Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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788 FXUS63 KLSX 280316 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1016 PM CDT Thu Jun 27 2024 .KEY MESSAGES... - Multiple rounds of showers and thunderstorms are expected Friday morning through Saturday evening. There is a low chance that some of the storms could become strong to severe Friday night and again Saturday afternoon and evening, with the main threats being damaging wind gusts and large hail. - Near normal to below normal temperatures are expected through the weekend, with heat returning toward the middle of next week. && .SHORT TERM... (Through Late Friday Night) Issued at 250 PM CDT Thu Jun 27 2024 Current water vapor imagery shows ridging across the Plains, with northwesterly flow over the CWA. An expansive surface high remains centered over the Great Lakes and extends southwestward into our CWA, keeping our weather relatively cooler and calmer compared to as of late. With mostly clear skies and ample sunshine, temperatures have warmed further than this point yesterday, but still at or below climatological normals. Tonight, the previously mentioned ridge will flatten and broaden across the region as a warm front lifts through the Upper Midwest. Warm advection associated with this front will spawn scattered showers and thunderstorms mainly north of I-70 and particularly over northeastern Missouri and west-central Illinois. This activity is expected mostly Friday morning, but isolated convection could continue through the afternoon in this part of the CWA. The warm air advection will bump-up temperatures relative to today, but cloud cover associated with the convection will keep temperatures from increasing no more than a couple of degrees over today. The shortwave and surface system responsible for Friday night`s convection has slowed further, with the main focus for robust convection remaining over the Central Plains. As it moves eastward into the CWA very late Friday night/early Saturday morning, it is expected to be in a decaying state given the lack of instability across the area. Additional convection is expected along the nose of the low-level jet mainly north of I-70 late through the overnight period. Again, with relatively low instability (roughly at or below 1,000 J/kg of MUCAPE), wide-spread severe weather is not anticipated, but a storm or two could become strong to severe and be mainly capable of damaging wind gusts and large hail. Elmore && .LONG TERM... (Saturday through Next Thursday) Issued at 250 PM CDT Thu Jun 27 2024 Early Saturday, there is a chance (50%) that convection from Friday night will still be ongoing as the low-level jet continues to veer across the CWA. Though, this convection is currently expected to be weak and scattered, as the jet will be weakening and instability will be at a minimum. Aloft, a shortwave will be departing the region, leaving subtle height rises in its wake per deterministic and ensemble guidance consensus as a cold front slowly sags southward through the CWA through the day. However, given the morning convection, outflow may force the effective boundary further south than the synoptic front. The front or effective boundary will be among an environment characterized by SBCAPE of around 2,000 J/kg, an ideal amount for robust updrafts to form along the boundary. However, guidance consensus is that shear will be weaker (20, possibly 30 kts at best), and a majority of model soundings show some degree of capping or poor low-level lapse rates. These factors, along with rising heights, keep confidence low in the occurrence and coverage of convection during afternoon and evening on Saturday. If convection is able to form and become strong to severe, damaging wind gusts is the primary threat, but large hail could occur within the strongest updrafts. On Sunday, a trough deepening over the eastern U.S., will cause flow over the Middle Mississippi Valley to become increasingly northwesterly, pushing high pressure into the Midwest and the front well south of the CWA. This will usher in an unseasonably cool airmass for the area on Sunday, with ensembles continuing to cluster around 80 degrees across the CWA. Despite guidance consensus continuing to have an upper-level ridge building quickly into the Midwest on Monday, the surface high is slow to depart the Midwest, keeping temperatures relatively cool on Monday. However, this cool- down will be short lived, as guidance consensus has the ridge expanding across much of the southern CONUS and southerly low-level flow returning to the region. In turn, warm air is expected to advect into the region for the mid-week period, with ensemble means supporting above-normal temperatures. How long this heat will last is uncertain, as guidance indicates that an upper-level trough will impede on this ridge and attempt to send a cold front through the Midwest. However, guidance varies on the strength of the ridge vs. trough, and therefore, how much southward progression the front makes. The current forecast for Wednesday leans more toward a slower FROPA, which is supported by a majority of guidance. Elmore && .AVIATION... (For the 06z TAFs through 06z Friday Night) Issued at 1009 PM CDT Thu Jun 27 2024 Increasing mid/high level cloudiness is forecast overnight tonight into the day on Friday. An area of rain and a few thunderstorms is expected to impact parts of northeast Missouri and west-central Illinois through the daylight hours, but may stay just northwest of KUIN. Better chances of showers and thunderstorms come late Friday evening into the overnight hours, but largely just after 0600 UTC Saturday. Gosselin && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX