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El Nino Update Issued 10-14-09
There has been much media coverage recently on the El Nino that has formed for this fall and what it means for weather patterns for this winter for the Northwest. Here is a very brief analysis.
El Nino is basically above average sea surface temperatures off the equatorial waters of the Pacific and in particular South America. In general and I do mean in a very general sense the Northwest tends to have above average temperatures and below average precipitation when there is an El Nino. These averages are usually the case for strong El Ninos.
Current El Nino
On the attachment number 1 note the current sea surface temperatures anomalies (SST) for September are close to 1 C above average in the area near the equator at 150 degrees longitude. This is the key area that is observed for trends in El Nino and with this at 1 C above average this is considered slightly above average El Nino. The interesting part is this SST has remained unchanged for the last month if one looks at the SST for October 2009. Is this a trend??
The El Nino Forecast (see attachment 2) indicates what 14 different forecast models predict in terms of El Nino. Half of the models have the SST at 1 C above average through December of 2009. However, 4 models have that at number at only 0.50 C above average which is a weak to neutral El Nino. Finally, one model brings the number to 2 C above normal which would be an above average El Nino. Thus, there is not a strong consensus here.
Prior El Ninos and Sea Surface Temperatures
On attachment 3 please note the El Nino of the winter of 1997-98. This was a strong El Nino and had 3 degrees C above average by September 1997. Also note from a strong El Nino for attachment 4 which is from the 1982-83 winter and this had a value of 3 to 4 C degrees above normal. Remember the value now for the same region along the equator is close to 1 C degree above average.
Snow from Prior El Ninos
On attachment 5 there is listed snowfall from Paradise Ranger Station at Mt. Rainier elevation 5500 feet. Although not a comprehensive analysis we have listed three seasonal snowfalls during three strong El Ninos and for two of them snowfall was close to average and the other 85% of normal. Data for the lower elevations sites was not readily available but from this analysis of Paradise one needs to be careful of making conclusions from correlation on El Ninos and snowfall.
2009-2010 Snow Season
The Climate Prediction Center (CPC - US government) which tends to lean heavily on their correlation between an El Nino in place and with that pattern the CPC predicts that the Northwest will get above average temperatures and below average precipitation. Thus, this would lead to a below average snowfall. The International Research Institute (IRI) has the same conclusion as well. The IRI is a cooperative between the US government climate programs and Columbia University.
What is my analysis for snowfall?? I’m assuming that this El Nino will be weak so I think there will be slightly below average snowfall at 5000 foot level for the West Cascades and Olympics. Thus 80 to 100% of normal for these regions. For the East Cascades perhaps closer to 75 to 85% of normal snowfall. However, if the El Nino starts to strengthen than I would lower the snowfall amounts
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