Bimonthly daily extremes in excess of the lower estimate for 20-yr events for September-October 2017
reported the 7th highest national count of 20-yr daily extreme precipitation events for this season since 1901 (17.9% of 886 available stations). Combined with the 4th ranked total in 2016 (18.3%), this tally increased the overall trend from 3.5% per century through 2015 (p=99.9%) to 4.0% per century through 2017 (p=99.99%). The national outcome was anchored by diverse regional totals with 33.3% in the Southeast (4th highest), 25.0% in the Pacific Northwest (6th highest), and 24.0% in the Northeast (15th highest). Coming on the heels of 22.3% in 2016, the Southeast now shows a significant upward trend through 2017 (+5.2% per century; p=95%), compared to +4.3% per century through 2015 (p=87%). On the other hand, the Pacific Northwest is still not showing a significant upward trend, despite a record-breaking total of 45.1% in 2016 followed by this year's 25.0%. Finally, the Northeast confirms a strong upward trend that was already +8.9% per century through 2014 (p=99%), only to rise to +10.2% per century through 2017 ((p=99.8%), after recording 28.8% in 2016 as well. In contrast, the West only reported 2.9%, not nearly as bad as those 30 prior cases with only 0.0% (since 1901), but a far cry from 36.6% in 2016, and not associated with any significant trend. Other noteworthy regional totals include 19.7% in the Northern Rockies (11th highest) and 21.2% in the Midwest (9th highest). Both of these regions have not recorded a significant trend in this season (upward or downward).
There were three "Billion-Dollar Disasters" ( https://www.ncdc.noaa.gov/billions/events
) in this season, continuing an extremely damaging Atlantic hurricane season. Hurricane Irma hit the Southeast, in particular Florida, killing at least 97 people, and causing $51B in damages. The high regional fraction of station with 20-yr daily extremes in the Southeast (33.3%) is clearly associated with that storm in particular. Hurricane Maria devastated Puerto Rico with at least 65 fatalities as a direct consequence of the storm, and over $90B in damages, while its precipitation extremes are not part of the dataset used here. In contrast, the excessive rains of 2016-17 in Northern California were followed by severe drought conditions during the early fall that set the stage for extremely damaging wildfires in October and led to 4 of the 20 most destructive in the state’s history ( https://en.wikipedia.org/wiki/2017_California_wildfires
), killing at least 40 people and causing damages in excess of $10B.