The 17th warmest and driest September in Oklahoma since records began in 1895 allowed drought to flourish during the month. Categorized as “flash drought,” its rapid onset and intensification occur when abnormally high temperatures and below normal precipitation persist for an extended period. Most often a warm season phenomenon, abundant sunshine and strong winds can also aid in its progression. Those are precisely the conditions Oklahoma experienced when previously abundant rains tapered off during early August and sweltering heat returned shortly thereafter. That weather pattern continued until relief finally arrived on the month’s final two days, bringing widespread rains and more seasonable temperatures.
According to the U.S. Drought Monitor, more than 73% of Oklahoma was experiencing drought conditions by the end of September, a 67% increase since the end of August and the state’s highest percentage since Feb. 20, 2018. Of that 73%, 49% was considered moderate drought, 21% severe, and 3% extreme. The Drought Monitor’s intensity scale slides from moderate-severe-extreme-exceptional, with exceptional being the worst classification. Several Mesonet sites had only received a hundredth of an inch of rain for the month before relief arrived on the 29th. Tulsa had gone 80 consecutive days without at least a quarter-inch of moisture before its streak was interrupted on that same date. Reports received by the Oklahoma Climatological Survey from across the state detailed dry stock ponds, cattle receiving supplemental feed months earlier than normal, and flagging crops due to the arid conditions. The USDA reported 79% of the state’s topsoils were “short to very short” of moisture on Sept. 26, a 52% increase since the beginning of August. The late-month relief was expected to reduce Oklahoma’s drought footprint on the first U.S. Drought Monitor report of October.
According to preliminary data from the Oklahoma Mesonet, the statewide average temperature finished at 76.3 degrees, 3.4 degrees above normal. Triple-digit temperatures were more common early in the month, although they occurred as late as the 20th. Buffalo’s 107 degrees on Sept. 11 led the Mesonet’s high temperature readings, with Eva and Boise City’s 38 degrees capturing the lowest temperature prize. Heat index values amongst the Mesonet’s 120 sites rose to 105 degrees or above 228 times during September, topped by Idabel’s 111 degrees on Sept. 1. The statewide average January-September temperature remained on the cool side at 62.8 degrees, 1 degree below normal and ranked as the 51st coolest on record.
The statewide average rainfall total was 1.37 inches as measured by the Mesonet, 1.95 inches below normal. Despite the late-month moisture, nearly the entire state suffered deficits of 1-3 inches during September. Fifty-two Mesonet sites failed to record at least an inch of rainfall, and only 29 reported at least 2 inches. The May Ranch Mesonet site in far northern Woods County recorded the highest total at 4.58 inches. Goodwell had the month’s lowest total with 0.05 inches. The January-September statewide average remained below normal by 0.68 inches at 27.92 inches, the 54th wettest such period on record.
Hope for further drought relief could arrive in October according to the outlooks from the Climate Prediction Center with increased odds of above normal temperatures and precipitation for much of the United States, including Oklahoma. The odds for a wetter October are a bit lower for the Panhandle. CPC’s October drought outlook indicates many of the areas impacted by dry conditions during September will see improvement or removal of drought by the end of October, save for the western Panhandle where drought is expected to persist or intensify.