Friday, September 14, 2012

Long Range Lookout: Traffic Jam Over North America

This is a new section I call the 'Long Range Lookout', where we take a look at what the long range models are showing, and what it means for the future.

This is a 500mb height anomaly map for September 22nd. We can see a very well established blocking pattern across North America, with two major ridges positioned over eastern Alaska into northwest Canada. This partially extends into the western US. Another major ridge stands in the waters to the south of Greenland. These two ridges create a pattern favorable for major troughing across the south central Canada/Upper Midwest regions in response to this atmospheric traffic jam.

I marked two arrows going in a circular pattern over the area where the trough is positioned. Looking at the forecast in a few days prior to this particular image, it is almost like an assembly-line system of new disturbances being generated across central Canada that get caught in the traffic jam and are forced south into a spinning, stationary trough as is pictured above. This will enable frequent visits of unseasonably cold air into the Midwest and Northern Plains as a result of the troughs pulling down cold air from Canada.

Also, take note of a disturbance to the south of the ridge positioned over Alaska and Canada. I did draw an arrow that is directed into the Southwest. Storm systems in this position commonly will take the route I outlined, and at this time, it looks to be the only route. As a result of the ridge in the Western US, the disturbance will be forced south and make landfall in the Southwest. As is typical of an El Nino, this system will likely move through the Southern Plains, but that's when things get tricky. The question is, will the major troughing pattern be able to pull in the disturbance traversing the South, thus bringing precipitation into the Mississippi Valley? Should that happen over and over again, we could have this winter's storm track on our hands. However, the probability of such a strong regime of blocking is low as far as lasting long goes.

Notice the red circle marked off the Southeast. This is a feature you should all get to know and love, especially the Midwesterners. This ridge can strengthen during the winter, and push into the Southeast. If the ridge pushes into the Southeast, storms can be directed on a northward path, following the track of the Chicago Blizzard in 2011. A glance at forecast hours near the image above shows the ridge south of Greenland pressing south, resulting in another major trough pushing the circled ridge west. If this would play out in the winter, this could have very interesting implications for those in the Midwest and Ohio Valley, as far as snowfall potential goes.

That's this evening's Long Range Lookout. As time goes on, I hope to expand this to include more forecasts and more notes that can be taken from said forecasts.

Have a great evening!


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.