What happened to our winter this year?

Here’s a closer look at this early active spring and the meteorological and climate factors at play:

Across New England, near or new records were set for warmest winter. And we were in good company. The entire continental US experienced the warmest winter ever recorded. National average temps were 32 degrees, about 5.3 degrees above normal. New England was particularly mild, with Vermont and New Hampshire setting new records.

Massachusetts, Connecticut, Rhode Island, and Maine didn’t quite set records but they got close. The cities of Burlington, Vt., and Concord, N.H., pushed 10 degrees or more above average, while Boston, Worcester, Providence, and Portland, Maine, averaged 5 degrees warmer. Transitioning from a shorter and warmer winter has spurred spring into action, especially with the end of February averaging 5-10 degrees above average for many communities.

Snowfall has been scarce throughout much of New England this past winter. When there are prolonged periods of snow on the ground, air temperatures lean colder.

Snowpack is historically present this time of year across the region, including into Southern New England. However, so far this March, you’ll have to drive well north to northern Vermont, New Hampshire, and Maine to see any snow cover, another indicator of an early onset of spring.

And snow depth in those areas aren’t as impressive this year. Currently, only mountainous New Hampshire and Maine have over a foot of snowpack, versus several feet according to the yearly average.

Warm winters can influence insect patterns. They are cold-blooded by nature and frigid wintry conditions affect their ability to survive. So, insects prepare to make it through by finding warmer places like burrowing into the soil or finding homes to sneak into.

Temperatures around 50 degrees are needed for insects to continue developing through their life cycles after their winter hiatus. Ticks, mosquitoes, and spiders are just a few species that respond rather rapidly to warmer temperatures. Much of Southern New England has seen temperatures running 4-8 degrees above average, exceeding the 50-degree mark numerous times in recent weeks. Not to mention, overnight temperatures have been consistently elevated as well.

With all of those factors at play, many insects are coming out to play. Their return is yet another indicator that Spring may have arrived early.

“Like humans on nice days, insects are ready to take advantage, and be out as long as the temperature enables metabolic activity,” said Piera Siegert, state entomologist for New Hampshire, in an interview.

“First leaf” is the emergence of the initial small leaves on trees and plants, and it’s a telltale sign that spring has arrived. Temperatures, sunlight, and precipitation all play an important role in determining when these first leaves begin to emerge. With an historically warm winter in the books, expanding daylight, and above-average precipitation across New England of late, a scattering of plant-life is starting to leaf out.

Much of the country has experienced early “first leaf.” Some areas are nearly three weeks ahead of schedule, including an expanding belt from the Plains, east through the Ohio Valley and into the Northeast.

The early leaf trend looks certain to overtake New England in the coming days. You can anticipate a shade of maroon across the treescape, symbolizing spring’s arrival days ahead of schedule.

Another way to determine the arrival of spring is forecasting when plants and crops may begin to grow. This is mainly done through what’s known as “growing degree days.” In the simplest terms, this measures how much heat a plant or crop receives daily, and over time. The more growing degree days that accumulate, the more favorable conditions are for development.

In Boston, for example, plant life has had more days supporting growth than not this time of year — about 12 more days. Meaning, there is more energy that should signal plants to come to life earlier.

We’re coming out of a winter that was heavily impacted by El Niño, a pattern that increases sea surface temperatures within the equatorial eastern and central Pacific Ocean. During El Niño events, the jet stream is pulled further south, influencing storm tracks and bringing disruptive weather across the southern half of the United States. At the same time, this change in pattern promotes warmer-than-average temperatures across the northern tier of the country, including New England.

The current El Niño event has been very strong, driving records globally for both sea surface temperatures and air temperatures. Though El Niño is currently waning, it’s still present and could end up in the Top 5 strongest El Niño events since tracking began in 1950. In Boston and other New England regions, this El Niño winter was exceptional in driving temperatures. Check out the details of this El Niño winter versus others, with New England temperatures 3-7 degrees above average.

The Northern Hemisphere polar vortex is a broad, bitter air mass circulating around the Arctic, influencing the weather patterns we feel at the surface. It is present year round, but strongest (and coldest) during the winter season. When the winter polar vortex is particularly strong, it keeps the frigid air in place near the Arctic and minimizes bursts of polar air from slipping south into the United States. We know these “bursts” as cold snaps.

This past winter, the polar vortex was strong and stayed mostly together, meaning we didn’t see too many moments of bitter air releasing toward the south. Even when we did, events were brief as the polar vortex came back together quickly. This led to milder air staying in place across much of the Lower 48 for longer periods of time.

The North Atlantic Oscillation (NAO) has also played a role in the current warmer weather conditions, especially in New England. The NAO index measures changes in pressure systems over the North Atlantic, and this past winter has been mostly positive. When the NAO index runs positive, this results in trends of warmer temperatures and drier conditions along the East Coast. Check out the graph below, especially after mid-January.

All of these elements have come together to signal that spring is among us. But as is typical with New England weather, there is always the chance for that one last push of wintry weather -- or at least a nudge before a very laid back Old Man Winter goes back to sleep.

Ken Mahan can be reached at ken.mahan@globe.com. Follow him @kenmahantheweatherman.