22 January 2011
Another Deep South Snow and Northeast Blizzard? This time the NAO is innocent!
Posted by Dan Satterfield
Here in the Eastern USA this winter will be remembered for many years if not generations. A white Christmas in North Alabama and Georgia is a less than once in a century event. The two East Coast blizzards the past 4 weeks have been amazing.
Now, don’t shoot the messenger but another one MIGHT be brewing.
We are in a different pattern now than the strongly negative NAO of a few weeks ago. This pattern is called a positive PNA pattern. It is characterised by a strong ridge of high pressure aloft over the Western USA and a deep trough of low pressure over the Eastern U.S. This NW flow of air over much of the Eastern half of North America brings cold and sometimes snowy weather to the east coast.
An easy way to think of it is this: The NW flow of winds in the upper atmosphere steers the surface low pressure and high pressure systems which bring cold air and occasional storms. While the pattern has changed, the results are the same! Cold and snowy in the Eastern USA and i Eastern Canada too.
Europe however has gotten a big break. The positive PNA pattern has brought much warmer weather there. The Christmas cold is just a bad memory in the UK now. Greenland has seen it’s mild weather replaced by the normal 20- 40 below they are used to.
The more immediate problem for forecasters like me is what is going to happen early next week. There are strong hints from some of the numerical models of a major snow event from the Deep South to New England Tuesday into Wednesday. The key word here is SOME of the models.
The latest runs of the NOAA long range model tonight show little snow over the SE on Tuesday and not much of a storm in the NE for Wednesday either. The Euro model is much different. It has a major storm developing for Alabama and Georgia followed by a major blizzard for New York and Boston. The end result is a stripe of snow from Columbus, Mississippi to Bar Harbor, Maine!
So which is right? Here are some thoughts on that.
The Euro model has been very consistent over the past few runs and this argues for it to be the better model. It also has a newer more modern and accurate initialisation procedure. Remember GIGO? Garbage in=Garbage out, and this certainly applies to weather forecasting. The fact that we cannot tell a weather model the starting weather EXACTLY, everywhere on Earth, is one big reason forecasts beyond 7 days have little skill.
That said, the higher resolution short range models are hinting that the ECM is overdone and the NOAA GFS may be closer to reality. In short, the responsible forecaster will mention the possibilities of a storm next week, but refrain from issuing any snowfall forecasts until there is better agreement among the numerical guidance. That’s just what I did on the evening broadcast Friday night.
Not everyone has been that responsible. There are a lot of arm chair meteorologists (and some on TV) who look at an NWP run and start the rumour mill in full motion! I’m already fending off emails asking about a heavy snowstorm with 7-12″ of snow! There are also some folks who love watching it and learning the art and science of forecasting, and I really enjoy interacting with them on social media and email.
As far as this crazy winter goes, PNA may actually stand for- Possibly Not Over!
does the PNA or other teleconnections have any links to pineapple express like events?
The pineapple express is a stream of tropical moisture that is often seen stretching from the Tropical Pacific to California and into Western Canada. They tend to occur more n El Nino winters and are associated more with another type of pattern called the Madden julian Oscillation. A positive PNA would be favourable for them as well.
I was under the impression that a positive PNA was generally associated with El Nino conditions, but we’re in the midst of a moderate/strong La Nina. How do reconcile that?
It does tend to happen more often in an El Nino. Another part of this surprising winter.