Just like each of us has our own personality, each nor’easter is different, with its own idiosyncrasies and mood. The rapidly intensifying storm taking shape off the Mid-Atlantic and southern New England on Wednesday, for example, is unusually feisty, with a mean streak.
For days now, computer models have locked onto the potential for an unusually widespread display of thundersnow — lightning produced during a snowstorm — due to the strong lift that this storm is going to produce from eastern Pennsylvania and New Jersey northward across parts of New York State, on up into southern New England.
These signs have come in the form of meteorological products that are virtually indecipherable to the typical weather consumer, such as model forecast soundings and projections of how much “convective available potential energy,” or CAPE, will be present at times.
But these indicators have been blinking red, signaling that very heavy precipitation rates, accompanied by lightning and thunder at times, is likely to occur on Wednesday into Wednesday night, potentially in big cities like Philadelphia and New York.
The atmospheric lift associated with these storms is key to the forecast for the New York City area, because surface temperatures at the start of this storm have been above freezing at the coast.
Thunderstorm outlook for March 7, 2018, showing thundersnow potential in Mid-Atlantic and southern New England.
Meteorologists forecasting up to a foot of snow in New York are counting on a process known as dynamical cooling, initiated by intense precipitation, to turn the rain to snow and cause it to pile up quickly. In fact, if some of the model projections come to fruition, the evening commute could be crippled in the New York City area, as well as Philadelphia and parts of Connecticut.
The ingredients for thundersnow are usually different compared to what cooks up a more common summertime storm. For example, summer storms typically form on warm, humid days, and snowstorms obviously lack such heating at ground level.
But what they lack in heat they make up for in lift, specifically narrow layers of rapidly rising air. Such lifting is typically triggered by dynamics associated with rapidly intensifying storm systems like the one undergoing bombogenesis, or a process of rapid intensification, south of Long Island on Wednesday.
It’s this strong lifting, usually in a narrow layer of air below 25,000 feet, that generates thundersnow.
Computer model runs show strong areas of lift, associated with the formation of fronts aloft, that will move across the Mid-Atlantic and Northeast through Wednesday afternoon. Meteorologists on Twitter have been impressed by the thundersnow potential.
Finally, here is a huge warning sign for later. This morning we are seeing significant CAPE values and convective lifting in the mid levels. If this translates this afternoon in precipitation rates, you have 3″+ snowfalls over these regions. pic.twitter.com/EdbhA9KDdw
— NY NJ PA Weather (@nynjpaweather) March 7, 2018
Snowstorms tend to have flat, layered clouds, with most of the precipitation formed in the so-called snow growth layer at relatively low levels of the atmosphere.
For thundersnow to occur, the right ingredients need to come together to lift some of these flatter clouds and mold them into protrusions that some meteorologists call turrets.
Such turrets protrude like a fist above the the broad and flat layers of snow-producing clouds, driven to greater heights by lifting from approaching weather features. These weather systems may be a deep dip in the mid-level jet stream, known as a trough, that causes air to rise ahead of it, or ascend from the formation of frontal zones within storm systems, known by the fancy term “frontogenesis.”
Both of these factors are present in abundance on Wednesday. For example, this chart of mid-level frontogenesis shows a clear signal that this will occur over the Mid-Atlantic and southern New England during this storm.
However, also making Wednesday’s storm unique is the presence of an abundance of extremely cold air aloft, which will make it possible for vertically growing clouds to take on a shape that more resembles summertime thunderstorms than more typical thundersnow events. Such storms could produce more lightning than usual, and, as the Washington Post‘s Capital Weather Gang noted, the possibility of hail falling at the same time as heavy snow.
When the right conditions are present, these turrets can form what’s known as elevated convection (elevated because the cloud is not based near the Earth’s surface). Within these clouds, large snowflakes mix with icy pellets known as graupel, and electrical charges can build up from static electricity as these particles collide in the turbulent atmosphere.
This static electricity buildup eventually can trigger occasional cloud-to-cloud or cloud-to-ground lightning strikes.
Areas of thundersnow are typically associated with extremely heavy snowfall rates because the extra lift in such snowbands enhances the production of snowflakes. This is partly why weather geeks love thundersnow so much, it combines heavy snow with lightning and thunder, a combo of the seasons, if you will.
Interestingly, because thundersnow occurs as a result of elevated convection, the thunder typically sounds different than the rumbles from an average summertime thunderstorm. Thundersnow tends to be somewhat muffled, influenced by its altitude, the cold air the sound waves are traveling through, and the snow on the ground.
Also, most cloud-to-ground lightning discharges during thundersnow events tend to be triggered by human-made structures, like tall skyscrapers like One World Trade in New York, or massive broadcast towers, for example. Such towers can poke into the base of the clouds and cause a buildup of electrical charges.