A visible satellite image of a low-pressure system approaching the Bering Sea on December 30, 2020.
2021 will begin with a roar in the northern Pacific as a growing low-pressure system reaches incredible intensity over Alaska’s Aleutian Islands. The system could bottom-out on Friday with a minimum central pressure below 930 mb. Even though this storm will have a minimum pressure that rivals the strongest hurricanes, the two types of storms couldn’t be more different. The process that goes into creating such a powerful system is as fascinating as the storm itself.
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The National Weather Service in Anchorage says a storm hasn’t grown this strong in the Bering Sea since 2014. The latest forecast from the Ocean Prediction Center shows the immense low-pressure system achieving its peak intensity on Friday morning with a minimum central pressure of about 927 mb.
The NOAA Ocean Prediction Center's surface forecast for the northern Pacific Ocean for the morning ... [+]
That’s an impressive intensity for any low-pressure system. For some perspective, a minimum pressure below 930 mb is about what you would expect from a scale-topping hurricane. Hurricane Laura made landfall in Louisiana this past August with maximum sustained winds of 150 MPH and a minimum central pressure of 938 mb. Hurricane Iota, the Atlantic season’s only category five storm, reached a minimum pressure of 917 mb at its peak.
Even though the Alaskan low is slated to grow as strong as some of the most powerful hurricanes this season, extratropical cyclones and tropical cyclones are more like cousins than identical twins.
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An extratropical cyclone is our everyday low-pressure system that features frontal boundaries rotating around the center of the storm. Think of a dynamic winter storm rolling across the country—you can see heavy snow to the north, clear and frigid skies to the west, and severe thunderstorms embedded in warm and humid air to the south.
A tropical cyclone, on the other hand, is a low-pressure system that features warm and humid air throughout the entire storm. Tropical cyclones have a tight core of powerful winds surrounding its eye, the strength of which gradually tapers off with distance from the center of the storm.
The key difference between the two types of low-pressure systems is how they draw their energy.
Tropical cyclones develop from clusters of persistent thunderstorms. The updrafts feeding these thunderstorms draw air up and away from the surface. A center of low pressure will form at the surface if the process continues undisturbed. Stronger storms lead to stronger lows, which in turn will trigger stronger thunderstorms to continue the feedback loop. The only limitation to the strength of a storm is the strength and persistence of the thunderstorms around the eye of the storm.
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A weather model graphic showing the jet stream over the northern Pacific Ocean on December 31, 2020.
While a tropical cyclone essentially powers itself, an extratropical cyclone is almost entirely reliant on upper-level winds for development. Strong winds in the upper levels of the atmosphere converge and diverge with the twists and turns of the jet stream. Divergence, or winds fanning out and spreading apart, leaves less air in the upper atmosphere, forcing air from the surface to rush upward to fill the growing void. Stronger jet streams can result in stronger divergence, which can generate stronger low-pressure systems at the surface.
That’s what we’re going to see in the northern Pacific Ocean later this week. The west-to-east oriented jet stream will grow incredibly strong on Thursday and Friday, packing winds stronger than 200 MPH at its greatest extent. This will lead to intense divergence that will power the low-pressure system over the Aleutian Islands to an intensity rarely seen in this part of the world.
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While the Bering low will have a minimum pressure similar to what we see in the strongest hurricanes, it won’t have winds nearly as strong as one of those scale-topping storms. The energy in an extratropical cyclone is diffused across the storm, leading to a widespread area of strong winds rather than a tight core of dangerous winds like we would see in a hurricane.
This storm will bring rough conditions to the Aleutian Islands and shipping lanes in the Bering Sea. Sustained winds stronger than 60 MPH are likely across a large area surrounding the storm, potentially generating 50-foot seas and wind damage to communities on the Aleutian Islands.
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