Post by Bennett D. Ebberly on Nov 8, 2003 16:00:58 GMT -5
Winter Outlooks - More Than Just El Nino
7AM EST, November 8, 2003
By WeatherBug Meteorologist, Justin Consor
Weather folklore has long fascinated people whose livelihood depended on the weather. For centuries farmers used signals from animals, plants and celestial bodies to divine a forecast for the upcoming winter, but in recent times the interest in such "folk" forecasts has surged dramatically.
There is absolutely no scientific evidence that shows these folklore forecasts are accurate or possible. It is much more logical that such signals simply respond to past weather patterns. However, interest in such lore continues.
No matter what means you use, attempts to forecast day-to-day weather beyond about two weeks in advance are largely futile.
Every year, several almanacs issue winter outlooks, often with specific forecasts for specific days. These factors are based on a wide assortment of factors, from the behavior of woolly bear caterpillars to numbers of acorns to cycles of planets and the sun.
One bit of folklore states that when woolly bear caterpillars in one`s location have more black than white segments, the winter will be harsher than usual.
From 1948 to 1955, biologist Charles Curran ran the only scientific study ever performed to test this hypothesis. Curran gave up when he discovered two groups of caterpillars living close to each other that had nearly opposite predictions for the upcoming winter.
The colorful panoply of weather folklore also includes the idea that several signals together - a bumper crop of acorns, as well as animals putting on heavy coats or storing food earlier than usual - mean a more severe winter.
The legend surrounding Groundhog Day is based on a similar kind of lore. Supposedly if the sun shines on February 2nd, with the groundhog thus seeing its shadow, there will be six more weeks of winter.
It is more likely to be sunny during wintertime in the U.S. when there is a cold arctic air mass versus a warmer one, but these cold snaps typically last a few days and have little connection to long-term weather conditions. Groundhog Day survives today mainly because it is a fun tradition that generates a lot of media attention.
Even when forecasts are based on more widely studied meteorological factors, they run into a fundamental limitation. Dr. Edward Lorenz from MIT showed that forecasts are highly sensitive to the current conditions, e.g. the temperatures, wind and pressure.
Lorenz showed that when weather forecast models are run, small errors in observations and short-term forecasts become multiplied.
When we look at day-to-day model forecasts out to around two weeks in the future, no matter how good the model is, the errors grow so large that the models produce no better forecasts than would flipping a coin.
Furthermore, the current weather data that the forecast models use to make forecasts is not nearly comprehensive enough:
About 72% of the world is water and on a typical day, only 1100 ships and buoys submit weather reports. That`s 1 weather report for about every 130,000 square miles of water.
On land, there are 6000 official World Meteorological Organization weather stations throughout the world, while AWS WeatherNet stations number about 6500 in the U.S. and southern Canada. That`s 1 weather report for every 15,000 square miles of land.
However, the situation is not as bleak as Lorenz may have envisioned. Meteorologists have been making long-range outlooks (beyond 2 weeks into the future) regularly since the 1960s.
The National Weather Service issues forecasts going out 12 months in advance. Their outlook for this winter (Dec-Feb) is the thumbnail image for this story.
Forecasters use a number of factors to come up with such outlooks:
Sea surface temperatures. For the U.S., certain ocean areas are important to look at:
Equatorial Pacific Ocean from Peru to Indonesia - "El Nino" develops here when the waters warm at least 1 degree above normal for periods of several months to as much as 5 years, and this has major impacts on storminess and temperatures throughout the winter. "La Nina" is its opposite, with cooler waters than normal.
The Pacific Ocean north of the equator - There is a large-scale ocean warming on about a 25-year cycle called the Pacific Decadal Oscillation. In the "warm phase", El Ninos are favored.
Atlantic Ocean - warmer water in high latitudes near Greenland and Iceland with cool Atlantic water from the U.S. east Coast to southern Europe historically favors more severe winters in the eastern U.S. The water temperatures relate to changes in the strength of areas of high and low pressure. These periodic pressure changes are called the "North Atlantic Oscillation".
The Arctic. The degree of sea ice buildup in the Arctic may explain a linkage between temperatures and pressure in the polar regions of Canada and those in the entire U.S. This is called the "Arctic Oscillation".
High altitude winds - Winds about 60,000 feet up in the atmosphere switch from easterly to westerly or vice versa every 13 or so months. Climate researchers have shown that these wind patterns often alter expected weather patterns associated with El Nino and La Nina.
Snow cover - More snow cover building up in fall and early winter in Canada and the northern U.S. typically leads to more severe cold blasts descending southward from the Arctic toward the U.S. Also, the exact location of the deepest snow cover is often important for which part of the U.S. gets hit by the worst of the arctic air.
Correlations to similar patterns in previous years - forecasters analyze the current weather pattern and look for years where several of the above factors match up.
In addition to these factors, forecasters use sophisticated models that predict climate going out several months in advance.
The National Weather Service issues its long-range outlooks in terms of probability, e.g. there is a 65% chance that temperature will be above normal in December.
However, when media meteorologists issue their interpretation of such outlooks, a high probability of above normal temperatures is often translated into "temperatures will be much above normal".
All in all, a successful winter outlook requires a careful assessment of several climate factors that tend to cycle back and forth over time. The challenge is that our ability to gauge when these cycles will flip and how they will interact is relatively limited.