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Monday marks the first day of meteorological spring. Here’s what that means.

The days get longer and warmer, but the potential for severe thunderstorms grows

Cherry blossoms in the Kenwood neighborhood in Bethesda on March 27, 2020. (Bonnie Jo Mount/The Washington Post)

Winter is over — sort of, anyway. The atmosphere doesn’t own a calendar, and is always prone to some humbling surprises. But Monday marked the first day of meteorological spring, the initial step to longer, brighter and warmer days.

Growing daylight and warming temperatures are expected across the nation in the coming months, signaling a habitual, but at times symphonic atmospheric transition from the doldrums of winter to the promise of summer.

D.C.-area March outlook: Expect a mild and rainy month after our cold February

Of course, every season comes with a price tag, and we can expect an uptick in tornadoes, severe thunderstorms and flooding.

It’s all thanks to the large-scale dynamics of the atmosphere as several key weather systems realign themselves into a springlike pattern.

What is meteorological spring?

Conventional springtime doesn’t start until March 20 and ends June 20, the day of the summer solstice. March 20 is the equinox, or the point at which the sun’s most direct rays cross north of the equator, and day and night are in balance across the globe. The summer solstice marks the start of astronomical summer and is the longest day of the year.

From a weather standpoint, however, those bounds don’t accurately capture when springtime really falls. Brian Brettschneider, an Alaska-based climatologist, ran the calculations to see when spring actually occurs in most American cities. He defined winter as being the coldest three-month period of the year and summer as the warmest, with spring and fall occupying the times in between.

He found that, generally speaking, March, April and May are generally when the most springlike weather is present. That’s been the long-standing definition of meteorological spring. Summer arrives June 1, at least according to meteorologists.

A pattern shifting to springtime

The mechanisms of the seasonal switch are already in motion as spring begins to arrive. At the North Pole, the stratospheric polar vortex is shrinking, leading up to an imminent dissipation. That’s the second layer of the polar vortex — the more concentrated, higher-altitude one with a tighter, rapidly spinning circulation. It vanishes once sunlight returns to the Arctic each spring, warming the upper atmosphere and eroding the frigid pool of chilled air that makes up the stratospheric polar vortex.

It doesn’t return until September or October when sunlight once again disappears from the North Pole as the sun’s most direct rays retreat south of the equator. That’s when polar night sets in the high Arctic.

At the same time, warmth is beginning to collect over the Gulf of Mexico, spilling onto the shoreline as it slowly gathers oomph. In the coming weeks, it will spread north in periodic insurgences, working to scour out entrenched cold air left behind by winter.

The seasons will battle and the air masses will clash in the form of strong to severe thunderstorms. At the same time, the jet stream — a river of swiftly moving air in the upper atmosphere — will follow the shift in seasons northward. That northward movement of the jet stream means fewer shortwaves, or lobes of low pressure, will swing across the southern United States with cold winter air; the pattern has already quieted down, at least for the time being.

What to watch for

While warm air works to become established over the Lower 48, cold air is still hanging around at higher altitudes. That means the atmosphere will become increasingly unstable, or prone to rising air. Such instability gives rise to heavy downpours and thunderstorms.

The jet stream, meanwhile, is still in proximity — its fierce high-altitude winds providing the needed dynamics to make thunderstorms rotate. Atmospheric scientists are already predicting an active tornado season across much of the South and the Tennessee and Mississippi valleys.

The same storm systems could bring periods of flooding, some significant, where repeated disturbances bring waves of heavy downpours. May is the wettest month of the year in Nashville. Flooding is a seasonal occurrence along the Ohio, Tennessee and Mississippi rivers, where upstream snowmelt and spring showers overwhelm the ground’s ability to handle water.

The good news

Despite the meteorological challenges that arise with the changing seasons, there’s plenty of good news, too. Many places in the Lower 48 will witness days grow by up to four hours, longer evenings and warmer weather affording opportunities for after-dinner recreation and outdoor barbecuing.

Boston will see an increase of three hours and 49 minutes of daylight; Washington will enjoy an uptick of three hours and 21 minutes. It’ll be about an hour and 59 minutes in Miami.

Busy tornado season projected across the southern U.S. this spring

Denver’s daylight will grow by three hours and 27 minutes over the course of spring.

All the while, temperatures will mirror the spiking daylight. In Birmingham, Ala., the average temperature climbs by 23 degrees during spring. In Chicago, temperatures should rise by about 21 degrees, and in Seattle by 15 degrees.

Simply stated, the forecast ahead is bright. The worst of winter is over, and summer is just around the corner.