First African Tropical Wave of the Year to Get Designated an 'Invest': 96L

By: Jeff Masters , 3:21PM,GMT on July 27,2016



 

One of the strongest tropical waves of the 2016 African monsoon season moved off the coast of Africa on Wednesday morning, and has the potential to develop into a tropical depression in the coming days as it tracks westwards at 15 - 20 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, and something we'll see a lot more of as once the Atlantic hurricane season hits high gear during the mid-August through late September peak of the season. Satellite loops on Wednesday morning showed 96L had only a limited amount of heavy thunderstorms, which were poorly organized and had no signs of a surface circulation. Wind shear was moderate, near 10 - 20 knots, and sea surface temperatures (SSTs) were warm, near 28°C (82°F), which was about 1°C (1.8°F) above average.


Figure 1. MODIS satellite image of Invest 96L taken on Wednesday afternoon, July 27, 2016. Image credit: NASA.

Forecast for 96L
Steering currents favor a westerly motion for 96L, with the system slowing down in forward speed late this week and reaching a point near 40°W, midway between the Lesser Antilles Islands and Africa, on Monday. The 8 am EDT Wednesday run of the SHIPS model predicted modestly favorable conditions for development through Friday, with wind shear in the moderate range, 10 - 20 knots, and warm SSTs near 28°C. However, on Saturday and Sunday, 96L will encounter cooler waters, with temperatures a marginal 26°C. The SHIPS model also predicts that wind shear over the weekend will rise to the high range, above 20 knots, and the atmosphere will get very dry, due to an intrusion of the Saharan Air Layer (check out this animation of the 10-day African dust forecast from NASA.) These unfavorable conditions would stymie any development of 96L, but forecasts of dry air and wind shear this far into the future are unreliable.

The Wednesday 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 Wednesday 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 about 40% of its twenty ensemble members predict that a tropical depression would form this weekend or early next week midway between Africa and the Lesser Antilles. Most of these forecasts had the storm dying out the middle Atlantic, due to unfavorable conditions, and none had it becoming a hurricane. Less than 10% of the 50 members of the 00Z Wednesday European ensemble model forecasts showed a tropical depression forming in the Atlantic over the next ten days. In their 2 pm EDT Wednesday 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 apart by wind shear and dry air well before reaching the Lesser Antilles Islands by the middle of next week. Should 96L overachieve, the next name on the Atlantic list of named storms is Earl.


Figure 2. Hurricane activity in the Atlantic is typically low through the end of July, but climbs steeply once we reach the third week of August.

Eastern Pacific gets its fifth hurricane of the year: Frank
The Eastern Pacific is a month ahead of schedule for hurricane activity, thanks to the intensification of Hurricane Frank into a Category 1 storm on Tuesday. There was one other active storm in the basin on Wednesday--weakening Tropical Storm Georgette, which topped out as an impressive Category 4 storm with 130 mph winds on Monday morning. Georgette was the seventh named storm to form in the Eastern Pacific this month, tying the July record for named storms set in 1985. Since July 2, the Eastern Pacific has had Tropical Storm Agatha, Category 4 Hurricane Blas, Category 2 Hurricane Celia, Category 3 Hurricane Darby, Tropical Storm Estelle, Category 1 Hurricane Frank and Category 4 Hurricane Georgette. This puts us far ahead of climatology: the Eastern Pacific usually does not see its seventh named storm until August 7, its fifth hurricane until August 26, and its third major hurricane until September 20. An average season has 15 named storms, 8 hurricanes, and 3 major hurricanes. There may be a Tropical Storm Howard joining the parade by next week: In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave a new tropical disturbance about 700 miles south of Manzanillo, Mexico 2-day and 5-day development odds of 0% and 50%, respectively. This storm is expected to move west-northwest and not impact Mexico.

Jeff Masters