Tropical Storm Fiona formed at 11 p.m. EDT Wednesday east of the Leeward Islands, and will be a heavy-rain threat to those islands beginning on Friday. As of 11 a.m. EDT Thursday, Fiona was packing 50-mph sustained winds and was about 500 miles east of the Leewards, moving west at 14 mph.

Fiona’s appearance on September 15 (in UTC time) comes more than two weeks later than the usual August 29 appearance of the season’s sixth named storm, according to the 1991-2020 climatology. Typically, by September 15, the Atlantic has spawned 8.9 named storms, 3.9 hurricanes, and 1.7 major hurricanes. This year, we are at six named storms and two hurricanes (neither of them major), with an accumulated cyclone energy (ACE) index of just 43% of average.

Fiona is surrounded by a large area of dry air, and satellite images Thursday afternoon showed that Fiona did not look healthy: The circulation center was exposed to view, with the storm’s heavy thunderstorms restricted to the east side of the center. This was occurring because strong upper-level winds out of the west were creating a moderate 10-20 knots of wind shear, and driving dry air on the west side of Fiona into its core. However, sea surface temperatures (SSTs) were plenty warm, at 29 degrees Celsius (84°F) – about 0.5 degree Celsius (0.9°F) above average for this time of year. The first hurricane hunter mission into Fiona is scheduled for Thursday afternoon.

Forecast for Fiona

Fiona is predicted to have marginal conditions for strengthening through early next week, as persistent moderate wind shear of 10-20 knots is expected to drive dry air into the core of the system. If Fiona remains weak, the system will track faster and more to the south; a stronger storm will have a slower and more northward track.

Moreover, any portion of Fiona’s track over Puerto Rico or Hispaniola will induce weakening because of land interaction with the mountains on those islands.

Fiona could potentially enter the southeastern Bahamas early next week, posing an eventual threat to Bermuda or the U.S. East Coast later in the week. The long-range forecast is complicated by a powerful ex-Typhoon Merbok in the Pacific (see below), which will merge with the jet stream by Friday, perturbing it significantly. This interaction cannot be predicted with much accuracy this far in advance, leading to significant uncertainties about where the troughs and ridges that will steer Fiona will be placed.

Merbok to affect Alaska on Friday as powerful post-tropical storm

After becoming one of the fartheast-northeast typhoons on record, Merbok will quickly evolve into a post-tropical cyclone on Thursday before intensifying in the Bering Sea and bringing ferocious conditions to western Alaska. Merbok will sweep through the Aleutians on Thursday night as a post-tropical storm – bringing wind gusts of up to 90 mph – then churn across the western Bering Sea on Friday as a hurricane-strength system, approaching the Bering Strait between Alaska and Russia by Saturday.

Winds wrapping around post-tropical Merbok will gust to more than 60 mph over large parts of western Alaska, and heavy rains are likely. The storm will help push Anchorage toward what’s likely to be its wettest year in data going back to the 1950s.

The storm will also bring Nome, Alaska, one of its largest storm surges in decades. The National Weather Surface on Thursday morning predicted the surge will top out at 11.3 feet on Saturday afternoon, September 17. That surge would exceed the 8.73-foot surge in a destructive Bering Sea storm in November 2011 and would approach the 13-foot surge observed in a sequence of major storms in November 1974.

With post-tropical Merbok arriving much earlier in the season than those two storms did, there will be little or no sea ice protecting the Bering Sea coast.

A new typhoon threatens Japan: Nanmadol

In the northwest Pacific, Typhoon Nanmadol is a threat to develop into a major typhoon that will affect southern Japan this weekend. At 8 a.m. EDT Thursday, the Joint Typhoon Warning Center (JTWC) pegged Nanmadol’s top winds at 80 mph (1-minute average), moving west at 6 mph. The Japan Meteorological Agency put Nanmadol’s intensity at 75 mph (10-minute average sustained winds).

Nanmadol had favorable conditions for intensification, with sea surface temperatures of 29-30 degrees Celsius (84-86°F), light wind shear of 5-10 knots, and a moist atmosphere. The Joint Typhoon Warning Center, in an 11 a.m. EDT Thursday advisory, called for a period of steady intensification, with Nanmadol topping out as a category 4 typhoon with 140 mph winds at 12Z Saturday. Weakening is expected thereafter as the typhoon interacts with the landmass of southern Japan, before an expected Sunday landfall as a category 2 or 3 typhoon on Kyushu, the southwesternmost of Japan’s four main islands. Satellite images showed Nanmadol as a sizable typhoon, likely to bring torrential rains and damaging flooding to a large portion of southern Japan this weekend.

Shanghai cleans up from impacts of Typhoon Muifa

Typhoon Muifa affected the world’s third most populous city, Shanghai, and its 25 million residents with an unusual direct landfall on Wednesday night local time. Muifa made its initial landfall in Putuo on Zhoushan Island, China (about 50 miles south-southeast of Shanghai), at 20:30 Beijing time (10:30 a.m. EDT), causing widespread damage to trees and power lines. The storm quickly moved off the island and headed toward a second landfall on the north side of Hangzhou Bay (the south side of Shanghai’s urban core), becoming the strongest typhoon to reach the Shanghai area in records dating back to the establishment of the People’s Republic of China in 1949. The scope of the damage to Shanghai is still unclear.

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