A historic cold snap set the tone for a cool September, which saw one of the earliest intrusions of winter weather in state history. An unusually strong cold front blasted through the state September 8-9, sending temperatures plummeting up to 50 degrees lower than the previous day’s highs. Lowest maximum and minimum temperature records were shattered across the western half of the state. High temperatures on the ninth struggled to a chilly 40 degrees at Boise City and Kenton following lows of 33 degrees. Those maximum and minimum temperatures were the lowest on record for that early in the fall season in Oklahoma. To find the previous instance of the earliest high temperature of 40 degrees requires a journey back to 1945 when Boise City hit 40 on September 28, a full 19 days later than the new September 2020 record. While wind chills remained in the 20s and 30s across the northwestern quarter of the state, heat index values soared close to 100 degrees in the far southeast, which missed out on the early winter feel altogether. Temperatures moderated through the rest of the month, failing to reach the depths of that early cold snap again. Severe weather was almost non-existent during September, although a brush with Tropical Storm Beta provided an unneeded dose of moisture to the far southeast later in the month.
According to preliminary data from the Oklahoma Mesonet, the statewide average temperature for the month was 69.9 degrees, 2.4 degrees below normal to rank as the 13th coolest September since records began in 1895. Summer weather was present, but certainly not common. Several stations reached 102 degrees on the 25th for the highest readings for the month, although the 120 Mesonet sites recorded only nine triple-digit temperatures for all of September. It was a particularly cool month for south central and southwestern Oklahoma. Both fell below normal by more than 3 degrees to rank as their eighth coolest Septembers on record. The year-to-date statewide average was 64 degrees, 0.7 degrees above normal to rank as the 35th warmest January-September on record.
There was a tremendous difference in rainfall during the month between the northern and southern halves of the state – save for the far southwest, which took dry to another level. From Interstate 40 south, rainfall amounts ranged from 2-5 inches above normal with localized larger totals. Totals north of I-40 fell 1-2 inches below normal. Overall, the September statewide average was 3.81 inches, 0.28 inches above normal and the 43rd wettest September on record. Talihina led the month with 13.2 inches of rain, but 30 Mesonet sites had at least 6 inches during September. The January-September average was 31.69 inches, 3.3 inches above normal to rank as the 24th wettest such period on record.
The cool weather helped preserve rains earlier in the summer and minimize drought expansion, but there was also little in the way of drought improvement. Adding to Oklahoma’s drought worries, below average sea surface temperatures in the Equatorial Pacific could lead to more dry times through early 2021. According to the Climate Prediction Center (CPC), La Niña conditions developed during September and are likely to continue through the winter, prompting the agency to issue a La Niña Advisory. This unhelpful El Niño counterpart can push the jet stream farther to the north across the North American continent, leaving the southern tier of the United States – including Oklahoma – warmer and drier than normal during the cool season. CPC’s October and October-December outlooks reflect La Niña’s influence with increased odds of above normal temperatures and below normal precipitation. CPC’s October drought outlook indicates possible drought expansion across western Oklahoma, while the October-December drought outlook shows that drought expansion extending across nearly all of the state, save for the southeastern corner. Possible implications for the state due to La Niña include further intensification of the current drought and an enhanced wildfire season. CPC forecasters caution that each La Niña is different, and not all impacts occur during every episode – their probabilities are increased, however.