Near record number of tropical storms cause alarm in Asia and USA
Wed 12 Sep 2018

A near record number of simultaneous typhoons, hurricanes and tropical storms currently nearly circle the globe from the Sea of China, across the Pacific and Atlantic to close to the African mainland. Two of these are in the highest categories on the Saffir-Simpson scale and are expected to strengthen.

The UK Met Office Twittered a good animation on 10 September showing where they were then. Their strengths have varied since, but all the storms are still active. Typhoons, hurricanes and Cyclones are all the same beasts, while Tropical Storms are below cyclone strength but can still produce damaging winds, and Tropical Depressions are another step down again and both can deliver flooding rain.

From the west, Tropical Storm Barijat is moving westwards to the south of Hong Kong, then will skirt the south China coast during Thursday and Friday before moving into northern Vietnam. It will be China's 23^rd typhoon or storm this year. Rain will be the main problem, though wind will gust 80-90km/h for the next few days. It hasn’t reached Category 1 and isn’t expected to.

Low-angled light of approaching sunset sharpens both the dramatic and the delicate cloud features of Super Typhoon Mangkhut at 1710 AEST on 11 September. Guam is shown on the far right. JMA Himawari 8

Typhoon Mangkhut, at category 5, is classified as a super typhoon and threatens the northern Philippines, Hong Kong and mainland China. The Joint Typhoon Warning Center (JTWC) at 1900 AEST today 12^th described it as moving west at 19km/h and being "a symmetric and highly consolidated system with deep feeder bands spiraling tightly into a sharply-outlined 30NM [55km diameter] eye". Sustained winds are currently estimated to be 275km/h with gusts to 335km/h, and the size of the typhoon, measured across its area of gale-force and higher winds, is just over 800km although the total system width is much larger. Its latest estimated central pressure is 905hPa.

JTWC expect Mangkhut to clip the northern tip of Luzon, north of Manila and south of Taiwan, late Saturday morning AEST with winds gusting to 270km/h. It then weakens due to interaction with land and reducing upper outflow, passing close to Hong Kong on Saturday afternoon AEST with gusts to 200km/h, and crossing the coast of China about 12 hours later. The Global Disaster Alert and Coordination System (GDACS) estimates 42.6 million people will be exposed to risk from the typhoon.

Moving farther east in the Pacific, Olivia, a Category 4 hurricane on 7 September, has fortuitously weakened to Tropical Storm status as it grazes the south of Hawaii today, but heavy rain is still expected. Farther east again, Paul, a Tropical Storm well west of Mexico, has weakened to a Tropical Depression as it moves slowly west.

Category 4 Hurricane Florence in the western North Atlantic is looking to be the most dangerous hurricane to hit the US east coast in living memory. The sea surface temperatures that it passes over right up to landfall are 1 to 3° above normal, giving it abundant energy and huge amounts of moisture. The upper winds above the hurricane are also favourable for it to maintain its power, and likely strengthen during Thursday 13^th, but will cause it to slow considerably late Thursday 13^th local time as it approaches the North and South Carolina coasts. It seems likely to make landfall overnight Thursday/Friday. Like Mangkhut, it is an enormous cyclone (compare its size with Florida, at left). GDACS estimates 1.3 million people will be exposed to risk.

Hurricane force winds are expected as Florence arrives on shore, but the gravest dangers come from the storm surge and rainfall. Storm surges of 1 to 3m are forecast on the southern side of Florence, locally higher and driven well inland by hurricane-driven waves. The surges come on top of a general 30cm rise in water level in the past 30 years due global warming, and will be worsened if time of arrival coincides with high tides. Mandatory evacuations of over a million people have been ordered in the last few days, mainly in coastal parts of South and North Carolina and Virginia, and a massive, co-ordinated effort is under way to get people out of the area.

The unusual circumstances that slow the hurricane in its approach may cause it to stall inland after it arrives, prolonging torrential rain. At present, the National Hurricane Center (NHC) is forecasting 500 to 750mm with isolated totals of 1000mm along coastal North Carolina, with 125 to 250, isolated 500mm, in South Carolina and W and N North Carolina. The NHC says "This rainfall would produce catastrophic flash flooding and significant river flooding." Should Florence stall inland, amounts could be much greater. In similar circumstances after Hurricane Harvey struck Houston in August last year, falls of 1100 to over 1500mm were recorded during the week after landfall, with the highest coming in at 1538.8mm. This and another very close reading were the highest single-storm readings ever recorded across the 50 United States, and helped Houston on its way to a one-in-a-thousand year flood.

The World Meteorological Organisation (WMO) issued this news bulletin late today 12^th, giving additional information on Florence and Mangkhut. The SMH [partial paywall] also covered the two in this story.

Moving east again, Isaac, also fortunately, has weakened from Category 1 on 10 September to a Tropical Storm, but will still bring rain and sustained winds of 80-100km/h to the Leeward Islands on Thursday 13^th with heavy rain threatening up to 840,000 people according to GDACS. These were the islands that bore the brunt of multiple major hurricanes last year.

Finally Helene is currently Category 2 as it curves from NW to NE through the eastern Atlantic, but is likely to be comfortably within Tropical Storm or Depression status by the time it moves west of the Azores in about 5 days.