Weather Lingo: ACE

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.

Source: CSU

(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.)

Weather Lingo: The Brown Ocean Effect

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.

Credit: NBC/TWC

Weather Lingo: Invest

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

Weather Lingo: Bombogenesis

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.

Credit: TWC

Weather Lingo: The Benchmark

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

Weather Lingo: Rain Shadow

The world of weather has some interesting words and phrases. One of these is “Rain Shadow”.

While it sounds rather poetic, a rain shadow refers to the land area on the leeside of a mountain that is exceptionally dry. Mountains act as barriers for weather systems traveling in a region’s prevailing winds, forcing them to drop most of their moisture on the windward side before they can pass.

As an air mass rises up and over a mountain, it enters an area of lower atmospheric pressure where it expands and cools. As a result, the moisture it contains condenses, clouds form, and precipitation falls. After the air mass moves over the mountain, it starts to descend the other side. The air is warmed by compression and the clouds dissipate. This means little to no rain falls on the leeward side.

Rain shadows are found all over the world, from the Tibetan Plateau in Asia to the Atacama Desert in South America. Here in the US, Death Valley is a famous example as it lies in the rain shadow of four different mountain ranges.

The Rain Shadow Effect. Credit: Kagee Commons