Tropical Storm Earl On The Horizon

A strong tropical wave that moved off the coast of Africa on Wednesday morning has become more organized over the far eastern Atlantic, and has the potential to develop into a tropical depression in the coming days as it tracks west-northwestward at 10 – 15 mph into the middle Atlantic.

NHC designated this disturbance Invest 96L on Wednesday morning–the first “Invest” of the year for an African tropical wave.

Satellite loops on Thursday morning showed 96L had a compact area of heavy thunderstorms, Some spin was evident in the cloud pattern, and low-level spiral banding features had begun to appear. Wind shear was a light 5 – 10 knots, and sea surface temperatures (SSTs) were warm, near 28°C (82°F), which was about 1°C (1.8°F) above average. Water vapor satellite imagery showed that the eastern tropical Atlantic was quite moist, with the dry air of the Saharan Air Layer (SAL) several hundred miles north of 96L. These conditions are favorable for development of a tropical depression.

Steering currents favor a west to west-northwesterly motion at 10 – 20 mph for 96L over the next five days, and the storm should reach a point near 40°W, midway between the Lesser Antilles Islands and Africa, on Sunday.


Models show conflicting forecast

The Thursday morning operational runs of our three reliable models for predicting tropical cyclone genesis, the European, GFS and UKMET models, all supported some limited development of 96L, but stopped short of predicting it would become a tropical depression. The 00Z Thursday run of the GFS ensemble forecast, done by taking the operational high-resolution version of the model and running it at lower resolution with slight perturbations to the initial conditions in order to generate a range of possible outcomes, had more than 50% of its twenty ensemble members predict that a tropical depression would form this weekend or early next week in the eastern Atlantic.

Most of these forecasts had the storm dying out the middle Atlantic, due to unfavorable conditions, and none had it becoming a hurricane. Between 10 – 20% of the 50 members of the 00Z Thursday European ensemble model forecasts showed 96L becoming a tropical depression. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave 96L 2-day and 5-day development odds of 30% and 40%, respectively.

Though the long-range uncertainty on what 96L might do is high, one reasonable scenario is for the system to steadily grow in organization the next few days, come close to or achieve tropical depression status by Saturday, then get ripped up by wind shear and dry air well before reaching the Lesser Antilles Islands by the middle of next week. Should 96L become a tropical storm, the next name on the Atlantic list is Earl.

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