Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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098 FXUS63 KLSX 270330 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1030 PM CDT Wed Jun 26 2024 .KEY MESSAGES... - Multiple rounds of showers and thunderstorms are expected Friday and Saturday. Some of the storms Friday night will be strong to severe, with damaging wind gusts and large hail being the main threat. - After a period of below normal to near normal temperatures through this weekend, heat returns early to mid next week. && .SHORT TERM... (Through Thursday Night) Issued at 220 PM CDT Wed Jun 26 2024 Water vapor imagery currently shows a shortwave passing over the CWA, which is continuing to aid in forcing mostly stratiform rain over southeastern Missouri and southwestern Illinois along and ahead of a cold front that is roughly bisecting the CWA from the northeast to the southwest. Given the cloud cover and rain along and south of the front, the effective boundary has temperatures actually cooler south of the front than north of the front. All this translates to that temperatures are running at to below normal this afternoon, with maximum temperatures reaching the mid 70s to around 80 north of I-70 and in the low to mid 70s south of I-70. As the shortwave progresses eastward through the remainder of the afternoon, rain chances will diminish west to east, with rain exiting the CWA to the southeast by late this afternoon/early this evening. Thanks to the cool and cloudy weather today and temperatures cooling into the low to mid 60s tonight, patchy fog can`t be ruled out tonight. This will especially be the case across southeastern Missouri and southwestern Illinois where rain has been falling through much of today. Tomorrow, a trough will deepen over the eastern U.S., keeping northwesterly flow aloft over the region and high pressure in the Midwest. The result is temperatures remaining at or below normal, though a couple degrees warmer than today due to clearer skies and ample sunshine. Elmore && .LONG TERM... (Friday through Next Wednesday) Issued at 220 PM CDT Wed Jun 26 2024 A potentially complicated convective scenario is expected across the region on Friday, with multiple rounds of convection possible. During the early morning hours, a subtle shortwave will move through the Central Plains and into the Midwest ahead of a trough digging into the Northern Plains coincident with warm front lifting through the Middle Mississippi Valley. Warm air advection associated with the front will lead to a cluster of convection that will move out of the Central Plains in the morning and into the Middle Mississippi Valley through mid-day. Both deterministic and ensemble guidance have the bulk of the convection passing just north of the CWA, with more scattered coverage further south across much of the CWA. In the wake of the warm front and early convection Friday evening and overnight, guidance consensus is that the atmosphere will destabilize across the Mid to Lower Missouri Valley among 35-40 kts of 0-6 km bulk shear. During the evening, a surface low and associated cold front will move into this airmass and force convection that will likely be supercellular in nature at least initially due to the shear magnitude and profile. Given the current expected track of the surface low and placement of the more potent airmass, the greatest chance for scattered severe storms is across portions of central Missouri Friday night. Initially storms will be capable of all severe hazards: large hail, damaging wind gusts, and tornadoes. However, convection will likely grow upscale into clusters and lines that will pose mainly a damaging wind threat. Given the trajectory of the surface low and quasi-zonal surface flow aloft, there is much uncertainty regarding the progression of the cold front through the CWA Friday night into Saturday. Faster solutions have the front clearing much of the CWA if not the entire CWA prior to peak heating on Saturday, while slower solutions have the front draped west to east somewhere across the CWA during peak heating and among an unstable airmass. A majority of guidance favors some flavor of the latter solution, which is reflected in the current forecast. This solution may support at least a low chance for severe storms, but this threat is highly conditional in what happens Friday and Friday night and is uncertain at this lead time. On Sunday, an upper-level trough will amplify over the eastern U.S. with northwesterly flow developing over the Middle Mississippi Valley, ushering high pressure into the region and pushing the front well south of the CWA. This will bring in an unseasonably cool airmass, with ensembles clustering around 80 degrees area-wide. How long this cool down lasts is uncertain, as guidance diverges on the eastward progression of the upper-level trough and how quickly ridging builds into the area early next week. A quicker solution favors temperatures warming on Monday relative to Sunday, while a slower solution favors similar temperatures Monday compared to Sunday. A majority of guidance has been trending toward the latter solution, which is reflected in the current forecast. Regardless, ensemble consensus is that the heat will return by mid-week as ridging builds across the Southeast. Elmore && .AVIATION... (For the 06z TAFs through 06z Thursday Night) Issued at 1026 PM CDT Wed Jun 26 2024 No real changes for the 6Z TAF package. A clear sky and light/variable winds are forecast overnight tonight. Light northeasterly winds are forecast on Thursday with FEW-SCT diurnal cumulus (bases 4-6 kft AGL). Any cumulus should dissipate near/just after sunset Thursday evening as winds turn more to the east. Still cannot rule out some river valley fog potentially impacting KSUS/KJEF/KCPS overnight tonight into early Thursday morning. However, some deeper mixing during the afternoon after it cleared out along with some dry advection suggests this is not a likely scenario. Better chances of river valley fog continue to be across southeast Missouri which was cooler today and where cloud cover departed later on in the day. Gosselin && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX