For those of you who saw my Summer Weather Preview, you already know there's a bunch of factors that I use in a seasonal forecast, and also saw the various limitations associated with an upcoming seasonal forecast compared to that of the upcoming weekend.
I want to split up my preview into a few different sections so that I can take some time to explain the different ingredients that go into the cauldron of a winter weather forecast. On the one hand, it's a more difficult forecast because you have the additional difficulty of figuring out what portion of the precipitation will come as snow, but it's also a little easier because the temperature difference across the northern and southern parts of the country are larger-- which means more true storm formation rather than these spotty pop-up small-scale things we have to deal with all summer long.
Air And Ocean Circulation Patterns
For a seasonal forecast, you have to concentrate on elements of the Earth system that change more slowly than the day-to-day weather; any signal that can rise above the noise. In the winter, these signals are a little more obvious than in the summer, because the influences are greater. There are two main large-scale influencers of the weather that can translate to what we experience locally. These are called 'teleconnections', because we're looking at something that's far away and connecting it to what we're seeing locally.
One is called "ENSO", or El Nino-Southern Oscillation, and the other is called the "NAO", or North Atlantic Oscillation.
The El Nino is the most famous, and the most influential on the Earth. It concentrates on the sea-surface temperature anomaly (departure from normal) in the south Pacific Ocean. The image at left describes a unusually strong El Nino (top Earth) and one from an equally good La Nina (bottom Earth). Once spotted, these ocean temperature patterns can persist for months on end, making their weather influences a handy tool to understand for the purposes of prediction. When the ocean temperatures in this region are warm relative to normal, it is called an "El Nino" phase, while the colder than normal are called "La Nina" phases. It was previously thought that these two influences showed up in fairly regular intervals, but now it's understood to be more complex than that. There are even periods of neutrality, where neither one or the other is dominant for a period of several months.
Studies have been conducted to understand the local influences of El Nino and La Nina phases for the US, using past cases that researchers have identified going back several decades (these are from the Climate Prediction Center at NOAA).
|El Nino - Winter Temperature Anomalies (°C)||La Nina - Winter Temperature Anomalies (°C)|
These demonstrate a little counter-intuitivity, as typically one associates El Nino with a warmer climate and La Nina with a colder one, but yet we see the opposite on average when it comes to tri-state weather (though it should be quickly pointed out that we're right on the edge of zero-net-anomaly in either situation). One of the primary reasons for the deviations are the positioning of the jet stream, and therefore precipitation patterns.
|El Nino - Winter Precipitation Anomalies (mm)||La Nina - Winter Temperature Anomalies (mm)|
El Nino, as far as snow-lovers are concerned, have a silver lining in that it can generate a southern storm track that brings some big nor'easters up the seaboard. It's precisely the opposite with La Nina conditions, both for the southeast and the Ohio Valley. Note that these represent only anomalies, so each year can certainly provide excitement no matter what the ENSO environment (especially when the entire seasonal snowfall comes in a single event or two-- which can happen).
Now comes the question of snowfall. It's not enough in a winter forecast to identify where the most precipitation is going to fall. Now you have to phase temperature and precipitation together. NOAA has identified many El Nino and La Nina years and compared their representative snowfalls compared to average, and identified anomalies that way. Here's what they have come up with:
|El Nino - Winter Snowfall Anomalies (inches)||La Nina - Winter Snowfall Anomalies (inches)|
You can clearly see the influence of the southern storm track in El Nino years, whenever they line up with cold air, generating snowfall. It doesn't take much down there to get a whole season's worth of snow, but that's not the same going up into New England. Speaking of New England, notice how these maps are implying that El Nino and La Nina are both bigger snowfall makers. This is the methodology used to come up with these maps. The research into the true impacts of El Nino and La Nina is still on-going. Naturally, we'd love to have hundreds of different El Nino and La Nina years to analyze, but that's just not the case. Also, not every year is either one or the other (most are 'neutral', and therefore not easily pushed to either side).
The North Atlantic Oscillation, or "NAO" is an atmospheric pattern that occurs over the Atlantic Ocean between Iceland and the Azores. Because this involves the air and not the ocean, this is a much more temporary pattern, and as such is more difficult to pin down months in advance. However, this particular pattern is an even stronger contributor to winter weather patterns from the Appalachians eastward than the ENSO pattern. It has two 'modes', a positive and negative mode. The negative is the one you want for snowfall. It all comes down to whether or not the jet stream will be forced to have higher amplitudes, and the frequency of 'blocking' high pressure systems notorious for keeping the big storms going over the same area.
In terms of snowfall anomaly, the positive phase correlates to lower snowfalls, while the negative phase correlates to increased snowfalls. Basically, it's the NAO that will edge the ENSO to its snowier or less snowier components. That's why purely for purposes of snowfall the two must be taken in tandem. For example, in order for the Mid-Atlantic (east of the mountains) to have a good snow season during a La Nina episode, there must be a negative NAO in place. Other regions can more or less try to forge possible above-normal snowfalls in all ENSO scenarios, but the NAO is always a dominant contributor when it comes to snowfall (it contributes the cold and the blocking).
So Where Do We Stand..?
Now that we've seen the two main teleconnections to our local winter experience, it's time to take a look at where they are, and to the best of our knowledge where they may be going (where applicable, click for a larger image).
ENSO - Observed And Forecast
|NAO - Observed And Forecast|
As a case in point, at this point last year, we had a neutral NAO and a modest La Nina (and coming off a mountain snow event in the first week of October). Things were looking good. Then, though the La Nina held, the NAO went resoundingly positive (+2 by mid-December). It stayed that way through January, and we hardly saw any snow until one tiny blip at the end of the season-- which was pretty crappy for snow overall. This year we're starting out in the weak El Nino territory with most trajectories going downward. If all the models are taken into account, one might as well call it a marginally neutral ENSO forecast. Now, the NAO has been negative, and decently so (El-Nino / Neg-NAO is pretty good around here for snow, if you haven't noticed so far in the 2012-2013 season). However, notice the much shorter time frame on the model. Unfortunately, the NAO has not been as predictable beyond several weeks, though recent studies have implied a rough cycle. We're into a pretty good dip though, so I doubt that's going to hold much longer (especially considering the up-and-down pattern that preceded it).
The National Weather Service took a few looks at this pattern and pretty much bowed out, declaring "equal chances of anything" for this upcoming winter. We're going to try to do better-- or at least it's entertaining to try.
The next thing we're going to look at is the most likely storm tracks for the 2012-2013 season. I'm talking about the actual rain/snow bringers, not the piddly things. Until then...
P.S. If you have any questions about this topic, post them below. If you want to know specifically what this will translate to in terms of the Winter 2012-2013 snowfall forecast, you will have to be patient, because it's coming :-)
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