A pair of side-by-side tropical cyclones is roiling the Northwest Atlantic this weekend. After rocketing from tropical storm to Category 4 strength on Friday, Hurricane Humberto – safetly out to sea from North America’s perspective – could hit Cat 5 strength before Sunday, pushing big surf from the U.S. East Coast to Bermuda. Update: As of 5 p.m. EDT Saturday, Humberto is now a Category 5 hurricane, with top sustained winds of 160 mph.
Meanwhile, newborn Tropical Depression Nine will take its timing churning through The Bahamas this weekend, likely becoming a tropical storm and perhaps a hurricane as it grinds toward the Southeast U.S. coast early next week. Whether it moves inland, or lurks just offshore for days, torrential rains are an increasingly good bet over parts of coastal Florida, Georgia, South Carolina, and/or North Carolina. A tropical storm warning was in effect for much of The Bahamas, and a tropical storm watch was out for much of the central Florida coast.
Humberto puts on a show at a safe distance, but it will swing near Bermuda early next week
The sun dawned Saturday on a phenomenally-well-structured Hurricane Humberto. On Friday, Humberto took advantage of textbook ingredients – light wind shear, unusually warm surface waters, deeper ocean heat, and a moist atmosphere – to vault through Saffir-Simpson hurricane categories in the space of 12 hours. As of 11 a.m. EDT Saturday, Category 4 Humberto was centered about 720 mi (1160 km) south-southeast of Bermuda, heading west-northwest at 8 mph (13 km/h) and packing top sustained winds of 145 mph (230 km/h).
Humberto went through an eyewall replacement cycle on Friday night, but it came through the process – which can often dent a hurricane’s strength by as much as a Saffir-Simpson category – largely unscathed. The surrounding environment remains very supportive, so there’s every sign that Humberto could get one more solid burst of intensification over the weekend, and it wouldn’t take much to propel it to Cat 5 strength. The 12Z Saturday runs of NOAA’s HMON, HAFS-A, and HAFS-B intensity models all push Humberto into that rarefied Cat 5 range by Sunday, as does the 11 a.m. EDT Saturday forecast from the National Hurricane Center (NHC). Beyond that point, one or more additional eyewall replacements could occur, leading to some strength fluctuations. Wind shear may increase above 20 knots (23 mph) by Monday as Humberto embarks on a classic recurvature path that’s predicted to take it west of Bermuda. Only a small rightward path nudge could bring considerably more impact, and there’s enough spread in the ensemble model forecasts (see Fig. 2 below) to make it wise for Bermudans to prepare accordingly. On the current forecast track from NHC, at least tropical-storm-strength winds will be likely in Bermuda (see Fig. 2 below).
One of many questions surrounding TD 9: will torrential rains move inland over the Southeast U.S. or stay offshore?
This weekend marks the first time in a decade – since 2015 – that the United States has made it as late as September 27 without a single hurricane landfall. That streak may or may not continue depending on whether Tropical Depression 9, moving across The Bahamas over the weekend, intensifies to hurricane strength, and whether it moves inland over the Southeast U.S. or lingers just offshore before heading out to sea, a scenario that’s been looking increasingly likely. The next name on the Atlantic list is Imelda.
As of 2 p.m. EDT Saturday, TD 9 was centered about 120 mi (190 km) south-southwest of the Central Bahamas, heading northwest at 6 mph (9 km/h) with top sustained winds of 35 mph (55 km/h). A hurricane reconnaisance flight was en route to investigate TD 9.
Satellite imagery at midday Saturday showed TD 9 boasting a large shield of scattered but intense showers and thunderstorms (convection). Upper-level outflow was increasing, and there were some hints of rotation around the developing surface center.
TD 9 will likely be a tropical storm as soon as Saturday evening, as forecast by NHC, and once it is, larger-scale conditions are in place for strengthening through the weekend, apart from moderate to strong wind shear around 15 knots through Sunday. A very moist atmosphere will nourish TD 9’s expanse of convection as the system passes over very warm waters both at the surface (sea surface temperatures of 30-31 degrees Celsius or 86-88 degrees Fahrenheit) and at depth.
Intensification may be more gradual for TD 9 than for Humberto, especially given its sprawling structure, but NHC predicts it to reach hurricane strength by early Tuesday, when it will likely be decelerating off the Southeast U.S. coast. The big question is TD 9’s future beyond that point. The key factors include:
- How quickly it moves north. An upper low across the Southeast could pull TD 9 inland, as shown by some model ensemble members, but that scenario looks increasingly less likely, especially if TD 9 continues to move slowly.
- How much it interacts with Humberto. By Sunday and Monday, TD 9 and Humberto will be moving north or north-northwest side by side, perhaps closely enough to trigger the Fujiwhara effect. When that happens, the overall movement of both storms can be modulated by rotation around a central point between them. In this case, the effect would tend to propel Humberto a bit more quickly toward the north and to slow down TD 9’s northward progress. This could force TD 9 to linger near or over the Southeast coast for one or more days. This poses a real concern for excessive rain, especially near the coast.
Since the location of any stall of TD 9 remains uncertain, it’s too soon to know whether the heaviest of these rains – which could easily exceed a foot or more in a stalling scenario – would be focused offshore or just onshore. It’s less likely, though not impossible, that TD 9 would get pulled inland far enough to deliver the heaviest rain to the Southern Appalachians, but several inches there would still be possible.
Between the effects of Humberto and TD 9, coastlines from the U.S. East Coast to The Bahamas, Bermuda, and the north-facing Greater Antillles can expect multiple days of high surf, beach erosion, and life-threatening rip currents.
Jeff Masters contributed to this post.
Republish our articles for free, online or in print, under a Creative Commons license.
Great Job Bob Henson & the Team @ Yale Climate Connections Source link for sharing this story.
