Currently showing: Climate/natural disasters > Floods/storms

27 Jan 15 15:15

Starting in the evening of Saturday, January 24th,output from weather forecast models started to show a significant snow event for much of the Northeast, including the cities of New York, Hartford,Providence and Boston. Snowfall totals were forecasted in terms of feet, not inches, and the media seized on the idea that this could be a "historic and unprecedented" storm. On Sunday, January 25th, blizzard warnings were issued for much of the Northeast, including the 5 boroughs of New York City and Boston. By Monday, January26th, travel bans were put in place in 13 counties in New York, New Jersey, Connecticut and Massachusetts, flights were canceled, airports were shut down, both Amtrak and commuter railroads suspended their service, and the city that never sleeps saw its 24 hour subway system grind to a halt.

…And then on the morning of Tuesday, January 27th, many, mainly New Yorkers, awoke to the news that the snow totals could be measured with a ruler and not a yardstick, and social media came alive with the question, "Did the forecast bust?"

As someone who grew up in New Jersey, experiencing my fair share of nor'easters, my answer to the "busted forecast" question is,"Not entirely." For starters, it takes a certain set of conditions for a nor'easter to form. Nor'easters develop in the late autumn, winter and early spring months, deriving their energy from the temperature difference between the cold continent and the warm Atlantic Ocean, and the jet stream, the river of air racing around the midlatitudes at 30,000 feet. They are born off coastal Carolina, move to the northeast, and, as the name implies, drive strong northeast winds into the mid-Atlantic and New England coasts. Although nor'easters are coastal storms, they are not hurricanes; they are dynamically different, and also are far more frequent. It is not uncommon to see 20 or more nor'easters off the coast in any year.

Blockbuster nor'easters almost all feature the same atmospheric and oceanic conditions: A warm Gulf Stream flowing off the North Carolina coast, a strong jet stream aloft, with the strongest winds situated to the southwest, a negatively tilted trough, meaning that the "dip," in the atmospheric circulation is tilted from northwest to southeast, and a high pressure system to the north, near the Canadian Maritimes and Nova Scotia (it's worth noting that the atmospheric set-up in the midlatitudes was conducive for a nor'easter during Hurricane Sandy, thus explaining the unique characteristics and track of the storm at the time of landfall in New Jersey).

Because the set of conditions which turn a pesky coastal storm into the "Blizzard of the Century," are highly specific, forecasting nor'easters is not the easiest science. For starters, any slight change in any of the aforementioned conditions can dramatically change the timing of nor'easter development, or the rate of storm intensification. Additionally, any shift in the position of the jet stream or the high pressure system over Canada can lead to a shift in the final track of the nor'easter. A westward shift in track will change a paralyzing snowstorm into a soaking but manageable rainstorm, while an eastward shift in track can change snowfall totals dramatically. Finally, because nor'easters are rapidly developing weather systems, and are intensifying as they are affecting the coast, many forecasts are made and decisions are taken before the storm actually forms.

With these considerations in mind, what happened in the case of the January 2015 storm? Initial forecasts, from both the American and European model, showed the storm taking that ideal track up the East Coast for a large snowstorm in New York and Boston. However, the origin of the storm was an Alberta Clipper moving in from the Midwest, not a low pressure system in the Gulf of Mexico. The northern origin of the storm meant that intensification would not truly begin until it approached Long Island. So, even though the initial track forecast led to high snow totals, the origin of the storm over land (a dry system) and the northern track, meant that the New York metro area would be on the southwestern flank of the snow, and that across a very narrow region, the forecasted snowfall totals varied tremendously. Given the atmospheric conditions for this particular event, the forecast already contained uncertainty.

The tight gradient in snowfall forecasts meant that a slight shift to the east in track, or a slight delay in the timing of the intensification, would lead to much lower snowfall totals in New York City. As Monday went on, the American forecast model started pulling the storm east, showing smaller snowfall totals in the NYC metro area. The European model stayed on track, showing large snowfall totals over New York City. Given that both forecast models are reliable and that the European model pegged Hurricane Sandy 8 days in advance, many forecasters split the difference, and erred on the bullish side of moderate to high snowfall totals. New York and New Jersey responded accordingly, putting plans in place to prepare for snowfall removal and emergency response.

By Tuesday morning, it was clear: The American model had more accurately forecasted this particular nor'easter. Furthermore, the storm did not intensify as rapidly as originally anticipated. Snowfall totals in New York were not 2 feet.

However, the world does not revolve around New York City (although in my mind, it sometimes does). Looking at this particular storm from a regional perspective, the event was forecasted very well. Blizzard conditions and several feet of snow were forecasted in Long Island, Massachusetts, and Rhode Island, and New England truly bore the brunt of this storm, as forecasted. Furthermore, the NYC metro area still received snow; enforcing a travel ban and suspending mass transit allowed the roads to get cleared, tracks to get de-iced and for plows to get their jobs done without accidents and motorists littering the road. Unfortunately, nor'easters don't do what we want and don't necessarily adhere to the forecast that has been generated.

The Blizzard of 2015 is an important reminder as to how sensitive snowfall totals are to the final track and timing of intensification; a shift of 30 nautical miles, a mere small fraction of the Earth's surface, can produce dramatically different snowfall totals locally than initially anticipated.

(Image: Looking south along Avenue A from East 14th St. in Manhattan on January 27, 2015)

Category: Climate/natural disasters: Floods/storms

Location: New York, NY, United States


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