Heating Up The Pattern Longer Term, JOESTRADAMUS Ponders A Split Flow
El Nino is heating up. The North Atlantic Oscillation is not heating up. The Eastern Pacific Oscillation (EPO) is heating up as is the Pacific North Atlantic Pattern or PNA. What does all this mean?
All these terms can drive one crazy. El Nino we all should know as this is the warming of the Pacific Ocean waters in the tropics from the Coast of South America near the equator and extending out across the Pacific. An active El Nino makes for a busy subtropical jet stream or a strong flow of moisture from the tropics across North America. We haven’t seen much of one, at least not yet. Today’s longer range weather forecast models however show the subtropical jet heating up
What was remarkable today was the fact that the subtropical jet stream was shown to be active on all three models. They are marked by the black arrows across the Pacific and into the Southern United States. The flow is called a split flow because you can see a second dominant jet from Canada on each model. Two separate jet streams that seem to come together in the east. Subtropical jet brings in the moisture. Canadian jet brings in the cold air. Result Snowstorm! Well not exactly.
Even though the three models are similar in look, there are practical differences with each one. The issue in situations like this is which jet dominates. If the northern jet is too strong then it overwhelms the east with cold air and everything winds up getting suppressed to the south. If the subtropical jet is too strong then the reverse happens and you wind up with not enough cold air and the precipitation, especially this time of year is mostly rain except well inland. It is a delicate balance that needs to be achieved and it is also a short range weather issue and not a long range weather issue.
Now back to those NAO,AO,EPO,PNA. These indexes that forecasters look at are indicators of the state of the atmosphere. For the Atlantic we look for negative values which tell us that the air over the northern latitudes is relatively warm. Cold air is displaced southward while it is warmer than normal at the north pole and across the Arctic. This favors what we refer to as blocking. Weather systems basically are forced to track further south than normal and it is with blocking that the Northeast has seen blockbuster storms. These indexes, the NAO and the AO are forecast to be positive in late November early December. So we conclude that if there is no blocking it can’t be cold and it can’t snow in the east..right?…well no.
On the other side of the equation is the Pacific indexes namely the PNA and the EPO which are essentially the same type of measures on the Pacific side. Long term analysis has shown that a positive PNA (which we are forecast to go into) and a negative EPO (which we are forecast to go into) are dominate and TRUMP the NAO and the AO. This was exactly the setup here the last 2 winters. We wondered aloud as forecasters how is this cold and snow happening without an NAO? Well it happened because the other two patterns are far more dominant then the Atlantic indexes which are much more volatile.
All of this is for the period coming up after Thanksgiving and into early December. This is at least what the plan is according to Today’s models. The Pacific indexes are far more dominate and less volatile then the Atlantic ones so they can stay positive or negative for a longer period of time.
One of the big drivers of the PNA last winter was the warmer than normal ocean water of the coast of the Canadian Northwest and the Gulf of Alaska. That pool of warmer than normal water temperature continues with no sign of it breaking down anytime soon. You can see the strong El Nino as well in the dark red. The key for the winter is whether the area out in the Central Pacific remains strong or continues to strengthen. Latest data continues to indicate that it is indeed strengthening in the Central Pacific.
So if we have the same set up as last winter…with the added influence of the El Nino, could this mean the beginning of a rather interesting period of weather coming up very soon? Well lets see what the models do over the next several days as far as the long range is concerned and lets see what kinds of adjustments they make. As we said earlier things may be heating up.