Hurricane Irene left a path of destruction along the east coast of the US over August 27 and 28. Because of its impact on populated areas, Irene received an unusual amount of media coverage, and the projections of its progress became a good case study of the current state of hurricane forecasting, one that highlights the remaining challenges. In this article, we'll have a look at the National Weather Service's National Hurricane Center Irene Advisory Archive to highlight the challenges involved in predicting a hurricane’s behavior.

Hurricane Irene was first spotted on August 20, when the US Air Force’s Hurricane Hunter plane found a “small low-level circulation center just southwest of a large convective burst.” A hurricane forms when a large number of cumulus clouds form in a small area, which typically happens over a body of warm water. When a large amount of heat is injected into the atmosphere from the water vapor’s latent heat release, the buoyant air displaces the surrounding air and lowers the surface pressure. As a result, the air parcel starts spinning to balance the pressure gradient with Coriolis Effect—this is how a tropical cyclone is formed. A tropical cyclone is the general name for hurricanes, typhoons, and tropical depressions. The names differ depending on where they form and how strong they are.

Challenges at the start

The formation phase is actually the hardest part of hurricane forecasting. The necessary ingredients for forming a tropical cyclone are: 1) a warm body of water; and 2) small vertical wind shear. A warm body of water is necessary to sustain cumulus convection, and the wind shear must be weak so that the heat released by the cumulus cloud stays in the same area and accumulates until the air parcel starts spinning.

However, even when both of these conditions are satisfied, predicting where a convective event will start is very hard. It's kind of like predicting which direction a standing pencil is going to fall. A standing pencil is an unstable system, so predicting that it is going to fall is easy—the hard part is knowing which way its going to fall.

That doesn't mean that nothing about it is unpredictable. The tropical Atlantic develops an unstable airmass every summer, and statistical prediction of approximately how many hurricanes will form each season has become fairly accurate, but exactly where those hurricanes will form remains hard to predict. After Irene formed, given the observed sea surface temperature, it was also fairly easy to predict that it was going to continue to receive energy from the warm tropical water. That energy set it on the path to reaching hurricane strength.

A sense of direction

After a cyclonic circulation like a hurricane forms, its motion is controlled by two effects. The first is the prevailing background wind. When a tropical cyclone is close the equator, it rides the trade wind, which blows westward. When a hurricane is in the mid-latitudes, the steering force comes from the eastward jetstream.

The northward forcing on a hurricane is called the beta-drift; in the northern hemisphere, it acts in the poleward direction for a counter-clockwise vortex such as a hurricane, and southward for a clockwise circulation. The exact opposite effect was displayed by the Great Dark Spot of Neptune during Voyager 2’s flyby—the Great Dark Spot is an anticyclone, thus it was observed to be slowly drifting toward the equator.

When a hurricane forms in the tropical Atlantic, it first rides the westward trade wind as it is forced by the beta-drift forcing; that’s why they generally move toward the northwest. As the beta-drift pushes the hurricane into the mid-latitude jetstream, its path starts curving eastward so the heading changes to north and then northeast.

The exact timing of this change in the hurricane’s path depends on the interaction between the background wind and the hurricane itself. Complicating matters further, the strength of the beta-drift also depends on the background wind. For example, Jupiter’s Great Red Spot is a strong anticyclone, but because it is trapped between two strong, stable jetstreams, it has been around for the last 400 years. On Earth, the mid-latitude jetstream has a strongly meandering course, so its fluctuating nature introduces a challenge in predicting a hurricane’s behavior.

These complications are noted in the NHC discussion on August 24, when it mentioned that Irene was expected to cross the subtropical ridge, which is the boundary between the regions affected by the trade winds and the mid-latitude jet.

Once it catches the jet, a hurricane's ride on the background wind is dependent on many factors, including the vertical structures of both the hurricane and the jetstream, and small eddies contained within these features—many of these details are currently not resolved in observational data. These details are represented differently in various forecasting models, and the small differences can be enough to make the predictions diverge. That's why the NHC discussion refers to many different models, including the GFDL model, the WRF, UK Met Office Model, and the ECMWF model.

At the end, NHC takes predictions from these various models into account to produce a forecast envelope. The NHC advisory first mentioned the need for issuing a hurricane watch for the US east coast at 11pm EST of August 24.

After Irene crossed the subtropical ridge and started riding the jetstream, its behavior became markedly more predictable, which you can see in the NHC's archived forecast animation—you can see that the hurricane’s predicted track is constantly adjusted while it's still east of Florida, but after it made a landfall in the North Carolina coast, Irene is pretty much following the predicted track.

So, the current research on hurricane forecasting largely focuses on the interaction between the hurricane and the background flow—in particular, where the effect of the mid-latitude jet takes over from the trade winds. The effect of global climate change on these storm tracks is also a hot topic, because the location of subtropical ridge is affected by the changing climate. Thus, future hurricanes may reach areas previously not affected by such storms.