Cold air from Canada and warm, wet air from the ocean crash together. They start to spin like water going down a drain. 🌀
The spin goes counterclockwise, the opposite way from a clock's hands.
Along the coast, the wind blows in from the northeast. That's how the storm got its name: a nor'easter!
The warm ocean air carries lots of water. When it gets pushed up over the cold air, the water falls out. On the cold side it's snow ❄️. On the warm side it's rain 🌧️.
The storm's pressure number (mb) tells you how strong it is. Lower means stronger. If it drops 24 in one day, scientists call it a bomb cyclone 💣.
Weather forecasters watch a spot in the ocean called the benchmark. When a storm passes right there, big cities like New York and Boston usually get lots of snow.
Low pressure + Coriolis. Air flows toward the low, but Earth's rotation deflects moving air to the right in the Northern Hemisphere, so it circles the low counterclockwise. Surface friction turns it about 20–30° inward across the isobars, which is the inflow you see spiraling in.
Why "nor'easter." A cold Canadian high (H) sits north of the storm. Squeezed between the high and the low, the pressure gradient drives strong winds from the northeast along the coast. Tap the map to measure them.
Cyclogenesis on a baroclinic zone. The storm feeds on the temperature contrast between Arctic air over land and the Gulf Stream. Right now that's 33°C (59°F). Bigger contrast = more energy to release.
Moisture. Warm air holds more water: about 7% more per °C (Clausius–Clapeyron). Air over a 23°C Gulf Stream can hold 17.6 g of water per kg. Condensing it releases latent heat, which deepens the low even faster.
Central pressure vs. time (this run). Red band = steepest 24 h fall.
Lowest pressure–
Steepest 24 h fall–
Bomb rule (Sanders & Gyakum, 60°N)24 mb / 24 h
Same rule scaled to latitude 40°N–
Verdict–
Bombogenesis means the central pressure falls at least 24 mb in 24 hours (Sanders & Gyakum, 1980). The 24 mb rule is set for 60°N; because the Coriolis parameter is smaller farther south, the fair threshold scales with sin(latitude), so at 40°N it is about 18 mb.
The benchmark (40°N, 70°W). Forecasters in the Northeast watch whether a low passes near this point southeast of Nantucket. There, New York to Boston usually sits in the cold northwest quadrant under the comma head, where moist air is lifted over cold air in the deformation zone and makes the heaviest snow bands. Closer to the coast, warm ocean air floods in and it rains. Farther out, the snow stays at sea.
Isobars are drawn every 4 mb. Where they crowd together the pressure gradient force is large and the wind is strong.
Model: Holland (1980) gradient-wind vortex (a smoothed Rankine vortex) plus a geostrophic Canadian high, friction-turned 20–30° and slowed near the surface. Temperature and moisture are carried by the wind on a 0.4° grid (semi-Lagrangian, 1-hour steps), warmed and moistened by the ocean. Precipitation = moisture × lift in the comma head, warm conveyor belt, and cold front. Deepening rate grows with the temperature contrast and Gulf Stream moisture.