Autumn 2017 ranked 114 (where 123 is warmest and 1 is coolest) given the 1895-2017 U.S. climatology. Warmer than normal temperatures occurred in each state except the northern Rockies into the northwest Great Plains. Record warmth was observed in the Southwest U.S. and New England States. As always, anomalous heat was at least partially attributed to dry soil moisture affecting much of the nation.
The primary contributor to the drier climate (which enabled the anomalous heat) was the warmer than normal North Atlantic basin (as defined by the Atlantic multi-decadal oscillation). The warmer than normal North Atlantic increased the strength of the subtropical ridge causing dryness and attendant heat in the eastern U.S. The heat in the Southwest U.S. was attributed to a stronger than normal subtropical ridge extending northeastward from anomalous warm waters of the Southeast Pacific Ocean.
ENSO decelerating from weak El Nino toward La Nina also helped the anomalous heat strengthen during autumn.
Given the La Nina climate a dry pattern in the southern U.S. is expected and indeed has developed during autumn. Note that Arkansas observed their driest autumn on record.
Pictured: State rankings for percent of normal rainfall occurring during meteorological autumn 2017 given the 1895-2017 climatology.