Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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164 FXUS63 KLSX 191123 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 623 AM CDT Thu Sep 19 2024 .KEY MESSAGES... -Several opportunities for rain tonight through next week. The best chances of showers and storms will be Saturday night through Sunday night. Otherwise, while any rain is beneficial, the chances for significant drought relief remains low. -Well above normal temperatures through Saturday, then cooler weather for the last half of the weekend and into next week. && .SHORT TERM... (Through Late Friday Afternoon) Issued at 310 AM CDT Thu Sep 19 2024 With an upper level ridge across the region, a shortwave over the Northern Plains will continue to slowly lift northeastward into southern Canada, thus dry and hot conditions will prevail across our area today. Highs will be in the upper 80s to low 90s under mostly sunny skies. In the meantime, a cold front associated with the Northern Plains system will move into northwestern Missouri this afternoon. As the front begins to move into northeastern Missouri this evening, will see increasing chances of showers and storms ahead of it. However, the main energy with the front will stay north of the forecast area. Also, a southwesterly low level jet will develop, but it will be fairly weak (around 25kts) and veer to the west by Friday morning. So precipitation coverage will be widely scattered with the best chances over portions of northeast Missouri and west central Illinois. The latest CAMs still have some differences in timing and location of the rain, but confidence is high that the best period of precipitation will be between 11 pm and 7 am. Despite MU CAPEs around 1000-1500 J/kg over portions of central/northeast Missouri late this afternoon, 0 to 6km bulk shear will be fairly weak (25kts) with ML lapse rates less than 6C/km. The instability will weaken fairly rapidly with loss of daytime heating, thus do not expect any severe storms in our area this evening. Otherwise, the front will gradually wash out as it sinks south during the day on Friday. Also, the upper level ridge will remain anchored overhead. Therefore, expect another well above normal day with highs in the upper 80s to low 90s. Some concern that activity could fire back up Friday afternoon. The latest HREF ensemble has mean MU CAPEs in excess of 1000 J/kg, mainly south of I-70. However, there is little to no convergence left along what remains of the frontal boundary. So for now will keep dry conditions in this area. Byrd && .LONG TERM... (Friday Night through Wednesday) Issued at 310 AM CDT Thu Sep 19 2024 Even though the upper level ridge will remain overhead through the weekend, the trough over the western CONUS will begin to lift out towards the Central Plains Friday night. By early next week the upper level ridge will begin to be suppressed to our south, allowing a series of shortwaves to slide through the region with rounds of showers and storms. Even though the latest cluster analysis still has various timing, location and strength issues with how the upper level pattern evolves through next week, the latest deterministic models are in decent agreement that a strong wave will move through Saturday night into Sunday. Even though the region remains cutoff from Gulf moisture, strong southwesterly flow will usher in monsoonal moisture into the region by Saturday. So showers and thunderstorms are likely (60-80%), especially along and north of I- 70. Beyond Sunday night, with such uncertainty on how the system will develop, only expect low rain chances (roughly 30-40%) Monday through Wednesday. As for temperatures through the extended period, one more day of above normal highs on Saturday despite increasing cloud cover. Then as the ridge is suppressed to our south beginning on Sunday, temperatures begin to cool to near normal. With some uncertainty on how the upper level pattern will evolve, especially early next week, the latest ensembles have a fairly wide spread between the 25th and 75th percentiles (at least 10 degrees) for temperatures Monday through Wednesday. Byrd && .AVIATION... (For the 12z TAFs through 12z Friday Morning) Issued at 622 AM CDT Thu Sep 19 2024 VFR flight conditions through the forecast period with light south winds. Otherwise, as cold front approaches region, could see some isolated storms over portions of northeast MO and west central IL, so added vicinity thunderstorm mention at KUIN after 05z Friday. Byrd && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX