When AccuWeather.com releases its seasonal forecasts, people ask how meteorologists can predict the weather so far in advance. What are these long-range forecasts based on? A great deal of research.
Paul Pastelok, leader of the AccuWeather.com Long-Range Forecasting Team, said that the team forecasts a season after studying the complex interaction of current ocean, land and atmosphere conditions around the globe.
For weather enthusiasts, here's a list of some of the weather indices used by the AccuWeather team: ENSO, PNA, NAO, AO, PDO, MJO, ocean temperatures, soil temperatures and analog years.
La Niña and El Niño
The weather patterns across this fluid, ocean planet are highly interconnected. For example, Pacific Ocean temperatures (La Niña/El Niño) are a key factor in forecasting weather patterns.
"We look at past La Niña and El Niño seasons, looking at the strength and trend of those El Niños or La Niñas," Pastelok said.
A La Niña occurs when sea surface temperatures across the equatorial central and eastern Pacific are below normal. El Niño occurs when these sea surface temperatures are above normal. The greater these temperatures depart from normal, the stronger the La Niña or El Niño.
Both phenomena have a significant influence on the jet stream and overall weather patterns across the globe. While no two El Niños or La Niñas are exactly the same, general trends have been observed in the influence they have on the weather in the U.S. and elsewhere throughout the world.
Long-range forecasters also look for past years in which similar trends occurred. Meteorologists call these years "analog years". An analog year is a past year where there were similar temperatures in the ocean and pressures in the atmosphere to the current conditions.
"No year is ever the same, but you can get the general trends and patterns," Pastelok said.
Models are only part of the long-range forecasting recipe. Long-range forecasters also study recent trends in the weather, looking at temperature, rainfall, drought, snow cover and more. Trends can be recent months, year-to-date or even over many years.
"Models are projected in the future. But if they're not capturing what's going on now, they can't predict what will happen in the future properly," AccuWeather meteorologist Meghan Evans said.
How a Long-Range Forecast Can Bust
There are big risks when a forecaster makes a long-range forecast. The conditions of the atmosphere and the oceans constantly change. An example of a major oceanic change might be a sudden flip from La Niña to El Niño.
"[The oceans and the atmosphere are] not steady state," AccuWeather meteorologist Jim Andrews said. "If the changes are significant... then it can throw everything out of whack."
Non-events are also problems for long-range forecasters, like if a forecast blocking pattern doesn't happen.
"Either way," Andrews said, "you've hung your forecast on certain conditions and to have them happen or not happen can easily send the forecast astray."
Meteorologists can measure the accuracy of the long-range forecasts to see if it was good. Meteorologists gauge if temperatures were in fact above or below normal and whether precipitation and snowfall were above or below normal. General trends can be verified.
However, how people perceive the season often carries more weight than the numbers.
"It's so difficult because it boils down to perceptions," AccuWeather.com Expert Senior Meteorologist Bob Larson said.
For example, with winter, "you can predict a milder-than-average winter with less snow than average and have it verify," Larson said. "But if you have a big Thanksgiving snowstorm, a Christmas blizzard and a New Year's Eve ice storm, that's what people remember. They remember those big storms, not the weather in between, and think it was a terrible winter. Their minds drift to memorable events."
Temperatures will be a few degrees below average across the UK this weekend, but largely dry conditions are expected.
After no rain for almost a month, Santiago braces for rain early in the week. Cool air follows, spreading into Chile, Argentina and Uruguay mid-week.
There is a significant chance that Jimena will turn back toward Hawaii and threaten the islands during the second week of September.
An unusually strong push of cool air for early September will move southward along the Atlantic Seaboard into the Labor Day weekend before July-like heat returns by next week.
Steering winds could take Ignacio, as a remnant storm, into the southeastern arm of Alaska or British Columbia during the middle days of next week.
Strong thunderstorms will roll across the Upper Midwest while rain and strong winds roar through the Northwest this weekend.
Los Angeles, CA (1988)
110 degrees -- all-time September record.
Washington, DC (1939)
"Once in a hundred-year rainstorm" 4.40 inches in 2 hours at the Washington Zoo.
Minneapolis, MN (1941)
Tornado - 5 dead - $450,000 damage.