The American GFS model simulates an intense low-pressure system approaching Canada. (WeatherBell)
An offshore storm exploded in intensity as it slammed Newfoundland on Tuesday with hurricane-level force. Though the system wasn't a tropical one, it contained damaging gusts over 75 mph and offshore winds exceeding 120 mph, rivaling a Category 2 hurricane -- thanks to something called a "sting jet."
The storm was expected to mostly stay offshore. But what produced this extratropical cyclone?
Conditions that formed the storm
On Monday morning, a weak wave of low pressure was present off North Carolina's Outer Banks. That surface low became energized as a potent upper-air disturbance worked overhead. Air was efficiently lifted up and away from the surface low, leaving less air in the middle. Within 24 hours, the system's air pressure dropped by more than 7 percent -- meaning about 7 percent of the air is essentially missing from the middle of the storm.
The rate of intensification is twice what's required to classify it as a bomb cyclone -- an ominous meteorological term for a midlatitude storm with a certain threshold of dropping air pressure. For this kind of storm with even greater intensity, some meteorologists have crafted informal terms like "double bomb."
That missing air at the storm's middle creates an inward vacuum effect of sorts, driving strong winds.
Some of the winds stem from the low level jet stream, or a river of warm air about a mile above the ground rushing into the storm out of the south. That would bring 60 to 70 mph winds to the coastal Avalon Peninsula, including around St. John, in the afternoon. An inch or two of rain was also expected possible.
The core of the low pressure was expected to come ashore Tuesday evening. Such a system may even feature briefly calm winds or clearing, reminiscent of the middle of a hurricane.
More significantly, however, was the anticipated development of a sting jet.
What is a sting jet?
It's a small but intense "stinger" of 100 to 120 mph winds. Sting jets occur on the back side of intense low-pressure systems. They're most common over the open ocean but can occasionally brush ashore or clip the Canadian Maritimes, affecting land.
Sting jets form in the comma head, or wraparound, of low-pressure weather systems. Strong winds aloft blow clouds into the low-pressure system's "dry slot," which is the wedge of dry air swept along into the circulation.
The moisture in the clouds evaporates, drying the air. Dry air is dense, so it sinks. In the process, that sinking air drags strong winds to the ground. That means the strongest winds may actually accompany clear skies!
Meteorologists are still unsure what other processes are at play and what exactly jump-starts the development of a sting jet. It is estimated that between 39 and 49 percent of the strongest low-pressure systems produce sting jets. They're most likely to form following a bout of "bombogenesis," or rapid strengthening, when a low-pressure cyclone is mature.
Sting jets have happened in the United States a couple of times, but no place is as prone to them as Western Europe -- particularly northern Spain, Ireland and Scotland. Sometimes, the winds barrel into England and Wales, too.
Atmospheric scientists didn't even know sting jets existed until the Great Storm of 1987, when a run-of-the-mill October storm unleashed 120-mph winds. Shoreham-by-Sea in Sussex, England, logged a 120-mph gust, while Pointe du Roc in Granville, in the Normandy region of France, gusted to 137 mph. The culprit was a sting jet.