The science behind the 'weather bomb' heading our way

A weather bomb is heading toward Nova Scotia and will bring a messy mix of rain, ice pellets and a whole lot of snow and strong winds on Thursday. 

The entire province is under weather warnings and Nova Scotia Power is urging customers to prepare for large-scale outages.

The storm had a "rather innocent start" as a weak, warm low-pressure area near the Bahamas on Tuesday, says CBC meteorologist Kalin Mitchell. However, it is now moving north, colliding with extremely cold air off the Eastern Seaboard of the United States.

"That is the fuel for a low-pressure system, and will strengthen into a major winter storm rapidly over the next 24 hours," Mitchell says. 

The fast drop in pressure — a fall of 24 millibars in 24 hours — is known as "bombing out," producing a "weather bomb," he says. That results in a storm that develops "explosively" — quickly and with great power. 

Some forecasts predict the central pressure of the storm to drop near 950 millibars "which is virtually unheard of in the Atlantic outside of a hurricane," Mitchell says. 

Winter hurricane? Not quite

Mitchell says some forecasts are comparing the storm to a "winter hurricane," but there are some important differences. 

Hurricanes get their power from warm ocean waters but Thursday's storm gets its power from the clash between the very cold Arctic air sitting over eastern North America and the warm air contained within the system that's moving north from the Bahamas. 

Though the pressure is the same as a hurricane, this storm is not as compact and does not spin as quickly. It's that compactness of a hurricane that gives it its devastating strength. 

"Think of a figure skater spinning on ice with their arms outstretched, if that figure skater then tucks their arms in their speed of rotation will increase," says Mitchell. 

Hurricane or not, wind will be a big factor with this storm, he says. Nova Scotia can expect gusts of between 80 and 110 km/h across almost all of the province late Thursday afternoon and evening. The Annapolis Valley will be slightly more protected from the winds but is still expected to see gusts of between 70 and 100 km/h.

Environment Canada warns there could be gusts as high as 140 km/h over the southwest of the province. 

Freezing temperatures to follow

The storm will likely result in a widespread loss of power, a disheartening prospect for those who endured days, in some cases, of power outages following the Christmas Day storm.

Though temperatures will be around zero or above during the storm, a polar vortex will plunge temperatures into the minus teens afterwards. 

Mitchell says a polar vortex is a semi-permanent, cold-air mass that circles over the Arctic. Under the right conditions, this swirling mass of air breaks down and settles in the southern areas of Canada and northern parts of the U.S. All this means a very cold weekend, something those without power need to take into account.

According to Mitchell, daytime highs on both Saturday and Sunday are expected to be near –10 C with the overnight lows well into the minus teens. 

What you can expect in your area

• Halifax and Lunenburg counties: Snowfall, wind and storm surge warnings. Snow will begin Thursday morning with 10 to 20 centimetres expected inland and five to 10 centimetres near the coast. Heavy snow will mix with ice pellets and rain in the afternoon. It's expected to changeover to rain in the evening, Mitchell says.

• Yarmouth, Shelburne and Queens counties: Rainfall, wind and storm surge warnings. The area will likely bear the brunt of the wind, with gusts anywhere from 80 km/h to as high as 140 km/h. Those counties may see two to 10 centimetres of snow, followed by 20 to 30 millimetres of rain.
• Digby and Annapolis counties: Snowfall and wind warnings with heavy snowfall expected to begin in the morning. Environment Canada is forecasting between 15 and 25 centimetres of snow for those areas before changing over to rain in the afternoon. Winds will be "weaker" than the rest of the province, with gusts between 70 and 100 km/h expected.
• Kings, Pictou, Antigonish and Guysborough counties: Snowfall amounts between about 15 and 30 centimetres are expected. Winter storm warning with winds gusting between 80 and 110 km/h and gusts as high as 130 km/h in Inverness County.

• Hants, Colchester and Cumberland counties: Snow beginning Thursday morning, giving about 10 to 20 centimetres of snow before mixing with ice pellets and changing over to rain late Thursday afternoon and night.
• Cape Breton: Snow late Thursday morning with 10 to 20 centimetres over much of the island. Less snow, about five to 10 centimetres, near the Atlantic Coast. Snow will then mix with ice pellets in the afternoon.