Many of Oklahoma’s most extreme rainfall events have occurred during the fall, the result of a rare conjunction of meteorological ingredients converging over the Southern Plains. Those ingredients – the remnants of a pacific tropical system, a stalled front, and abundant moisture from the Gulf of Mexico – came together late in the month to produce massive rainfall totals across south central Oklahoma. The Oklahoma Mesonet site at Fittstown recorded 14.2 inches of rain on Sept. 21, the second-highest daily total observed in the state since individual station records began in the 1880s, and the highest total in the 25-year history of the Mesonet. Enid remains in the top historical spot with 15.68 inches on Oct. 11, 1973, while the Mesonet’s previous record of 12.42 inches at Burneyville on April 29, 2009, was easily bested. There were numerous unofficial and radar-estimated reports of 15-20 inches in the Pontotoc County area during the storm. The rain totals represent a greater than 1,000-year 24-hour event according to the Oklahoma Conservation Commission and USDA-Natural Resources Conservation Service. Widespread flooding was reported across the southern half of the state, and the National Weather Service (NWS) issued a flash flood emergency for Pontotoc County and the surrounding area.
Heavy rains during the first week combined with that later storm to produce a statewide average total of 5.21 inches, a surplus of 1.68 inches, to rank as the 19th wettest September since climate division records began in 1895. Much of southern Oklahoma received between 8-10 inches of rain, although the far southwest struggled to reach 4 inches. South central Oklahoma’s average of 11.04 inches was 7.11 inches above normal, the wettest September on record for that section of the state. The northern third of the state did not fare as well with 1-2 inches commonly reported. The northeast corner’s average of 2.9 inches was 1.6 inches below normal, 48th driest on record. Fifteen of the Mesonet’s 120 stations recorded at least 10 inches, led by Fittstown’s 18.75 inches, and another 48 sites received at least 5 inches. The NWS cooperative observer at Pontotoc reported 20.89 inches for the month. Kenton had the lowest total at 0.26 inches. Statewide, August and September combined to produce the eighth wettest such period on record at 9.21 inches, 2.63 inches above normal. The average for the first nine months of the year rose to 29.15 inches, 0.76 inches above normal to rank as the 40th wettest January-September on record.
The Mesonet’s 120 sites recorded only five triple-digit temperatures during September, versus 10 readings in the 30s. The state’s highest temperature of 100 degrees was observed at five different locations, the last at Talihina on the 19th. Barring a rare triple-digit temperature during the final three months, that was undoubtedly 2018’s last such occurrence. The Eva Mesonet site recorded the lowest September temperature of 36 degrees on the 22nd. Despite the lack of extreme heat, the statewide average temperature still managed to finish 1.1 degrees above normal to rank as the 59th warmest September on record. The positive temperature anomaly was due mainly to September’s minimum temperatures, which were nearly 4 degrees above normal.
Drought took a large step back for the second consecutive month. Drought coverage dropped from 55 percent of the state at the beginning of August to 9 percent at the end of September. Only two small core areas of drought remained – across far southwestern Oklahoma and a smaller area centered on eastern Osage and southern Washington counties. The October outlooks from the Climate Prediction Center (CPC) indicate increased odds of above normal temperatures across the entire state, but especially eastern Oklahoma, and above normal precipitation. Given those outlooks, CPC’s October Drought Outlook sees improvement across the remaining drought areas in Oklahoma by the end of October.