Jack Frost Strikes Back

Forty-nine inches of snow sit on the frozen tundra of West Virginia. Baltimore and its suburbs are snowed in beneath a record snowfall. More than 2,000 flights have been canceled. Several abominable snowman sightings have been reported in central New Jersey. In short, this weekend’s massive snowfall has caused problems for pretty much everyone along the Eastern seaboard.

The Washington Post called it “epic” and “a revelation of unending snow globe enchantment,” which merely hints at the long-lasting effects of the storm which hit Williamstown on Monday. Ultra high-speed winds, freezing rain and more snow than you can shake a stick at kept most common citizens housebound during a frigid, frosty Winter Carnival and holiday weekend.

More than 20 storm-related deaths have been reported since Friday, when the winter storm that engulfed the region over the weekend began. Its consequences continue to resonate – the District of Columbia will be shut down for several days, while travel has been rendered all but impossible in most corridors. Even Williamstown, which avoided the brunt of the accumulation over its Winter Carnival weekend, was struck today, forcing the College to send home all non-essential workers due to the snow.

Complicating the matter is the fact that the impact of the storm on business was dulled by the Presidents Day holiday; many workers unable to leave their homes were not scheduled to work in the first place. With major metropolitan areas attempting to return to normal today, much of the most devastating havoc might not yet have been wreaked.

A winter that many in the meteorological community believed would be tame for much of the country has turned out to be anything but. In December, the National Weather Service released their winter outlook, which called for above normal temperatures over the Northeast and northern tier states for the duration of the winter, with near normal precipitation over much of the United States, except for the southeast, where they predicted more precipitation than normal.

Instead, this winter has brought significantly colder-than-normal temperatures to much of the country east of the Rockies (ironically, the Northeast especially), and above-normal snowfall to much of the Southeast, especially the Carolinas and Georgia, all of the mid-Atlantic states and much of the Northeast. Indeed, some places like Washington, D.C., Philadelphia and New York City have seen anywhere from 100-150 percent more snow than normal. Can we blame this on El Niño? The answer is an emphatic yes and no.

El Niño-Southern Oscillation, or ENSO for short, refers to the coherent, large-scale fluctuation of ocean temperatures, rainfall, atmospheric circulation, vertical motion and air pressure across the tropical Pacific. Over the past four months, we have seen marginal to at-times moderate El Niño conditions. That means that over the past several months meteorologists have noted abnormally warm sea surface temperatures over much of the eastern tropical Pacific.

“Typical” El Niño events tend to exhibit an eastward extension of the jet stream to the extreme eastern Pacific. In other words, the southern “branch” of the jet stream over the United States tends to be more active than usual, transporting both Pacific and Gulf moisture to the south and southeastern states, and occasionally up the eastern seaboard via coastal storms known as Nor’easters.

This has indeed been the case over the past two to three months, as the southern branch of the jet stream has been particularly energetic, bringing more rain than normal to the southeast, and leading to more frequent – numerous, in fact – coastal snowstorms developing off the eastern seaboard. So we can say for a fact that above normal precipitation over the eastern United States has, in fact, been a direct result of El Niño conditions and a more active (moist) southern jet stream, but what about our below-normal temperatures?

The problem with blaming our cold weather this winter on ENSO is that typically El Niño actually leads to warmer-than-normal temperatures over the northern tier states and the Northeast. Indeed, government meteorologists and climatologists were likely primarily basing their warmer-than-average winter temperature prediction in the Northeast on past El Niño events that have brought mild winters to the region.

However, the winter so far has featured numerous, persistent bitter-cold Arctic air outbreaks; in fact, average temperatures over the past three months in Albany, N.Y. – the closest official reporting station – have averaged close to four degrees colder than normal. That might not sound like a lot, but to climatologists, this represents significant, abnormal cold, and it demonstrates the sustained colder-than-normal temperatures we’ve seen in the Berkshires and much of the northeast over the past three months. So then if not El Niño, then what?

Other seasonal modes of climate variability that impact temperatures over much of the United States – the Northeast, especially, however – include the Arctic and North Atlantic Oscillations (NAO). According to Dr. Joe D’Aleo, chief meteorologist at Weather Services International, “the weather patterns in the far North Atlantic play a key role in the winter temperatures and precipitaiton in the eastern United States.”

The NAO has two phases – positive and negative. A negative NAO is climatologically conducive to increased cold and storminess over the eastern United States. Somewhat related to the NAO is the Pacific/North American Oscillation (or PNA). The positive phase of this oscillation tends to feature a ridge of high pressure over the western United States and a strong trough of low pressure over the eastern United States; in turn, Arctic air from Canada (and even Siberia) is allowed to sink southward into the Northeast.

Indeed, this winter so far has featured an exceptionally strong and persistent negative NAO and positive PNA. (In fact, at one point in the middle of January, when Williamstown experienced its coldest weather of the winter, the NAO value was the most negative that had ever been recorded!) The point here is that some climate signals tend to override other signals. In this case, it follows that a very strong NAO and PNA pattern has greatly overshadowed the effects of an only marginal to moderately-strong El Niño.

This week’s “Blizzard of 2003” perfectly demonstrates the atmospheric dynamics that have been at play during most of the winter thus far. An active southern branch of the jet stream provided significant Gulf and Atlantic moisture and energy for a disturbance (area of low pressure) to develop over the southeast and redevelop off the Virginia coast as a Nor’easter. However, a very strong dome of Arctic high pressure over the northeast – courtesy of the aformentioned persistent trough over the eastern United States – supplied plenty of cold air up and down the east coast and also slowed down the storm considerably (since storms – low pressure – slow down when they encounter such strong high pressure).

In effect, El Niño provided the moisture and energy for the storm, and the NAO and PNA oscillations provided cold air to support snow as well as acting to slow down the snowstorm. The result was a snowstorm of historic proportions that dumped 15-30″ of snow over most of the mid-Atlantic and northeast states; the most snow in more than 50 years in some locations. Washington, D.C., for example, received more than their entire winter average snowfall of 14.7″ in a 36 hour period. And Williamstown, too, received significant snowfall, as well as blizzard-like conditions of gusty winds and blowing and drifting snow. This weather prognosticator believes there should be a foot or more of snow by the time you read this.

It is the middle of February and Williamstown has now surpassed its average seasonal snowfall of 70″. Indeed, more snow will surely fall over the rest of the winter, though there are some indications that the bitter cold may not be quite as persistent over late February as it has been over most of the past six weeks. A cold, snowy winter it has been; a mild, tame “El Niño winter” it has not.

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