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