Thunderstorms pose a number of familiar hazards, such as lightning and hail. The lesser-known downburst, however, is also a serious threat to life and property.
A downburst is a strong downward current of air that causes damaging winds on or near the ground. They initiate high up in the atmosphere, where relatively dry air is entrained inside of an intense thunderstorm. The dry air evaporates some of the storm’s raindrops, which has a cooling effect. Since this cooler air is denser than the warm air that surrounds it, it sinks rapidly toward the surface. When it hits the ground, it spreads out radially – in straight lines in all directions. Reaching speeds in excess of 100mph, a downburst will knock down trees and other obstacles leaving a trail of debris all facing the same direction.
These straight-line wind events, according to the NWS, can vary in size and duration. When they cover an area less than 2.5 miles, they are referred to as microbursts. These typically last between 5 and 15 minutes. Larger events, known as macrobursts, affect an area greater than 2.5 miles and last from 5 to 30 minutes.
While short-lived, these powerful winds can pose a threat to property on the ground as well as airplanes in the process of taking off or landing.
March, a transitional month between winter and spring, is well known for its winds. In large cities like New York, however, the wind can be accelerated by something called the Venturi Effect.
Tall buildings and straight, grid-like streets essentially create man-made canyons that affect how the wind moves through a city. Funneled through the buildings, the wind is constricted and forced to speed up. The same process is seen when you put your thumb over the mouth of a hose to create a choke point and make the water flow faster.
The Venturi Effect is named for Giovanni Battista Venturi, an 18th-century Italian physicist.
The winter season can produce a number of different types of storms. One of these is a nor’easter.
These intense systems generally affect the east coast of the United States from the mid-Atlantic to New England. They traditionally develop when a strong area of low pressure to the south moves up the coast and meets cold air pushing down from Canada. With a plentiful supply of moisture from the Atlantic, these storms are notorious for producing copious amount of precipitation. The exact type – rain or snow – depends on the temperature at the time of the storm. They are also known for their strong onshore winds that can cause coastal flooding and beach erosion.
Spinning counterclockwise, these storms take their name from the steady northeasterly wind they produce.
The calendar says January, but it felt more like spring in New York City on Tuesday. The temperature soared to 60°F in Central Park, a staggering 22°F above average for this time of year and 2°F shy of the record high for the date.
This unseasonable warmth is part of the “January Thaw” currently taking place in a large part of the eastern United States. After an extended cold blast, this is a period when winter’s grip relaxes a bit and temperatures rise at least 10°F above normal for a few days. It does not necessarily occur every year, but when it does, it is usually in mid to late January, hence the name.
With temperatures in the 50s since Saturday, many New Yorkers have been out in the parks enjoying the break from the cold. It is, however, still January. So, keep your winter gear handy.
Melting ice on The Lake in Central Park, NYC. Credit: Melissa Fleming
January is usually the coldest month of the year – the so-called dead of winter. There are times during the month, however, when temperatures soar well above average. This is known as a “January Thaw”.
This type of weather phenomenon typically occurs after an extended cold blast and is marked by temperatures at least 10°F above normal for few days. It does not necessarily occur every year, but when it does, it is usually in mid to late January, hence the name.
Credit: Melissa Fleming
There are a number of different ways to gauge a hurricane season. One of these is the Accumulated Cyclone Energy index, which is widely referred to as ACE.
It expresses the combined intensity and duration of individual cyclones and provides a measure of activity for an entire hurricane season. For a single storm, it is calculated by summing the squares of the maximum sustained wind speeds measured every six hours while they are at least tropical storm strength. This number is then divided by 10,000 to make it more user-friendly. Overall, the stronger and longer-lived a storm is, the higher its ACE value.
The ACE for a season is the sum of the ACE values from individual storms that occurred that year. NOAA considers a season with an ACE of 111 or higher to be above average, while an ACE of 66 or lower is regarded as below average.
The Atlantic hurricane season runs from June 1 through November 30.
(Note: Year to date the Atlantic Basin has had 13 storms with a combined ACE of 202 and there are still two months left in the 2017 hurricane season.)
Tropical cyclones are fueled by warm ocean water and typically peter out over land. Sometimes, however, their lives are extended by something called the “brown ocean effect”.
This is a phenomenon where a storm derives energy from the evaporation of abundant soil moisture deposited by previous rainfall. Essentially, the saturated soil mimics the role of the ocean allowing a tropical cyclone to maintain its strength or even intensify after making landfall.
For the brown ocean effect to occur, according to a NASA funded study by Theresa Andersen and Marshall Shepherd of the University of Georgia, three criteria need to be met:
- The soil needs to contain copious amounts of moisture.
- Atmospheric conditions near the ground must have tropical characteristics with minimal variation in temperature.
- Evaporation rates must be high enough to provide the storm with sufficient latent heat that it uses for fuel, at least 70 watts averaged per square meter.
Although this process supplies less energy than the ocean, it is enough to sustain a storm for a longer period than normal over land. It was first noticed in 2007 after Tropical Storm Erin made landfall in Texas and then intensified as it traveled inland. It formed an eye over Oklahoma and unleashed a massive amount of rainfall.
Storms that are impacted by the brown ocean effect maintain a warm-core and are known as Inland Tropical Cyclone Maintenance and Intensification events (TCMIs). While rare, they are most common in the US, China, and Australia.
The Hurricane Season officially began in June, but August is when things typically start to ramp up. It is also when the word “invest” (short for an investigative area) becomes more prevalent in weather forecasts.
When the National Hurricane Center (NHC) wants to take a closer look at an area of disturbed weather that could possibly develop into a tropical cyclone, it designates it as an invest. This opens up specialized resources, such as computer models and satellites that provide forecasters with additional data on the area in question. That said, an invest does not always become a tropical system.
More than one invest can exist at any given time, so the NHC has a special way to identify them. They are numbered from 90 to 99, followed by a letter. If the invest is in the Atlantic, the letter will be “L” and if it is in the Eastern Pacific, it will be an “E”. The numbers can be reused throughout a season, as necessary.
The Central Pacific Hurricane Center and the Joint Typhoon Warning Center follow a similar system. Letters for the basins they cover include “C” for the Central Pacific, “W” for the Western Pacific, “A” for the Arabian Sea, and “B” for the Bay of Bengal.
If an invest in any basin develops into a tropical storm, it is reclassified and given a name from that season’s pre-determined list.
An example of how the National Hurricane Center monitors and forecasts the development track of an Invest. Red is the current Invest 99L and Yellow is 90L Credit: NHC
The weather world has some interesting words and phrases. One of these is “bombogenesis”.
Sounding rather ominous, it is a combination of the words cyclogenesis (storm formation) and bomb. It refers to the explosive or rapid intensification of an area of low pressure. More specifically, it means the central pressure of a storm system drops at least 24 millibars in 24 hours.
Air pressure is measured in millibars (mb) and the lower it is, the stronger the storm.
Taking place along steep temperature gradients, bombogenesis is most common along the east coast where cold continental air masses meet the relatively warm waters of the Gulf Stream. Disturbances in the jet stream above this type of temperature contrast help the air to rise and the pressure to drop.
This process can develop any time of the year but is most likely between October and March. When a system “bombs out” – a variation on the original phrase – strong winds, heavy precipitation, and even lightning can be expected. Nor’easters often become “weather bombs” – another popular variation – as they move up the coast.
Just like real estate, weather is all about location. In the northeastern US, a special set of coordinates known as “The Benchmark” (40°N 70°W) can help identify the type of impacts a winter storm will have on the region.
When a low-pressure system travels west of this position, coastal areas will see more rain than snow as the storm pulls relatively warm marine air onshore. Further inland, where the air is colder, snow is more likely.
If a storm tracks east of the benchmark, it is essentially moving away from land and less warm air is pulled onshore. Some light snow will fall along the coast, but usually not very much.
When a system moves directly through the crosshairs of the benchmark, coastal communities in the region can expect a major snow event. This is exactly what happened with the storm on Thursday that dumped heavy snow across the area.
The 40/70 Benchmark. Credit: Wx4cast