A hurricane warning is up for Puerto Rico, now in the sights of an intensifying Tropical Storm Fiona that could bring 10 to 20 inches of rain. Meanwhile, Japan is bracing for Typhoon Nanmadol (see below), which is on track to be one of the nation’s most destructive storms in years.

Fiona packs a surprising punch in the Leewards

Guadeloupe was hammered unexpectedly hard by Fiona. Although Fiona was still somewhat disorganized as it passed through, intense showers and thunderstorms (convection) blossomed overnight, dumping huge amounts of rain to the island nation. The Neuchâteau station on Guadeloupe’s southeast coast picked up 423 millimeters of rain, or 16.65 inches.

At least one death was reported in Guadeloupe, and more than 100,000 customers lost electricity. Residents were told by authorities to stay home on Saturday morning while damage was assessed.

Satellite image of Fiona
Figure 1. Visible satellite image of Hurricane Fiona at 1635Z (12:35 p.m. EDT) Saturday, September 17, 2022. (Image credit: tropicaltidbits.com)

Forecast for Fiona

At 11 a.m. EDT Saturday, Fiona was about 130 miles southeast of St. Croix, moving west at 8 mph with top sustained winds of 60 mph. Fiona’s circulation was quite large on Saturday morning, but it was not yet fully aligned vertically. This lack of full vertical alignment can leave the center of an organizing tropical cyclone vulnerable to relocation if strong convection alerts the balance of forces. Such a relocation happened with Fiona on Saturday morning, shifting both the initial position and the forecast track a bit eastward.

The upshot: Fiona is now expected to move over or near western Puerto Rico on Sunday, perhaps as a hurricane. On such a track, most or all of the island would be on the stronger right-hand side of Fiona, so torrential rains will likely pose a serious flood threat. Parts of Puerto Rico were still abnormally dry as of last Tuesday, according to the U.S. Drought Monitor, but Fiona will produce more than enough rain for significant flash flooding and mudslides. The National Hurricane Center (NHC) is forecasting 12 to 16 inches, with local maxima of 20 inches possible. Puerto Rico is still in a fraught multiyear process of upgrading its electrical grid – the lights went out on Saturday during a Fiona briefing from the governor – so power supply is a concern as well.

Most or all of the Dominican Republic will be on Fiona’s weaker left-hand side. Still, parts of the eastern DR could get up to 12 inches of rain.

By Monday, Fiona will have moved into the open Atlantic north of the Antilles. There is high uncertainty on Fiona’s strength at this point, depending on how much Fiona can organize and how much that organization is dented by land interaction. Regardless, Fiona appears set to become a powerful Atlantic hurricane next week as it heads northwest and then north. Moderate wind shear of 10-15 knots on Sunday and Monday will be countered by an increasingly moist atmosphere (mid-level relative humidity increasing to 60-65 percent), very warm sea surface temperatures (SSTs) of 29-30 degrees Celsius (84-86 degrees Fahrenheit), and substantial oceanic heat content. The HWRF and HMON intensity models both project Fiona to be at category 3 strength by Wednesday.

It now appears increasingly likely that a break in the broad midlatitude ridge that stretches from the eastern U.S. across the Atlantic will allow Fiona to recurve into the Northwest Atlantic. Only a handful of ensemble members from the 06Z ensemble runs of the European and GFS models continue to indicate a threat to the U.S. East Coast (see Figure 2 below). The odds of a hurricane strike are rising for Bermuda, though.

Ensemble output from models
Figure 2. Ensemble output from the ECMWF (left) and GFS (right) model runs at 06Z Saturday, September 17, 2022. (Image credit: weathernerds.org)

Elsewhere in the Atlantic

NHC was tracking one other system, located in the central tropical Atlantic, that could take on some organization by the middle of next week. Even if it does, it will most likely aim for the same break in the ridge that Fiona is targeting and stay well out to sea. NHC gives this system a near-zero chance of development through Monday and a 20% chance by Thursday.

A rare emergency warning for Japan for Typhoon Nanmadol

In the northwest Pacific, huge and powerful category 4 Typhoon Nanmadol is poised to land a devastating blow to Japan on Sunday. The Japan Meteorological Agency has issued a rare emergency warning for violent winds, high waves and a storm surge for Kagoshima Prefecture in southwestern Japan, where Nanmadol is expected to make landfall at approximately 12Z Sunday. The level 5 alert is the highest on the emergency scale, and is only used for a situation of unprecedented danger.

Above: The Meteorological Research Group of the University of the Ryukyus has done two reconnaissance flights into Nanmadol. Here is the translation of this Tweet: “It is the second day of observation of Typhoon Nanmadol. We succeeded in reaching the center today as well. This is the wall cloud seen from an altitude of about 14km. The slope of the wall cloud looked like a precipitous cliff.”
Super Typhoon Nanmadol forecast
Figure 3. Forecast for Super Typhoon Nanmadol from the Joint Typhoon Warning Center at 11 a.m. EDT Saturday, September 17, 2022.

According to the Joint Typhoon Warning Center (JTWC), Nanmadol had weakened slightly since attaining its peak intensity of 155 mph winds at 2 p.m. EDT Friday. At 8 a.m. EDT Saturday, Nanmadol was a slightly weaker but still formidable category 4 typhoon with 145 mph winds, and was moving north-northwest at 12 mph. The noon EDT Saturday update from the Japan Meteorological Agency put Nanmadol’s intensity at 120 mph (10-minute average sustained winds), with an exceptionally low central pressure of 910 mb.

Satellite images Saturday afternoon (U.S. EDT) showed a very large and imposing storm with a prominent 13-mile-diameter eye surrounded by very cold cloud tops. Nanmadol’s hurricane-force winds extended out up to 85 miles from the center, and tropical storm-force winds extended out up to 270 miles. This large wind field was generating huge waves up to 48 feet high, according to the JTWC. Moreover, these winds will be able to pile up a massive storm surge capable of causing devastating destruction.

Nanmadol had favorable conditions for intensification on Saturday, with sea surface temperatures of 29-30 degrees Celsius (84-86°F), light wind shear of 5-10 knots, and a moist atmosphere. However, the ocean had less heat content than on Friday, and the Joint Typhoon Warning Center, in an 11 a.m. EDT Saturday advisory, called for steady weakening before landfall, because of higher wind shear, cooling sea surface temperatures, and interaction with the landmass of southern Japan. Nanmadol is predicted to be a borderline category 2 or 3 storm at its expected Sunday landfall on Kyushu, the southwesternmost of Japan’s four main islands.

Predicted wind speed and sea level pressure for Nanmadol
Figure 4. Predicted wind speed (colors) and sea level pressure (black lines) for Typhoon Nanmadol at 5 a.m. EDT (9Z) Sunday, September 18, from the 6Z Saturday, September 17, run of the HWRF model. The model predicted Nanmadol would make landfall near Makurazaki, Japan, putting Kagoshima Bay in the right-front quadrant of the storm. Nanmadol was predicted to be a category 3 typhoon with 115 mph winds and a central pressure of 928 mb. (Image credit: Tropical Tidbits)

Potentially a historic typhoon for Japan

The massive storm is forecast (Figure 3) to bring at least tropical-storm force winds to all of Kyushu and to more than 95% of Honshu, Japan’s main island, as the storm takes a worst-case track along the length of both islands. Torrential rains and damaging flooding will affect a large portion of southern Japan this weekend. Storm surge and wind damage will likely be severe near Nanmadol’s landfall location in Kyushu, and the typhoon very likely will lead to a multi-billion-dollar disaster for Japan.

The greatest concern for storm surge will be in Kagoshima Bay, a long, narrow bay that can act as a funnel that concentrates storm surge. The city of Kagoshima (population 600,000) lies on the west side of the bay. Kagoshima Bay is predicted to experience the powerful right-front quadrant of Nanmadol when the typhoon is predicted to be near category 3 strength (Figure 4). The Makurazaki Typhoon of September 17, 1945, which took a track similar to that expected for Nanmadol, generated a storm surge in excess of 2 meters at the head of the bay, causing severe damage to the west side of the bay. The 1945 typhoon made landfall near Makurazaki with a central pressure of 916.7 mb; the Japan Meteorological Agency is predicting that Nanmadol will have a central pressure of 920 mb at landfall near Makurazaki.

Tracks of six costliest typhoons
Figure 5. Tracks of the six typhoons that have caused at least $6 billion in damage (2022 USD) in Japan, as rated by EM-DAT, the international disaster database. #Nanmadol will threaten to join this list. (Image credit: NOAA Historical Hurricanes Tracks tool)
Costliest typhoons, 1945-2021
Figure 6. Costliest western Pacific tropical cyclones in world history, as rated by EM-DAT. Six of the top-10 most expensive storms have been in Japan.

Tropical Storm Lester making landfall on Mexico’s Pacific coast

Tropical Storm Lester formed on Friday morning in the waters near Mexico’s Pacific coast, and will have a very short life: at 11 a.m. EDT Saturday, the center of the storm was near the coast of Mexico, and was expected to dissipate by Saturday night as it moves inland over the high terrain of Mexico. Lester had winds of just 40 mph, but will be a very dangerous heavy rain threat. Localized flash flooding and mudslides may occur from rains of 8 to 12 inches, perhaps up to 16 inches, are predicted over coastal Michoacan and Guerrero states.

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Bob Henson is a meteorologist and journalist based in Boulder, Colorado. He has written on weather and climate for the National Center for Atmospheric Research, Weather Underground, and many freelance...

Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...