Dorian is shaping up to be a major threat to the Southeastern United States

[Karlston]

Dorian is shaping up to be a major threat to the Southeastern United States

Current forecast calls for a Category 3 hurricane to hit near Kennedy Space Center.

Enlarge / A NOAA satellite image from 11am ET shows the position of Tropical Storm Dorian near Puerto Rico.

NOAA

Tropical Storm Dorian appears to pose an increasing threat to the Southeastern United States, potentially including significant landmarks such as Disney World and the Kennedy Space Center.

 

As of Wednesday morning, Dorian was nearing hurricane strength, with sustained winds of 70mph. The storm's center should pass just to the east of Puerto Rico today and then have as much as four days to strengthen over open ocean before approaching the Florida coast.

 

The National Hurricane Center has ratcheted up its intensity forecast for Dorian, such that it is now predicted to come ashore as a Category 3 hurricane on Monday morning, near Kennedy Space Center on Florida's Atlantic coast.

 

The intensity forecast has really ramped up for a couple of reasons. First of all, the storm is no longer expected to interact with the mountainous terrain of Hispaniola. Its movement is also slower, meaning it will have several days over the very warm waters near the Bahamas, with moderate wind shear. Finally, the upper-atmosphere pattern is very favorable to intensification.

Enlarge / The GFS forecast model ensemble predictions for Dorian show a range of possibilities for the track into Florida, and beyond.

Weathernerds.org

 

In terms of forecast track, there are some questions about the overall flow pattern in the upper atmosphere, so the landfall location carries more uncertainty than usual. A final landfall remains possible from north of Miami to Jacksonville. There are also questions about where the hurricane moves after it crosses the Florida peninsula. Dorian now seems more likely than not to reemerge in the Eastern Gulf of Mexico and eventually turn north, perhaps making a second landfall anywhere from the Florida Panhandle to Southeastern Louisiana.

Effects

The immediate concern is heavy rainfall over Puerto Rico and the US Virgin Islands, which could lead to flash flooding. Longer-term, the Florida coast faces the potential for storm surge and strong winds. Both of these will depend heavily on the intensity that Dorian reaches, and where the storm makes landfall, as effects are more significant to the right of the center.

 

As it now appears that Dorian will make a landfall at nearly a 90-degree angle to the Florida coast, storm surge effects will be amplified as the counter-clockwise motion of the storm's winds push water directly onshore.

Enlarge / Very early precipitation accumulation forecast for the next seven days.

Pivotal Weather

 

In terms of winds, for now, Dorian is a relatively compact storm, so its worst winds may remain confined to within 50 or 75 miles (80-120km) of its center. Wherever the center crosses the Florida peninsula, it will have the potential to cause significant damage.

 

A final concern is heavy rainfall. The steering currents by this weekend, and into early next week, are not overly pronounced. A slower-moving storm means that some areas of the Southeastern United States—Florida, Georgia, Alabama, or the Carolinas—would see a large amount of precipitation and flooding. It is impossible to say at this time where the worst of this inland flooding will occur, but it likely will be somewhere to the right side of the storm's track, although not necessarily particularly close to the center.

 

 

 

Source: Dorian is shaping up to be a major threat to the Southeastern United States (Ars Technica)

[steven36]

[Karlston]

Yeah, the forecast for Hurricane Dorian is a mess

It's as if the world's best model just threw up its arms in despair.

Enlarge / 5pm ET Thursday official track forecast for Hurricane Dorian.

National Hurricane Center

Hurricane-track forecasting has gotten pretty good over the last couple of decades—so much so that some hurricane scientists believe we are close to reaching a limit on predictability. However, there are still outlier storms that are difficult to forecast, and Hurricane Dorian is definitely one of those cases.

 

This uncertainly is only going to amplify the misery of Floridians seeking to prepare for or evacuate from what is likely a major hurricane bearing down on the state. In this case, the problem may be further worsened by premature confidence in where Dorian will go: as of Thursday evening, the one thing we do know about this storm is that we don't know where it's going to go.

 

Spaghetti plots of forecast tracks are commonly shared online, both in news stories and on social media, and they're sometimes useful. With Dorian, a plot like the one below from Thursday evening, which shows the official forecast track from the National Hurricane Center (in black) as well as other models, is actually very deceiving.

Hurricane Dorian track guidance from Thursday afternoon.

ucar.edu

From the plot above, if we dismiss TCLP and CLP5 (which you should, because they are not really models at all but persistence and trajectory tracks), there is a nice clustering of models bringing Dorian to the Central Florida coast on Monday. The agreement is quite good.

 

But don't believe it. To get a better idea of where Dorian is likely to go over the next week or so, a better practice is to look at ensemble forecasts from the global models. They do a good job of capturing the range of outcomes based upon slightly different initial conditions. For this article, we'll look at the Thursday 12z ensemble output from the European model. (In this case, 12z means the models were initialized at 12:00 UTC Thursday and finalized about six hours later).

12z ensemble output for the Hurricane Dorian.

Weathernerds.org

What is important to note about this plot is that it only shows the track of Dorian out to 120 hours, so that means its position as of Tuesday at 8am ET. Notice that there is a remarkable variance in the location of the "center" of Dorian in these roughly four dozen ensemble members. Yes, a reasonable amount of the ensemble members bring Dorian to the coast between Sunday and Tuesday, but some are far, far away. In some scenarios, Dorian turns north before even reaching Florida.

 

This uncertainty meshes with what we're actually observing about the atmospheric conditions that will help steer Dorian—namely that the steering currents around the Bahamas are weak. Eventually, Dorian is going to turn northward and traverse around the western periphery of a ridge of high pressure over the Atlantic, but it's difficult to say when that turn will come. In the meantime, the storm is going to move slowly.

 

The 5pm ET Thursday track forecast from the National Hurricane Center shown at the top of this post—which is absolutely the authority for such things—captures this ambiguity. It encompasses the entire Florida peninsula within the five-day cone of uncertainty.

 

All of this is singularly unhelpful to the residents of the Sunshine State, who are staring down at the possibility of a major hurricane making landfall somewhere along the peninsula in three to four days. The time for preparations and evacuations is now, but it is hard to offer much in the way of specificity about where the worst storm surge, damaging winds, and inland rainfall will occur. Certainly a slow-moving storm will make the latter variable, rainfall, worse.

 

 

For what it's worth, the European model provides ensemble tracks for the storm 10 days out. If we look at the 240-hour tracks, the confusion only increases. Weather enthusiast Jack Sillin captured this nicely on Twitter this afternoon, highlighting several of the variables in what remains a complicated forecast.

 

So yeah, Dorian's forecast is a mess. Beware of anyone who tries to tell you otherwise over at least the next day or so.

 

 

 

Source: Yeah, the forecast for Hurricane Dorian is a mess (Ars Technica)