Saturday, September 15, 2012

Has A Strong Trough Hit The North US Before, and What Does It Mean For Fall?

Model forecasts are pointing towards a monster trough entering the North Central US in the next couple of weeks. This trough is forecast to sit and be pretty much stationary over the region for a bit. This is not something we see too often, so we look to see if it's happened before. So, have we seen this before?

This is a chart of September 22nd in 1983. It shows 500mb vorticity on the top left, with 1000-500mb thickness and high/low pressure markings in the top right. The bottom two boxes contain different relative humidity values. Specifically, the 700mb RH on the left and 850mb on the right.

As you can see, a very strong disturbance is stationed over the north central US, pulling in below-freezing air into the Midwest. This below-freezing air is shown as the 540 red dashed line dipped into the Midwest and Northern Plains. It looks like some snow may have fallen over parts of the region above the 540 line.

So, what does 1983 have anything to do with 2012?

Analogs can be pretty powerful. Taking a look a week or two past this date, and here is what I find in the atmosphere after this date:

•Disturbance moves towards the Southwest and rapidly strengthens.
•Heat re-surges into the Plains and Midwest.
•Progressive disturbances move thru the US/Canada border.
•Another strong disturbance brings below-freezing air into the north central US on October 13.

What can one draw from all of this?

If we are to look at the ENSO phase, we see a strong El Nino in early 1983 progressing into a La Nina in the late fall and winter months, so this won't exactly play out. But, should this strong disturbance that is forecast to hit the North Plains and Midwest verify, 1983 could hold some hints. I do not doubt that the heat resurges into the nation, but a more active jet stream is not out of the question.

Time will ultimately tell, but analogs can be one of the most powerful tools forecasters can use.


No comments: