Friday, September 14, 2012

Using the CFS to Pick Up on the Upcoming Fall Pattern

If you are in to long range forecasting, you know that the CFS model can be a pretty tough model to handle. On one hand, the forecasts can be ridiculous at times. At others, the CFS pays out pretty fairly. Me? I think we can check out what the upcoming fall pattern will hold for us by looking at more short range forecasts.

I was able to get my hands on a few perturbations of CFS 40 day precipitation forecasts. For those wondering what a perturbation is, think of it as an ensemble forecast- each member is from the same model, but starts with slightly different conditions.

These four forecasts are the four perturbations of the CFS 40 day precipitation forecast. We can use this to help define a future fall pattern, and possibly, find some hints to winter.

Analyzing this closely tells me that precipitation will be fairly heavy across two areas: The Southeast and up the East Coast, as well as the Midwest. In cases like these, I prefer to average out all forecasts to get the best possible solution. If we do that, we find the Midwest and East Coast to both be in above normal precipitation sectors into the end of October, suggesting a storm track(s) may be setting up in those areas.

However, forecasts are simply forecasts. We need some more solid evidence to prove if these areas could receive above normal precipitation in the next month or so. Let's look at observed conditions across summer in comparison to fall of this year.

This is the observed precipitation rate from June 1st to August 1st, typically observed as summer in popular culture. Over the summer, we saw a very dry northern Plains and Midwest, as well as parts of the Southeast. Even the Central Plains ended up on the dry end of the spectrum. On the other hand, the Gulf Coast got fairly wet, especially in Texas. The Northwest also breached the above normal line.

Overall, summer was a pretty dry period for the nation, with the jet streams more or less diverting storms into Canada or into the Gulf of Mexico, both of which observed above normal precipitation anomalies.
Now, let's take a glimpse into what August 1st thru September 1st has brought us.

Only a month has passed, and a drastic change has taken place for precipitation anomalies. We are now seeing a wetter then normal East Coast, as well as an above normal precipitation anomaly from the lower Mississippi Valley into the Midwest. This is a drastic change from only a month or two ago, when we were seeing extreme drought conditions prevailing throughout much of the nation. Is it a sign of the upcoming fall? Considering we are technically in an El Nino, this is a sign of a El Nino, as far as the wet East Coast goes. A different story must be presented for the Midwest, however, as that region is usually dry in an El Nino.

September is a transition month into fall, and everything must be watched closely for signs of what's to come as far as winter weather is concerned.


1 comment:

Anonymous said...

"as well as an above normal precipitation anomaly from the lower Mississippi Valley into the Midwest"

I think these areas have above normal precipitation due to Hurricane Isaac. I don't know for sure, but I don't THINK this is necessarily due to an El Niño event. I think we can all safely say that an El Niño hampers tropical activity in the Atlantic, usually. In the winter season, it's a different story. I'm just questioning if we can use Isaac's rain as an accurate piece of evidence regarding the El Niño event, since we are still in the summer season (Although quickly moving into fall). If anything, the high tropical activity gives its point to a La Niña. That's just my thinking process. Thanks, Andrew.

- RW