Japan’s remote Ishigaki Island, part of the Yaeyami chain at the southwest end of the Okinawa group, took a direct hit from slow-moving Typhoon Muifa on Monday local time. Fortunately, the typhoon had weakened substantially from its category 3 peak achieved over the weekend, arriving at category 2 strength, and the winds on the stronger east side of Muifa’s eyewall bypassed the island. Top sustained winds at Ishigaki were around 56 mph in the southern eyewall, while the heaviest rains and squalls were in the northern eyewall.
As of 8 a.m. EDT Monday, Muifa remained a category 2 storm with top sustained winds of 100 mph. Muifa was embarking on a second eyewall replacement cycle Monday, which will help limit the typhoon’s ability to take advantage of warm sea surface temperatures (around 28 degrees Celsius or 82 degrees Fahrenheit) and still-moderate wind shear (10-15 knots). The atmosphere is only moderately moist (mid-level relative humidity around 60%). Overall, the environment should allow Muifa to hang together, perhaps gradually weakening over the next couple of days. Update: As of 8 a.m. EDT Tuesday, Muifa remains a category 2 storm, now with top sustained winds of 105 mph, still on track to weaken and head toward the Shanghai area as detailed below.
Muifa is on track to pass over or near Shanghai late Wednesday, probably as a borderline tropical storm/category 1 typhoon, and hug the coast of east-central China as a tropical storm, possibly passing near Beijing around Friday before it dissipates entirely. Most of the coastline will be on the weaker left-hand side of Muifa, so winds should not be a major threat, but widespread torrential rain is a distinct concern. Totals of 3 to 10 inches are possible in and near Shanghai, with 3 to 6 inches or more also possible as Muifa encounters the Shadong Peninsula.
Next up in the Northwest Pacific: Merbok
The Northwest Pacific continued its revival after an unusually quiet August with the formation of Tropical Storm Merbok on Sunday. As of 11 a.m. Monday, Merbok was located in the remote Northwest Pacific about 350 miles northwest of Wake Island, packing top sustained winds of 50 mph. Merbok should remain harmless to land interests, moving north and eventually northeast while topping out as a category 2 storm. Update: As of 8 a.m. Tuesday, Merbok was an intensifying tropical storm with top sustained winds of 70 mph. A new system, Tropical Depression 16W, will intensify over the Northwest Pacific over the next several days and may approach southern Japan as a significant typhoon by the weekend of September 17-18.
Despite the active week now under way, the Northwest Pacific is having a notably tranquil typhoon season overall – a common outcome during La Niña years. The amount of accumulated cyclone energy (ACE) in the Northwest Pacific for the year through September 12 was little more than half of the basin’s long-term average to date, according to statistics kept by Colorado State University.
Quiet in the Atlantic
On Monday morning, the National Hurricane Center was monitoring two Atlantic tropical waves for development. Neither wave has much model support for development, and both will struggle with the hostile conditions that have done in so many tropical waves this year.
A tropical wave midway between the coast of Africa and the Lesser Antilles on Monday afternoon was the more impressive of the two, with a large area of disorganized thunderstorms showing a little bit of rotation. This wave was under light-to-moderate wind shear of 5-15 knots, but was surrounded on three sides by very dry air. The wave will progress west to west-northwest at 15 to 20 mph this week, and will likely bring rain showers and gusty winds to the Leeward Islands beginning on Thursday night or Friday. In its 8 a.m. EDT Monday Tropical Weather Outlook, NHC gave the wave 2-day and 5-day odds of development of 0% and 20%, respectively. Update: As of 8 a.m. EDT Tuesday, this wave – now classified as Invest 96L – has 2- and 5-day development probabilities of 30% and 40%.
Another tropical wave near the coast of Africa on Monday afternoon was also predicted to move west to west-northwest at 15-20 mph, and it will pass through the Cabo Verde Islands on Tuesday. Dry air and high wind shear will make this wave struggle to develop, and in its 8 a.m. EDT Monday Tropical Weather Outlook, NHC gave the wave 2-day and 5-day odds of development of 0% and 20%, respectively. Update: The 2- and 5-day odds remain 0% and 20% as of 8 a.m. EDT Tuesday.
With the season slightly more than half over, there have been five named storms, two hurricanes, and no, zero, intense hurricanes – the first time since 2014 that no intense hurricane has been observed by the season’s halfway point. The 1991 – 2020 climatological averages for this point in the season are 8.5 named storms, 3.5 hurricanes, and 1.6 major hurricanes. The accumulated cyclone energy is currently 47% of average. The long-term runs of the GFS and European model show little change to the hostile conditions for hurricane development over the coming week, and it is a reasonable bet that there will be no named storms this week.
Historically, the first half of September is the busiest part of the Atlantic hurricane season, and it is uncommon to go more than a week without a named storm during early- to mid-September. The last season with a long break at this time of year was 2016, when a nine-day gap occurred from September 3, when Hermine became post-tropical, to September 12, when Tropical Storm Ian formed.
A new tropical depression expected in the eastern Pacific
An area of low pressure located along the coast of southern Mexico was producing disorganized cloudiness and showers on Monday afternoon. This system is forecast to interact and merge with another tropical disturbance off the southwestern coast of Mexico around midweek, and form into a tropical depression by the end of the week while moving slowly westward. Heavy rains from the disturbance are likely to affect the coast of southern and southwestern Mexico beginning on Wednesday. In its 8 a.m. EDT Monday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day odds of development of 10% and 70%, respectively. Update: As of 8 a.m. EDT Tuesday, the 2- and 5-day odds of this disturbance – classified as Invest 94E – are now 30% and 80%. Another disturbance just to the west of 94E has 2- and 5-day odds of development of just 10% and 10%.
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