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Don't let your guard down: Only takes one storm to make a hurricane season memorable


Gina Ressler
Meteorologist

Tuesday, September 3, 2013, 2:00 PM -

Quiet Start to the Season

For the first time in 11 years, the month of August came and went without a single hurricane in the north Atlantic. This has only happened six times since 1944 (1967, 1984, 1988, 2001, 2002, 2013) which was the year when Hurricane Hunter aircraft began gathering reconnaissance data. In an average year the area should see at least two hurricanes by September. The latest it has gone without an Atlantic hurricane was in 2002, when the first hurricane -- Hurricane Gustav -- formed on September 11. Interestingly enough, Hurricane Gustav went on to make landfall as a Category 1 hurricane near St. Esprit Island on the south coast of Cape Breton, Nova Scotia.

The 30-year seasonal average for the Atlantic basin is 12 named storms, six hurricanes, and three major hurricanes (Category 3 or higher).

There have been six named storms so far this season in the Atlantic basin -- Andrea, Barry, Chantal, Dorian, Erin, and Fernand -- which in terms of pure numbers is about average for this time of year. In fact, the season even got off to an early start when Tropical Storm Andrea formed in early June. Andrea made landfall along the Florida panhandle, then tracked up the east coast where the remnants brought heavy rainfall and minor flooding to Nova Scotia.

Since then, the Atlantic hurricane season has been largely underwhelming. Meteorologists often use the term Accumulated Cyclone Energy (ACE) to describe the activity level of individual tropical cyclones or entire tropical seasons. The ACE uses the square of the wind speed over a storm’s lifetime to approximate the activity level or damage potential. In other words: the more intense or long-lived the tropical cyclones, the higher the total ACE. This year, not only has total ACE in the Atlantic basin been well below climatology (around 22 per cent of normal), but August 2013 had one of the lowest ACE values on record. So, even though we’re on track in terms of the number of named systems, the total activity level this season has been well below average.

Why the lack of activity?

Back in the spring, lead forecasters from NOAA and Colorado State University were predicting an above-average season. (NOAA’s forecasts are based mainly on ENSO conditions, multi-decadal signals, and dynamical models, while CSU uses a statistical prediction scheme.) Since then, both parties have reduced their projections slightly, while still calling for an active season.

Yet the tropics are still quiet. What gives? If the season was supposed to be active, how can we explain the lack of hurricanes?

Let’s review the ingredients needed for tropical cyclone (TC) development:

  • Warm sea surface temperatures (SSTs), typically greater than 26.5°C.
  • Low wind shear, since strong winds aloft can disrupt organized patterns of convection.
  • High near-surface relative humidity.
  • An unstable air mass which favours the development of convection and thunderstorms.
  • Some kind of “trigger” to produce convergence at the surface, for example, tropical waves off of Africa, weak fronts or troughs.

El Nino years are typically associated with higher wind shear, and thus, fewer Atlantic hurricanes. (This was the case in 2002.) La Nina years are associated with a greater number of Atlantic hurricanes. This year, we’re in ENSO-neutral conditions, which is neither favourable nor unfavourable for TC development. Earliest this year, some models were suggesting a return to La Nina conditions, but now it looks like we'll stay in a neutral pattern for the rest of the year. This is part of the reason why forecasters have reduced their projections.

There have been small issues with sea surface temperatures and wind shear this year, but one major problem has been the amount of dry, stable air affecting the tropics, which effectively shuts down any TC development. The influx of dry air off the Sahara through much of August has, no doubt, had a huge effect on the level of activity so far.

Satellite image of the Saharan Air Layer (SAL). The yellow and red shading indicate the presence of dry and/or dusty air in the lower to middle levels of the atmosphere. (Courtesy: Cooperative Institute for Meteorological Satellite Studies / University of Wisconsin-Madison)

Satellite image of the Saharan Air Layer (SAL). The yellow and red shading indicate the presence of dry and/or dusty air in the lower to middle levels of the atmosphere. (Courtesy: Cooperative Institute for Meteorological Satellite Studies / University of Wisconsin-Madison)

Looking ahead

The beginning of September is peak hurricane season in the Atlantic, and the basin usually remains active well into October. So even though it’s been a slow start to the season, there’s plenty of time left for the activity to pick up.

The official hurricane season for the Atlantic Basin (the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico) is from 1 June to 30 November. The peak of the season is from mid-August to late October. (Courtesy: NOAA)

The official hurricane season for the Atlantic Basin (the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico) is from 1 June to 30 November. The peak of the season is from mid-August to late October. (Courtesy: NOAA)

Conditions are slowly becoming more favourable for TC development. The amount of dry Saharan air has been reduced somewhat since mid-August, although there are some hints that another plume may affect the tropics in the next week.

That said, there’s another important indicator that hints that things may pick up soon -- the Madden-Julian Oscillation (MJO). The MJO can be thought of as a “pulse” of cloud and rainfall that propagates eastward along the equator. The MJO influences tropical cyclone activity in both the eastern Pacific and Atlantic basins. The MJO has been stuck in an unfavourable pattern for the last few months, but recently, the pattern has been amplifying. This isn’t a guarantee that we’ll see an increase in TC development soon, but it’s beginning to look more likely.

Let’s look at the current conditions in the Atlantic. The National Hurricane Center (NHC) in the US has highlighted three areas of interest. All three have a low chance of tropical cyclone development in the next 48 hours, but the NHC is giving the area of showers in the eastern Caribbean Sea a medium chance of development (around 50 per cent) in the next five days. Could this be our first hurricane? It’s too soon to tell, since intensity forecasts are notoriously unreliable, but it’s something to watch.

(Courtesy: NOAA's National Hurricane Center)

(Courtesy: NOAA's National Hurricane Center)

What this means for Atlantic Canada

At least four tropical storms track through Canadian waters each year, with about one or two tracking over Canadian soil. (The storms that affect Canada may no longer be fully tropical by the time they get here.) It is important to note that seasonal predictions of tropical activity are not landfall predictions. Whether or not Canada will have active year depends just as much on storm tracks as it does overall Atlantic activity. In fact, there is very little correlation between the number of storms that form in the Atlantic and the number that make their way into Canadian waters.

The bottom line for Canadians is that it only takes one storm to make a season memorable. There's still plenty of time left in Atlantic Hurricane Season, and damaging storms can happen at any point between now and late October.

But for now, we wait.

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