Damp and Cool November; Second-Wettest Autumn in U.S. History

Six eastern U.S. states joined Texas in slogging through their soggiest autumn in records going back to 1895, according to monthly and seasonal data released Friday by the NOAA National Centers for Environmental Information (NCEI).

Every state from the Great Plains to the East Coast, except for Florida, had a wetter-than-usual autumn. Record-wet autumn conditions were recorded in Maryland, New Jersey, North Carolina, Rhode Island, Texas, Virginia, and West Virginia. Drier-than-average conditions were restricted to Florida and the West; Oregon saw its 12th-driest fall on record.

Figure 1.  Statewide rankings for average precipitation for September-November 2018, as compared to each autumn since records began in 1895. Darker shades of green indicate higher rankings for moisture, with 1 denoting the driest month on record and 124 the wettest. Image credit: NOAA/NCEI.

As is often the case, hurricanes played a big role in wet conditions across the eastern U.S. Chief among them was slow-moving Florence, which dumped 10” – 20” rains over a broad part of the Carolinas in early September. Florence set all-time records for heaviest rainfall from a tropical cyclone in both North Carolina (35.93” at Elizabethtown) and South Carolina (23.63” near Loris).  Hurricane Michael also passed over the Southeast in early October, though Michael’s rapid motion made it a less prodigious rain-maker than Florence.

Remarkably, Texas saw its record-wet autumn without any direct hits from Atlantic tropical cyclones. Much of the rain came from weak upper-level disturbances intersecting with surface boundaries amid a very moist atmosphere. In early October, the remnants of former Category 4 Hurricane Sergio moved from the Northeast Pacific across northern Mexico and brought heavy rain to Texas, triggering major flooding in the state’s Hill Country.

Figure 2. Precipitation in Texas between September and November 2018 smashed a 99-year-old record for the highest observed in any autumn in records going back to 1895. Image credit: Jon Erdman, weather.com, via NOAA/NCEI.

Temperatures were near average for the contiguous U.S. as a whole in autumn 2018, but that average hides some stark regional differences. The Great Plains, Upper Midwest, and Maine were all significantly cooler than normal—North Dakota had its 12th-chilliest autumn on record—while mildness predominated over most of the South and East. It was one of the ten warmest autumns on record in California, Delaware, Florida, Georgia, Maryland, and North and South Carolina. Reflecting the mild, humid conditions that prevailed, average minimum temperatures were the highest on record for any autumn in Delaware, Maryland, and West Virginia.

Figure 3. Statewide rankings for average temperature for September-November 2018, as compared to each autumn since records began in 1895. Darker shades of red indicate higher rankings for heat, with 1 denoting the coldest month on record and 124 the warmest. Image credit: NOAA/NCEI.

A chilly, moist November

Last month was the 31st-wettest and 27th-coolest November on record for the contiguous U.S., according to NCEI. Temperatures were below average across most of the nation east of the Rockies, with Florida and the West Coast the only places significantly warmer than average. The states of Arkansas, Illinois, Mississippi, Missouri, and Oklahoma each had a top-ten-coolest November.

Over the 30 days ending on December 1, the United States saw a preliminary total of 1526 record daily lows and only 273 daily record highs, according to NCEI’s U.S. Records database. The year 2018 is still on track to wind up with far more record highs than lows, however: 22,469 vs. 13,590, as of Dec. 1.

Figure 4. Statewide rankings for average temperature for November 2018, as compared to each November since records began in 1895. Darker shades of red indicate higher rankings for heat, with 1 denoting the coldest month on record and 124 the warmest. Image credit: NOAA/NCEI.

It was the wettest November on record for Delaware, Maryland, and Massachusetts. Except for Florida and Maine, every state east of the Appalachians—plus Ohio and West Virginia—had a top-ten-wettest November.

Figure 5. Statewide rankings for average precipitation for November 2018, as compared to each November since records began in 1895. Darker shades of green indicate higher rankings for moisture, with 1 denoting the driest month on record and 124 the wettest. Image credit: NOAA/NCEI.

Record-high continental snow cover in November

Chilliness and moisture teamed up across both the United States and Canada in November to produce the most extensive snow cover in North America for any autumn month in satellite data going back to 1966, according to the Rutgers Snow Lab. As reported by weather.com, the continental departure from average snow cover for November—about 861,000 square miles, more than three times the size of Texas—was the largest departure from average for any month of the year in the 52-year Rutgers data base.