As Harvey moves away from the Gulf Coast, a new storm is brewing in the Atlantic and forecasters are monitoring the potential for yet another storm in the Gulf of Mexico.
Hurricane Irma is expected to become a formidable cyclone, and its path puts the Caribbean and the Southeast U.S. coast at risk. It’s too soon to predict exactly where it will track once it reaches the Caribbean. But this time of year, history has taught us that these kinds of storms — beasts that form just off the coast of Africa, near the equator — are not to be ignored.
Irma is forecast to intensify significantly: There’s essentially zero vertical wind shear, very warm water temperatures, humid air and a robust preexisting circulation. In layman’s terms, conditions are ripe. Irma is expected to become the season’s second major hurricane — Category 3 or stronger. Irma should be in the vicinity of the Lesser Antilles in about a week.
The U.S. coast should be on high alert not just for Irma, but for other storms that could spin up quickly, closer to land.
Specifically, we’re concerned about a possible storm in the western Gulf of Mexico — near Texas — early next week. It seems likely that something will form, and it could have an impact on the Texas coast or areas farther east, sometime between Wednesday and Friday. It does not appear at this time that it would be strong, but even weak systems can produce torrential rainfall and flooding.
Here’s how the National Hurricane Center characterized this threat in their Thursday forecast:
An area of low pressure could form over the southwestern Gulf of Mexico by the weekend. Development, if any, of this system is expected to be slow to occur as the low moves slowly northward. If this system does develop, it could bring additional rainfall to portions of the Texas and Louisiana coasts. However, any rainfall forecast is uncertain at this time range, and it is too soon to determine any specific impacts. Interests in these areas should monitor the progress of this potential system for the next few days.
Hurricane Irma — still far out in the Atlantic — will remain safely over the open ocean for the next five or six days, but what people are most interested in is what is to follow. Models lose quite a bit of skill beyond a week, so their long-range output is used to look at trends and probabilities.
The two leading global models have rather different outcomes through the next 10 days. The ensembles — multiple runs of the same model but with slightly different configurations to simulate realistic uncertainty — have little overlap. This gives forecasters much less confidence in long-range outlooks.
What the ensembles do tell us: Anyone from Belize to Bermuda and points in between should be monitoring the situation closely. Also, one should be wary of relying on a single model or a single run, no matter how dramatic they may sometimes be.
Adding the intensity information to the track, you can see an ensemble-based probability of a major hurricane being near a location through the next 10 days. This example from the European model is a recent example of that. Keep in mind that it will evolve over time.
— Todd Crawford (@tcrawf_nh) August 31, 2017
The atmospheric steering currents that will dictate Irma’s track depend on how strong it is. A stronger “deeper” storm is steered by different layers of the atmosphere than a weak, shallow storm would be. But as is often the case, models don’t even agree on what the exact environmental pattern will be.
In the maps below, which show a forecast of where Irma could be next Thursday, the hurricane is in different positions and has different intensities. But what’s most important at this time is the huge difference in the colored contours, which represent mid-atmospheric ridges and troughs.
The European model, left, has a much stronger ridge over Irma, which forces it hundreds of miles farther south than the GFS solution. The GFS forecast will allow Irma to recurve to the north and likely miss land. The European forecast environment will absolutely impact land and possibly the United States.
All we can do now is wait and watch this storm carefully.
More Washington Post reporting on Hurricane Harvey