


Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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693 FXUS63 KLSX 212308 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 608 PM CDT Sat Jun 21 2025 .KEY MESSAGES... - Dangerous heat will continue Sunday through at least Tuesday. Heat index values of 100+ are expected each afternoon. && .SHORT TERM... (Through Late Sunday Afternoon) Issued at 218 PM CDT Sat Jun 21 2025 A strong upper level ridge is moving across the Mississippi Valley today. The ridge will settle over the eastern United States by early Sunday morning and remain nearly stationary into next week. Low level southwest flow continues to funnel 70+ degree dew point air from the southern Plans and Gulf Coast up into the Lower Missouri and Mid Mississippi Valley. This pattern will persist tomorrow, and Sunday looks like a near carbon copy of today. Forecast soundings are showing deep mixing into a relatively dry layer aloft, so afternoon dew point temperatures may be a couple of degrees lower. However, dangerous heat index values in excess of 100 degrees continue to be likely Sunday afternoon and the Extreme Heat Warning and Heat Advisories will continue unchanged at this time. Carney && .LONG TERM... (Sunday Night through Next Saturday) Issued at 226 PM CDT Sat Jun 21 2025 ...Corrected... The primary concern moving into the medium range remains excessive heat. The upper level ridge remains over eastern U.S through much of the week, although it is forecast to weaken slowly in the Wednesday through Saturday time frame. LREF mean and GFS/ECMWF deterministic models show 594+ dam heights at 500mb locked in over the Mid Mississippi Valley through at least Tuesday, so the atmosphere will have the potential to be just as warm through Tuesday as it is today and Sunday. With that said, there is some question about how high the heat index will get both days as forecast soundings show deep mixing which could cause surface dew point temperatures to drop several degrees during the afternoon. There`s also the possibility of some afternoon thunderstorms, primarily on Tuesday which could also break the heat. While some deterministic models, most notably the GFS, do print out QPF Tuesday afternoon over our area, I find it difficult to imagine there will be as much convection as advertised under the influence of the ridge. While I cannot totally rule out precip on Tuesday, I think the wetter members of the NBM are influencing the deterministic PoPs and temperatures causing them to be too high and too low respectively. Have therefore cut PoPs to no higher than 20% on Tuesday, and leaned on the 75th percentile for highs on both Monday and Tuesday. This will bump heat index values up a little from previous forecast and lends a little more confidence in continuing the heat headlines. The potential for precip and cloud cover increases as the ridge breaks down during the latter half of the week, so will stick with the NBM`s deterministic mean highs and PoPs through the end of the period. Carney && .AVIATION... (For the 00z TAFs through 00z Sunday Evening) Issued at 604 PM CDT Sat Jun 21 2025 Southwesterly winds will continue to draw hot and humid conditions into the region with few mid/high clouds on occasion. Gust subside tonight before returning over central MO and west-central IL. For metro terminals, KSTL was the only site including a prevailing group for gusts, largely accounting for the broader, open airfield. While surrounding sites may push 20 knots, the stronger winds will remain west and slightly lower than today`s winds. Maples && .CLIMATE... Issued at 1225 PM CDT Thu Jun 19 2025 Record maximum temperatures at St. Louis, MO (STL), Columbia, MO (COU), and Quincy, IL (UIN) along with the year of the record: | 6/21 | 6/22 | 6/23 | 6/24 | 6/25 -------------------------------------------------------------------- STL | 99 (1988) | 102 (1930) | 101 (1930) | 102 (1988) | 103 (2024) | | | | | COU | 98 (1988) | 100 (1988) | 100 (1988) | 103 (1988) | 102 (1988) | | | | | UIN | 100 (1988) | 100 (1930) | 100 (1934) | 99 (1988) | 102 (1931) Record high minimum temperatures at St. Louis, MO (STL), Columbia, MO (COU), and Quincy, IL (UIN) along with the year of the record: | 6/21 | 6/22 | 6/23 | 6/24 | 6/25 -------------------------------------------------------------------- STL | 81 (1901) | 80 (2016) | 80 (2010) | 80 (1914) | 82 (1988) | | | | | COU | 76 (1937) | 77 (1944) | 76 (2010) | 77 (1937) | 77 (1988) | | | | | UIN | 77 (1937) | 78 (1944) | 80 (1944) | 79 (1931) | 80 (1937) && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Heat Advisory until 9 PM CDT Tuesday for Audrain MO-Boone MO- Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO- Iron MO-Knox MO-Lewis MO-Lincoln MO-Madison MO-Marion MO- Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike MO-Ralls MO-Reynolds MO-Saint Francois MO-Sainte Genevieve MO-Shelby MO-Warren MO-Washington MO. Extreme Heat Warning until 9 PM CDT Tuesday for Jefferson MO- Saint Charles MO-Saint Louis City MO-Saint Louis MO. IL...Heat Advisory until 9 PM CDT Tuesday for Adams IL-Bond IL-Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-Macoupin IL- Marion IL-Montgomery IL-Pike IL-Randolph IL-Washington IL. Extreme Heat Warning until 9 PM CDT Tuesday for Jersey IL- Madison IL-Monroe IL-Saint Clair IL. && $$ WFO LSX