First a bomb cyclone and an atmospheric river. Now a double Fujiwhara effect. What it all means.
As an intense week of weather wraps up for the Pacific Northwest and Northern California, Mother Nature has one last, nasty trick up her sleeve: the Fujiwhara effect.
It involves two atmospheric disturbances doing an intricate dance -- or in this case, two storms taking turns lashing the Northwest while rotating around a central point. And this rare phenomenon is actually happening in two places at once this week: in the Northwest and the Northeast.
The animation below of air pressure across several days showcases the unusual phenomenon. See how the initial storm, which was the bomb cyclone, and a second storm pivot around each other?
The Pacific Northwest isn't the only region of the United States experiencing this effect -- it's simultaneously occurring in the East this week, as two snowy systems collide and eventually join forces over Canada, shown in the bottom panel of the animation above.
The swiveling of the storms resembles a synchronized pair of figure skaters out on a rink, each mirroring the other's sweeping movements.
This rare meteorological phenomenon may have been made more likely by a loopy jet stream pattern near Alaska, with a vast area of high pressure near the state.
The Fujiwhara effect, explained
The Fujiwhara effect involves two atmospheric disturbances pinwheeling around one another. One storm can be bigger, smaller, weaker or stronger than the other, but they must both be low-pressure systems. In other words, the Fujiwhara effect can't happen between a low-pressure system and a high.
The disturbances can be two hurricanes doing an intricate dance or, in this case, two midlatitude cyclones taking turns battering the Northwest while rotating around a central point.
Sometimes the storms merge into one, bigger storm, and other times the effect can alter the course of one or both of the systems involved -- which ultimately affects the weather we experience.
In the Northeast, a strong storm is swallowing a weaker storm. The two storms are expected to merge and cause stormy weather in Atlantic Canada this weekend.
In the Northwest, which involves one record-strong storm and another moderately intense system, the Fujiwhara effect will cause another round of heavy rain, snow and wind Friday.
While they sometimes look similar on satellite imagery, midlatitude cyclones are fueled by differences in temperature, whereas hurricanes siphon their energy from warm ocean water -- two distinctly different processes.
Why it matters -- more rain, wind and snow
The Fujiwhara effect will contribute to another surge of rain, wind and snow Friday in an already sodden and weather-weary corridor from Northern California to Washington.
The National Weather Service in Seattle was warning of another round of strong winds that could cause more power outages after half-a-million customers lost power in Washington state this week. Powerful winds were also expected in coastal Oregon on Friday.
In Northern California, there was a moderate risk (level 3 of 4) for excessive rainfall Friday as an atmospheric river aimed for the region for one last day.
Meanwhile, in the Northeast, residents of New York, New Jersey and Pennsylvania were digging out from a half-foot of snow, as the other Fujiwhara-influenced system tracked into the region Thursday night while intensifying.
Winter storm warnings were still in effect Friday morning as some schoolchildren celebrated their first snow day of the season north of New York City, according to local reports.
The eastern storm was forecast to move into Atlantic Canada on Saturday while it continued to bring rain and snow to northern New England.
Early next week, another storm was forecast to affect the West Coast before it moved into the central states by midweek. Unsettled conditions, along with the potential for more snow, continued to look possible on Thanksgiving Day and Black Friday in the Midwest and East.
Not just meteorological jargon
While it may seem that meteorologists are pulling new descriptors out of their hats each week, terms like "bomb cyclone," "atmospheric river" and "Fujiwhara effect" have been around for a while, and are rooted in science.
The Fujiwhara effect is named for Japanese meteorologist Sakuhei Fujiwhara, who was the director of the Central Meteorological Observatory of Japan from 1941 to 1947. Usage of the phrase dates back more than 70 years in scientific literature.
The term bomb cyclone has been around for more than 40 years, since it was introduced by Frederick Sanders and John Gyakum in 1980.
Bomb cyclones are midlatitude cyclones -- not hurricanes -- that undergo explosive intensification within a 24-hour period.
Even the name atmospheric river is more than 30 years old, as coined in 1992 by Newell et al., though originally it was called a tropospheric river. They were initially described as narrow but long filaments of water vapor that persist for many days. Now we call them rivers in the sky, and appreciate the important role they play in transporting water to almost all corners of the world.
If you feel so inclined, play a game of meteorological trivia at the Thanksgiving dinner table next week -- whoever offers the best guess at the definitions wins the last slice of pumpkin pie.
No, Uncle Joe, a bomb cyclone isn't the pile of dishes in the sink at the end of the night.