Posted by: pyeager | October 22, 2009

El Nino Still Weak–What Does it Mean?

The latest discussion from NOAA (National Oceanic and Atmospheric Administration) indicates that the El Nino remains weak (ENSO Diagnostic Discussion) as of early October. While that may (or may not) be of interest on its own, what’s more important is what implication the strength of the El Nino has on the upcoming weather across the United States. (It’s an important factor globally, but this blog focuses on U.S. weather).

The uncertainty of the ultimate strength of the El Nino means that the accuracy of winter forecasts based on the El Nino must be read with caution.

El Nino Forecast to Strengthen, But…

The computer models are currently forecasting that the weak El Nino will strengthen–becoming either a moderate or perhaps a strong El Nino.

Computer model projections of strength of El Nino (image courtesy of Climate Prediction Center)

Computer model projections of strength of El Nino (image courtesy of Climate Prediction Center)

However, these very computer models, including the government’s Climate Forecast System (CFS) model, have been too aggressive in predicting the strength of the El Nino so far this year. This was noted in the October ENSO discussion as well and is indicated by the next image:

National Center for Environmental Protection's CFS (Climate Forecast System) model, indicating the over-prediction of the strength of the El Nino

National Center for Environmental Protection's CFS (Climate Forecast System) model, indicating the over-prediction of the strength of the El Nino

Winter Forecast Accuracy

I’m not one to criticize anyone who makes a long-range forecast because they’re so quickly dismissed, require tremendous amount of skill (if done properly), and most important, I don’t make any; however, we have to look objectively at the forecasts. If the El Nino does not strengthen to a moderate level, does that put the accuracy of the forecasts at risk?

A strong El Nino produces a more active southern branch of the jet stream, which is why so many of the winter forecasts (such as NOAA Winter Outlook and AccuWeather Winter 2009-2010 Winter Forecast) are predicting more precipitation than normal across the southern tier of the country. If the El Nino never becomes strong enough to produce that effect, then the forecasts are in jeopardy.

AccuWeather.com (AccuWeather Winter Forecast), to be fair, never forecast a strong El Nino to occur (and bases the forecast on other factors as well–the discussion of which is generally relegated to premium customers); however, their forecast is based, at least in part, on the weakening of a moderate El Nino during the winter, resulting in more snow-producing storms along the Eastern Seaboard. What if the El Nino never strengthens to a moderate level to begin with? Will southern moisture and storms not be available for the northern branch to merge with?

Now that it’s October (and some people have already had accumulating snow!), we’ll know soon enough–and I’ll make sure to look at the strength of El Nino next month.

–Paul Yeager

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