Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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608 FXUS63 KLSX 250843 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 343 AM CDT Tue Jun 25 2024 .KEY MESSAGES... - The heat continues for one more day. Heat index values of 105+ are likely across parts of central, east central, and southeast Missouri as well as southwest and south central Illinois. The Heat Advisory remains in effect with no changes needed. - Thunderstorms are expected to form this afternoon/evening along a slow moving cold front over Iowa, and move into our area during the late evening or overnight. Some storms could produce damaging winds of 60 mph or greater. A few could also produce some 1 inch hail. The highest threat of severe storms will be across northeast Missouri and west central Illinois, with a diminishing threat as the storms move farther south. - The cold front passes more slowly than expected in previous forecasts. While We`ll see relief from the heat on Wednesday, it will be more from clouds, and showers and thunderstorms, particularly in the afternoon south of I-70 where a few strong and possibly severe storms may develop. && .SHORT TERM... (Through Late Wednesday Afternoon) Issued at 341 AM CDT Tue Jun 25 2024 The Heat Advisory should be in good shape with the latest temperature guidance giving us upper 90s and even some 100-101 highs in the St. Louis Metro Area. The one fly in the ointment is the potential for mixing this afternoon. The RAP and to a lesser extent the GFS forecast soundings show deep mixing (over 700 mb on the RAP!) which drops surface dew point temperatures into the 60s across parts of southeast and east central Missouri into southwest Illinois. In this scenario the heat index would be closer to the air temperature. Though we technically wouldn`t make it to advisory criteria, the air temperature would likely be hotter than forecast, and the potential health effects would be the same, so I`ve made no changes to the advisory at this time. The heat and humidity will produce strong instability over northern Missouri into Iowa this afternoon ahead of the cold front which will ultimately bring us relief from the heat. The GFS and RAP show 4500 to 5000+ J/Kg MLCAPE late this afternoon across Iowa. They both develop convection and push it south slowly ahead of the front. Most CAMs have not been handling the ongoing convection across the Upper Midwest well, but the ARW WRF does seem to have a clue, which is making me lean on its solution. It develops several clusters of convection along the front which grow upscale into a QLCS that races south-southeast through the forecast area. Timing may not be perfect, but the evolution makes sense. The RAP and GFS never show more than about 2000 J/Kg MUCAPE Tuesday night as the line moves through, and deep layer shear is only about 15-25kts. That being said, we could see some damaging wind, particularly across northern portions of the area as the QLCS moves out of Iowa. Not sure how far south the wind threat will continue, but the storms should weaken as they move south away from the greatest instability. The latest guidance continues to show a slower solution for the FROPA which will bring us relief from our hot spell. This looks like it`s primarily due to a slower and sharper upstream short wave moving across the northern Plains and Upper Midwest late tonight into early Wednesday morning. The stronger wave forces low level cyclogenesis over the eastern Plains to along the Missouri/Iowa border. The surface-850 low closes off and takes its time meandering southeast through Missouri and southern Illinois Wednesday. While the front doesn`t make it all the way through the area until some time during the late afternoon/early evening, clouds and convection should keep temperatures in the 80s. Speaking of convection, both the RAP and to a lesser extent the GFS show instability increasing again in the afternoon across southern Missouri and Illinois ahead of the front. The RAP is showing 3000+ J/Kg of MLCAPE with deep-layer shear of 30+ kts, so there will be another threat for severe storms Wednesday afternoon ahead of the front. Carney && .LONG TERM... (Wednesday Night through Monday) Issued at 341 AM CDT Tue Jun 25 2024 Not much has changed for the remainder of the forecast. After a cooler Thursday under the influence of high pressure centered over the Great Lakes, southerly flow returns on Friday and we warm back into the 90 on Friday. Another cold front develops over northern Plains as a short wave aloft moves across the US/Canada border late Friday and the cold front associated with this system dives through the Mid Mississippi Valley Saturday. Temperatures ahead of the front on Saturday are forecast to reach into the 90s again, although ensemble guidance is showing a pretty wide spread across the area due to differences with the speed of the front. The 25th to 75th percentile high temperature spread on the LREF is 9 degrees at Lambert on Saturday, and the NBM forecast is warmer than the 75th percentile, so the current forecast may be a little hot. I did not adjust it this morning due to the uncertainty in the speed of the front at this time. One way or another, Sunday should be much cooler with another strong high pressure system moving into the Midwest and continuing to the east over the Great Lakes on Monday. Carney && .AVIATION... (For the 06z TAFs through 06z Tuesday Night) Issued at 1043 PM CDT Mon Jun 24 2024 VFR conditions are expected through much of the period as we await the potential for thunderstorms develop late Tuesday. Focus continues to be on fine tuning thunderstorm potential Tuesday afternoon, evening, and into the overnight period. A number of the hi-res guidance has either backed away from or delayed afternoon convection. Trends do not provide any higher confidence (actually less) in where isolated to scattered thunderstorms could develop during the afternoon, if any do. Thunderstorms over the Upper Midwest tonight will need to be watched to determine where, or if, resulting outflow will make it far enough south to assist in development. Any daytime thunderstorms would be after 18z through 00z. Otherwise, the primary impacts will hold off until later in the evening into the overnight hours. Another complex of thunderstorms is expected originate somewhere over southwest Iowa, which the pushes southeast into northern Missouri and west-central Illinois. The breadth of the complex will be such that all TAF site will eventually be affected with KUIN first in line, followed by central MO and metro terminals. Trends seems to be delaying it`s arrival by an hour or two at any given site, placing the complex just northwest of KUIN by 06z tomorrow night, KCOU/KJEF around 09z and metro terminal 10z and afterward. The other question is just how long in maintains intensity this far south. This complex, especially while mature, could result in high winds of 60-70 mph, heavy rain, and frequent lightning. This potential is highest around KUIN/KCOU/KJEF, but become questionable further south and east. Any direct impact could result in MVFR/IFR visibility and localize IFR cigs. Maples && .CLIMATE... Issued at 353 AM CDT Mon Jun 24 2024 With hot temperatures forecast, here are some of the daily high temperature records: MON JUN 24 TUE JUN 25 ST LOUIS 102 in 1988 102 in 1954 COLUMBIA 103 in 1988 102 in 1988 QUINCY 99 in 1988 102 in 1931 && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Heat Advisory from noon today to 8 PM CDT this evening for Audrain MO-Callaway MO-Crawford MO-Franklin MO-Gasconade MO- Iron MO-Jefferson MO-Lincoln MO-Madison MO-Montgomery MO- Osage MO-Reynolds MO-Saint Charles MO-Saint Francois MO- Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO- Warren MO-Washington MO. Heat Advisory until 8 PM CDT this evening for Boone MO-Cole MO- Moniteau MO. IL...Heat Advisory from noon today to 8 PM CDT this evening for Bond IL-Clinton IL-Fayette IL-Madison IL-Marion IL-Monroe IL- Randolph IL-Saint Clair IL-Washington IL. && $$ WFO LSX