By Deborah Zabarenko
WASHINGTON (Reuters) - As much of the United States basks in summer-like temperatures, weather and climate experts said this year's warm winter could mean early corn planting, a risk of killing frost for apricots and a baby boom for tree-chomping bark beetles in the West.
The winter of 2011-12 was the fourth-warmest in the 117-year record, according to the National Oceanic and Atmospheric Administration, which uses meteorological winter, which ended on February 29. Astronomical winter runs until early March 20, when spring arrives at 1:14 a.m. EDT.
"Spring has been incredibly warm," said Accuweather senior meteorologist Paul Walker. "The weather pattern is anywhere from four to six weeks ahead."
All four of the warmest winters occurred within the last 20 years, said Brad Rippey, a U.S. Agriculture Department meteorologist. U.S. winter temperatures overall have increased by about 2 degrees F (1.1 degree C) since the 1890s, he said.
And while there is usually no correlation between an especially mild winter and a hot summer, says Carl Erickson of Accuweather, the extended U.S. forecast for the next 10 days at least shows no sign of a cool-down. Erickson notes we could still be in for a cold snap in April.
The U.S. winter that wasn't has been driven by a climate feature called the North Atlantic Oscillation, which has sent an area of deep low pressure across the North Atlantic ocean, Rippey said.
This created a so-called "vacuum cleaner effect" that sucked cold air away from the Arctic, into northern and eastern Canada and into the North Atlantic without delivering any of it to the contiguous 48 states.
BLOOMING TOO SOON
Although lovely and welcome to many, short mild winters and early springs improve conditions for pests, wreaking havoc for crops, native plants and pollinating insects.
Larry Salathe, a USDA economist, said the early warmth could mean Midwest farmers will get an early start on corn planting, possibly translating into lower corn prices, assuming fields dry out and rains are moderate.
What's good for corn could be bad for the wheat crop in the southern Plains states, Salathe said, depending on how much moisture that area gets - with warm weather, these crops will need additional rain.
Reservoirs in Western states may already be low because there was less snow than normal this winter, but recent rains have replenished some water supplies, Salathe said.
Early warmth also encourages some crops, including fruit trees like cherries and apricots, to bloom before their normal time, putting them at risk if a killing frost comes. The earlier the heat, the more risk, according to David Inouye, a biology professor at the University of Maryland.
"As spring starts earlier, the date of the last hard frost is not changing," Inouye said, so there is a longer period between the time the first blossoms appear on fruit trees and wildflower plants and the frost-free date, increasing the chance that buds and flowers could be killed by frost.
Continued warm winters also increase the risk of bark beetles, which have blighted millions of acres (hectares) of Western forests in recent years.
In the past, the tree-bark eating beetles were killed off during cold winters, said Inouye, who does field research in Colorado. Without temperatures of minus 30 degrees F (minus 34.4 degrees C) or lower, bark beetles thrive, producing two insect generations a year instead of just one, Inouye said.
That means there could be up to 60 times as many insects attacking trees in any given year, researchers at the University of Colorado Boulder found.
And in the East, warmer winters increase populations of woolly adelgid, an invasive insect pest originally from Asia that attacks hemlock forests.
Normally killed off by cold winters, the insect's population has been growing and spreading in milder weather, the biologist said, and now threatens hemlock in Maryland.
OUT OF SYNC
Unseasonable warmth can put plants and migrating pollinators out of sync, Inouye has found. In Colorado, he said, broad-tailed hummingbirds normally spend winters in Mexico and return to the United States for the summer, in time to feed on nectar in certain plants.
Recently, though, the plants are flowering before some of the birds arrive, meaning less nectar for the hummers and less pollination for the wildflowers, which then produce fewer seeds.
There are obvious advantages to an early spring, Inouye said: "One of the consequences is we get to enjoy an earlier spring and that could be considered a benefit to mental health."
But for Robert DeFeo, chief horticulturist at the National Park Service, the unusual warmth has meant repeatedly revising his forecast for the peak bloom of Washington D.C.'s iconic cherry trees, now expected on March 20-23, compared to the average peak bloom date of April 4.
Unseasonably warm days and mild nights have spurred budding on the trees that are the centerpiece of the National Cherry Blossom Festival, now in its 100th year. There's no stopping the blooms at this point, DeFeo said.
"The cherries don't slow down at night, so in one day you get two days' development," he said.
(Reporting by Deborah Zabarenko; editing by Todd Eastham)