Cpv17 wrote:Ralph's Weather wrote:The ensembles are showing the -EPO/-PNA/=NAO pattern that can be great around here for early Dec. I still am targeting the 12/3-5 period for a possible Texas winter storm.
Could you or anyone else explain to me what these oscillations are and what regions they cover? I’m trying to learn more about all these different oscillations.
Ptarmigan pretty much covered what they are, but I'd like to add a few notes. All being the different modes of ridge-trough pattern at 500mb where the jet stream is.
EPO - it is the cold/or warm loading pattern for North America as a whole. A negative (-)EPO is essentially a block over the northeast Pacific Ocean. What this does is directs Marine (warmer in winter) Pacific air up to the pole instead of North America and allows the continent to cool. The positive (+)EPO is the opposite and has a trough in the same region flooding North America with warmer marine air. You will hear the term (zonal flow) quite often and that's what it is. Nearly all severe Arctic outbreaks in Texas are associated with historic -EPO episodes.
NAO- It is the far North Atlantic from Eastern North America to Europe. There are two variants an east based NAO (Scandinavia) and west based NAO (eastern Canada-Greenland). When the NAO is negative the pattern over North America is blocked. This slows down the weather pattern over the continent and often you have bigger storms as they are not shredded in faster flow and can deepen. It also has a tendency to lock in place whatever air is present. The AO is often an extension of the NAO and both likes to follow one another. A huge part of the Arctic is made up of the North Atlantic Ocean.
PNA is the steering pattern of the US in general. It directs flow of cold air generated by the -EPO/-NAO/-AO. A +PNA means ridging over the western US at varying degrees, and a -PNA is the opposite with trough in the west and ridge in the east.
These are often the most talked about indexes along with AO which covers the Arctic. There is also the WPO (west Pacific Oscillation) that is the sister mode for west Pacific in East Asia (Siberia) adjacent to the EPO. A -WPO/-EPO is often the requirement for cross-polar flow from Siberia to the US.