Another nor'easter set to cripple travel, trigger thundersnow

UPDATE: March 7, 2018, 9:22 p.m. EST The storm has ended in the immediate New York City area, leaving behind a wide range of snow totals, from about 2 feet to the west of the city, to anywhere between 4 and 10 inches in the city itself. Many communities in New Jersey, New York, and Connecticut are without power due to the heavy wet snow snapping trees and bringing down power lines. The storm continues to intensify as it moves northeast into southern New England. Heavy snow, including thundersnow, is falling in Connecticut and moving into the rest of New England. More power outages, thundersnow, and snarled travel is all likely overnight and into Thursday.

UPDATE: March 7, 2018, 12:49 p.m. EST Heavy snow is falling from Philadelphia to New York City, and is starting to move into southern New England as well. Thundersnow has already occurred in New York City, with more likely to come in many areas as the storm rapidly intensifies. The National Weather Service has increased the snowfall forecast for New York City to about 1 foot, and amounts have been raised for the Boston area too, as the storm track becomes more clear.

A major developing storm is taking shape along the Mid-Atlantic coast, and it's proving to be quite the illusionist. Residents of New York City woke up to a mixture of light rain and snow, and questioned whether the storm forecast was a "bust."

But computer model guidance has been consistent in showing extremely heavy snowfall moving into the Philadelphia metro area, northern New Jersey, New York City, and then up into Connecticut and Massachusetts beginning Wednesday afternoon. This will occur as the storm "bombs out," or undergoes the process of rapid intensification known as bombogenesis.

In other words, the storm's effects as of Wednesday morning are not WYSIWYG (what you see is what you get), to use an old school internet term. Instead, it's trying to lure folks into a false sense of complacency, draw them into work or school, and then make the evening commute impossible.

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Model projections, for example, show an unusually strong signal for thundersnow across the Mid-Atlantic and Northeast associated with this storm. Thundersnow is just what it sounds like, a thunderstorm occurring while it is snowing, rather than raining. The exact characteristics of thundersnow events are different from summertime thunderstorms, particularly in the location of unstable air within the atmosphere.

Thundersnow tends to occur when layers of unstable air form in the mid-levels of the atmosphere, several thousand feet up, whereas summertime storms involve surface-based instability. This is why you can see the dark clouds rolling in during summer storms, whereas thundersnow seems to come out of nowhere -- it's taking place higher in the atmosphere.

In meteorologist-speak, this storm has strong dynamics associated with it. In normal person vernacular, that means this storm means business, and will bring surprisingly heavy rates of snowfall once it gets going in earnest by early to mid-afternoon in the big cities.

The snowfall forecast has been a major challenge, particularly in New York and Boston, where relatively mild air from the ocean could keep snow from sticking or turn precipitation over to rain for a time. However, dynamical cooling from strong lifting of the air aloft and heavy precipitation dragging down cooler air from above should overcome milder temperatures in most areas. That happened in Washington, D.C. on Tuesday night, when rain suddenly switched over to big, fat flakes of snow and stinging sleet accompanied by thunder.

Consider, though, the snowfall forecast for New York, where media outlets and the Weather Service are expecting a big difference in accumulations between JFK International Airport in Queens, which is right on the ocean, and Central Park. The Weather Channel, for example, has been predicting 12 to 18 inches will fall in Central Park, while just about 6 inches will fall at JFK, which is located only about 15 miles.

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The fact that forecasters can now zero in on transition zones with such precision is an example of how far meteorology, aided by powerful computing technology, has come in recent years. However, poor communication of large forecast ranges has led to some media headlines that ruffled feathers in the meteorology community, since large ranges in forecast accumulations is not the same as saying "we just don't know."

The storm presents a major communications challenge for weather forecasters to make clear that the evening commute in major cities could be downright impossible due to heavy snowfall rates. During past storms, New York City has ground to a sudden halt once snowfall rates reached 2 to 3 inches per hour for even just a couple of hours, putting lives in danger and complicating commutes of all kinds.

Similar events in Washington, D.C. have proven that storms involving a benign morning commute followed by a harrowing evening commute can result in stuck cars and commuters overnight.

New York City's public schools remained open on Wednesday, for example, suggesting that the authorities may have underestimated the storm's potential to cause havoc.

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This storm is striking New England right on the heels of an even stronger and slower-moving storm that battered the region beginning on March 1.

Coastal flooding from that storm caused serious damage in eastern Massachusetts, including breaching seawalls, weakening coastal homes, and flooding numerous homes and businesses. While this storm won't be as bad, coastal flood warnings are in effect for eastern Massachusetts, as strong onshore winds will ensure that a remarkably prolonged period of flooding continues.

In addition, thousands of people still don't have their power back after the previous storm, either, and this event will bring even more wet snow that could take out trees and power lines. This could extend power outages -- particularly in inland areas that pick up the most snow -- from days to more than a week.

This is a developing storm and story. Check back here throughout the day for updates.