Invest 94L Off the Coast of Africa May Slowly Develop

The first African tropical wave worthy of being classified by NHC as an area of interest (an "Invest") has emerged from the coast of Africa, and lies a few hundred miles southeast of the Cape Verde Islands. Invest 94L has conditions that favor some slow development over the next few days. Satellite images show that 94L has a decent amount of spin and a small but growing area of heavy thunderstorms. Wind shear off the coast of Africa is moderate, 10 - 20 knots, ocean temperatures are a warm 28°C, and the atmosphere is reasonably moist. These conditions should allow for some slow development as the system heads west at 15 mph the next two days. However, 94L faces a rugged path ahead of it. The 8 am Wednesday run of the SHIPS model predicted that wind shear would remain moderate over 94L this week, but a massive area of dry, dusty air from the Saharan AIr Layer (SAL) is dominating most of the tropical Atlantic, from the coast of Africa into the Central Caribbean. As 94L moves into the Central Atlantic this weekend, the storm will encounter increasingly dry, stable air, making development difficult. If 94L does manage to make it to the Caribbean, the high wind shear that has dominated the region all summer will likely tear the storm apart. The ensemble runs of the GFS and European models--done by running the models at lower resolution and varying the initial atmospheric conditions slightly to generate an "ensemble" of twenty potential weather situations (fifty for the European model)--do have a number of their 20 - 50 runs that develop 94L into a tropical depression, but none of these solutions have the storm that develops making it as far west as the Lesser Antilles Islands. The operational high-resolution versions of our three top models for predicting genesis of tropical cyclones--the GFS, European, and UKMET models--show only weak development of 94L over the next few days. In their 8 am Wednesday Atlantic Tropical Weather Outlook., NHC gave 94L 2-day and 5-day odds of development of 0% and 20%, respectively.


Figure 1. Invest 94L between the coast of Africa and the Cape Verde Islands as seen by the MODIS instrument on NASA's Aqua satellite on Wednesday morning, July 29, 2015. Image credit: NASA.


Figure 2. Saharan Air Layer (SAL) analysis for 8 am EDT Wednesday, July 29, 2015, from the University of Wisconsin CIMSS shows plenty of dry air dominating the tropical Atlantic.

Hawaii should watch Invest 91E
In the Eastern Pacific, Tropical Depression Eight-E is encountering dry, stable air and should dissipate well before reaching the Hawaiian Islands. Hawaii should be more concerned with Invest 91E, which both the GFS and European models show getting within 500 miles of the islands by next Wednesday. In their 5 am PDT Wednesday Tropical Weather Outlook, NHC gave 91E 2-day and 5-day development odds of 60% and 90%, respectively.

In the Western Pacific, all looks to be quiet until at least Saturday, when the European model predicts a new tropical depression will form about 1500 miles east of the Philippines' Luzon Island.


Figure 3. Tropical Cyclone Two in the Bay of Bengal as seen at 05 UTC July 29, 2015, from the MODIS instrument on NASA's Terra satellite. JTWC gave top winds of 40 mph to the system, which was expected to move slowly onshore over Bangladesh. Image credit: NASA.

Bay of Bengal's Tropical Cyclone Two generating heavy rains
The Indian Meteorological Department (IMD) and Joint Typhoon Warning Center are issuing advisories on Tropical Cyclone Two in the Indian Ocean's Bay of Bengal, which is bringing heavy rains to portions of India, Bangladesh, and Myanmar. Tropical cyclones embedded within India's monsoon rarely grow into strong tropical storms, but can be prodigious rain makers. India's monsoon has been 12% below normal in rainfall as of July 22, so the country could use more rainfall--though perhaps not in the concentrated manner a monsoon tropical storm typically delivers, causing dangerous flooding rains. Tropical Cyclone Two is expected to move slowly onshore over Bangladesh and weaken over the next few days.

Jeff Masters