Wednesday, December 12, 2012

Long Range Lookout: Stormier Pattern On The Way

The pattern for the next while is looking much stormier in the near future.

Above is a 4 panel forecast of two teleconnections, the North Atlantic Oscillation (NAO) on the bottom half and the Pacific North American (PNA) index on the top half. The left half forecasts are from the ESRL/PSD, a physics-based branch of the government's weather services, and the right half forecasts are from the NCEP, which gives us the GFS model. If you didn't catch that, here's a breakdown:

Top left: ESRL/PSD PNA Forecast
Top right: NCEP PNA Forecast
Bottom left: ESRL/PSD NAO Forecast
Bottom right: NCEP NAO Forecast

The Pacific North American index involves what the atmosphere does in the northeast Pacific and the western coast of North America. When we see a stormy pattern in place over these regions, we call such a pattern a negative PNA, due to the below normal height anomalies in this region. In a similar sense, when high pressure dominates that same region, we call that a positive PNA. A negative PNA will bend the jet stream to give the storms to the Plains and the Deep South regions, frequently initiating high pressure system formations over the Central US. A Positive PNA will bring about an opposite response to high pressure (HP) over the West, and will have the stormy pattern evolve over the East US. These forecasts call for a deep negative PNA, which typically is not that favorable for cold in the East. We'll discuss that prospect later, but keep in mind that negative PNA.
The North Atlantic Oscillation involves the presence of a high pressure system over Greenland (negative NAO) or the presence of a low pressure system over Greenland (positive NAO). In the negative NAO, the jet stream will buckle into the Northeast to allow storms and cold to thrive in that region. The positive NAO denies this region any of these benefits. The ESRL and NCEP are varying on their forecasts, but I have a feeling that the NAO will stay negative, so HP systems caused by the -PNA may be deterred slightly by the -NAO.

This is the Arctic Oscillation (AO) forecast. In its negative phase, you will see high pressure forming over the North Pole, displacing cold air and letting it flow south into North America. The positive phase of the AO brings about a very strong polar vortex (low pressure system that sits on the North Pole), which locks up the cold air in north Canada. Multiple models and ensembles are predicting that the AO will go positive for a while before dropping negative during the second to last week of December, finally trying to push positive in the days leading up to New Year's. There seems to be good agreement among the models, so cold shots are likely to be shooed away in the positive phase and enhanced in the negative phase.

All in all, a negative PNA, weak negative NAO and rollercoaster AO support a much stormier pattern but not a huge potential for a cooldown. Unless we see a sturdy -NAO and -AO, don't expect to see any sustained cold.

Andrew

14 comments:

Anonymous said...

Yeah... we shall see about that.

Anonymous said...

Hey Andrew!

Thanks for all you do.

Now the 18-19 winter storm os begining to look like a bust, is there anything on the models for anytime afterwords. Maybe a secondary storm?

Also, do you have any more info on the potential new years storm?

Thanks!!

Anonymous said...

This winter will be similiar to last winter.But wait a minute before anybody jumps down my throat.I will explain what probably will happen.First off,we are having an up and down pattern right now.This will continue through the winter!The only difference between last winter is this winter will have colder shots of air,but they will be very,very,very,very,short lived.Yes we will have snow.As for the snow sticking around for long,it probably will not happen.As for going long range with a winterstorm,we should proceed with caution.

Anonymous said...

You could not of had said it better anonymous above me.

Anonymous said...

Back in the 1980's and 1990's it was funner when a storm came.Many may ask why.Well i'll tell you why.Everyone went with short term 5-6 day forecasts.Not long range.And yes the models were way more accurate back then.I grew up in that period of time.That's how I know!

KakHome said...

Anonymous 3 (saying this winter similar to last) So basically you are saying there will be back and forth (no sustained patterns) but at least the chance for a few more short-term cold hits than last year, not a cold winter but these hits perhaps lowering the average a little versus last year?

Rod Houston said...

This winter is looking like last years winter.The only thing that is different from last winter is it's going to be more stormy but most of these storms will be rain events because it will be too warm for snow.There's no signs of the PNA going positive and the AO is up and down just like last year and the NAO is also up and down.This winter may be the worst winter over.

Anonymous said...

you guys are the laughing stock of this post. andrew may not have a very good track record but he is the one sticking his neck out, you arent.

Anonymous said...

Andrew,

Any thoughts as to when we might see some cool air spilling down in Washington State?

KakHome said...

Rod the NAO was NOT up and down last year, it was positive the whole season. Also Siberia had forecast a mild winter. This year, it doesn't, for February at least. Just because winter doesn't start like 2010-11 did in the East doesn't mean there will be no winter! It's December 12th, not March 12th. That didn't mean a thing last winter, as it turned out, but you CANNOT use ONE year to come up with a complete forecast. This is like trying to make inferences on a population based on a statistical sample with one data item.

And no, saying there is no sign of the PNA flipping in the next 3 weeks doesn't cut it. That's barely even into January.

I understand there are still tremors left from the shellshock that was last winter, but while that will by nature make opinions and forecasts more pessimistic, they are of little value when they're driven by mood and not by facts.

WX BRIAN said...

Rod - It was very stormy (in Ohio Valley) last year, we had above average precip, and way above average rain...
Anonymous below Rod - Agreed.

KakHome said...

Anonymous below Rod - Why do you say the commentors aren't sticking their neck out?

Anonymous said...

So far Northern California near Sacramento weather this year is totally different compared with last year. Last year rain was very sparse until March. This year the rains have been heavy and stormy and we are 300% above this time last year. Good news for all of the lakes.

Anonymous said...

Andrew do you think you will have another update by tomarow? there are a few folks here that are concerned about next week, they dont want another Sandy, some more info for me to help downplay the situation would be appreciated, thanks