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A Cape Verde Wave in the Central Atlantic

By: Bob Henson 3:41 PM GMT on July 17, 2015

The ferocious wind shear associated with El Niño has remained far enough west to give a tropical wave in the central Atlantic at least a slender chance of development. Invest 93L was located near 10.7°N and 38.4°W at 8:00 am EDT Friday, moving west at about 18 mph. Peak winds are close to 30 mph, with disorganized showers and thunderstorms evident on satellite imagery within a fairly large envelope of moisture. 93L has formed a bit early for systems in the Cape Verde region, which typically peaks in production during August and September. An upper-level anticyclone over 93L has resulted in low wind shear of 5 - 10 knots immediately over the circulation. Water temperatures are slightly below average across the deep tropical North Atlantic, but at around 26 - 27°C (79° - 81°F), they are just warm enough to support development along the track of 93L. Track models generally take 93L toward the northern Leeward Islands over the weekend before projecting a gradual recurvature. Statistical models bring AL93 up to minimal tropical storm strength, but the dynamical models most reliable for predicting tropical genesis are failing to develop AL93 substantially. Any embryonic system will soon ingest drier air toward the west, not to mention struggling against powerful wind shear that now tops 50 knots around the Leeward Islands. NHC gives the system 10 percent chance of tropical cyclone development in the next 48 hours and a 20 percent chance in the next five days. Given the currently favorable upper-level conditions and marginally warm water, I would give 93L a modest chance of becoming a minimal-strength named storm for a day or two.

Figure 1. An infrared satellite image from GOES-East of Invest 93L, taken at 1515 GMT (11:15 am EST) on Friday, July 17. Image credit: NOAA/NESDIS.

Pacific tropics calming down after a hectic few days
Tropical action in the hyperactive 2015 Pacific season looks set to take something of a breather over the next several days. Former Typhoon Nangka is now a weak tropical storm in the Sea of Japan after having swept over the Japanese islands of Shikoku and western Honshu. At least two deaths and 32 injuries have been reported. The nation’s rail lines and air traffic were seriously disrupted by Nangka, which came ashore as a minimal Category 1 typhoon around 11:00 am local time on Thursday. The remnants of Nangka are projected to arc eastward close to the north end of Japan’s Honshu island this weekend. To the southeast, Tropical Storm Halola is still fighting moderate to strong wind shear, but it should enter a more favorable environment over the next couple of days and gradually intensify to typhoon status. Halola could take a swipe at Japan next week before recurving.

Figure 2. Waves crash against the coast of Katsurahama on the island of Shikoku, western Japan, during the approach of Typhoon Nangka on July 16, 2015. Image credit: Kyodo News via AP.

Figure 3. A channel of upper-level moisture extended from Tropical Storm Dolores into Arizona and New Mexico on Friday morning. Image credit: Stu Ostro, The Weather Channel.

In the Northeast Pacific, Tropical Storm Dolores weakened below hurricane strength overnight, with colder waters and greater wind shear lying in wait on its northwestward path. The National Hurricane Center expects Dolores to weaken further over the next several days. Moisture associated with Dolores will continue to mix with monsoonal moisture moving toward the southwest U.S., bringing a threat of heavy showers and thunderstorms to Arizona on Friday and Saturday and a rare chance of July sprinkles over the Los Angeles area. Flash flood watches are in effect for much of Arizona and far southeast California. The remnant circulation of Dolores itself is predicted to stall early next week several hundred miles southwest of Los Angeles, but slugs of moisture from Dolores and the larger monsoonal pattern may get drawn as far north as central California, perhaps bringing an enhanced chance of thunderstorms there. Farther out to sea, weak Tropical Storm Enrique continue to spin harmlessly, while NHC is giving a tropical wave south of Mexico a 30 percent chance of development between Sunday and Wednesday.

I’ll have a post later today on the 2015 State of the Climate release, and a special report this weekend on progress in severe weather modeling and prediction. Jeff Masters and I will both be back on board next week.

Bob Henson


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.