A Warm Kickoff to Autumn Across Eastern U.S.

Last month was the ninth warmest September for the contiguous 48 U.S. states in records going back to 1895, according to the monthly national climate summary released by NOAA on Wednesday. While temperatures were close to average from the Great Basin to the West Coast, the central and eastern U.S. had an uncommonly warm segue to meteorological autumn. The states of Iowa, Louisiana, and every state east of the Mississippi except for Tennessee and South Carolina had a top-ten-warmest September, and it was the hottest September on record for Ohio. Average minimum temperatures (nighttime lows) were the warmest for any September in Virginia, Maryland, and Delaware, as well as the District of Columbia.

This toasty month followed the fifth hottest U.S. summer on record. September 2015 was the second warmest on record for the contiguous U.S., and it’s now been a full decade since the nation last saw a cooler-than-average September (2006).


Figure 1. Statewide rankings for average temperature during September 2016, as compared to each September since 1895. Darker shades of orange indicate higher rankings for warmth, with 1 denoting the coldest month on record and 122 the warmest. Image credit: NOAA/NCEI.


Figure 2. Statewide rankings for average precipitation during September 2016, as compared to each September since 1895. Darker shades of green indicate higher rankings for moisture, with 1 denoting the driest month on record and 122 the wettest. Image credit: NOAA/NCEI.

Precipitation in September: Another mixed bag
The national patchwork of sogginess and drought that emerged this past summer underwent some shifts in September. Even before Hurricane Matthew arrived, the Atlantic states from South Carolina to Delaware got much-needed moisture in September (too much, in some cases). It was also a very wet September from the northern Rockies to Upper Midwest. The states of North Dakota, Delaware, and South Carolina each had a top-ten wettest September. Northeast Iowa and parts of adjoining states were walloped by thunderstorm complexes in late September that led to record flood crests in some areas. More than 5000 homes were affected in the Cedar Rapids area, where the flood crest exceeded the 100-year recurrence interval. A massive sandbagging effort in the Cedar Rapids area--which had been devastated by a flood in 2008--helped reduce the damage somewhat this time around.


Figure 3. Floodwaters in late September 2016 surround the Veterans Memorial Building, which was heavily damaged by the city’s devastating 2008 flood. The building housed the Cedar Rapids City Hall from 1927 until 2008. The new location of City Hall, several blocks away, was closed after basement flooding occured during the September 2016 flood. Image credit: City of Cedar Rapids.


A regional drought intensified from New York through New England, with Maine having a top-ten-driest September. As of October 4, the coastal stretch from southeast and central Massachusetts to central Maine was in extreme drought, with moderate to severe drought covering most of the rest of New York and New England, according to the weekly Drought Monitor from October 6. (Rains of 2” - 4” associated with Matthew over Rhode Island and southeast Massachusetts may put a modest dent in the drought there.) It was also unusually dry across the lower Mississippi Valley, Colorado, and the West Coast states of Oregon and California (where September is normally a fairly dry month).


Figure 4. Hannah Swanson, assistant wine maker, samples some of the grapes being harvested at Fresh Tracks Farm Vineyard & Winery in Berlin, VT, on October 3, 2016. The dry summer weather was ideal for growing grapes in some spots in the Northeast, but the drought in southern New England and parts of New York may have decreased the crop. Image credit: AP Photo/Lisa Rathke.

Mediocre is better than awful: California wraps up water year 2015-16 and looks ahead
Precipitation for the water year 2015-16 (extending from October to September) wound up very near average for California--a relief after four years of unremitting drought. However, the statewide water-year total obscures the drying effects of continued very warm temperatures, as well as a marked a north-to-south divide: rains were generous from central California northward, while southern California largely missed out on El Niño’s bounty. Downtown Los Angeles picked up only 5.85” for the water year, compared to an average of 11.09”. San Francisco racked up a respectable 23.10”, less than half an inch below the long-term average of 23.65”.

California’s reservoirs, concentrated in the north half of the state, were only at about 80% of average capacity on September 30 (see Figure 5 below), thanks in large part to an earlier-than-average meltout of a less-than-extravagant snowpack. The state’s forests are continuing to suffer from the effects of a half-decade of drought, which has led to an estimated 66 million dead trees. “The accumulated effects of drought and warmer temperatures are likely to leave forests susceptible to diseases, pests, and further drought conditions,” wrote Jay Lund in the California WaterBlog. “There is little that water managers can do to affect drought impacts to forests, although this might be one of the drought’s biggest and most long-lasting effects.”

What about water year 2016-17? Californians are understandably leery about drawing conclusions from El Niño and La Niña, given the disappointing outcome of El Niño in Southern California this past water year. For what it’s worth, NOAA has cancelled its La Niña watch for 2016-17. Last month it pegged the odds of La Niña at around 40% for this autumn, dropping to around 30 - 35% by winter. While below 50%, those odds are far from nonzero. Temperatures in the benchmark Niño3.4 region of the tropical Pacific are down to -0.9°C below average, according to Monday’s weekly update from NOAA, and the cooler-than-average waters now extend below 100 meters (330 feet) throughout most of the central and eastern tropical Pacific. Also consistent with La Niña: Rainfall has increased across Australia, Southeast Asia, and the Maritime Continent.

Nicole continues to intensify en route to Bermuda
Hurricane Nicole is barrelling toward a potential direct hit on Bermuda. Nicole's top sustained winds were up to 110 mph as of the 5 pm EDT advisory from the National Hurricane Center, and Nicole is predicted to become a Category 3 storm tonight before weakening slightly as it approaches the island on Thursday. Direct hurricane strikes are quite unusual for Bermuda, and it's also uncommon to have a hurricane intensifying so close to an encounter with the island. We'll be back with a full update with Nicole on Thursday morning. See our post from Wednesday morning for more background on Nicole and on Bermuda's hurricane history.

Bob Henson


Figure 5. California reservoir levels as of September 30, 2016. Values shown for each reservoir are percentage of capacity (left/blue value) and percentage of the historical average for the time of year (right/red value).