Ask Weather Dan: Pulling out the crystal ball (Part 2)

Richmond had a crazy winter and a crazy summer. Here Weather Dan looks into some serious data and lets us know if we can expect the same weather drama in the not-too-distant future.

October 8, 2010; 11:30 AM • by Weather Dan

In our last installment, I talked about NOAA’s Climate Prediction Center and their forecast for the upcoming months.

I also mentioned that there were some other atmospheric patterns that influence seasonal weather. I called them oscillations before – which is true, because they tend to oscillate between two extremes – but a better name for them would be “teleconnections.”

The important thing to take away here is that it’s not just the fluctuation of conditions at a certain location that is significant; it’s how the changes in certain parameters at certain locations affect global weather patterns.

Sound familiar? It might – you’re probably already familiar with a cycle known as El Niño.

It’s full name is the El Niño – Southern Oscillation (ENSO). It’s focus is two components: the sea surface temperatures of the equatorial region of the Pacific Ocean (the El Niño portion), and the mean sea level air pressure at two stations: Tahiti, in French Polynesia, and Darwin, Australia (the Southern Oscillation portion). Specifically, the NOAA/NWS Climate Prediction Center focuses on a region known as the 3.4 region, which is an area bordered by the 5-degree north and south parallels, and between 120 and 170 degrees west longitude.

Every four to five years, the ocean temperatures in the Pacific cycle between warm (El Niño) and cool (La Niña) phases. Fluctuations in ocean temperatures and air pressure over the equatorial Pacific has impacts on global weather patterns; areas that see high rainfall in El Niño years may see very little in La Niña years.

Here’s what the situation looks like currently:

The top image shows current sea surface temperatures (SSTs) for the Pacific, while the bottom image shows SST anomalies, or the difference between the actual temperature and the average temperature for that location.

That long arm of cool water stretching west from South America is the indication that we’ve been entering a La Niña period; in fact, El Niño dissipated in May this year, and we’ve been transitioning into a strong La Niña period over the summer.

Typically in a winter La Niña pattern, Virginia sees conditions that are warmer than normal, and winter storms like the ones we saw last winter (during a strong El Niño) can be deflected away from the Mid-Atlantic, resulting in drier than normal conditions. It isn’t always the case, however. Climatologically speaking, there’s not a strong wet or dry trend when it comes to Richmond winters during La Niña. In fact, the during years which experience a strong La Niña, precipitation is still slightly higher than average.

But ENSO isn’t the only influence on Richmond’s winter weather. The North Atlantic Oscillation (NAO) is a measure of the difference in the intensity between an area of high pressure permanently located in the Atlantic Ocean west of the Azores and an area of low pressure permanently located over Greenland. The NAO is measured in terms of negative and positive phases; the positive phase features a strong dome of high pressure and a strong, deep area of low pressure, while the negative phase indicates weak areas of high and low pressure.

Last winter featured a particularly negative NAO phase, which normally brings us more cold air and thus more snow events. Unlke ENSO, which fluctuates over months or years, the NAO fluctuates more often and is slightly more sensitive. However, for the latter half of September, the NAO has remained close to neutral.

Last winter, the NAO was strongly negative (at or about a value of -2) for most of meteorological winter (the months of December – February). We know that when the NAO is in a negative phase, it leads to increased snowfall across the southern and eastern parts of the US, and when coupled with strong El Niño from last winter, we definitely felt it.

As for what’s to come this year, it’s hard for me to say. The NAO fluctuates on a much larger scale than ENSO, and thus it is harder to create a useful forecast of NAO values more than about two weeks out. If the trend of a neutral NAO continues into the winter, then the La Niña will likely dominate the weather pattern, and we’ll see warm, slightly dry weather. However, I doubt we’ll see a neutral NAO through the entire winter; pattern shifts will occur as the NAO fluctuates. The NAO has remained slightly negative through most of the summer, and I see no reason for this pattern to be disrupted.

While the ENSO and the NAO are the two predominant players in Virginia’s winter weather, there is still a third: the Arctic Oscillation (AO). The AO is similar to the NAO;however, rather than measuring the difference between static areas of high and low pressure, the AO is a measure of sea level pressure variations north of 20 degrees north latitude – or, in the Arctic.

The AO features a similar positive-negative rating just like the NAO, and just like the NAO, was quite negative through last winter. A strongly negative phase of the AO brings the polar jet stream farther south, allowing cold arctic air to intrude into the Mid-Atlantic and southeastern US. In the positive phase, the polar jet stays farther north, and wintertime temperatures stay more moderate. As with the NAO, the AO fluctuates over a short term, and can’t be reliably forecasted out more than 14 days.

Let’s take a look at what’s been happening lately:

Again, like the NAO, the AO has been trending between neutral and slightly negative. (Sensing a pattern yet?)

What does all this mean? And more importantly, what will this winter be like?

This is my first shot at a seasonal forecast, and admittedly, I’m not necessarily very confident in it.

That being said, here’s what I think. I don’t believe the NAO and AO will stay neutral through the winter; the cold air brought in by the pressure fluctuations will balance some of the warmth seen by the strengthening La Niña and the general pattern of climate warming seen over the last couple decades, and ultimately, temperatures for the season will wind up slightly above average. Snowfall will also be near or slightly above average. Richmond’s annual average snowfall is 11.9”, and the average high and low temperatures are 48.1 and 29.5, respectively. I don’t think we’ll see a repeat of last winter, where we saw 28” of snow officially.

If only it were as easy as pulling out a crystal ball…

October 8, 2010 11:30 AM 2