Tropical Depression Fifteen Forms in the Far West Atlantic

Tropical Depression Fifteen has formed in the far Western Atlantic, forecast to move out to sea over the next 5 days with minimal impacts to any landmasses. This is the fifteenth tropical cyclone of the 2020 Atlantic Hurricane Season and could become the fourteenth named storm. If it does, it will be called Nana. Presently, the earliest 14th named storm in the Atlantic Basin is Nate, which formed on September 6th, 2005.

This tropical cyclone is not forecast to impact Trinidad, Tobago or the Lesser Antilles.

At 5:00 PM AST, the center of Tropical Depression Fifteen was located by an Air Force Reserve Hurricane Hunter aircraft near latitude 32.6 North, longitude 76.5 West. The depression is moving toward the northeast near 12 MPH (19 KM/H) and a motion toward the northeast or east-northeast is expected for the next few days.

Maximum sustained winds are near 35 MPH (55 KM/H) with higher gusts. The depression is forecast to become a tropical storm on Tuesday, but little, if any, additional strengthening is forecast Tuesday night or Wednesday.

The estimated minimum central pressure based on the Air Force Reserve reconnaissance data is 1009 millibars.

Tropical Depression Fifteen Forecast Track as of 5:00 PM AST Monday 31st August 2020. (National Hurricane Center)
Tropical Depression Fifteen Forecast Track as of 5:00 PM AST Monday 31st August 2020. (National Hurricane Center)

Watches & Warnings

Tropical Depression Fifteen Watches and Warnings as of 5:00 PM AST Monday 31st August 2020. (National Hurricane Center)
Tropical Depression Fifteen Watches and Warnings as of 5:00 PM AST Monday 31st August 2020. (National Hurricane Center)

There are no coastal watches or warnings in effect.

Hazards Affecting Land

Tropical Depression Fifteen probabilities for tropical-storm-force winds as of 5:00 PM AST Monday 31st August 2020, as well as the most likely time of arrival of tropical-storm-force winds. (National Hurricane Center)
Tropical Depression Fifteen probabilities for tropical-storm-force winds as of 5:00 PM AST Monday 31st August 2020, as well as the most likely time of arrival of tropical-storm-force winds. (National Hurricane Center)

SURF: Swells generated by the depression are affecting portions of the coast of North Carolina, especially along the Outer Banks. These swells are likely to cause life-threatening surf and rip current conditions through tomorrow. Please consult products from your local weather office.

This system is of no direct threat to the Windward Islands, including Trinidad and Tobago.

Tropical Depression Fifteen Forecast Discussion

Tropical Depression Fifteen off the U.S. Mid-Atlantic Coast (Weathernerds)
Tropical Depression Fifteen off the U.S. Mid-Atlantic Coast (Weathernerds)

Satellite images indicate that the area of low pressure offshore of the Carolinas has had convection organized in bands since before dawn, and scatterometer plus an Air Force Reserve Hurricane Hunter mission data confirm that the circulation is closed. Thus, this is now a tropical depression, and the initial wind speed is set to 30 kt in accordance with 25-30 kt ASCAT-A data plus buoy 42001 readings which earlier had an adjusted 10-m peak of 30 kt.

The depression is moving northeastward at about 10 kt. The system should gradually turn toward the east-northeast by Wednesday due to it moving around the northwest side of the subtropical ridge, then move eastward in a few days around the flat ridge. By late week, the cyclone could slow and eventually turn back toward the northeast around a rather strong mid-latitude high-pressure system over the northeast Atlantic. There is considerable spread in the guidance, which really seems to depend upon whether the system stays coherent, like the official forecast, or would become a shallow low-level swirl by 120h and end up slower and south of the forecast track. This forecast is near the corrected-consensus guidance, leaning toward the ECMWF-based models, and it should be considered of low confidence.

Gradual strengthening is expected over the next day or so while the depression remains in a low-to-moderate shear environment. Although the depression is expected to be traversing the warm Gulf Stream for the next several days, wind shear is expected to greatly increase by Wednesday, which should limit intensification. In fact, there’s some chance the system could decay and lose any deep convection in rather strong shear in a few days. However, since it likely will be moving near the Gulf Stream, I suspect it will continue to pulse thunderstorm activity and stay alive throughout the period. The NHC intensity forecast is near or just above the model consensus on that reasoning, closest to the HWRF model. The cyclone could become extratropical (or a remnant low) by the end of the forecast, but this is very uncertain.

Forecast Discussion by Forecaster Blake from the NHC.

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