2017 Hurricane Season

2017 Hurricane Season

By Stéphane Gentile posted Sunday 28th May.

With the start of the Atlantic Hurricane Season fast approaching, we take a look at a few facts about Hurricanes and what we might expect for 2017.

When is 2017 Hurricane season?
The 2017 hurricane season will officially begin on 1st June, 2017, and end on 30th November 2017, although as we’ll see further on, not all hurricanes and development always adhere to the bounds of this “season”.

So, what is a hurricane?
First of all, a tropical cyclone is a rapidly rotating storm system characterized by a low-pressure centre, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain.

Hurricane KatrinaSatellite image taken of Hurricane Katrina at peak intensity in the Gulf of Mexico on August 28, 2005. Image credit: NOAA

To be classified as a hurricane, a tropical cyclone must have maximum sustained winds of at least 74 mph (64 knots or 119 km/h) (Category 1). The highest classification in the scale, Category 5, is reserved for storms with winds exceeding 156 mph (136 knots or 251 km/h).

What ingredients do these storms need to form?
The six basic ingredients for tropical cyclone formation are:

1. A pre-existing disturbance (cloud cluster) in the lower-half of the troposphere with low-level cyclonic spin and convergence (as in African Easterly Waves). This is why many storms have their origin near Cape Verde Islands.
2. Sea-surface temperatures of at least 26.5 degrees Celsius (80 degrees Fahrenheit). A deep warm layer of water beneath the ocean surface (of at least 50 meters or so) can be helpful, but is not necessarily required.
3. A location (usually) at least five degrees of latitude away from the equator.
4. Values of vertical wind shear (variation in wind direction and or speed with height) between the lower and upper part of the atmosphere that are relatively low (generally less than 10 meters per second).
5. A middle atmosphere that is relatively moist as the incursion of dry air aloft can weaken or prevent deep convection.
6. An atmosphere that favours deep convection.

What hazards are associated with hurricanes?
The major hazards associated with hurricanes are:

• Storm surge and storm tide

Storm Surge is an abnormal rise of water generated by a storm’s winds and can reach heights well over 20 feet and can span hundreds of miles of coastline.

Storm surge

damage Damage from the storm surge of Hurricane Katrina in Gulfport, Mississippi. Credit: FEMA / Mark Wolfe

Storm tide is the water level rise during a storm due to the combination of storm surge and the astronomical tide.

• Heavy rainfall and inland flooding (by far the deadliest of all hazards)

• High winds

• Rip currents

• Tornadoes

How was the 2016 season?
The 2016 Atlantic hurricane season was the first above average Atlantic hurricane season since 2012, producing a total of 15 named storms, 7 hurricanes, among which 4 major hurricanes (Gaston, Matthew, Nicole and Otto).

Below is a chart showing the track of all tropical storms, hurricanes and major hurricanes that formed during the course of last year.

2016 Storm tracks Image credit: NOAA

What one can notice is the fact that most systems recurve in the Atlantic Ocean and never reach the eastern US seaboard or the Caribbean.

As mentioned earlier, development doesn’t always adhere to the norm and 2016 saw the season kickstart with a rare January storm, Hurricane Alex, which was the first to develop in the month since 1938. The 2016 season also yielded some other rarities, for instance Hurricane Matthew which became the southernmost Category 5 hurricane on record in the Atlantic basin, having undergone explosive intensification in the Caribbean Sea and went on to be classed as the ninth costliest Atlantic Hurricane on record.

See below the temporal distribution of the other events:

Storm DIST2016 Image credit: Wikipedia

Sadly, 743 fatalities were reported and with the total damage estimated at over $16 billion.

What are the preliminary forecasts for the 2017 season?
Ahead of and during the season, several national meteorological services and scientific agencies forecast how many named storms, hurricanes and major hurricanes will form during a season.

See below predictions of tropical activity from some sources:

2017updatedforecasts Legend: Tropical Storm Risk (TSR), Colorado State University (CSU)
The Weather Company (TWC), North Carolina State University (NCSU) National Oceanic and Atmospheric Administration (NOAA)

The numbers quoted above are mostly near the 30-year average for the Atlantic basin, but with NOAA forecasting an above-average season.

Again, 2017 saw an early start to the season with the first tropical storm (Arlene) active between 19th and 21st April. Below is the list of the other names:

2017Names 2017 Atlantic hurricane season names.
Image credit: Weather.com

There is no strong correlation between the number of storms or hurricanes and U.S. landfalls in any given season. One or more of the 12 named storms forecast to develop this season could hit the U.S., or none at all.

El Niño’s potential re-development is among the factors the forecast takes into account. So why is a tropical Pacific phenomenon important for hurricanes in the Atlantic?

With El Nino predicted return this year, what impact does El Nino / La Nina have?
El Niño is a periodic warming of the central and eastern equatorial waters of the Pacific Ocean and tends to produce areas of stronger wind shear (change in wind speed with height) and sinking air in parts of the Atlantic Basin.

ElNino Effects of El Niño in the eastern Pacific, Caribbean and western Atlantic Ocean
Image credit: Weather.com

These effects are known to be detrimental to either the development or maintenance of tropical cyclones.

At the moment, the National Oceanic and Atmospheric Agency (NOAA) puts the odds of El Niño’s development at 50 percent during August-December.

As a result, the latter portion of the season *could* be less active if El Niño conditions develop. But it’s unclear how much and how soon any type of atmospheric response there would be if El Niño did materialise.

It is interesting to note from the graph below that when the counterpart of El Niño (La Niña) occurred over the 1900-2016 period, more than twice as many major hurricanes made landfall in the US.

ElNinoLandfalls Mean Annual United States Landfalling Major Hurricanes, by ENSO phase.
Credit: Philip Klotzbach (Meteorologist at CSU)

In addition, the table below clearly shows that most costly damages from hurricane landfall in the US occur with La Niña and neutral conditions in the tropical Pacific:

CostsNumber of damaging events that exceed certain thresholds by phase of the ENSO cycle.
Credit: NOAA

What other factors, if any, influence development?
Dry air and wind shear are detrimental to tropical storm or hurricane development no matter whether El Niño is present or not.

As per point 5 of the ingredients above (need for moist atmosphere), any Saharan Dust blown by weather systems from West Africa across the Atlantic would hinder storm development.

North Atlantic Ocean water temperatures are colder than average and tropical Atlantic sea-surface temperatures have also cooled, both of which cause atmospheric conditions to be unfavourable for the development and strengthening of Atlantic hurricanes.

With the growing expectation of El Nino to make a return and with MeteoGib supporting the Team Britannia UK Global Challenge, we will be paying particular interest to this year’s Atlantic season as well as developments across the Pacific.