The weather system forecast to slam into Britain has been named Ciara, and is expected to unleash heavy rains and strong to damaging winds. Ciara will have its origins from the storm in the Southeast United States, which has generated tornadoes, flooding rain and heavy snow.
The stormy weather pattern over the North Atlantic is related to an unusually strong polar vortex, which is bottling up frigid air over the Arctic rather than releasing much to the south and, at the same time, helping to generate extreme weather in the high latitudes.
Bomb cyclone in the North Atlantic
The bomb cyclone expected to develop east of Greenland will be one of the mechanisms to shove the U.S. storm and atmospheric river toward Europe. It is presently just a weak area of low pressure between the Azores and Newfoundland, but will explosively intensify over the next 36 hours.
By Saturday morning, the storm could have an air pressure of 932 millibars — compared with the 934 millibars achieved by Category 5 Hurricane Matthew — as it meanders east of Greenland. That’s about a twelfth less than ordinary atmospheric pressure. And the lower the pressure, the stronger the storm.
While this storm will probably produce hurricane-force winds as it sits east of Greenland, the winds won’t be as strong as Matthew despite the lower pressure. Why? The winds a storm packs are a product not of the air pressures but of how quickly air pressures change over distance. It’s sort of like sledding: The speed your sled travels doesn’t depend on the height of the hill but rather how steep it is. Because storms at the high latitudes are large, they spread their wind energy over a greater area.
Commercial airliners breaking the speed of sound?
The bomb cyclone will drag the jet stream into a favorable location to promote speedy flights east across the Atlantic. The jet stream, akin to a river of swiftly moving air in the upper atmosphere, is sometimes used as a sort of highway for aircraft looking to tap into a free turbo boost to their destinations.
By Sunday, an atmospheric squeeze play between the Greenland storm and a high-pressure system to the south will drape the jet stream directly from the Northeast United States to Britain and the vicinity. Flight-level winds funneled through the jet stream could top 250 mph.
That means any planes that ride with the jet stream could cruise at velocities greater than the speed of sound. That includes commercial airliners. How is that possible? It’s because only the plane’s ground speed will surpass the 767 mph speed of sound but not its air speed.
It’s just like being on a moving walkway at the airport. Your motion adds to the motion of what you’re riding — in this case, the floor. You’re not actually walking any faster, but you’re traveling farther because something else is contributing to your total motion.
We wrote about this when a Boeing 787 achieved a ground speed of 801 mph in a record-breaking jet stream over Pennsylvania. On Dec. 4, an American Airlines flight from Tokyo to Los Angeles also hit 800 mph.
An atmospheric river in Europe
Atmospheric rivers are narrow channels of tropical water vapor that are carried away from the equator by strong storm systems. More than 80 percent of the moisture in an atmospheric river is stored within the lowest 10,000 feet of the atmosphere — making them superb heavy-rain producers.
The bomb cyclone and attendant jet stream will help yank the atmospheric river, present along the East Coast of the United States, all the way over to Europe by late this weekend. The humidity felt in Britain on Sunday will have originated near Mexico’s Yucatán Peninsula! The atmospheric river acts as a conveyor belt shuttling moisture north. And it could set the stage for very heavy downpours.
Storm Ciara to hit Britain
Ciara, immersed within the atmospheric river, will eventually steal some of the energy from the bomb cyclone, surfing the jet stream, and eventually blasting Britain with heavy rain, high waves and strong winds. The worst weather will affect northern areas. The downpours could be unusually heavy, thanks to the atmospheric river along it.
A yellow weather warning is in effect for localized rainfall totals exceeding three inches, rather atypical for storms in Britain. The heaviest will fall from Sunday morning through the evening.
The coastlines of Northern Ireland and Scotland could see wind gusts of 70 to 80 mps, with a few inches of snow in the higher elevations.
How this relates to the polar vortex
What’s been fueling this wicked jet stream and subsequent meteorological mania? Blame the polar vortex. A boring polar vortex.
In fact, the polar vortex has been unusually stable lately, as indicated by a near-record-setting “Arctic oscillation.” That means the core of the cold and strong winds associated with the vortex are relegated to the northern reaches of the Arctic.
The vortex is strong, and there are no signs of it being knocked out of kilter anytime soon. If and when the vortex is disrupted, if would probably result in a more meandering jet stream, perhaps increasing the chance of a major snowstorm for snow-starved areas of the Mid-Atlantic and coastal New England.
For now, though, the stable vortex favors a jet stream farther to the north, with the most active weather in the northern hemisphere relegated to the high latitudes.