Posted by: pyeager | November 22, 2009

December Forecast

When talking about long-range forecasts, as I have often in this blog (long-range forecasts), I’ve noted that we meteorologists base long-range forecasts on more than just whether an El Nino is or isn’t occurring, and I wanted to give an example of one of those other tools today.

Let’s take a look at the NCEP coupled forecast system forecast (is that one too many forecasts?) for December. If it’s right, then much of the United States is in for quite a month!

What It Isn’t

The coupled forecast system is unlike the GFS model I referred to in my series of forecasts about the weather for Thanksgiving Day (Thanksgiving Day Forecast–No Big Storm). The GFS generates a specific daily forecast, hour after hour after hour, and extends a couple of weeks out, typically becoming more and more inaccurate with time since mistakes are magnified by the use of the previous forecast information as the basis for the next forecast iteration.

What It Is:  Specific Long-Range Model

The coupled forecast system is specifically used to generate long-range forecast trends, rather than specific hour-by-hour or day-by-day information (similar to a climate model).

I’ll give details about how the model works in a future post, but the point here is that it generates a general forecast, not a specific forecast. In other words, it forecasts temperature and precipitation deviations from normal, for example, and not the amount of precipitation location/amount on a particular day.

Since that’s what the model is programmed to do, rather than continually generate new forecasts based on the output from a previous forecast time, it doesn’t multiply its forecasting errors. It may not be right, but a bad forecast for one time period doesn’t negatively affect the output for the next time frame, making it possible to create forecasts for months in advance.

Much Colder than Normal in December

The latest coupled forecast system run is predicting a colder-than-normal December in much of the country, with the cold air encompassing the eastern two-thirds of the country. The map is below, but it’s difficult to see since the forecasts for many months are included on one image–and the background is of the entire globe. Go to their site–from the link above or by clicking on the image for more information.

The blue areas indicate colder than normal, and the red is warmer than normal.

Monthly coupled forecast system forecast


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