Combine a very energetic jet stream disturbance with warm, Gulf Stream water, a deep sub-freezing air layer, and tropical moisture, and what you get is an East Coast snowstorm for the record books. Over the coming hours, these ingredients will conspire to generate a textbook snow-maker.
The storm is called a nor’easter, or coastal low — a wintertime breed of mid-latitude cyclone powered by contrasting air temperatures. The Arctic air mass that recently invaded the mid-Atlantic, combined with an unseasonably warm Gulf Stream just offshore, will provide the necessary temperature contrast.
Shown below is the GFS model simulation of surface pressure and winds, valid at 7 a.m. Saturday morning. The storm center is depicted by a red L. Note the ridge of cold high pressure — a blue H — to its north.
In addition to funneling cold air south into the storm, the ridge acts to enhance the pressure gradient (solid black lines). A pocket of 55-60 mph northeasterly winds (magenta) sits just offshore (with gusts to near hurricane-force, 75 mph) – driven by the tight pressure gradient.
The central pressure at 7 a.m. Saturday is forecast to be 992 mb. The storm looks like it will be intensifying rapidly at this point, while exiting to the northeast. It will have formed over Cape Hatteras during the previous 12 hours – essentially jumping across the Appalachians from a position in the Tennessee Valley.
This type of pattern is called a “secondary low” or “Miller Type-B” pattern – a classic, textbook scenario for the biggest snowstorms.
A deep dive in the jet stream
While the temperature gradient provides energy, the upward motion needed to trigger and sustain intense low pressure at the surface comes from the jet stream’s undulating winds.
Shown is a deep, closed low over Appalachia, and a classic “S-shaped” configuration to the jet axis. Air rises vigorously as it streams through the “S,” where the curve changes from counter-clockwise to clockwise. A sort of turbo-boost is facilitated by two interacting, fast pockets of wind called jet streaks. One is labeled 120 knots, the other 160 knots.
The zone of juxtaposition (white ovals) is where these jet streaks are coupled – directly over the “X” which marks the surface position of the low. Here, air rises exceptionally fast – another hallmark, dynamic setup for the classic snow-maker.
Like a yin and yang, the figure below shows the storm’s thermal field at 5,000 feet (850 mb) – the level in the clouds where snow is manufactured. On the back side of the coastal low, a deep subfreezing air layer (temps down to -7 C [19 F] over Washington, D.C.) ensure the refrigeration for efficient snow production. The freezing line sits close to the Bay, where heavy, wet snow and sleet are expected to mix in on Saturday.
Tropical conveyor belt
Finally, snow requires water vapor, and the figure below indicates a veritable pipeline of Caribbean-sourced moisture entering the nor’easter from the south. Magenta colors indicate total precipitable water in excess of two inches. This is an integrated measure of water vapor mass through the troposphere. It’s as if the storm is greedily sipping from a nearly infinite reservoir of high humidity air!
These four key ingredients will conspire to create an epic storm, one that has shown remarkable congruence in the model predictions for several days. There are important details we have not discussed, including the possibility of one or more heavy snow bands that will likely set up, and really amp up accumulation in localized areas. We’ll cover these and other interesting facets in a follow-on story.