“The climate has more mimicked what you’d expect to see in the Deep South and along the Southeast Atlantic Coast,” said Greg Carbin, a meteorologist with the National Weather Service. “It is quite eye-opening.”
As Washington has notched nearly double its normal amount of precipitation in the past year, what is driving this exceptional rainfall? And are these recent tropical conditions the new normal thanks to climate change?
The short answer is that the historic rainfall over the past year is somewhat of a random occurrence. It is mostly a result of weather patterns that have frequently arranged themselves, by chance, in an optimal way to squeeze water from the sky.
Yet, at the same time, this record-wet year has occurred against a longer-term backdrop of climate warming and increasing precipitation extremes. In other words, climate change probably intensified the rain and increased the chance it would become a record breaker.
Generally, a whole combination of factors, not any one thing, has spurred the onslaught of deluges, said Carbin, chief of the Weather Service’s forecast operations branch. “It seems we’ve brought together a number of favorable ingredients to support abnormally high precipitation,” he said.
For one, high pressure has frequently positioned itself just off the East Coast. The clockwise circulation around this zone of sinking air has pumped vast amounts of moisture inland, sourced from both the Atlantic Ocean and Gulf of Mexico.
At the same time, storm after storm has tracked near the Mid-Atlantic, tapping into this abundant moisture supply. Carbin attributed the stormy pattern, in part, to a second zone of high pressure positioned near Alaska linked to some of its record warmth. This pressure system has diverted the jet stream, the atmosphere’s super highway for storms, southeastward toward the eastern United States. Weather fronts situated near the jet stream have repeatedly visited the Washington region, dispensing both steady cold season downpours and summertime torrents.
Last hurricane season also pumped up rainfall totals, as the remnants of Florence and Michael brushed the region.
But, unlike areas hit head-on by hurricanes, Washington’s rainfall did not come disproportionately from a single storm.
“The rain fell so consistently over a lot of smaller and midsize events,” said Chris Strong, a meteorologist with the National Weather Service office serving the Washington region. “It’s not like we had a big tropical event like Houston [in 2017 from Harvey] with 40 inches of rain.”
Strong said El Niño, the episodic warming of ocean water over the tropical Pacific Ocean, may have enhanced precipitation some.
During El Niños, the storm track into the West Coast often shifts south, allowing weather systems to draw moisture from the subtropical Pacific and Gulf of Mexico as they track across the country. This tends to increase precipitation across the South and into the Mid-Atlantic, especially during the winter.
But Carbin said he had “not seen a lot of evidence” that El Niño has had a big impact on precipitation, noting it’s been a rather weak event.
The configuration of weather systems, perhaps with some assistance from El Niño, seem to have been the main drivers of the excessive precipitation. But what about the role of climate change?
Rising temperatures could certainly have contributed to the exceptional precipitation over the last year. A warming climate intensifies the water cycle by speeding up evaporation, which puts more water in the air for storms to draw from.
The U.S. National Climate Assessment published last year showed rather marked increases in the very heaviest precipitation events in the eastern United States in recent decades.
Carbin said his analysis of precipitation supports an increase in large amounts between June through August. He also noted an overall increase in precipitation amounts in the eastern United States in the past two decades relative to the 1980s and 1990s.
Washington has seen an uptick in the frequency of rainfall events of at least 0.5, 1.0, and 2.0 inches since 1950, according to an analysis from Climate Central, a nonprofit science communications group based in Princeton, N.J.
Examining Washington’s annual precipitation since 1945, as measured at Reagan National Airport, we do see a small upward trend (occurring at a rate of about three inches per century). But it’s also clear that 2018′s precipitation total is an outlier, way out of step with the subtle increase in precipitation seen in recent decades.
In other words, while Washington is witnessing an increase in heavy precipitation events and perhaps overall rainfall, data do not suggest what we’ve endured over the last year is becoming the new norm.
But it is fair to say that warming temperatures will continue to increase the number and intensity of heavy precipitation events we experience and lead to a gradual rise in annual precipitation.
For now, let’s enjoy the chance to dry out. The Weather Service’s 8- to 14-day outlook favors below normal precipitation in the Mid-Atlantic.