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Danny intensifies into first hurricane of the 2015 season

Visible satellite image of Hurricane Danny from 7:45 am EDT.

After its skeletal appearance on Wednesday, Danny has exploded to life overnight and is now a hurricane with sustained winds of 75 mph. Hurricane Danny is the first hurricane so far this season, and right on time — the median date of the first hurricane formation is Aug. 16.

Hurricane Danny’s forward speed is just 12 mph, meaning it’s still about four days away from reaching the eastern Caribbean if it maintains this speed. The official forecast from the National Hurricane Center calls for Danny to maintain its hurricane strength through Sunday as it approaches the Lesser Antilles islands.

The Hurricane Center suggests Danny will continue west toward the Caribbean over the weekend, reaching the Lesser Antilles on Monday and then weakening to a tropical storm. By Tuesday, Danny could be close to Puerto Rico, though there’s still a lot of uncertainty about whether Danny will be able to maintain any intensity through early next week.

The center’s forecast cone is designed such that there is a 67 percent probability that the storm’s center will remain within the cone — the wind and rain impacts often extend beyond the cone. The cone grows larger at longer forecast times because of increased uncertainty.

Recent satellite images, including microwave imagery, show that Hurricane Danny developed an eye on Thursday morning. While visible and infrared satellite images allow us to see only the cloud tops, microwave images allow us to peer down into the storm’s underlying structure due to microwaves’ longer wavelength. In the image below, a visible satellite image is shown in the background, and microwave data are overlaid in color to highlight areas of strong thunderstorm activity. The ring in the middle is a nascent eye wall.

However, Danny is still very close to a significant plume of dry, stable Saharan air, which is depicted in the image below by the warm-colored shading. Danny is located in the lower right part of the map. This could go either way — a hurricane can either maintain its isolation from the disruptive dry air, or it can entrain it and weaken. The difference in Danny’s appearance yesterday (paltry and weak) and today (strong and burgeoning) illustrate the two paradigms perfectly.

Since it looks like Hurricane Danny has successfully isolated itself from the Saharan air to its north, it should be free to take full advantage of the otherwise favorable conditions for further intensification — at least for the time being. The hurricane is also extremely small in size, which tends to allow for more rapid changes in intensity, up or down. As long as the storm is so close to that dry air, it’s likely that Danny will just see short periods of moderate intensification.

The track forecast contains quite a bit of uncertainty. The two leading global models, the European ECMWF model and the U.S. GFS model, do not agree on Danny’s future — and that goes for their deterministic runs as well as their ensembles.

The “deterministic” run is the one meteorologists tend to refer to when they’re talking about the models. It’s commonly seen on track maps, and it is the single high-resolution run from that particular model.

But modeling groups also typically run an “ensemble,” which is a bunch of runs (usually 10 to 50) that are similar, but not identical, to the deterministic run. Ensembles allow for errors in storm intensity, position and size, as well as environmental differences and even different model configurations.

The two maps here are from a recent ECMWF and GFS run and show track forecasts color-coded by storm intensity.

The ECMWF favors a weak storm that stays south, while the GFS is generally stronger and farther north, on average. It is extremely important to recognize that model runs are not official forecasts — they are just possible scenarios, and they might not even include what will actually happen with the storm. Never place high confidence in a single run from a single model. The forecasts you see from the National Hurricane Center are designed to be steadier in time; the center tries to make only slight changes to the track and intensity forecasts, even when the model guidance may be bouncing all over the place.

Given what we know now, it is still far too uncertain for anyone beyond the Leeward Islands to be on alert for Danny. The storm won’t reach the Leeward Islands for another four days, and a lot can change in the meantime.