Winter Weather Forecasting; An Inside Look
October 30, 2007
by WeatherBug Sr. Meteorologist Mark Lee
The Climate Prediction Center has issued its 2007/2008 Winter Weather Forecast and it shows a warmer and drier winter on tap for most of the U.S. But what are the factors that impact winter weather forecasting? What on the surface seems to be a simple forecast actually takes quite a bit more time and knowledge than you might think.
El Nino Southern Oscillation
You can`t do a winter forecast without looking at the affects El Nino and La Nina. El Nino produces warmer than normal water temperatures in the Eastern Pacific, while La Nina produces just the opposite. Together, they are called "ENSO." ENSO stands for "El Nino Southern Oscillation" and not much was known about this until the 1982 & 83 winter, when El Nino caused devastating weather across the U.S. and the world.
After this devastating El Nino event, scientist knew they needed a better understanding of how the ocean interacted with atmosphere conditions in the eastern Pacific, so they set out to learn. Today, thanks to research, we now know El Nino brings much warmer winter weather to a large part of the U.S. along with very wet conditions to the South. In contrast, La Nina pulls the storms farther north into the Pacific Northwest thus bringing drier conditions to the South along with near normal temperatures to a large part the country. The exception is the Pacific Northwest where the increase in storms causes an increase in clouds and thus cooler temperatures.
So we know ENSO plays a major role in putting together a winter weather forecast, but there are other factors to consider. For instance, before starting a forecast, it`s critical that we understand where we are right now.
The Southeast and Southwest have been under extreme drought conditions this year, while the South-central U.S. has been very wet. The soil conditions in these areas are and will play a major role on region for the rest of the fall and winter.
Hot spells, for instance, become even hotter when soil conditions are dry while moist locations will be cooler. By considering the current soil conditions along with the effects of ENSO you can start to get an idea what this coming winter will look like.
For instance, we know ENSO is in a weak La Nina state right now and we also know soil conditions across a large part of the country. So what happens when you combine the two? Since the La Nina weather pattern steers storm systems farther north, it would make sense that drought condition across the south would likely continue leading to above normal temperatures.
In preparing a winter forecast, it`s not enough to just look where we are now. It`s just as important to look where we`ve been in the past and not just a few days either. By going back in the weather record books, scientists are able to see past weather that is similar to today`s patterns.
By looking at what kind of weather we experienced then, gives us an idea of what kind of weather we will see this year. This year, scientists are going back to look at past years with severe drought conditions in the Southeast and Southwest and combine this with a weak to moderate La Nina to try and gauge this seasons weather. Typically, in the south, a La Nina pattern combining with drought conditions often leads to a continuation of dry weather and above normal temperatures.
Siberian Snow Cover
Another factor meteorologists consider when putting together a winter seasonal forecast is mid October snow cover and not snow cover here, but rather snow cover in Siberia. This snow cover acts as a "giant freezer," the deeper the snow depth the deeper the freeze. How far the snow field expands southward is also important because with snow on the ground, the more likely an "arctic outbreak" will reach the United States un-modified.
In conclusion, the biggest influence on winter weather forecasting by far is ENSO and its two extremes, El Nino and La Nina. But other factors, such as climatology, soil conditions and snow cover also play an important role. All of these will likely lead to a soggy Northwest, with unseasonable warm and dry weather across the Southeast.
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