A cool and somewhat raw weekend continues in New England as Hurricane Joaquin remains well off the coast of the United States heading northeast. This storm will make a relatively close pass to Bermuda and will at least impact the island with very high surf and heavy rain and wind. There will be only some big waves and rip currents in New England as it passes.
I think there are two other big stories unfolding with the weather. The first is the damage and loss of life in the Bahamas from hurricane Joaquin. We are just getting some pictures from Long Island in the Bahamas where most of the roads are still impassible. It will take a couple of days before the full extent of the damage is known. The hurricane sat over that island for over 24 hours relentlessly pounding its residents with ferocious winds and torrential rains. It will take years for that area to recover.
Unprecedented In Modern Records
The second major story is the rainfall across the southeastern United States, especially South Carolina. Here, a river of moist air has been funneling copious rainfall into the region for days with precipitation that has amounted to 12 to 16 inches in some areas.
This situation isn’t over. There could be easily another 5 to 8 inches of rain in some areas by Monday, making this an historic event. The term historic is often used a lot to describe meteorological events over the past couple of decades, but this event will truly warrants the term.
Why This Is Occuring
An oversimplified reason for the rain is a stalled out low pressure system and associated through funneling moisture into that part of the coastline. The transport of moisture is part of something called an Atmospheric River. I’ll go into more detail shortly. Some interaction from hurricane Joaquin is likely playing a role, but isn’t the main reason for the rain. It’s tough to know what would happened if the hurricane hadn’t formed, because everything globally is so interconnected. There are still many ways we have yet to understand these connections. When you see the damage unfolding in South Carolina and other adjacent states from all the rain this weekend and in the days before and after, you now know the reason and it’s not a hurricane.
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On Friday, Dr. Marshall Shepard, president of the American Meteorological Society, wrote about “atmospheric rivers” (AR’s) in Forbes.com. AR is a term used to describe how moisture is transported from the tropics to areas outside and can bring small to large amounts of precipitation with it. His article in Forbes can be found here. In it he discusses the origins of an AR’s and how this is playing a role in the Carolina event.
The National Oceanic and Atmospheric Administration (NOAA) Atmospheric River Information Page defines Atmospheric Rivers as:
“relatively narrow regions in the atmosphere that are responsible for most of the horizontal transport of water vapor outside of the tropics. While ARs come in many shapes and sizes, those that contain the largest amounts of water vapor, the strongest winds, and stall over watersheds vulnerable to flooding, can create extreme rainfall and floods. These events can disrupt travel, induce mud slides, and cause catastrophic damage to life and property. However, not all ARs cause damage – most are weak, and simply provide beneficial rain or snow that is crucial to water supply”
The images below show several AR’s from a few years ago. Since they are colorized, the warmer tones represent higher levels of moisture. The moisture is transported northward from the tropics and looks like a tongue or finger coming off the main area of moisture near the equator. These AR’s can bring small amounts of beneficial moisture or in the case of what’s happening this weekend, too rain causing millions of dollars in damage.
Snowstorms and AR’s.
About a year ago an article was published in Geophysical Letters, the title was “The role of atmospheric rivers in anomalous snow accumulation in East Antarctica.” The article looked at two extreme snowfall events from May 2009 and February 2011, which showed an atmospheric river (AR) signature. Using the enhanced integrated water vapor (IWV), meteorologists observed concentrated long narrow bands of moisture stretching from subtropical latitudes to the East Antarctic coast. It is theorized these bands played a major role in the heavy snow accumulation.
In our part of the world, snowstorms and their formation have been studied for decades. Once of my favorite books on the subject is Northeast Snowstorms by Paul Kocin and Louis Uccellini. The book discusses moisture transport and energy in detail. Some of our largest snowstorms likely have some connection to an AR or a piece of one. Look at the water vapor image from the Valentine’s Day snowstorm in 2014.
There’s at least a passive if not direct connection to the tropics and how that moisture interacts within nor’easters like this can play a role in how much precipitation we receive. The coming years will undoubtedly bring much more research into the world of the AR and how they interplay with other atmospheric phenomenon.