Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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952 FXUS63 KLSX 121742 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1242 PM CDT Wed Jun 12 2024 .KEY MESSAGES... - Temperatures will warm well above normal to end this week and start next week. The warmest conditions are expected Thursday, Sunday and Monday with heat index values reaching 100-105 in some locations. - There is a threat for severe thunderstorms Thursday evening. The primary threats will be large hail and damaging winds. && .SHORT TERM... (Through Late Thursday Afternoon) Issued at 325 AM CDT Wed Jun 12 2024 Warming trend continues today as southwesterly low level flow ushers in 850mb temperatures into the mid to upper teens, then surge to between 20-25C on Thursday. So expect widespread highs in the 90s by Thursday. In the meantime, as dewpoints rise through the period, expect hot and humid conditions with heat indices around 100 degrees by Thursday for the first time this summer. Otherwise, the main forecast issue in the short term is the increasing chances of showers and storms, especially by Thursday evening. With the upper level ridge over the western CONUS slowly sliding to the east, flow aloft becomes northwesterly overhead. So as systems slide west to east along the U.S./Canadian border, each successive system will ever so slightly push the cold front southward towards the forecast area. A few of the latest CAMs indicate that far northern portions of the forecast area could see showers and storms move in towards daybreak on Thursday. However, confidence is low as a majority of the latest forecast soundings depict a decent cap, limited moisture, and no upper support, so kept forecast for tonight/early Thursday morning dry at this time. During the day on Thursday, a majority of the deterministic models, even the latest NAM, as well as ensembles now have similar timing and placement of the cold front, just north of the IA/MO border by 18z Thursday. South of the front, surface dewpoints in the upper 60s to the mid 70s will pool along and ahead of the cold front. This will likely result in moderate instability with ML CAPEs 3000+ J/kg. However, capping is forecast to be in place and there is some uncertainty of when it will weaken enough to allow convection to initiate. At this time, the latest forecast soundings indicate that the cap will break by early evening, so have increasing POPs after 21z Thursday. Now as far as convective mode, could initially see supercell development, especially because of the high ML CAPEs, 0- 6km bulk shear 30-40kts, and ML LR 7.5-8C/km. So the main threat will be large hail initially then as activity transitions to line segments as it moves east across the region, damaging winds become the main threat. However, as the cold front and associated activity pushes south through the evening, the shear lessens as well as the instability. Thus there is still some uncertainty on exactly how far south strong to severe storms will make it before waning. For now best chances will be along and north of I-70. Byrd && .LONG TERM... (Thursday Night through Tuesday) Issued at 325 AM CDT Wed Jun 12 2024 For the remainder of the forecast period, the main issue will be the well above normal temperatures as well as heat indices meeting and/or exceeding 100 degrees. By Friday morning, the cold front will be south of the forecast area and stall out, then it will move back north as a warm front on Saturday. So temperatures on Friday will be a bit cooler, but still above normal in the upper 80s to low 90s. However, there is some uncertainty on exactly where the cold front will stall out. If it remains over southern portions of the forecast area, dewpoints in the 70s along and south of the front could make it rather uncomfortable in this area as winds will also be rather weak and not provide any relief. Beyond that, the upper level ridge will build into the region with increasing southerly surface flow as the front moves back north as a warm front on Saturday. So a resurgence in hot and humid weather returns for the rest of the weekend and into next week. As for any precipitation chances, some guidance suggests a few showers and thunderstorms along the warm front as a weakening upper shortwave trough over the central Plains lifts northeast into the Great Lakes region Saturday. A majority of the latest deterministic and ensembles keeps the best chances of showers and storms just off to the west and northwest of the forecast area at this time. Beyond Saturday, there are considerable timing, strength and location differences among the deterministic models on how the pattern evolves and additional chances of showers and storms. For now we remain dry through early next week with the next chance for rain by Tuesday. Byrd && .AVIATION... (For the 18z TAFs through 18z Thursday Afternoon) Issued at 1241 PM CDT Wed Jun 12 2024 High pressure and northwesterly upper flow will result in VFR conditions through the period. Light southerly flow will become light and variable before returning to 5-10 knots Thursday afternoon. Maples && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX