The Arctic Oscillation and Winter 09-10: A Piece of the Puzzle

This latest entry is going to focus on the past winter (since we're now in meteorological spring!) and one of the main drivers in what made this one so memorable in much of the East and South.

If you'll recall a couple weeks back, I posted a lengthy blog trying to debunk people using this winter to prove or disprove the theory of anthropogenic global warming. I mentioned in that entry that one of the biggest issues about people trying to use this year as proof of anything was that this was the first time we've had both an El Nino and the Arctic Oscillation at such sustained strong levels since at least the 1960s, and even that's a loose match.


What the Heck Is It?

Here's a quick primer on the Arctic Oscillation (AO). The AO is basically a gauge of blocking in the upper levels of the atmosphere over, not surprisingly, the Arctic. Blocking is pretty much what it sounds like. The overall weather pattern in a blocking pattern becomes "blocked up," meaning certain aspects sustain themselves for days, if not weeks at a time. When the Arctic Oscillation is in its negative phase, a large area of high pressure in the upper atmosphere basically sits over the Arctic. Underneath the high, temperatures get warmer than normal and the cold air is forced to be displaced. Traditionally, it's shunted southward, right into the Central and Eastern US in North America and straight into the heart of Europe and parts of Asia. You're basically forcing the coldest air, relative to normal, to leave the Arctic and it oozes south. The further south you live in the US, generally, the colder, relative to normal, it is.

Here's a visual primer on the AO. The top part is the positive phase of the AO. The bottom part is more representative of this winter and the negative phase of the AO.

So How Odd Was This Winter?

If you click the chart above, you'll see a running graph of monthly Arctic Oscillation values since 1990. Notice how things tank at the end of the chart for this winter. So we're automatically at the most sustained negative levels of the AO that we've seen anytime in the last 20 years.

People may argue, "Well, look at the AO and how it's tanking. That's not normal. That's clearly an indication that something is changing in the Arctic." Well, the picture's a little different when you look at the AO since 1950. Click the image below to enlarge.

Notice that since 1950, we've had several instances where the AO has tanked. For my purposes, my definition of "tanked," will be when the average AO monthly value ends up under -3. It happened in the winters of 1962-63, 1965-66, 1968-69, 1976-77 and 1977-78. We came close in 1984-85 and 1985-86. Unfortunately, data only exists back to 1950 and we can't see any serious long-term trends.

But it is interesting how there was a build up in the 1950s and by the mid-1960s, the AO tanked and held low for 5-8 years, followed by a break from ~ 1971-1976. We tanked again in 76-77 and stayed generally low until 1986. We started seeing more negatives in the AO around 1995-96 and continuing periodically through this year, when it tanked. I'm wondering if we aren't entering a period of 4-8 winters where we're going to see the AO stay at a very negative level, helping to supply more cold than we've been accustomed to seeing lately.

At any rate, with the AO as negative as it was this year, we pretty much had (with a few exceptions) an almost continuous supply of cold air draining out of the Arctic and helping to supply sustained cold weather for the Plains and Midwest (contrary to typical El Nino conditions). It also helped supply enough cold in the East (especially the further south Mid-Atlantic, Southeast and Southern Plains...areas that normally would have been borderline for rain/snow) to keep many storms this winter as all snow. The plentiful amounts of moisture were primarily courtesy of El Nino. The mechanism to help keep it snow, was likely due largely in part to the pattern over the Arctic.

Remember, the AO is just one piece of a much, much larger puzzle. But the main point is, while it's unusual for us to have seen the AO this negative in recent years, it does fall within the realm of reason of the last 60 years.

A Spring Outlook

Well, I took a crack at winter this year a little more rigorously than in year's past, and I'll go over the results of that forecast sometime in the next week or two. Disappointed on the whole regarding the specific forecast, but I took quite a bit from this winter and following it on a larger scale day to day that will help me in the long run I think.

So with some of that in mind, I decided I would take a shot at Spring. Not an easy outlook, but I did find a few things to look at. We still have an El Nino in the Pacific, but it has begun the typical seasonal decline we typically see in late winter/early spring. Whether it completely fades away or not is another question entirely (mainly a question for summer anyway). El Ninos are difficult to try and make predictions for. But the computer model projections shown above indicate that at the very least a weak El Nino should be in place through all of spring.

Additionally, I did some research on some of the extreme atmospheric blocking we've seen this winter and if it correlates to anything in Spring, and there seems to be some agreement for blocking to at least maintain itself through mid-April in some fashion, albeit weaker. There were a handful of years where blocking really went to the opposite side of the scale starting as early as the first week in March. We are heading in that direction in the short term (not entirely going to the other side of the scale, but weakening considerably.

I looked at a number of other things as well, mainly experimenting to see if I find a useful method. I basically found the years where a number of trends were similar and bunched those years together, filtered out any that were completely opposite in the Pacific (La Nina instead of El Nino for instance) and used those as base analogs. I then looked at some things exclusive to the East and some things mainly dominant in the West and put some extra weight on those years for those regions and blended my analog ideas together for a national map.

So with that said, below you'll find my March, April and May temperature outlooks (click them to enlarge).

Additionally, I'm thinking wetter than normal conditions will persist in the South initially, though the focus may shift into the Southern Plains and Central Mississippi Valley this month, with continued generally wet conditions on the West Coast, Southwest and in the Mid-Atlantic/South Florida. There was a signal for a considerably drier than normal Gulf Coast as well, but I'm not entirely sure I believe it. We'll see. There was also a signal for a drier than average Ohio Valley and Northeast as well. Again, we'll see.

In terms of severe weather, climatology would favor active severe weather in the Southeast in a spring like this. In terms of tornadoes specifically, here are some findings based on the years I'm looking at:

- Well below the rolling three year average for March and April (138/194 tornadoes)
- And near to slightly below the rolling three year average for May (305 tornadoes), with a slight risk of above.
- Late April to Late May being the most active periods. The first period of activity looks to be around March 10th perhaps, with the models indicating a potentially potent Mid-Continent storm.
- Perhaps a greater than usual risk for significant tornadoes in the Southeast

That's my call and we'll see how it goes!

Mid-Week Storm, Spring Outlook Preview

Well it looks like we're going to get a good storm this week, but the problem is that it's probably going to be a miss for most areas in the Northeast. The best chance for any snow with this storm is going to be from Atlanta north into the mountains of North Carolina, as well as far Eastern North Carolina and Southeast Virginia. There's a secondary chance that snow will be able to extend onto the Jersey and Delaware shores, as well as potentially Eastern Long Island and Cape Cod late Tuesday night into Wednesday morning...later up north. A weak disturbance will provide the interior with some scattered snow/rain showers on Wednesday.

There are a couple things holding this back. The pattern overall is too progressive...it's moving forward too much. You're not able to phase together the disturbances shifting East. The storm from last week is just now starting to exit, which is a bit too late to get the players on the field lined up properly to score a storm further inland. And the large ridge of high pressure in the upper atmosphere out West is centered over Wyoming and Montana. For a good coastal storm, that needs to be centered closer to Boise, ID. It's becoming too late in the game for this to really change.

So my overall feeling for plowable snow is:

North and West of I-95: Limited to No Risk
Jersey/Delaware Shore: Low Risk
Eastern Long Island: Low Risk
Cape Cod: Low to Medium Risk
Southeast VA/Eastern NC: Low to medium risk
Central/Southern NC through ATL: Medium risk

The biggest problem in the Southeast is that it's now March and you have warmer surface temperatures and a stronger sun than you had even 2-3 weeks ago. It can snow and it can accumulate in the South in March, but I'm not sure this is going to be the best setup for it.

I'll post an update on the evening data when it rolls in.

In other news...

Tomorrow, I'll post my Spring Outlook with a brief overview of what I think we can expect for March, April and May. Just a preview, I don't think winter is entirely over yet, but we're certainly running out of time. The big question is whether parts of the East can break the funk of the last couple April/Mays. I'll try and answer that for you tomorrow.