Summary of Oppressive Heat and Humidity August 20-26, 2023

Overview

A large ridge of high pressure aloft set up over the center of the country around August 20th and lead to a week-long heatwave across our region. High temperatures were primarily only in the low to mid 90s most days, but the main headline was the extreme humidity levels witnessed over the entirety of this event. Portions of our region experienced very heavy rainfall amounts of 15 to 25 inches from July 1st through mid August. This set the stage for record levels of evapotranspiration from the vegetation and crops. Dew points, a measure of humidity levels, were routinely in the upper 70s to lower 80s, with several locations peaking in the mid 80s. Cape Girardeau and Paducah each tied their highest dew points on record of 83 and 82 degrees respectively. This caused heat index readings to peak between 105 and 120 degrees each day from August 20th through the 25th, and continued across southwest portions of the region on the 26th. The high humidity levels also led to dense fog formation several nights during this event, which was rather unusual for a prolonged heat wave.

Heat Index
Extreme Humidity
Records
Environment

Heat Index

The looping image below shows the highest heat index observed across our region each day from August 20th through the 26th. Further down, you will see a table with the highest values observed each day at select locations across our region. The highest reading was 130 degrees at the Perryville, MO airport. Most locations peaked between 115 and 120 degrees. There were several other sites that peaked slightly above 120 degrees though.

Location	Provider	Aug 20	Aug 21	Aug 22	Aug 23	Aug 24	Aug 25	Aug 26	Max
Cape Girardeau MO	ASOS	118	117	116	113	119	120	107	120
Carbondale IL	ASOS	111	117	111	108	110	114	92	117
Evansville IN	ASOS	104	106	106	107	111	114	M	114
Paducah KY	ASOS	108	110	111	114	113	117	107	117
Poplar Bluff MO	ASOS	114	112	113	108	115	113	110	115
Cairo IL	AWOS	113	112	M	M	117	118	110	118
Carmi IL	AWOS	115	114	111	110	118	119	96	119
Fort Campbell KY	AWOS	98	106	111	105	110	114	110	114
Greenville KY	AWOS	100	106	108	105	111	114	98	114
Harrisburg IL	AWOS	114	118	116	116	117	124	101	124
Henderson KY	AWOS	109	109	106	106	113	115	93	115
Hopkinsville KY 2 E	AWOS	99	113	111	110	113	113	98	113
Madisonville KY	AWOS	102	108	107	107	112	115	101	115
Marion IL	AWOS	117	117	113	110	113	123	90	123
Metropolis IL	AWOS	106	108	108	105	108	113	102	113
Mt Vernon IL	AWOS	114	117	113	111	117	118	85	118
Murray KY	AWOS	111	113	117	113	120	124	117	124
Owensboro KY	AWOS	102	108	106	105	114	115	96	115
Perryville MO	AWOS	125	130	122	117	121	125	90	130
Sikeston MO	AWOS	113	115	113	113	118	115	113	118
Sturgis KY 2 E	AWOS	108	110	107	105	110	115	97	115
Benton KY 4 N	KYMN	106	110	109	109	111	116	108	116
Cadiz KY 4 SW	KYMN	102	106	110	110	112	118	105	118
Calhoun KY 5 NW	KYMN	106	113	115	117	116	121	104	121
Dixon KY 1 NW	KYMN	110	110	109	112	115	118	101	118
Elkton KY 5 SW	KYMN	99	105	107	104	110	114	101	114
Greenville KY 6 N	KYMN	98	105	107	106	108	114	100	114
Henderson KY 5 E	KYMN	109	111	111	106	116	119	99	119
Hickman KY 2 E	KYMN	110	109	111	111	115	115	111	115
Hopkinsville KY 6 N	KYMN	100	106	109	110	112	120	102	120
La Center KY 5 NNE	KYMN	109	112	113	109	113	118	111	118
Madisonville KY 4 S	KYMN	100	103	106	102	108	113	101	113
Marion KY 4 NE	KYMN	107	108	110	110	112	117	104	117
Mayfield KY 6 SW	KYMN	106	108	109	110	111	115	110	115
Morganfield KY 4 E	KYMN	113	115	115	117	117	120	102	120
Murray KY 1 W	KYMN	107	109	111	111	112	117	108	117
Princeton KY 2 SE	KYMN	102	106	105	105	108	113	101	113
Bean Ridge IL	RAWS	111	113	114	113	113	117	108	117
Big Springs MO	RAWS	122	126	117	112	116	116	106	126
Crab Orchard IL	RAWS	117	121	118	114	119	123	104	123
Dixon Springs IL	RAWS	115	115	112	110	115	121	101	121
Doniphan MO	RAWS	114	118	112	109	114	114	110	118
Fort Campbell, KY	RAWS	99	109	118	116	114	119	109	119
Golden Pond KY	RAWS	103	108	111	114	113	116	107	116
Greenville KY	RAWS	101	108	109	107	111	116	102	116
Piedmont MO 20 ENE	RAWS	118	121	123	111	117	121	114	123

Extreme Humidity

Some of the highest dew points on record were observed across our region during this heatwave. Most locations reached 80 degrees at least once, and several locations reached this level for 7 consecutive days! Dew point temperature is a measure of humidity, with values greater than 75 degrees typically referred to as oppressive. The looping image below shows the highest dew point observed across our region each day from August 20th through the 26th. Further down, you will see a table with the highest values observed each day at select locations. The highest reading was 86 degrees in Perryville, MO and the McLean County Kentucky Mesonet site near Calhoun.

Location	Provider	Aug 20	Aug 21	Aug 22	Aug 23	Aug 24	Aug 25	Aug 26	Max
Cape Girardeau MO	ASOS	83	82	82	82	83	82	81	83
Carbondale IL	ASOS	80	81	81	80	79	82	77	82
Evansville IN	ASOS	77	78	78	77	78	78	M	78
Paducah KY	ASOS	79	81	79	82	80	81	78	82
Poplar Bluff MO	ASOS	81	79	80	79	80	78	77	81
Hopkinsville KY 2 E	AWOS	77	77	81	82	81	82	77	82
Sturgis KY 2 E	AWOS	77	77	77	79	79	79	73	79
Cairo IL	AWOS	81	83	M	M	83	82	80	83
Carmi IL	AWOS	82	82	82	80	82	80	77	82
Fort Campbell KY	AWOS	79	82	84	81	84	84	80	84
Greenville KY	AWOS	76	79	78	78	79	79	74	79
Harrisburg IL	AWOS	83	84	83	82	82	83	79	84
Henderson KY	AWOS	78	79	79	79	79	79	74	79
Madisonville KY	AWOS	75	81	81	80	80	80	76	81
Marion IL	AWOS	82	84	84	82	82	83	77	84
Metropolis IL	AWOS	80	82	80	81	81	80	80	82
Mt Vernon IL	AWOS	80	81	80	80	80	81	74	81
Murray KY	AWOS	82	84	84	84	84	84	84	84
Owensboro KY	AWOS	76	78	78	78	80	81	75	81
Perryville MO	AWOS	85	86	85	84	85	83	78	86
Sikeston MO	AWOS	81	79	81	81	81	79	79	81
La Center KY 5 NNE	KYMN	81	81	82	80	81	83	81	83
Benton KY 4 N	KYMN	79	80	79	79	81	81	79	81
Cadiz KY 4 SW	KYMN	76	79	81	80	81	83	79	83
Calhoun KY 5 NW	KYMN	79	84	85	86	83	84	81	86
Dixon KY 1 NW	KYMN	80	81	81	81	82	81	77	82
Elkton KY 5 SW	KYMN	76	79	80	79	80	81	77	81
Greenville KY 6 N	KYMN	75	79	79	78	79	79	78	79
Henderson KY 5 E	KYMN	80	81	81	81	82	82	77	82
Hickman KY 2 E	KYMN	80	81	81	81	82	82	81	82
Hopkinsville KY 6 N	KYMN	76	82	81	81	81	84	80	84
Madisonville KY 4 S	KYMN	74	80	80	77	79	79	75	80
Marion KY 4 NE	KYMN	80	82	81	80	81	82	79	82
Mayfield KY 6 SW	KYMN	79	82	80	83	81	81	81	83
Morganfield KY 4 E	KYMN	81	82	83	83	82	82	79	83
Murray KY 1 W	KYMN	79	80	80	81	81	81	80	81
Princeton KY 2 SE	KYMN	77	81	80	80	80	81	78	81
Bean Ridge IL	RAWS	79	81	81	81	82	81	78	82
Big Springs MO	RAWS	80	82	78	81	78	78	77	82
Crab Orchard IL	RAWS	83	83	81	85	82	84	80	85
Dixon Springs IL	RAWS	81	81	80	81	81	82	77	82
Doniphan MO	RAWS	81	80	77	79	80	78	77	81
Fort Campbell KY	RAWS	75	82	81	81	81	82	78	82
Golden Pond KY	RAWS	76	81	79	82	81	82	78	82
Greenville KY	RAWS	75	80	75	79	79	79	75	80
Piedmont MO 20 ENE	RAWS	81	82	81	82	80	82	79	82

Records

A list of all of the records and other notable statistics that were observed in Paducah and Cape Girardeau. (Click on the images to enlarge)

Environment

The upper level pattern at 500mb (roughly 18,000 feet above the surface) featured a large 600dam ridge that became established on August 20th across Missouri and Kansas. It reached its peak on August 21st before slowly shrinking through the 25th. Several upper air sites established their highest 500mb heights on record including Springfield, MO and Topeka, KS.


August 20 - 7 PM CDT	August 21 - 7 PM CDT	August 22 - 7 PM CDT	August 23 - 7 PM CDT

Map showing all of the National Weather Service Watches, Warnings, and Advisories that were in effect the afternoon of Wednesday, August 23rd. The magenta color across much of the central part of the country shows where the Excessive Heat Warnings were in effect on this date.

Additional Information

Chicago, Illinois Summary (Highest heat index on record of 120 degrees in Chicago)
Central Illinois Summary (Highest heat index on record of 121 degrees in Peoria)

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