A tropical wave located a few hundred miles west-southwest of the Cabo Verde Islands on Wednesday morning was growing more organized, and was designated Invest 91L by the National Hurricane Center Thursday morning. The wave was headed west-northwest at about 5-10 mph, and satellite imagery showed that it had a small area of heavy thunderstorms that had acquired a moderate degree of spin. The system was located far enough south of the dry air of the Saharan Air Layer to favor development, and sea surface temperatures near 28 degrees Celsius (82°F) and light wind shear of 5-10 knots were also favorable for development.
The wave has considerable support for development from the major models, with most of them showing a tropical depression or tropical storm forming by this weekend. The system is expected to follow a west to west-northwest track with a faster forward speed next week, potential passing through or just northeast of the Lesser Antilles Islands Sep. 10-12. The potential threat to the mainland U.S. after that time will not become clear for many days, and will depend upon the position and strength of a trough of low pressure expected to be near the U.S. East Coast late next week. If 91L becomes a hurricane early on, it would be more likely to recurve to the north and east and not make landfall in the U.S. A weaker storm would be less likely to recurve, and potentially be a greater long-term threat to the U.S.
In their 8 a.m. EDT Wednesday Tropical Weather Outlook, the National Hurricane Center gave 91L 2-day and 7-day odds of development of 50% and 80%, respectively. The next name on the Atlantic list of storms is Gabrielle.
Weakening Tropical Storm Lorena to bring flooding rains to Mexico and U.S. Southwest
Downgraded from a hurricane on Thursday morning, and now expected to stall before making landfall, Tropical Storm Lorena will still make its presence felt in northwest Mexico and the west end of the U.S. Sun Belt. Abundant moisture will stream inland from the decaying storm through Friday, bringing locally intense showers and thunderstorms.
Lorena peaked as a Category 1 hurricane with 85 mph winds on Wednesday night in the waters off the west coast of Mexico, but high wind shear of 20-25 knots and cool ocean waters less than 26 degrees Celsius (79°F) have taken their toll on the storm. At 11 a.m. EDT Thursday, Lorena had weakened to a tropical storm with winds of 70 mph (110 km/h), and was centered around 125 miles (200 km) west-southwest of Cabo San Lazaro, Mexico, heading northwest (parallel to the coast) at 8 mph (13 km/h).
Increasing wind shear and sharply chillier waters will likely weaken Lorena to a remnant low by Friday night, when it will be just off the sparsely populated west coast of Mexico’s middle Baja Peninsula. The biggest risk from Lorena and its remains will be localized flash flooding and mudslides. Ample moisture from Lorena and its eventual remnants will fuel widespread rains of 1 to 4 inches, perhaps up to 12” in isolated spots, from central Baja California into coastal parts of Mexico’s Sonora and Sinaloa states.
Showers and thunderstorms will be more sparse over the far southwest United States, but they could still be heavy in spots, especially from southeast California across southern Arizona on Thursday and into southern New Mexico on Friday. Flood watches are in effect for much of this area, and motorists and hikers should be aware of how quickly dry washes can fill with dangerous runoff.
Apart from flood worries, the moisture will be welcome: the weekly U.S. Drought Monitor released on Thursday shows nearly all of the region in the grip of severe to extreme drought.
Major Hurricane Kiko will rage in the Pacific for days to come
After surging to Category 4 strength on Wednesday and peaking overnight, Hurricane Kiko remained a formidable storm on Thursday. The hurricane underwent an eyewall replacement cycle that brought its top sustained winds down from an overnight peak of 145 mph. As of 11 a.m. EDT Thursday, Kiko had top winds of 130 mph (215 km/h). Kiko was centered far from land, almost 1500 miles (2400 kilometers) east-southeast of Hilo, Hawaii, and was moving west at 9 mph (15 km/h).
Kiko is gradually evolving into an annular storm—one dominated by a large core of intense showers and thunderstorms, with little spiral banding. This structure will serve Kiko well over the next several days, as annual storms tend to be more resilient. Wind shear should decrease from around 15 knots into the 5-10 knots range from Friday into Sunday, and sea surface temperatures below Kiko’s path will remain around 27 degrees Celsius (81 degrees Fahrenheit), so the storm could remain potent for several more days. The National Hurricane Center predicts Kiko to reach another intensity peak on Thursday night and then only gradually weaken through the weekend as it takes on a west-northwest bearing.
Odds are increasing that Hawaii could see at least some impacts next week from Kiko. Increasingly dry mid-level air and higher wind shear will invade the storm by Sunday and Monday, and significant weakening is a safe bet. However, NHC predicts that Kiko will remain a tropical storm by Tuesday, when it’s expected to be approaching the islands. There is higher-than-usual uncertainty in the angle of Kiko’s track by then, so Kiko could miss the islands entirely or could move over or near any of them. It would behoove people living in or traveling to Hawaii to keep an eye on Kiko’s progress over the next few days.
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