Trees near the Potomac River on April 23. (Kevin Wolf via Flickr)

Some years tree pollen explodes like a sudden thunderstorm, while other years it disperses itself more gradually.

Allergy sufferers may find this shocking, but there has been no real outburst of tree pollen this year. Instead, it has entered the air in two distinct but rather moderate waves, and its peak level this April ranks among the lowest on record.

The weather took tree pollen counts on a roller coaster ride this spring but ultimately moderated their extremes.

Tree pollen in 2016 compared to the long-term average (Susan Kosisky)

The first wave of pollen arrived in March, thanks to unseasonably warm weather. Temperatures leaped into the 80s, and the cherry blossoms peaked more than a week early.

“We had a huge spike in March,” said Susan Kosisky, chief microbiologist at the U.S. Army Centralized Allergen Extract Lab. “Our early pollinating tree counts were two to three times higher than normal daily averages.”

At the time, it seemed as if the pollen counts, about two weeks ahead of schedule compared to average, were on course to peak early.

But in early April, the D.C. area had a sharp cold snap and the pollen shut down.

When temperatures returned to the 80s the third week of April, pollen levels surged a second time, hitting their peak around the normal time.

But this year’s big April spike was rather small compared with recent history. When the count reached 1,596 grains per cubic meter of air April 21, it marked the second-lowest peak level in almost two decades of measurements.

Peak tree pollen counts 1998 to 2016 (Susan Kosisky)
Peak tree pollen counts 1998 to 2016 (Susan Kosisky)

Had it not been for the April cool spell, the pollen would have peaked earlier and counts would have potentially surged much higher, Kosisky said.

The cool spell essentially divided the tree pollen season into two distinct segments, the first — in March — made up of the early pollinators such as elm, maple, and cedar/cypress/juniper trees, and the second — in April — made up of later pollinators such as oak, pine, ash, sycamore, mulberry  and birch trees.

In many years, the early and later pollinators overlap more, leading to more acute spikes.

“That early heavy load from early March this year took away from what might have been added [later in the spring],” Kosisky said. “In many years, when it’s cool early in the season, you’re waiting and waiting [for warmer weather] and then it gets into the 80s, and the counts explode.”

Levels have been known to surge above 4,000 grains per cubic meter of air some years, like 2009 and 2010.

But this year, thanks to the more gradual, two-stage dispersion, the peak count achieved less than half of its potential.  And, now, levels are in decline.

“Tree season is going to start coming to a close,” Kosisky said. “The first week of May things typically start to drop off.”

Kosisky said the recent flurry of oak catkins, those yellow strands that accumulate all over roads and gutters, are  a sure sign the end of the worst of tree pollen season is upon us.

While tree pollen season is wrapping up, grass pollen season is just starting and peaks in mid-May. So keep the antihistamines close by.