What ‘La Nina’ means for hurricane season and Philly’s winter snow prospects
The tropical Pacific is cool, and that means more Atlantic hurricanes are likely, and during La Nina Philly has had some snowy winters.
The record-setting Atlantic hurricane season is likely to keep on setting records. And fast-forwarding a few months, it should actually snow again in Philadelphia this winter.
Based on past behavior, those are two likely by-products of the La Niña event — an anomalous cooling over about 500,000 square miles of the tropical Pacific — officially declared Thursday by the National Oceanic and Atmospheric Administration.
The Pacific sea-surface temperatures have been below long-term normals for months, and the government’s pronouncement meant that the coolness had lasted long enough to meet La Niña criteria.
A La Niña tends to suppress the upper-air winds that can snuff out burgeoning tropical storms in the Atlantic, and that’s one reason the Atlantic season already has set 13 records for early-forming storms and is about to run through the alphabet.
So far, 17 named storms, those with winds of 39 mph or higher, have formed, and more are brewing. The average for the entire June 1-to-Nov. 30 season is 11 storms.
“The shear’s been fairly low,” said Mike Halpert, deputy director of the Climate Prediction Center and a tropical-weather specialist. And with the La Niña due to persist into the winter, it’s all but certain to stay low.
The La Niña certainly will have some impacts on the North American winter, although it is impossible to predict precisely what those will be in the Philadelphia region.
Historically, La Niña-influenced winters have exhibited a certain strangeness and stubbornness.
What is a La Niña?
It is the twisted sibling of El Niño, which is an anomalous warming of sea-surface temperatures over vast expanses of the tropical Pacific.
It is one of three phases of what is known as the El Niño-Southern Oscillation; the third one is La Nada, in which the temperatures are near normal. The cycles usually switch places every few years.
Weather moves west-to-east, so what happens in the Pacific is crucial to weather in the United States.
When waters out that way are unusually cool or warm — right now they are just under 2 degrees Fahrenheit below normal — they interact with the overlying air, affecting upper-air patterns that steer storms and air masses.
During a La Niña, the jet stream winds that are the upper-air boundaries between warm and polar air, and are active storm highways, tend to retreat farther north.
What it could mean for Philly’s winter
Another snowless winter is not likely, even though the climate center says trends favor above-normal temperatures.
Halpert said the temperature outlook is more about the overall warming trend of recent decades than a La Niña.
The 21 La Niña winters dating to 1950 have been quite variable around here. The only total snow duds were the 4.0 inches in 2011-12 and the 6.3 inches in the winter of 2007-08.
This event so far is classified as “weak,” with ocean temperatures a few tenths of a degree below the “moderate” category. During the 15 weak and moderate La Niña winters, snowfall has averaged about 24 inches; the long-term normal is around 22.
That average was dragged down by the aforementioned winter of 2011-12, 4 inches total, and bumped up by the winter of 1995-96, when 65.3 inches was measured, at the time a Philadelphia record. That included the record 30.7 inches of snow on Jan. 7-8, 1996. Both seasons coincided with weak phases.
Temperatures have been a wintry mix of above and below normal during La Niña winters.
The one thing they have in common is a tendency for persistence, in that what happens tends to keep happening, the winter of 1995-96 being the classic example.
The other case in point would be 2019-20, which occurred during an El Niño, which turned out to be a La Nada for snow. A grand total of 0.3 inches fell officially at Philadelphia International Airport for the entire season.
That persistence makes sense, said Halpert. “El Niño, La Niña are persistence themselves,” he said. Oceans have far greater attention spans than land masses and are slow to change phases.
That’s not to say that any one pattern is going to lock in for three or four months, he said. The weather also is driven by short-term developments, such as pressure changes over the Poles and the North Atlantic, which are difficult if not impossible to predict beyond 10 days or so in advance.
“My three favorite words, ‘natural climate variability,’” he said. “Things just happen.”