To Snow Or Not To Snow

That is the question as we head into December. Right now, we are dealing with a strong La Nina. So, are we going to get cold and is it going to snow?

The NAO is expected to go negative. Some of the Ensemble guidance takes the NAO down to around -3, -4, which is pretty strong. However, the NAO is east based. The NAO needs to retrograde westward, and bring the highest height anomalies over Eastern Canada. With the-NAO west based, the blocking would hold in troughs over the Eastern US. The AO is expected to go negative as well. 

Problem then becomes what becomes of the Pacific. The Pacific is going to be quite unfavorable over the next couple of weeks. Why? The PNA is expected to be negative. Because of the Pacific Jet crashing into the Pacific Northwest (NW Flow aloft)  any of those s/w's will knock down the Western Ridge or +PNA. The Pac Jet is a staple of La Nina however. Typically, you'd want troughing in the NE Pacific as that would help to build up the +PNA ridge. 

The s/w's that do come onshore will not be able to dig south enough. What happens is that the energy amplifies and phases west of the Mississippi River Valley due to the -PNA. So, those storms head into the Lakes, and brings SW flow aloft over the East which floods the East with warmth.

However, because of the -NAO, it leads to more of a zonal flow over the east. The SE ridge is very evident in La Nina years, and each time a storm moves out, the ridge builds again. Sometimes, those G/L Cutters run into strong blocking, and secondary off the coast. Typically, they redevelop too far north, and don't undergo bombogenesis until off the NE Coast. We call these storms Miller B's. 

If the NAO was positive, we'd be dealing with a blow torch and a lot of warmth. 

If the PNA was positive, those s/w's will be able to dig further south and east, and be able to phase east of the Mississippi. You won't have phased energy heading into the Lakes, but rather a Miller A which can run up the east coast.

What do we want to look for?

The Ensemble guidance as of today do have the PNA index rising which can correlate to big East Coast snowstorms. The AO and NAO are expected to remain negative for the time being, and we'd like to see the -NAO block retrograde into Eastern Canada. The NAO allows for troughing to stay over the east, as well as blocking to keep storms from being progressive, quick movers. It can also hold in surface high pressure to the NW to allow for low level cold air to bleed in.