Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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056 FXUS63 KLSX 271735 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1235 PM CDT Thu Jun 27 2024 .KEY MESSAGES... - Several rounds of showers and thunderstorms are expected Friday through Saturday. A few strong to severe storms are possible with damaging winds Friday night. Saturday`s storms could be stronger ahead of a cold front, although potentially less wide- spread than Friday night`s storms. Damaging winds will be the primary threat on Saturday. Large hail is also a possibility. - Below normal temperatures are expected on Sunday and Monday behind the front. Temperatures rise back above normal by the middle of next week. && .SHORT TERM... (Through Late Friday Afternoon) Issued at 252 AM CDT Thu Jun 27 2024 High pressure centered over the Upper Midwest will drift southeast over the Great Lakes today. The wind will veer to the northeast and then east as the high moves today. While the flow won`t be particularly strong, this will funnel cooler and drier air into the Mid Mississippi Valley. While it won`t be a "cool" day, it will be cooler and much less humid than earlier in the week. Guidance is in good agreement that highs will max out in the mid to upper 80s which is right around normal for late June. The high will continue east tonight into Friday, moving into New England, and then off the East Coast by Friday evening. Meanwhile, a mid-upper level short wave will move across the Rockies into the northern Plains tonight and Friday resulting in low level cyclogenesis. The low level jet ramps up over the Plains ahead of the system tonight which forces convection to develop. The showers and storms move east overnight into Missouri on Friday morning. Short range guidance is in good agreement that this broad area of showers and thunderstorms will move across Missouri into Illinois on Friday. Clouds and precip should suppress temperatures particularly across northeast Missouri and west central Illinois where the most rain is forecast to fall. A couple of days ago, we were expecting highs on Friday to rise back into the 90s across much of the area. With the inclusion of clouds and precip, our latest guidance keeps highs in the 80s. The cooler temps limit how unstable the atmosphere can get over our area Friday afternoon, and now most of the high instability stays over eastern Kansas and western Missouri. The models develop convection on the nose of a low level jet in these areas during the late afternoon or early evening, and move the area of storms east through the night. While precip does look likely for our area on Friday night, the strongest storms should stick to where the highest instability will be, over western Missouri. Several CAMs bear this out, showing that the highest simulated reflectivity will remain outside of our area. With all that said, Friday`s rain doesn`t materialize, we could get hot and unstable Friday afternoon which would produce stronger storms Friday evening. Saturday`s forecast will depend on a number of factors including, the position of the cold front, where the effective boundary from Friday night`s storms is located, and how much lingering clouds and showers are left over from Friday night`s convection to affect temperatures. The deterministic GFS and ECMWF show similar solutions with the cold front pushing south-southeast through Missouri and Illinois during the day. The GFS pools dew points ahead of the front showing a broad area of 75-78 degrees. This produces MLCAPE values in excess of 3000 J/Kg ahead of the front during the afternoon and early evening. The EC is more believable at 72-75 dew point, producing between 2000-2500 J/Kg. This agrees well with the LREF mean SBCAPE values for Saturday afternoon and evening so I`m tending to lean in that direction. Regardless, there will be more than enough instability to produce strong updrafts, however the shear will be marginal at best with most guidance members showing between 15-25 kts of deep layer shear. Even the stronger GFS only gets to around 30kts. Guidance is also showing some CINH above the front which could limit the coverage of storms, particularly since low level convergence along the front doesn`t look very strong. Overall, I think the severe weather threat for Saturday is limited, but probably higher than Friday night`s threat. If we realize that much instability, think the primary threat will be damaging winds. Can`t rule out hail entirely either, but it`s less likely than wind due to the warm temperatures forecast ahead of the front. Carney && .LONG TERM... (Sunday through Wednesday) Issued at 252 AM CDT Thu Jun 27 2024 Sunday and Monday look dry and cooler behind the front and high pressure crossing the Mississippi Valley into the Great Lakes. Bias corrected guidance looks like it`s affecting the NBM as it`s highs both days were slightly above the 75th percentile. With that said...it`s almost July...and the sun angle doesn`t get any higher. It`s also 4-5 days out, so there`s plenty that can still go wrong with the forecast. Winds turn back to the south Tuesday and persist into Wednesday bringing warmer and more humid air back to the Mid- Mississippi Valley, as well as a chance for more Thunderstorms. Carney && .AVIATION... (For the 18z TAFs through 18z Friday Afternoon) Issued at 1228 PM CDT Thu Jun 27 2024 Confidence is high in dry and VFR flight conditions through today and much of tonight for most of the local terminals. The exception to this is KUIN where scattered showers and thunderstorms may impact late in the period. These showers and thunderstorms are expected to remain north of the other local terminals, though very brief impacts are possible at any terminals. For terminals that are impacted by this convection, drops in visibility are expected. Elmore && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX