Super Typhoon Meranti Threatens China with Major Storm Surge Event

Super Typhoon Meranti barrels through the Pacific and will threaten China with a major storm surge event on Wednesday night.

In the pre-dawn hours local time Wednesday, Meranti approached the southern tip of Taiwan as a Super Typhoon with maximum sustained winds near 190 mph. This would place the storm well above the threshold for a category-5 hurricane in the Atlantic or Eastern North Pacific basins.

Super Typhoon Meranti was centered just south of Taiwan in the pre-dawn hours of Wednesday. Source: NOAA

Fortunately for Taiwan, Meranti's center of circulation is forecast to track south of the island. Nonetheless the powerful storm will inflict damage from extraordinary waves, wind and heavy rain/ mudslides.

The real concern for coastal flooding will come Wednesday night, as Meranti approaches China's coast as a Super Typhoon, with winds equivalent to a category-4 hurricane. The Joint Typhoon Warning Center predicts Meranti's maximum sustained winds will be 115 knots (132 mph) around midnight tomorrow local time, as the storm is centered just off the coast of China.

The U.S. Navy's Joint Typhoon Warning Center forecasts Super Typhoon Meranti to track towards China during the day on Wednesday. 

Meranti's storm surge potential will be a great concern in places like Xiamen, China. In 1983, Super Typhoon #4 took a similar track to Meranti and generated a 5.87-ft (1.79-m) storm surge in Xiamen (Liu 2002). The 1983 typhoon was weaker than Meranti, however, approaching the coastline with 80-knot (92 mph) winds.

Of course, typhoon size is a factor in storm surge height as well. If Meranti's size remains comparable or larger than the 1983 typhoon, and the winds remain stronger, the surge potential will be greater than in 1983, and could exceed 10 feet (3 m) in portions of coastal China. The country observes a storm surge of this height around once every three years; 15 surges exceeding 10 ft (3 m) were observed in the 50-year period from 1949 to 1998 (Tang et al. 2011).

Super Typhoon #4 in 1983 took a similar track as Meranti. Maximum sustained winds were lower than Meranti's forecast on approach to China. Source: Unisys Corporation.

Although such storm surge heights may not sound extreme by global comparison, the impacts of a coastal flood like this could be severe, given the dense coastal population in China. Even two decades ago, the 3.28 ft (1-m) coastal plain in China contains 73 million people (Han et al. 1995), enabling  low-magnitude surges to have major impacts.

Typhoon Fred in 1994 provided an excellent example of severe coastal flooding impacts in China from a moderate storm surge level. Even though the highest storm surge level only reached 8.83 ft (2.69 m) (Le, 2002), this typhoon destroyed 323 miles (520 km) of seawalls, inundated 189 towns and flooded more than 22 million people (Le, 2000).

China's densely populated coast is vulnerable to major impacts from even moderate storm surges. The city of Xiamen, pictured above, may experience a major storm surge, depending on Meranti's track. Image: realbusiness.co.uk

This should cause coastal residents to take precautions and share for a widespread storm surge from Meranti.

Coastal populations should also consider Meranti's timing when considering evacuation. Meranti's greatest surge potential will come around the time of landfall, which will be after midnight on Wednesday night. It is always harder for people to discern a flood situation during the dark of night, so it is wise for coastal residents to either evacuate or prepare for storm surge during the daylight hours on Wednesday.

REFERENCES

Han, M., J. Hou, and L. Wu, 1995: Potential impacts of sea-level rise on China’s coastal environment and cities: A national assessment, Journal of Coastal Research, 14, 79–95.

Le, K., 2000: An analysis of the recent severe storm surge disaster events in China, Nat. Hazards, 21, 215–223.

Le, K., 2002: Severe storm surge disasters and strategic measures, Mar. Forecasts, 19(1), 9–15.

Liu, J., 2002: Feature and Varied Rule of Typhoon Storm Surge along the Coast of South East China Sea. Marine Forecasts, 19, 1, 81-88.

Tang, L., J. M. Zhan, and Y. Z. Chen, 2011: Typhoon process and its impact on the surface circulation in the northern South China Sea, Journal of Hydrodynamics, 23(1), 95–104.

Hermine Produced the 5th-Highest Recorded Water Level at Cedar Key, Florida

Hurricane Hermine generated a widespread storm surge along Florida's West Coast, north of the Tampa/ St. Pete metro area. Although Hermine made landfall as a category-1 hurricane, it's relatively large size contributed to the high storm surge. As it approached the coast, 50-kt winds (57 mph) extended to the southeast for 120 miles.

Also, the region in which it struck is very efficient for generating large surges. The concave shape of Florida's Big Bend area, and the shallow bathymetry, or water depth, serve to enhance water levels.

The Weather Channel broadcast Hermine's storm surge inundation live on Facebook during the afternoon and evening on Thu Sep 1.

Water levels were particularly high to the east and south of Apalachee Bay, in areas like Cedar Key. The graph below plots Hermine's water level within a broader context of storm surge events at Cedar Key.

Hermine produced the 5th-highest recorded water level in Cedar Key, Florida.

Rapid summary of high water at Cedar Key:

The water level at Cedar Key reached 7.64 ft above the NAVD88 datum on Fri Sep 2 at 1:36AM EDT. This is:

​The 5th highest storm tide level out of 34 surges on record

The 4th highest storm tide level from a tropical system

​The highest recorded water level since 1993 (23 years)

Note: The storm surge level (actual - predicted level) exceeded 7.5 ft at Cedar Key

All eyes now turn to the Mid-Atlantic Coast, where both the European and GFS models predict that Hermine, or more likely an extra-tropical system, will park near the coast and possibly turn towards land over Labor Day weekend.

The GFS model run, hosted on the Tropical Tidbits website, displays a nearly stationary area of low pressure off the Delmarva Peninsula on Labor Day. 

This forecast is of great concern for coastal areas from Virginia through Massachusetts, as storm surge and waves will have ample time to build if this forecast holds.

Check back over the weekend, as I hope to provide some storm surge analysis for the Mid-Atlantic region.

New Coastal Flooding Products Help Us Track Hermine's Storm Surge

Tropical Storm Hermine is now a strong tropical storm, with maximum sustained winds of 65 mph, as of the National Hurricane Center advisory at 7:00AM Central time. Hermine is forecast to strengthen a bit more, possibly becoming a hurricane before it makes landfall near Florida's Big Bend/ Apalachee Bay.

Tropical Storm Hermine is forecast to strengthen as it approaches landfall near Florida's Big Bend/ Apalachee Bay. 

New coastal flooding products are helping us track Hermine's storm surge,

The National Hurricane Center's new Potential Storm Surge Flooding Map provides a map of water levels that have around a 10% chance of being exceeded. The map below shows that areas of Jefferson and Taylor counties, along the shore of Apalachee Bay, have around a 10% chance of observing 6 ft (1.83 m) or more of water above ground level.

The National Hurricane Center has released a new storm surge inundation product, which provides users with a map of potential water levels above ground

This tool has brought storm surge forecasting into the realm of geospatial analysis, using the SLOSH modeling expertise of the National Hurricane Center's Storm Surge Unit. This product will be very helpful for decision making as tropical weather systems approach coastlines.

The U-Surge project is also providing historic storm surge analog maps, which enable us to see a map of coastal flooding with previous storms that took similar tracks. This map of storm surge from Tropical Storm Alberto (2006) is available at Tropical Storm Hermine's U-Surge page.

Tropical Storm Alberto tracked into Apalchee Bay as a strong tropical storm with maximum sustained winds 70 mph (60 knots) in 2006. Alberto generated storm tide levels exceeding 8 ft (1.22 m) in Dixie County, and 2-4 ft in the Tampa area.

A map of Tropical Storm Alberto's storm track and maximum coastal flooding levels

With such analogs, it is important not to focus on the exact track or to consider the analog a forecast for the present storm. However, analogs provide us with insights into storm surge patterns that are evident with certain storm tracks.

For example, Alberto's map shows a clear distinction between high water levels to the east of the storm track, and relatively low water levels to the west. Although the storm made landfall more than 150 miles north of Tampa, it still generated a 2-4 foot storm tide in that area. We should expect coastal flooding in these areas with Hermine, as well.

Although several feet of storm surge may not sound threatening for places like Tampa, St. Petersburg and Clearwater, flood impacts are often exacerbated in metro areas as the storm surge impedes the drainage of heavy rain that falls on hard surfaces. Such flooding was evident during Tropical Storm Debby in 2012.

A combination of heavy rain and prolonged storm surge led to flooding in the Tampa metro area during Tropical Storm Debby in 2012. Photo: Jason Behnken

The combined impact of storm surge and heavy rain is a major concern for coastal cities. Thomas Wahl was the lead author on a paper published last year in Nature Climate Change that found an increasing trend in compound rain/ surge events for major U.S. cities.

Such compound flood events will become more severe over time even if storm frequency does not increase, due to rising seas associated with climate change.

Historical inundation graphs are another useful tool if you're interested to track Hermine's water levels relative to past coastal flood events. Hermine's U-Surge page provides historical graphs for Cedar Key and Apalachicola in this new product that has just been launched.

Track Tropical Storm Hermine's water level in the context of historical flood events and sea level rise at Hermine's U-Surge page.

Highest total water levels are likely to occur during late afternoon/ evening in Apalachicola. High tide occurs after 4PM, so total water levels may reach maximums between 4-6PM, as the storm is still approaching at the time of high tide.

Cedar Key has a low tide at 9PM, so expect water levels to remain nearly stationary through the evening, instead of dropping, like normal. Then expect water levels to rise rapidly after 9PM, as increasing tidal levels and storm surge combine to elevate total water level. Maximum water levels should occur after midnight, as strong westerly winds behind the back of Hermine continue to push storm surge as the 3AM high tide approaches.

In locations farther removed from Hermine's track, like Tampa, water levels will not rise so suddenly. Expect a gradual water level rise in the metro area, with flooding most likely associated with heavy rain bands that have trouble draining into the elevated coastal waters.