Saturday, September 22, 2012

Long Range Lookout: Active North US Pattern To Continue

In this week's 'Long Range Lookout' segment, we'll examine what propsects the already-active fall season for the North US has for continuing into the next few weeks.

This is an ensemble forecast for the East Pacific Oscillation, or EPO. In a positive EPO, we see a strong trough centered over the Gulf of Alaska, with a ridge of high pressure extending west from Baja California and stretching across much of the Pacific. Additionally, a cold north US is a common trait of a negative EPO.

Recently, the EPO has been in a negative state, and this has been shown by the appearance of a strong trough over the Gulf of Alaska and cool temperatures across the northern US. This bodes well with typical negative EPO conditions. I will explain how this plays into a bigger pattern in a little bit.

We are also watching the Pacific-North American Pattern, or PNA pattern. In a positive state, the PNA creates a strong ridge over eastern Alaska and much of western Canada. This ridge enables storms to jump over the ridge, plummet down towards the Southwest, and follow the jet stream into the southern Plains. From there, the storms take a path that bodes well for high precipitation across the Midwest, Ohio Valley, and Great Lakes.

In the most recent week, we have had a positive PNA. This has been well reflected by the presence of a strong ridge in northwestern North America. Storm systems have been hitting the Ohio Valley and Midwest at a fair pace as well, illustrating the effects of the positive PNA.

Now, both of the above indices can (and did) mix together. The two combined to produce a ridge in the West, and a stormy pattern in the East, as illustrated below. This chart was from September 20th.

University at Albany
If the ensemble forecasts from above do verify, we could again see strong storm systems hitting the North US and bringing very cold air down to the region. This also includes the risk of some severe weather in the Northeast, as cold air moves in on warmer air brought up by warm fronts. This could also be a crucial sign of the incoming new Lezak Recurring Cycle. Every little change could mean something bigger later on.



mike paulocsak said...

Hello Andrew & everyone else! We have seen more rain in the Ohio Valley this past month than we have seen since May.It will be interesting to see how long this pattern continues.

ERN WX said...

Mike, I see it going till winter easily.

Anonymous said...

Will Mn get more snow? Last year got only 20" total & our avg is about 45" thanks

Andrew said...

Mike: Very interesting observation! Many areas previously hit by the drought have been experiencing more precipitation as of late.

Anonymous: Likely above 20'', but above 40'' could be a stretch, depending on the ENSO state and the state of the AO/NAO.