Twin Invests 96L and 97L Worth Watching in the Atlantic

There's a new threat area to discuss today in the Atlantic: a tropical wave midway between the Lesser Antilles Islands and the Cabo Verde Islands that is headed west to west-northwest at 25 mph. This disturbance was designated Invest 97L on Thursday afternoon by NHC, and should arrive in the northern Lesser Antilles by late Saturday night. The Hurricane Hunters are on call to investigate the storm on Sunday, if needed. Satellite loops on Friday morning showed 97L had a modest area of heavy thunderstorms which were poorly organized, though there was some increasing spin evident in the cloud pattern. Wind shear was a light 5 - 10 knots, and sea surface temperatures (SSTs) was an adequate-for-developement 27°C (81°F), which was about 1°C (1.8°F) above average. Water vapor satellite imagery showed that 97L had a modest amount of dry air from the Saharan Air Layer (SAL) surrounding it, which was slowing development.


Figure 1. Latest satellite image of Invest 97L in the middle Atlantic.

Forecast for 97L
Steering currents favor very rapid west to west-northwesterly motion at about 25 mph for 97L though Monday, and storms that move this fast typically have trouble getting organized. This motion should take the disturbance through the northern Lesser Antilles Islands on Sunday morning, over the Virgin Islands and Puerto Rico on Sunday afternoon, and into the Dominican Republic by Sunday night. All of these areas should expect to see heavy rains of 2 - 4" and wind gusts of 30 - 35 mph as 97L passes, and the NWS may end up issuing a Flash Flood Watch for portions of the Virgin Islands and Puerto Rico this weekend. The 8 am EDT Friday run of the SHIPS model shows somewhat favorable conditions for development through Saturday night, with wind shear in the light to moderate range, 5 - 15 knots, a moist atmosphere, and SSTs near 27 - 27.5°C (81 - 82°F.) However, an unfavorable factor for development will be large-scale sinking motion over the tropical Atlantic over the next few days imparted by the passage of what is called a Kelvin Wave (see the tweet by The Weather Company's Mike Ventrice on this.) On Sunday through Monday, 97L is predicted to encounter high wind shear of 20 - 30 knots, which would thwart development. At that time, the system may be undergoing interaction with the rough topography of Puerto Rico and Hispaniola, will would also inhibit development. By Tuesday, 97L should be traversing Cuba, and will slow down to a forward speed of about 10 - 15 mph, taking it into Mexico's Yucatan Peninsula by Wednesday and into the Gulf of Mexico around Thursday.

The Friday morning operational runs of our three reliable models for predicting tropical cyclone genesis, the European, GFS and UKMET models, did not show 97L developing much. The 00Z Friday runs of the GFS and European model ensemble forecasts, 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 fewer than 10% of their ensemble members predict that 97L would become a tropical depression. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave 97L 2-day and 5-day development odds of 20% and 30%, respectively. When 97L reaches the Western Caribbean and Gulf of Mexico by the middle of next week, we will need to watch it, but the crystal ball is very murky on whether or not 97L might find favorable conditions for development then.


Figure 2. MODIS visible satellite image of 96L south of the Cabo Verde Islands taken on Friday morning, July 29, 2016. Image credit: NASA.

96L continues to grow more organized
A tropical disturbance that began as a strong tropical wave that moved off the coast of Africa on Wednesday morning continues to grow more organized over the eastern Atlantic, and has the potential to develop into a tropical depression by Saturday as it tracks west to west-northwestward at about 15 mph into the middle Atlantic. Satellite loops on Friday morning showed 96L had a compact area of heavy thunderstorms, and this activity was showing increasing organization. Plenty of spin was evident in the cloud pattern, and low-level spiral banding features were evident. 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.

Forecast for 96L
Steering currents favor a west to west-northwesterly motion at 15 - 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 night. The 8 am EDT Friday run of the SHIPS model predicted modestly favorable conditions for development through Saturday night, with wind shear in the light to moderate range, 5 - 15 knots, a moist atmosphere, and warm SSTs near 27 - 27.5°C (81 - 82°F.) However, by Saturday night, 96L will encounter cooler waters, with temperatures a marginal 26.5°C (80°F). The SHIPS model also predicts that wind shear over the weekend will become high, greater than 20 knots, and the atmosphere will get very dry, due to an intrusion of the Saharan Air Layer (check out the 10-day African dust forecast from NASA.) These unfavorable conditions would stymie any development of 96L. As 96L approaches the Lesser Antilles Islands later next week, the shear increases even further and the air grows drier, making 96L unlikely to be a threat to the islands.

The Friday morning operational runs of our three reliable models for predicting tropical cyclone genesis, the European, GFS and UKMET models, all supported continued development of 96L through Saturday. The 00Z Friday run of the GFS ensemble forecast had 30 - 40% of its twenty ensemble members predict that a tropical depression would form from 96L this weekend 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 30 - 40% of the 50 members of the 00Z Friday European ensemble model forecasts also showed 96L becoming a tropical depression this weekend. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave 96L 2-day and 5-day development odds of 40% and 50%, respectively.

A tropical cyclone-free Eastern Pacific for the first time since July 1
For the first time since July 1, there are no active tropical cyclones in the Eastern Pacific, thanks to the dissipation of Tropical Storm Frank on Thursday. The seven named storms this month tied a record set in 1985 for the most July storms on record in the basin. We have a chance to break the record with an eighth named storm, if it forms by Sunday: in their 8 am EDT Friday Tropical Weather Outlook, NHC gave a new tropical disturbance 850 miles south-southwest of the southern tip of Mexico's Baja Peninsula 2-day and 5-day development odds of 50% and 80%, respectively. This storm--which would be named Howard if it gets to tropical storm strength--is expected to move west-northwest and not impact Mexico.

This Sunday marks the 40th anniversary of Colorado's Big Thompson flash flood, one of the deadliest flooding disasters in U.S. history, with 139 people killed. Bob Henson will be back later today with a look back at this historic flood.

Wunderblogger Steve Gregory has more on the tropics in a Thursday afternoon post, HEAT WAVE ENDS AS MR & MRS ENSO FIGHT IT OUT.

The next name on the Atlantic list is Earl.

Jeff Masters