Tropical storm warnings were flying for the Leeward Islands, Virgin Islands, and Puerto Rico on Friday afternoon as Tropical Storm Fiona began spreading heavy rains into the islands. As of 11 am Friday, Fiona was packing 50-mph sustained winds and was about 135 miles east of the Leewards, moving west at 14 mph. The first heavy rain showers from the storm had already arrived in the islands, as seen on Guadeloupe speed camera.
Fiona is surrounded by a large area of dry air, and satellite pictures Friday afternoon showed little change to Fiona over the previous 24 hours: The circulation center was exposed to view, with the storm’s heavy thunderstorms restricted to the east side of the center. This situation occurred because a moderate 10-20 knots of wind shear was bringing dry air on the west side of Fiona into its core. However, there was more heavy thunderstorm activity near the core of Fiona than there was on Thursday, an indication that the shear had decreased some. Sea surface temperatures (SSTs), warmer than on Thursday, were about 29.5 degrees Celsius (85°F) – approximately 0.5 degree Celsius (0.9°F) above average for this time of year.
Forecast for Fiona
The primary threat from Fiona over the next three days will be heavy rains causing flash flooding. Eastern Puerto Rico (4-8 inches of rain predicted) and the eastern Dominican Republic (6-10 inches) are particularly at risk. Even moderate-strength tropical storms can cause devastating flooding when they drop torrential rains atop already saturated soils. Tropical Storm Erika in 2015, with peak winds that never exceeded 50 mph, brought up to a foot of rain to Dominica, causing catastrophic flooding that killed 35 people and did more than $500 million in damage. Erika was one of only two tropical storms to have her name retired.
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. Over the weekend, though, Fiona will be moving into a moister atmosphere with more favorable upper-level winds, which might allow the storm to become a category 1 hurricane before its expected landfall in the Dominican Republic on Sunday night or Monday morning.
This landfall is expected as a result of a change in the steering currents, resulting from passage of a trough of low pressure that will be moving off the US East Coast. If Fiona remains weaker than expected, this northwesterly turn will be delayed and less sharp than the National Hurricane Center is predicting.
Fiona’s encounter with the rugged terrain of Hispaniola will disrupt the storm, but the models insist that this disruption will be temporary, and the official NHC forecast brings Fiona to hurricane strength on Tuesday, when it is expected to be the near the Turks and Caicos Islands and southeastern Bahamas. However, Fiona could be much weaker then if her encounter with Hispaniola proves more disruptive than forecast.
Later in the week, Fiona poses a threat to Bermuda or the US East Coast. It remains to be seen if the trough of low pressure that turns Fiona to the northwest will be strong enough to recurve the storm to the northeast and out to sea, as the GFS model is predicting. The 0Z Friday run of the European model predicted that Fiona would not recurve next week, but would instead be trapped to the south of a ridge of high pressure that would build in to the north, forcing Fiona toward the US East Coast. Stay tuned.
A serious threat for Japan: Super Typhoon Nanmadol
In the northwest Pacific, Super Typhoon Nanmadol exploded into a mighty category 4 storm with 150 mph winds on Friday, and is poised to land a devastating blow to Japan over the weekend.
According to the Joint Typhoon Warning Center (JTWC), Nanmadol rapidly intensified by 70 mph in the 24 hours ending at 8 am EDT Friday, and was moving northwest at 9 mph. The 12 pm EDT update from the Japan Meteorological Agency put Nanmadol’s intensity at 110 mph (10-minute average sustained winds), with a very low central pressure of 925 mb.
Satellite pictures Friday afternoon (US EDT) showed a very large and imposing storm with a prominent 13-mile-diameter eye surrounded by very cold cloud tops. Nanmadol’s large size was allowing it to generate huge waves up to 52 feet high, according to the JTWC. Precipitable water imaging showed Nanmadol was pulling in moisture from a vast area of the northwestern Pacific, with a plume of water vapor more than 2,000 miles long feeding into the typhoon from southeast Asia and across the Philippines. This tremendous spigot of water vapor will enable Nanmadol to drop prodigious amounts of rain over Japan this weekend, and the rainfall forecast there is very concerning.
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 am EDT Friday advisory, called for Nanmadol to peak just below category 5 strength on Friday night, with 155 mph winds. Weakening is expected thereafter 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. The massive storm is forecast (Figure 2) 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 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 likely will be severe near Nanmadol’s landfall location in Kyushu, and the typhoon very likely will lead to a multi-billion dollar disaster for Japan.
Ex-Merbok brings high surf, high wind to Alaska
The post-tropical cyclone that was Typhoon Merbok just two days ago crossed the Aleutian Islands of Alaska and entered the Bering Sea late Thursday into Friday morning, bringing winds that tasted to 81 mph at Adak Island. Gusts of up to 100 mph are possible in the open reaches of the Bering Sea on Friday, and waves of up to 54 feet are expected. The west coast of mainland Alaska is plastered with coastal flood warnings and high-wind warnings.
As of Friday morning, Merbok’s central pressure had dipped to 937 millibars, a reading only achieved at sea level by intense tropical or extratropical cyclones. No other Bering Sea storm in September is known to have had a central pressure below 960 mb, National Weather Service (NWS) meteorologist Jonathan Christiest told CNN. The year-round record for the Bering Sea is 924 mb in November 2014, as noted by Christopher Burt in a review of the most intense Bering Sea storms.
Ex-Merbok’s powerful winds and rapid northward motion have pulled a ribbon of tropical moisture into subpolar latitudes (see tweet below). Precipitable water within this narrow ribbon, or the amount of water vapor above a given point, ranged above 1.5 inches in the southern Bering Sea, which is close to the average value for mid-September in Charleston, South Carolina.
At Nome, Alaska, the southerly flow from ex-Merbok will push a major storm surge on Friday night into the coastline. Depending on the timing, surge values projected to be between 11 and 12 feet will combine with astronomical tides to yield a storm tide predicted by NOAA on Friday to be as high as 12.24 feet overnight. Many streets and structures will be inundated, the NWS warned. The surge will likely well outpace the 8.6-foot surge from a destructive storm in November 2011, perhaps falling just short of the 13-foot surge observed in November 1974, the city’s strongest storm in modern records. Some level of surge may extend up the Kuskokwim River as far as Bethel, 50 miles inland from Kuskokwin Bay.
With post-tropical Merbok arriving so early in the fall season, there will be no sea ice to help protect the Bering Sea coast from surge and erosion.
Tropical Storm Lester heading toward Mexico’s Pacific coast
A new tropical depression that formed Thursday afternoon off the Pacific coast of Mexico became Tropical Storm Lester on Friday morning. Lester’s top sustained winds were only 40 mph as of 11 a.m. EDT Friday. A straightforward steering flow will push Lester northwest to an anticipated landfall late Saturday on the coast of Guerrero state, perhaps near or just west of Acapulco. This track and timing will give Lester only about 36 hours to strengthen, and easterly wind shear of 15-20 knots will keep any intensification slow. At least some strengthening is expected, though, given warm SSTs of around 29 degrees Celsius (84°F) and an extremely moist atmosphere (mid-level relative humidity of 85 percent). The most likely outcome, projected by the HWRF and HMON intensity models and reflected in the NHC forecast, is for Lester to make landfall as a strong tropical storm. Localized flash flooding and mudslides may occur from rains of 3 to 6 inches, perhaps up to 10 inches.
Another eastern Pacific tropical disturbance – Invest 94E, located just west of Lester – will draw on the same rich moisture and warm waters, but will be plagued by strong wind shear of 25-30 knots till at least Sunday. As the shear abates, 94E could reach tropical storm strength; however, it’s expected to remain well offshore.
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