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827 FXUS63 KLSX 211047 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 547 AM CDT Sat Jun 21 2025 .KEY MESSAGES... - A prolonged period of dangerous heat will continue today and last through at least Tuesday. Widespread heat index values of 100+ degrees are expected each day. && .SHORT TERM... (Through Late Sunday Afternoon) Issued at 248 AM CDT Sat Jun 21 2025 Upper level ridging is currently amplifying across the eastern 2/3 of the CONUS with a longwave trough spinning over the Pacific Northwest. As a result, the Mid-Mississippi Valley sits sandwiched between high pressure to our east and low pressure to our northwest. This places us in an area of enhanced southwest pressure gradient winds which will continue to advect warm air and moisture into the region today. This will cause hotter conditions than what we saw yesterday, with widespread 90s expected and mid-70s dewpoints equaling heat index values of 100 to 110 degrees areawide. Because of this, an areawide Heat Advisory and an Extreme Heat Warning for the St. Louis metro area will take effect at noon today. This morning will be our first low temperature > 70 degrees during the hot stretch. In the city's urban heat island, the low will remain above 75 degrees. These values will threaten if not break record warm minimum temperatures at STL, COU, and UIN (see CLIMATE section). Jaja && .LONG TERM... (Sunday Night through Friday) Issued at 248 AM CDT Sat Jun 21 2025 As mentioned previously, hot weather will continue Sunday through early next week as upper level ridging continues to build and the low-level pattern remains largely stagnant. Though it won't be as warm as today, widespread heat index values of 100 degrees are expected Sunday and Monday with low temperatures in the 70s. This combination of hot days and warm nights does not allow the body to gain the necessary reprieve it needs to withstand heat. Vulnerable populations and individuals without access to air conditioning will be at most risk of heat related illness and death during the coming days and need to take preventative actions against this. The length of the dangerous heat comes into question during the middle of next week. A shortwave trough ejects from the mid-level longwave referred to in the short term, causing low pressure in the northern Plains to become more progressive send a cold front our way. This cold front is forecast to stall to the north of us Monday night into Tuesday with convection along it staying in Iowa. However, if the aforementioned shortwave trough is able to sufficiently erode the northwest edge of the ridge, the chance for precipitation will increase, both from the front and from warm sector showers. Deterministic and ensemble guidance are beginning to lean toward an eroding ridge, with most guidance showing at least a chance of rain Tuesday afternoon. If this does come to fruition, the Heat Advisory and Extreme Heat Warning may conclude earlier than forecast. The ridge will continue to slowly erode and shift east through mid-week, albeit possibly more slowly than originally anticipated. With the possible exception of Tuesday, 100+ degree heat index values are still forecast for portions of the forecast area on Wednesday and Thursday. Confidence in this verifying is low at this juncture, but I feel it's important to mention given how long that would make the hot streak in some locations. Jaja && .AVIATION... (For the 12z TAFs through 12z Sunday Morning) Issued at 544 AM CDT Sat Jun 21 2025 Dry and VFR conditions with clear skies are forecast for the TAF period. Winds today will pick up from the southwest with gusts of 20 to 30 kts forecast. Winds will be strongest in northeast Missouri and west-central Illinois where the pressure gradient will be strongest. Gusts will diminish after sunset today. Jaja && .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 from noon today to 9 PM CDT Tuesday for Crawford MO-Franklin MO-Gasconade MO-Iron MO-Lewis MO-Lincoln MO- Madison MO-Marion MO-Montgomery MO-Pike MO-Ralls MO- Reynolds MO-Saint Francois MO-Sainte Genevieve MO-Warren MO- Washington MO. Extreme Heat Warning from noon today to 9 PM CDT Tuesday for Jefferson MO-Saint Charles MO-Saint Louis City MO-Saint Louis MO. Heat Advisory until 9 PM CDT Tuesday for Audrain MO-Boone MO- Callaway MO-Cole MO-Knox MO-Moniteau MO-Monroe MO-Osage MO- Shelby MO. IL...Heat Advisory from noon today to 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 from noon today to 9 PM CDT Tuesday for Jersey IL-Madison IL-Monroe IL-Saint Clair IL. && $$ WFO LSX