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| Stop complaining about spring snow – it's not that unusual | Stop complaining about spring snow – it's not that unusual |
| (10 days later) | |
| If you live in the northeast or the midwest, you're probably wondering where the heck spring is. Temperatures have been running well below average, and a few snowstorms have been added to the mix. Today for instance, Chicago and New York saw snow and the high temperatures in both cities were nearly 15F below average. | If you live in the northeast or the midwest, you're probably wondering where the heck spring is. Temperatures have been running well below average, and a few snowstorms have been added to the mix. Today for instance, Chicago and New York saw snow and the high temperatures in both cities were nearly 15F below average. |
| Just what is going on? | Just what is going on? |
| Our temperatures have been so cold mainly because of what can only be described as an historic blocking pattern. The arctic oscillation (AO) has been strongly negative over the past few weeks, which means that cold air from the north pole can come down and invade eastern North America. Combine that with a negative north Atlantic oscillation (NAO) (also known as high pressure over Greenland), and you've got the recipe for some temperatures way below average. | Our temperatures have been so cold mainly because of what can only be described as an historic blocking pattern. The arctic oscillation (AO) has been strongly negative over the past few weeks, which means that cold air from the north pole can come down and invade eastern North America. Combine that with a negative north Atlantic oscillation (NAO) (also known as high pressure over Greenland), and you've got the recipe for some temperatures way below average. |
| These patterns affect our British readers in much the same way – that's why many in the UK are wondering whether spring will ever come. But just how unusual is what we're experiencing? | These patterns affect our British readers in much the same way – that's why many in the UK are wondering whether spring will ever come. But just how unusual is what we're experiencing? |
| I looked back at the past 30 years of New York City records, which can be used as a baseline for the northeast's urban corridor, and what I found tends to confirm what we're feeling right now. I counted only four other years where the average March temperature was 39F or lower, like this year: 2005, 2001, 1993 and 1984. | I looked back at the past 30 years of New York City records, which can be used as a baseline for the northeast's urban corridor, and what I found tends to confirm what we're feeling right now. I counted only four other years where the average March temperature was 39F or lower, like this year: 2005, 2001, 1993 and 1984. |
| 2005 had a ridiculously low NAO (even lower than this year), as well as an above average Pacific North American (PNA) index. A positive PNA means that the low pressure system in the Aleutians of Alaska gains a lot of strength. The result is a ridge in the western US and a trough in the east. It helps to reinforce the effects of the NAO in this case. Most of this March the PNA has actually been negative, thus explaining the relative lack of snow in the mid-Atlantic; storms cannot form too far south, despite the cold temperatures. | 2005 had a ridiculously low NAO (even lower than this year), as well as an above average Pacific North American (PNA) index. A positive PNA means that the low pressure system in the Aleutians of Alaska gains a lot of strength. The result is a ridge in the western US and a trough in the east. It helps to reinforce the effects of the NAO in this case. Most of this March the PNA has actually been negative, thus explaining the relative lack of snow in the mid-Atlantic; storms cannot form too far south, despite the cold temperatures. |
| 2001 had a below average NAO and slightly above average PNA. Snowfall geeks wish the PNA were higher, given the fact that most of the mid-Atlantic just missed a historic snowfall, thanks to a storm that developed too far north. | 2001 had a below average NAO and slightly above average PNA. Snowfall geeks wish the PNA were higher, given the fact that most of the mid-Atlantic just missed a historic snowfall, thanks to a storm that developed too far north. |
| March 1993 actually had an above average NAO, but also an above average PNA. The real reason for that year's cold, though, was the Storm of the Century, which helped usher in temperatures far below normal across the eastern US. | March 1993 actually had an above average NAO, but also an above average PNA. The real reason for that year's cold, though, was the Storm of the Century, which helped usher in temperatures far below normal across the eastern US. |
| March 1984, on the other hand, was particularly cold simply because of the powerful average NAO and above average PNA combination. So yes, what's going on right now is unusual, in terms of cold air. It's not, however, anything to write home about, in my opinion. This should happen once every six years. | March 1984, on the other hand, was particularly cold simply because of the powerful average NAO and above average PNA combination. So yes, what's going on right now is unusual, in terms of cold air. It's not, however, anything to write home about, in my opinion. This should happen once every six years. |
| Part of the reason you might think this year is different is because we've been getting lucky, recently. Last year, the average temperature in New York City was 51F. Two years before that it was 48F. All other years for the past five have had averages of about 43F. Here's the problem: for 43F to be the average, there must be higher and lower average temperatures from year to year. If you have only above-average and average temperatures from year to year, the average rise. | Part of the reason you might think this year is different is because we've been getting lucky, recently. Last year, the average temperature in New York City was 51F. Two years before that it was 48F. All other years for the past five have had averages of about 43F. Here's the problem: for 43F to be the average, there must be higher and lower average temperatures from year to year. If you have only above-average and average temperatures from year to year, the average rise. |
| This March, the average temperature of 39F will certainly help keep the long-term average at its previous levels, but it's not anywhere close to balancing out the warmer weather of the past few years. Indeed, the winter as a whole will likely end up being just about average in the northeast. December was well above average, while January was so-so. February's slightly colder than average temperatures cancelled out January's warmth, while March pretty much cancelled out December's warmth. | This March, the average temperature of 39F will certainly help keep the long-term average at its previous levels, but it's not anywhere close to balancing out the warmer weather of the past few years. Indeed, the winter as a whole will likely end up being just about average in the northeast. December was well above average, while January was so-so. February's slightly colder than average temperatures cancelled out January's warmth, while March pretty much cancelled out December's warmth. |
| So sit back and relax; this winter has been about normal. Yes, it may have lasted a little longer, but it should all be over soon. | So sit back and relax; this winter has been about normal. Yes, it may have lasted a little longer, but it should all be over soon. |
| The first week of April will likely get an above normal day or two. The whole blocking pattern we're experiencing should break down by the middle of April, if not perhaps a little sooner. | The first week of April will likely get an above normal day or two. The whole blocking pattern we're experiencing should break down by the middle of April, if not perhaps a little sooner. |
| Just be happy that the PNA is not positive. If it were, we wouldn't just be dealing with cold. We'd be dealing with (more) snow. | Just be happy that the PNA is not positive. If it were, we wouldn't just be dealing with cold. We'd be dealing with (more) snow. |
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