LI winter outlook: Another walk on the mild side

Snow days — oftentimes a boon for Long Island school-aged children and a bane for their parents, could turn out to be a rarity this winter.

Chances are good the coming months will be on the mild side — just like last year.

“The warmer signal is probably slightly stronger in our more localized outlook,” Jay Engle, the Upton-based National Weather Service’s winter team leader, told reporters this week on a conference call as the agency assessed the regional winter outlook. 

Engle was contrasting that outlook with an earlier analysis for the entire nation issued by the weather service’s Climate Prediction Center.

The odds of a warmer December-to-February period from Maine to South Carolina, on west through Texas and a narrow band stretching to Arizona, are 40% to 50%, the center had estimated last month.

As for precipitation, the center had calculated there are equal chances of more, normal or less rain and snow, a forecast Engle said his office is “very much in agreement with.”

Driving the outlook is the La Nina pattern, one of the few systems whose long-term effects can be predicted. “That’s kind of like our starting point when we piece this thing together,” Engle said.

Forecasters also probed the polar vortex’s stability and analyzed previous winters that had developed the same way as the approaching one, which begins Wednesday with the winter solstice.

It will be the third successive La Nina winter, though this time, Engle noted, the system appears less intense.

During La Nina, the easterly trade winds around the equator strengthen, pushing warm waters of the Pacific toward Asia and letting the coldest layers rise to the surface off the Americas.

El Nino is the opposite weather pattern. 

During a La Nina winter, a high-pressure system in the northern Pacific can push the jet stream toward Alaska, while a low-pressure trough then helps drive that west-to-east flow to the South.

The result is a colder Upper Midwest, which may get more storms; the South usually is warmer, with fewer storms. And the mid-Atlantic region — like the filling in a sandwich — can lie between these two patterns.

Daily high temperatures in Islip average 47 degrees from early December to mid-March, according to the weather service, citing records dating back to 1963.

January often is the coldest month, with an average low of 27 and a high of 40.

The first month of the year also tends to be the snowiest, with an average of 5.4 inches.

Another key player in winter forecasts is the polar vortex. That's the term for the "strong westerly winds that zoom around the northern pole in the stratosphere every winter,” explained Amy Butler, a scientist with the University of Colorado Cooperative Institute for Research in Environmental Sciences, in a National Oceanic and Atmospheric Administration blog.

Spinning six to 30 miles above Earth, the polar vortex can break down when atmospheric waves as long as 1,000 miles travel up from the South. That's more likely during an El Nino winter, Butler said.

This year, a stable vortex is more likely, Engle said, so frigid air is much less likely to spill down into this area.

However, the New York region still could get cold snaps this winter.

“It just means we think the frequency of polar outbreaks will be diminished this year,” Engle said.

Despite advanced sensors, radar and computer models, forecasters still look at historical patterns, which often ruled predictions in earlier eras.

The winters that most resembled what New Yorkers can expect this time were the past two, along with 2017-2018, 2011-2012, and 2005-2006, Engle said.

There were some brutal winter storms in those years — including this past January, when a late-month storm blanketed Long Island MacArthur Airport with 24.7 inches of snow.

Despite all the modeling and monitoring, predicting how much snow will fall in a storm — let alone all winter — is exceptionally difficult; forecasts are only accurate a day or so before a storm arrives.

"Seasonal snowfall predicting — it’s next to impossible,” Engle said.