28 August 2019

Strengthening Dorian passing east of Puerto Rico today, Erin forms west of Bermuda

Tropical Storm Dorian remains the primary feature of interest in the Atlantic. It is strengthening in the northeast Caribbean and will pass close to Puerto Rico today as a borderline hurricane. As of 11am EDT, the sustained winds are up to 70 mph and it's centered over St. Croix.

On Tuesday, the storm took an unanticipated turn to the north, which not only threw off the model runs, but changed the bigger picture for the worse. Even today, the storm is tracking more north than most forecasts. In fact, it won't even hit Puerto Rico head-on but rather pass to its east, which means it will not get significantly weakened by mountainous Hispaniola OR Puerto Rico, and it dramatically increases the odds of a hurricane landfall in the southeast U.S.

The map below shows the full history of NHC track forecasts for Dorian, and if you look in the eastern Caribbean, the observed track in black has been consistently north of forecasts.  That also is reflected in the longer-range forecasts, which show a steady northward trend in Florida.

The environment ahead of Dorian is favorable for intensification, so with obstacles out of its way, it is poised to become the season's second hurricane.  In fact, model guidance has recently been quite aggressive with intensification in the coming 2-4 days, and a major hurricane (Category 3+) is not at all out of the question.  The NHC official forecast brings Dorian up to Category 3 intensity in four days, but that will evolve. Based on the current forecast, tropical storm conditions could reach Florida as early as Saturday afternoon. Anyone from south Florida up into North Carolina should take this threat seriously and stay aware of changes in the forecast.

There are a couple subtle features that will be responsible for steering Dorian in the coming days: a ridge near Bermuda (the stronger that is, the more westward the track will go) and a trough moving through the midwest and eastern US (the stronger that is, the more northward the track will go).  The balancing act between them is what governs a south Florida landfall, a north Florida landfall, or even a Carolinas landfall.  The difference between those scenarios is not at much as it sounds like -- because of the curvature of the coast. Never focus on a specific model or a specific run; there can be quite a bit of variability.

A suite of dynamical global and regional models (colored lines) and the NHC forecast (black line).
As always, rainfall will be a big deal when it comes to a tropical cyclone of any intensity. Hefty accumulations are possible over all of Florida in the coming days.

It has now been a week since we started watching a disturbance over the Bahamas that passed over south Florida last weekend, then became Tropical Depression 6 on Monday, and was upgraded again to Tropical Storm Erin on Tuesday night.  It has since been downgraded to a depression again. Erin is the season's 5th named storm.  It is a highly-sheared tropical depression centered about 430 miles west of Bermuda and is forecast to track off to the northeast toward Nova Scotia on Thursday.

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