Long-range Weather Forecasting
By Dr. Ed Brotak | Dec 7, 2017
Everyone wants to know whether they will have to deal with harsh winter conditions, and they like to know as soon as possible. This takes us into the realm of “long-range weather forecasting” and its most famous product, the “winter outlook.”
Although numerous people and groups put out winter forecasts, the official forecast comes from the National Weather Service’s (NWS) Climate Prediction Center. The meteorologists there produce long-range monthly and seasonal forecasts up to a year in advance, and these are updated every month. So, you can get a forecast for next winter by the end of the current winter. As you approach the time period in question, forecast confidence typically increases.
I should mention here that the NWS uses the terms “forecast” and “outlook” for different situations. Forecasts are more precise with specific temperature and precipitation probabilities given, but only go out to seven days. Beyond this time frame, the NWS uses the term “outlook,” and predictions are less precise — typically just comparisons to normal or average conditions.
For extended outlooks, like the one for the following winter, temperature and precipitation predictions are given as above or below normal, or as “equal chances” (of above or below normal). This can be inferred as normal or average since there are no strong indications of abnormal conditions. The final forecast for the coming winter is issued in mid-November.
So how do NWS meteorologists make long-range forecasts, especially the winter outlook? They use computer models. When forecasting out to 14 days, there are mathematical models that will actually depict weather systems that control our weather. But the errors with these mathematical models compound over time, and they are fairly useless after 14 days. Beyond that, meteorologists must rely on less accurate statistical models. They will look at past conditions and consider persistence and any noticeable trends.
With nearly 70 percent of the Earth’s surface covered by water, ocean temperatures are a major factor. If possible, they look for large-scale weather patterns — patterns that are capable of lasting for months and can control the weather. To a large extent, the winter outlook is based on the El Niño/La Niña (EN/LN) cycle. The media has often made mention of them in recent years, and meteorologists first expressed interest in them back in the 1980s. The reality is that this cycle has been going on for hundreds, maybe thousands of years, and we’ve only just realized it.
So what exactly is the EN/LN cycle? It’s a weather phenomenon that occurs in the Southern Hemisphere, specifically the tropical southern Pacific Ocean. Water along the coast of South America (particularly Chile and Peru) is typically cold. A cold current — called the “Peru Current” — flows south to north along the coast and brings with it colder water from the far southern Pacific. There is also “upwelling,” a process that stirs up colder waters from the bottom to the surface. You get a lot of fog but no rain, sometimes for years.
The cold ocean waters are filled with nutrients and teeming with fish, but every once in a while these waters warm dramatically. Skies clear initially, but then it can rain, and the fish vacate for cooler waters. This typically occurs around Christmastime, and locals started calling it “El Niño,” the child, for the baby Jesus. But this weather phenomenon wasn’t just local. It affected the ocean waters across the entire tropical Pacific, and, as it was later discovered, it affected weather conditions around the world.
Wind and water
In the tropics, the winds typically blow from east to west — the “Trade Winds.” Ocean currents, which are partially wind driven and partially produced by the same forces that move the air (temperature and the Coriolis Effect), also flow from the east to the west in the tropics. In the Pacific, warm waters along the South American coast are transported westward by the South Equatorial Current. This allows the cold current and upwelling to produce the cold coastal waters. But sometimes, the Trade Winds and tropical ocean currents weaken, so warm water begins to “back up” along the South American coast. This is the El Niño effect.
There is also a counterpart to El Niño — the La Niña. This occurs when the Trade Winds and ocean currents, especially the South Equatorial Current, become abnormally strong. The central and eastern Pacific regions are cooler than normal. Put EN and LN patterns together and you have what is called the Southern Oscillation, or the El Niño Southern Oscillation (ENSO). Sometimes the ocean temperatures are near average for some time, called “ENSO neutral.” A more exact cause of the ENSO remains unclear.
Around the world
With vast amounts of energy involved and ready interactions between the water and the air, these cycles actually influence weather patterns throughout much of the Western Hemisphere and beyond. The warmth of El Niño fuels a strong southern jet stream that produces an active southern storm track and blocks cold air from the north from entering much of the United States. La Niña brings a more variable northern jet and a northern storm track with occasional cold outbreaks.
Statistical analyses indicate that certain patterns of warm or cold and wet or dry have occurred in various parts of the U.S. in correlation with many El Niños or La Niñas. So, if you have El Niño or La Niña occurring, these weather patterns are what meteorologists use as a basis for long-range forecasts. For an El Niño, we can expect northern tier states to be warm and relatively dry, the southern states cooler than normal, and precipitation from southern California to the East Coast. For La Niña, almost the exact opposite is expected with warm, dry conditions to the south, and cool, wet conditions to the north.
The EN/LN cycle is anything but regular, though. One El Niño pattern can be followed by neutral conditions or even another El Niño. The strength of the actual event varies considerably. There are other weather influences that can override the El Niño or La Niña effects, and these other factors can’t be forecast months in advance.
The last two winters are good examples of what we’re dealing with. The winter of 2015 -’16 was highlighted by one of the strongest EN events in history, and the winter forecast followed the scenario given above. In the end, most of the country was warm, especially the Northeast and Alaska, which saw record-breaking warmth. But the precipitation pattern was not typical for El Niño. The Pacific Northwest was wet, as was much of the East including the Southeast. But from central California eastward to Mississippi, precipitation was below normal.
A weak LN was forecast for the 2016-’17 winter, and it did occur. The winter forecast followed the typical LN pattern, but the country was divided more between east and west than north and south in terms of winter weather. The West, especially the Northwest, was cold, and the East was warm. Much of the East, especially the mid-Mississippi Valley and the Florida Peninsula, was dry. With the exception of western Washington, precipitation was above normal in the West with much of it in the form of snow in the mountains. Most of California received copious amounts of rain and snow, ending long-standing drought conditions.
What to expect
What about this coming winter? The weak La Niña of 2016-’17 ended, and water temperatures in the tropical Pacific returned to normal through the summer. But to show how irregular this cycle is, another La Niña is currently developing. And this was not apparent in the summer, when the first winter forecasts started to appear.
The official winter forecast now follows the typical La Niña scenario: Warm in the southern two-thirds of the country and up into New England, and a normal cold winter to the north. Alaska is forecast to have another warm winter. Wet in the Northwest and the Midwest, dry in the Deep South and Florida, and normal precipitation expected elsewhere.
There are other major atmospheric cycles such as the Arctic Oscillation, which involves upper-level weather systems in high latitudes. It can have a tremendous influence on weather. But it has a shorter time span, sometimes just weeks, and is impossible to forecast far in advance. What can be said about the accuracy of long-range forecasts? The American Meteorological Society, a respected organization of professional meteorologists, says, “Presently, forecasts of daily or specific weather conditions do not exhibit useful skill beyond eight days, meaning that their accuracy is low. Skill in monthly and seasonal forecasts is extremely variable from period to period.”
What should one make of the winter outlook? These forecasts cover a lot of territory. In some places, they’ll be right and in other places, wrong. And it’s impossible to know ahead of time where the forecast will be accurate.
For 30-plus years, Ed Brotak taught thousands of college students about the weather and helped hundreds of them pursue a career in meteorology. He lives in Asheville, North Carolina, with his wife, also a meteorologist, and his two daughters, who vow never to be “weather weenies.” He still goes outside when he hears thunder.
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