supported by a shortwave sliding through the advancing mid-level trough. The MCS is expected to move through the area during the early to late morning hours. Where exactly the MCS tracks remains uncertain, some guidance sources push it along the I-70 corridor, while others push it across southern Missouri and Illinois. 1500-3000 J/kg of MUCAPE and strong shear throughout the profile combined with cold pool support from the MCS will allow portions of the MCS to become strong to severe. The main concerns with any area of the MCS that become severe will be damaging winds and embedded tornadoes, although large hail can`t be ruled out. The MCS will exit during the morning hours, and the path the MCS takes through the CWA will determine where the outflow is deposited, and thus where the best chance for afternoon re-development will be. Thunderstorms are expected to redevelop during the afternoon and into the evening as the mid-level trough pushes into the forecast area. Moisture and instability will advect into the forecast area Sunday behind the morning convection within the warm sector, but how much destabilization occurs in the wake of the MCS remains uncertain. Ensemble minimum surface based CAPE from the HREF shows 1000-2000 J/kg of instability present across the entire forecast area tomorrow afternoon, increasing confidence that at least some destabilization will be able to occur. Confidence is highest that the best storm environment will be along and south of the outflow boundary left over from the morning convection. Here instability will be high with MLCAPE values between 2000-4000 J/kg. Additionally, favorable bulk shear (30-50 kts of 0-6 km), and 7-8 C/km mid-level lapse rates will support severe thunderstorm development. Multicell clusters and bowing segments that become severe will be capable of significant damaging winds, as well as large hail and tornadoes. The NAEFS situational awareness table indicates integrated water vapor transport (ie moisture advection through the atmosphere) will be in the 90th percentile Sunday. The high moisture content combined with the potential for two rounds of thunderstorms and training thunderstorms introduces a heavy rain risk. There is a particular risk for heavy rain and localized flash flooding in areas that saw heavy rain tonight. The probability-matched mean QPF from the HREF indicate that with both the morning MCS and afternoon convection that QPF could exceed 2 inches/3 hours. These storms will track southeastward over the evening, chased by the attendant cold front of the previously mentioned surface low. Additional thunderstorm development along the front is not anticipated to be widespread given the weak convergence along the front. The front will exit the forecast area by Monday. MRM && .LONG TERM... (Sunday Night through Friday) Issued at 253 AM CDT Sat May 25 2024 Behind this system largely dry and near seasonable temperatures will persist into the second half of the work week under a northwesterly flow regime. Another pattern change will take place towards the end of the period as the mid-level ridge that will be over the Rockies during the work week shifts eastward and throws the mid-Mississippi Valley back into southwest flow aloft. This will bring above normal temperatures and chances for precipitation back to the forecast. MRM && .AVIATION... (For the 12z TAFs through 12z Sunday Morning) Issued at 541 AM CDT Sat May 25 2024 A patch of stratus continues to linger across the St. Louis metro, causing varying flight categories. This stratus is expected to continue to diminish this morning as the sun comes up, but I cannot rule out brief periods of IFR to MVFR ceilings before that happens. As such, I`ve added a tempo group at KSTL where the stratus edge is lingering. VFR flight conditions will persist through the remainder of the TAF period ahead of the next round of thunderstorms. Thunderstorms will move into the area late in the TAF period, but where exactly they move remains uncertain at this point. I have outlined in the TAFs the best timing for these thunderstorms, but details remain to be worked out. A few thunderstorms could be strong to severe at the end of the TAF period, with damaging winds and tornadoes the main hazards. MRM && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX