Atlantic Canada Nor'easter: Bomb cyclogenesis
Wednesday, March 26, 2014, 11:53 AM -
Over the past week, our meteorology team has been busy nailing down the forecast details for what will likely be the strongest storm for Atlantic Canada this year.
TUNE IN FOR LIVE COVERAGE: Chris St. Clair will on location in PEI, Chief Meteorologist Chris Scott and Meteorologist Mark Robinson will be monitoring the storm from Cape Breton and Nathan Coleman will be on location in Halifax.
Confidence is growing that this will be a powerful Nor'easter, undergoing what meteorologists call “bomb cyclogenesis”, or rapid intensification. East coasters are used to a handful of Nor'easters each winter, but this one will be a powerhouse. Some forecasters are already drawing comparisons between this week's storm and White Juan, the powerful Nor’easter that buried Atlantic Canada in snow 10 years ago.
Awesome visualization of forecast wind flow around this monster to be RT @SwiftChaser #atlstorm Yikes pic.twitter.com/nDH9mguYyK
— Chris Scott (@ChrisScottWx) March 25, 2014
What is “bomb cyclogenesis”?
Like this winter’s ubiquitous “polar vortex”, “bomb cyclogenesis” (or “bombogenesis”) is a real meteorological term that has made its way into the public lexicon thanks to some recent high impact storms and the power of social media.
The term “bomb cyclone” was coined around 1980 by Fred Sanders, one of the grandfathers of synoptic meteorology, and his student, John Gyakum, now a professor at McGill University.
ATLANTIC CANADA NOR'EASTER: Eight weather terms you'll be hearing this week
The most common definition of a “bomb” is an extratropical cyclone whose central pressure falls 24 millibars in 24 hours -- i.e. very rapid deepening. And if we’re getting technical, it should be noted that the criteria for bomb cyclogenesis varies by latitude, from 28 mb / 24 hr at the pole to 12 mb / 24 hr in the subtropics.
This week’s storm could deepen by as much as 40 to 50 millibars in 24 hours, depending on which model you choose. By Wednesday afternoon, our Nor’easter could have a central pressure as low as 950 mb as it swing north towards Cape Breton.
While Nor’easters themselves are common in the depths of winter, the strength of this week’s storm is incredible for late March. It’s tough to say exactly how rare this week’s storm is, since historical records aren’t complete. A search through reanalysis data provides some insight. Storms will a central pressure less than 968 mb are far more common in January or February than in March or April.
Based on initial research, a spring Nor’easter of this strength only occurs roughly once every decade or so.
Ingredients of a Nor’easter Bomb
Although strong Nor’easter bombs can generate winds speeds near hurricane strength, they derive their energy from a different source: the clash between warm and cold air.
NEXT PAGE: FIND OUT THE IMPACT THE STORM WILL HAVE ON ATLANTIC CANADA
Three critical ingredients came together this week:
1) Cold polar air
Strong Nor’easters get their energy from the clash between cold polar air and warm subtropical air. Certainly, there has been plenty of cold air to work with this season.
The latest blast of arctic air caused low temperature records to fall across Ontario and Quebec on Monday morning, before pushing farther south.
2) Strong sea surface temperature (SST) gradient
Nor’easters frequently form off the coast of Cape Hatteras, North Carolina due to one important feature -- the Gulf Stream. This important ocean current transports warm tropical water northward along the US eastern seaboard. Eventually the Gulf Stream collides with cold water originating from the North Atlantic. The stronger the clash, or SST gradient, the more “energy” is available to feed the Nor’easter.
This year, the Gulf Stream has been running a few degrees above normal, while the waters around New England and the Maritimes have been running a few degrees below.
When cold polar air (see #1) flows over the strong SST gradient towards warmer water, the lower levels of the atmosphere become unstable. This provides a prime setup for east coast bombs.
3) Upper-level disturbance
Once the clash of cold air and warm water is in place, bomb cyclogenesis is typically triggered by the passage of an upper-level trough. A strong upper-level trough will act as lifting mechanism. As the air lifts and the surface low strengthens, warm and cold fronts will develop. Warm and cold air advection around the surface low will work to further strengthen the upper-level trough. Through this positive feedback loop, a bomb cyclogenesis occurs.
Nor’Easter Impacts
Although heavy amounts of snow are expected, wind-related impacts will be the bigger issue.
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As the centre of the low approaches Cape Breton Wednesday afternoon, winds will ramp up across Atlantic Canada. Sustained winds will reach 70 to 80 km/h in coastal regions, with gusts of 160 to 180 km/h possible in the Wreckhouse area and western Cape Breton. High waves, dangerous storm surge, and coastal flooding are possible Wednesday afternoon through Thursday. Strong winds, combined with the heavy snow, will also cause blizzard conditions with low to zero visibility.
The Weather Network’s team of meteorologists will continue to provide updates throughout the storm.