Life-Saving Information about Your Home Elevation in Context of Coastal Floods

As Tropical Storm Cristobal approaches the northern Gulf of Mexico coast, the National Hurricane Center has predicted peak storm surge levels could reach 3-5 feet from southeast Louisiana, east of the Mississippi River levees, to Ocean Springs, Mississippi. Cristobal’s broad wind field is likely to push anywhere from 1 to 4 feet of storm surge along the Alabama and Florida Gulf Coast.

The Experimental Peak Storm Surge Forecast Graphic from the National Hurricane Center forecasts 3-5 feet of storm surge along the Southeast Louisiana and Mississippi Coast from Tropical Storm Cristobal.

It makes sense for coastal residents to compare these forecasted storm surge levels with their home elevations to assess flood risk. Many residents in flood-prone communities have elevation certificates, official documents provided by a licensed surveyor, architect or engineer, that provide the elevation of the lowest habitable floor in their home, as well as the ground beneath the building.

These residents often make the common mistake of assuming the elevation on the certificate measures height above Mean Sea Level (MSL), the approximate average between high and low tide. However, sea level is dynamic, changing levels both long-term, from sea-level rise, and short-term, from tidal cycles and daily weather. Therefore, elevation certificates are measured above a datum, a fixed line that provides a constant “0” mark above which we measure elevations.

An elevation certificate referencing elevations in feet above the NAVD88 datum.

The most common datum referenced on elevation certificates is called the Northern American Vertical Datum of 1988, abbreviated NAVD88. However, some certificates reference the National Geodetic Vertical Datum of 1929, or NGVD29. The datum is provided on the certificate, as seen in the example above, where the “top of the bottom floor” is listed at 11.1 feet above the NAVD88 datum.

Heavy rainfall and storm surge inundate a property near Bay St. Louis, Mississippi, in Tropical Storm Cristobal, June 7, 2020. Photo: Dr. Hal Needham.

So what does this mean for assessing your flood risk from an approaching storm like Tropical Storm Cristobal?

Scientists, surveyors and engineers are trained to convert datum elevations to water levels. After all, what you really care about is the level of water that may impact your building.

Generally speaking, MSL elevations, and certainly high tide elevations, are usually higher than the NAVD88 datum. The graphic below shows the datum conversions at the Bay Waveland Yacht Club tide gauge, near Bay St. Louis, Mississippi, maintained by NOAA Tides and Currents.

Datum conversions for the Bay Waveland Yacht Club near Bay St. Louis, Mississippi, provided by NOAA Tides and Currents. Graphic markups by Dr. Hal Needham.

The water levels in this graphic are measured above Mean Lowest Low Water, or MLLW. This level is basically the mean of the lowest tide levels. Note that the NAVD88 level is 0.7 feet above MLLW, but MSL is 0.9 feet above MLLW. Simple subtraction reveals that MSL is 0.2 feet, or 2.4 inches, higher than NAVD88. I put a yellow box around these values.

For prolonged coastal flood events with broad wind fields, like we might find with Tropical Storm Cristobal, referencing high tide levels may be more important than MSL, because a high tide cycle is more likely during the coastal flood event. That level is shown on the graphic as 1.64 feet above MLLW, or 0.94 feet (11.28 inches) above NAVD88.

Remember that your elevation certificate references height above NAVD88. High tide level is nearly one foot higher than this level, according to the datum chart. The differences between NAVD88 (what is referenced on your elevation certificate) and MSL or Mean High Tide level is site-specific and will vary from one location to the next.

Graphic explaining the two main causes of rising sea levels are melting land ice and thermal expansion of sea water. Source: Northern Gulf of Mexico Sentinel Site Cooperative.

However, before you get out your tape measure to calculate your exact elevation above the closest high tide level, we have one more factor to consider. Rising sea levels related to climate change, specifically from thermal expansion of warming water and melting of land-based ice, raise coastal water levels even higher above fixed datums, like NAVD88.

For example, the datum at Bay Waveland Yacht Club was established from averaging sea level data from 1983-2001. A 19-year averaging period like this is common to account for various tidal ranges related to lunar cycles. Consider that the mean year of this time period was 1992, nearly 30 years ago.

According to Renee Collini, Coordinator of the Northern Gulf of Mexico Sentinel Site Cooperative (NGOMSSC), sea level rise along the northern Gulf Coast has been noticeable. Bay Waveland has observed slightly less than 8 inches of sea level rise since 1978, while Dauphin Island, Alabama, has observed 8 inches since 1966. Rates in western Mississippi are slightly higher than eastern Mississippi and Alabama due to subsidence, or sinking ground. This means water levels today are even higher than the NAVD88 datum than are listed on the datum conversions graphic.

A GPS instrument measures ground elevation in Biloxi, Mississippi. Photo: Dr. Hal Needham.

Collini’s calculations fit my own field work observations I collected while measuring ground elevations along the coast in Biloxi, Mississippi. On my last day of field work before the coronavirus pandemic shut down society, I measured ground elevations at 18 sites, including water-level sites along the Gulf Coast and Biloxi Bay. This work was funded on a grant to CNC Catastrophe and National Claims through the NGOMSSC, in coordination with Dauphin Island Sea Lab.

As you can see in the photo above, the bottom of my GPS pole was located between the water level to the right and high tide line to the left. Note the debris pattern showing the outline of two high tide lines to the left. I estimate that the bottom of my pole was around 2-3 inches vertical distance below the highest tide line. The GPS elevation of my pole bottom read 1.53 feet above NAVD88.

These field measurements provide an approximation that high tide level on that March afternoon would have come to around 1.74 feet ( 1 feet 9 inches) above the NAVD88 datum. We can think of this as though the water level already has a 21-inch head start on flooding the land before a storm even arrives. And that doesn’t even include the contribution to waves that can push additional water on top of the storm surge.

Massive waves pushed by Hurricane Dorian reached the bottom of the pier at North Myrtle Beach, South Carolina, during September, 2019. Storm surge predictions usually do not include the contributions from waves. Photo: Dr. Hal Needham

These factors are important, and surprising, to many coastal homeowners, who assume the house level on their elevation certificate is the height above sea level. In many parts of the coast, high tide levels may be 1-3 feet higher than the NAVD88 datum, meaning the floor boards of people’s homes are 1-3 feet closer to the water level than they thought.

Considering the science behind these measurements and the accelerated rates of sea level rise along our coastlines, coastal residents will benefit from including a “buffer” both when they build new construction and when they assess the risk of flooding during an approaching storm, like Cristobal.

In the context of Cristobal, the 3-5 foot storm surge forecast from the National Hurricane Center does not mean we should expect water levels to stop at 5 feet above the mapped (NAVD88) datum. We should add approximately 2 feet above this level in coastal Mississippi (values will change with location), and then give a little more buffer to account for wave action that comes in on top of the storm surge. The big picture is that coastal residents should build in a considerable buffer when comparing their home elevation with forecast storm surge levels.

Biggest Threat from Cristobal is Heavy Rain along Northern Gulf Coast

Tropical Weather Update – Fri Jun 05 at 0900AM CDT

As Cristobal reorganizes into a tropical storm and tracks towards the northern Gulf Coast, the biggest threat will be heavy rain. My latest Hazard Area Likeliness (HAL) Index Map has now upgraded the likeliness for minor damage from rainfall (6″+) to PROBABLE for southeast Louisiana and the Mississippi Coast, depicted on the map below as red circles at Shell Beach and Gulfport.

Hazard Areal Likeliness (HAL) Index Map for Tropical Storm Cristobal. Updated on Fri Jun 05 at 0900AM

The National Hurricane Center has now released a rainfall potential map, using a forecast from NOAA’s Quantitative Precipitation Forecast (QPF) analysis. The map forecasts a wide swath of rainfall exceeding 4 inches (shown in yellow on the map below), for much of South Louisiana east through coastal Alabama.

TS Cristobal Rainfall Potential Map from the National Hurricane Center. Source: https://www.nhc.noaa.gov/refresh/graphics_at3+shtml/123700.shtml?rainqpf#contents

Due to higher than normal uncertainty in the storm track and the fact that wind shear could push the heaviest rain well east of the center of Cristobal’s circulation, rainfall forecasts could increase substantially to the east, particularly along the Florida Panhandle.

As Cristobal tracks off the Yucatan Peninsula and into the southern Gulf of Mexico, the system should reorganize later on Friday or Friday night. Once this happens, the track forecast should improve. The most likely forecast brings the center of circulation to near the South Louisiana Coast on Sunday night as a moderate tropical storm, as seen in the NHC tracking map below.

NHC Tracking Map for Tropical Depression Cristobal on Fri Jun 05. Link: https://www.nhc.noaa.gov/refresh/graphics_at3+shtml/123700.shtml?cone#contents

Given uncertainties in the track and the forecast for Cristobal to develop a broad wind field, HAL index values for minor damage from tropical storm force winds are now classified as POSSIBLE from Galveston, Texas, through Apalachicola, Florida, including inland locations like Jasper, Texas, Shreveport and Alexandria, Louisiana, and Jackson, Mississippi.

STORM SURGE POTENTIAL

The Big Pic on Storm Surge: Tropical Storm Cristobal’s disorganized structure will reduce surge impact along Gulf Coast. I’m expecting  a widespread 2-3 foot storm surge from South Louisiana to the Florida Panhandle, with isolated maximum surges exceeding 4 feet possible in extreme SE Louisiana and Mississippi.

Minor damage from storm surge exceeding 4 feet is unlikely along most of the Gulf Coast because Cristobal will likely be disorganized, with wind shear and dry air displacing the strongest wind field substantially east of the circulation center.

The map below shows the air pressure and wind vector forecast from the Euro model for Saturday evening (0700PM CDT). This model was initialized on Thursday evening at 0700PM CDT. Note that the strongest wind field is forecast to be offset substantially to the east of the circulation center.

Map of air pressure and wind vectors from http://www.tropicaltidbits.com. Graphical edits from Dr. Hal Needham.

Storm surge piles up more efficiently along the Northern Gulf Coast if a storm has a wind field that is more attached to the core circulation because of a two-step process. If a wind field is more attached to the core, a prolonged east wind sets up along the coast. As water deflects to the right of the wind direction in the Northern Hemisphere because of a process called Ekman Transport (related to the Coriolis Effect), this starts piling up a dome of water along the coast. Stronger winds near the eyewall then force that water ashore with a pronounced storm surge around the time of landfall.

We can think of this process as a 1-2 punch that storms use to efficiently generate surge in this region. It’s not too much of a stretch to compare this process with volleyball players who set up a ball only to see a striker slam it across the net. In a better-organized tropical storm or hurricane, prolonged east winds set up the ball and the eyewall strikes it home in a pronounced storm surge.

Well-organized Gulf storms deliver a 1-2 punch that is similar to a setup and spike in volleyball. We do not expect such results from Tropical Storm Cristobal this weekend. Image: Public Domain Files – non-copyright.

I don’t expect to see such efficient storm surge generation with Cristobal, and for that reason I have classified all locations along the Gulf Coast as UNLIKELY to see minor damage from a 4-foot storm surge, except for extreme Southeast Louisiana and the Mississippi Coast, where I’ve classified Shell Beach and Gulfport as a POSSIBLE chance for a 4-foot surge.

Nonetheless, Cristobal’s broad wind field will likely push a 2-3 foot storm surge along a wide swath of coastline. This surge level is high enough to cause minor coastal and beach erosion and inundate the lowest coastal roads.

HURRICANE PREPAREDNESS TIP #5 – HAVE A RADIO

I am providing seven hurricane preparedness tips in seven days. Today is tip #5- “Have A Radio”.

When I was in Panama City, Florida, during and after Hurricane Michael in 2018, I had a conversation with a man who was running a disaster relief center from his church. I asked him what he learned in the storm.

He told me, “Hal, I can’t believe I was blindsided this time. We’ve become so reliant on smartphones and I never thought of what I would do if we lost phone service.”

Extensive wind damage cut electricity and cell phone service during Hurricane Michael in 2018. This picture shows damage in Panama City, FL. Photo: Hal Needham

In Michael’s aftermath, not only was power cut for an extended time, but many people lost cell phone service for more than one week. Important disaster-related information are shared on the radio during these times. This includes information like curfew details, FEMA supply giveaways, Red Cross food and water distribution, advice on where water should be boiled and where to apply for storm relief from FEMA and other organizations.

If you live in within 100 miles of the U.S. Gulf or Atlantic Coasts, you should really have a battery-powered radio to keep you plugged in with crucial information should you lose electricity and/or cell phone service this hurricane season.

Tropical Weather Update Thu Jun 04 -0500PM CDT

Update on Cristobal’s Track and Intensity Forecast

Cristobal has been downgraded from a tropical storm to a depression, with maximum sustained winds of 35 mph. The center of circulation is drifting east-southeast at 3 mph. A turn to the east and then north is expected and forward speed should increase, bringing Cristobal’s circulation center over the warm waters of the southern Gulf of Mexico Friday or Friday night.

Updated track map from the National Hurricane Center

Environmental conditions over the Gulf of Mexico should be marginally favorable for intensification. While sea surface temperatures are warm enough to support strengthening, moderate wind shear and some dry mid-level air should inhibit some of the potential intensification.

As it is difficult to forecast the precise location where the center of circulation will reorganize in the Gulf, the track and timing forecasts have lower confidence than usual. The most likely forecast from the National Hurricane Center predicts that Cristobal will approach the Louisiana coast as a moderate (60 mph) tropical storm on Sunday afternoon or evening, although the location, intensity and timing are all subject to change. The track guidance has shifted slightly west since this morning.

Map of tropical-storm-force winds and likely arrival time from the National Hurricane Center.

According to the National Hurricane Center map above, the most likely arrival time of tropical-storm-force winds is Sunday afternoon or Sunday evening along much of the Louisiana coast.

Update on the Hazard Area Likeliness (HAL) Index Map

As Cristobal’s track guidance has shifted slightly west, the HAL index has been upgraded and downgraded for several cities. The likeliness thresholds are for minor wind and flood damage from tropical-storm-force winds (39 mph+), 6+ inches of rain and 4+ feet of storm surge. Here’s the evening update…

Updated HAL Index Map – Thu Jun 04 at 0500PM CDT

CHANGES FOR MINOR WIND DAMAGE

Downgraded from POSSIBLE to UNLIKELY:

Jackson, MS

Gulport, MS

Upgraded from UNLIKELY to POSSIBLE:

Shreveport, LA

Jasper, TX

High Island, TX

Galveston, TX

CHANGES FOR MINOR RAINFALL FLOOD DAMAGE

Upgraded from UNLIKELY to POSSIBLE:

Pensacola, FL

Destin, FL

CHANGES FOR MINOR STORM SURGE FLOOD DAMAGE

Downgraded from POSSIBLE to UNLIKELY:

Gulfport, MS

Upgraded from UNLIKELY to POSSIBLE:

Cameron, LA

I will continue to update the HAL index twice daily until landfall. I expect once Cristobal reorganizes in the southern Gulf of Mexico Friday or Friday night, the forecast accuracy, and the HAL index, will have higher confidence.

Have a great evening everyone!
-HH

Tropical Weather Update Thu Jun 04 0800AM CDT

TROPICAL STORM CRISTOBOL TRACK AND INTENSITY FORECAST

Tropical Storm Cristobol continues to track inland near the southeastern portion of the Bay of Campeche in Mexico. The storm center is drifting southeast at 2 mph and maximum sustained winds have weakened to 40 mph.

Tropical Storm Cristobol tracking map from the National Hurricane Center

Cristobol is forecast to be downgraded to a tropical depression today as it drifts east and then eventually north. Upper level steering will gradually guide Cristobol to track north, and the center of circulation should emerge over the southern Gulf of Mexico Friday or Friday night.

It is difficult to pinpoint where the center of circulation will set up when it reorganizes over the southern Gulf of Mexico. For that reason, the track forecasts have more uncertainty than usual.

It does appear that environmental conditions over the Gulf of Mexico should support some strengthening and the most likely forecast is that Cristobol approaches the northern Gulf of Mexico, most likely near South Louisiana, Sunday or Sunday night.

Most likely arrival time of tropical-storm-force winds from the National Hurricane Center

The most likely arrival time of tropical storm force winds is Sunday morning in SE Louisiana and Sunday evening for SW Louisiana, Mississippi and Alabama, according to the National Hurricane Center. This timing is subject to change.

The biggest threat from June tropical storms is flooding from torrential rains if the storm stalls. At this point it looks like Cristobol will pass through the northern Gulf without stalling, which is great news.

UPDATED HAZARD AREA LIKELINESS (HAL) INDEX MAP

I updated the Hazard Area Likeliness (HAL) Index Map this morning and made some edits. The biggest changes are found in South Louisiana. I moved several locations to places that are more susceptible to storm surge.

I replaced Lafayette with Cypremort Point, a small community on Vermillion Bay that observed a 7.3-foot storm tide last year during Hurricane Barry. I moved the Morgan City location east to Grand Isle, because Grand Isle is more vulnerable to storm surge. Finally, I moved the New Orleans location east to Shell Beach, a location extremely susceptible to storm surge, just outside the levees of St. Bernard Parish. Shell Beach approximates the storm surge levels approaching New Orleans from the east.

Hazard Area Likeliness (HAL) Index Map – Thu Jun 04 0800AM CDT

These changed locations enabled me to increase the likeliness of minor storm surge damage (4 foot surge) to POSSIBLE at Cypremort Point, Grand Isle and Shell Beach. I also classify Gulfport, Mississippi, as POSSIBLE.

The only changes to likeliness of minor wind damage is that Port Arthur, Texas has been upgraded from UNLIKELY to POSSIBLE since yesterday evening. The rest of Texas will UNLIKELY see minor damage from wind, rain or storm surge. If you’re in Texas and your local media person is hyping this storm, I suggest you turn off that channel and go back to watching Little House on the Prairie.

The final change this morning is that the likeliness of minor damage from heavy rainfall (6 inches +) has been downgraded to UNLIKELY for the four locations in the Florida Panhandle.

HURRICANE PREPAREDNESS TIP #4

I am giving seven hurricane preparedness tips in seven days. Today’s tip- take a walk around your property and look for any tree branches that could scrape your roof or siding in windy conditions. Also be aware of any overhanging branches that look weak or could possibly fall on your house when winds pick up. Even tropical storm force winds (39 mph +) are strong enough to bring down tree branches or move them erratically so they scrape your house.

A stately oak tree extends branches towards a house in Galveston, Texas. Photo: Hal Needham

Also remember that if your neighbor’s tree threatens to damage your home, you are not powerless. If the tree concerns you, have a friendly discussion with your neighbor. It’s possible that he or she didn’t notice or was unaware of the danger. In some cases the neighbor will pay to remove or trim the tree. I’ve also heard of neighbors splitting the cost of tree removal.

If your neighbor isn’t cooperative with your concern, taking photos of the tree and providing a certified letter of concern to your neighbor could help you immensely in the legal/ financial process if the tree were to fall on your house in the future. But if you don’t say anything, you may not have much of a case.

Thanks for reading everybody and have a great Thursday! Stay tuned for frequent updates, as Cristobol’s track, timing and intensity are all subject to change until it re-emerges into the Gulf of Mexico and reorganizes.

Despite Weakening, Tropical Cyclone Amphan Still Capable of Producing High-Impact Storm Surge

As Tropical Cyclone Amphan tracked towards the coast of India and Bangladesh in the Bay of Bengal on Tuesday, dry air and wind shear began weakening the storm. While this is good news that may serve to reduce some wind impacts, Amphan is still capable of producing a high-impact storm surge for three reasons.

Satellite image of Tropical Cyclone Amphan, provided by University of Wisconsin CIMSS.

Reason #1: Concave-shaped Coastline and Shallow Water Depth

The first reason is that the physical geography and coastal profile of the northern part of the Bay of Bengal is very efficient at generating high storm surges because of the concave shape of the coastline and the shallow water depth, or bathymetry.

The concave shape and shallow bathymetry, or water depth, in the northern Bay of Bengal, enhances storm surge levels. Graphic Link

This may seem counterintuitive, as most people expect locations with deeper offshore water to have a higher potential for coastal flooding. However, when tropical cyclones displace tremendous amounts of water into shallow areas with concave-shaped coastlines, the water gets trapped and has nowhere to go but “up”, as it floods the landscape.

The concave shape and relatively shallow water along the Georgia coast, including adjacent portions of Florida and South Carolina, make it more susceptible to coastal flooding. Graphic: Hal Needham

By contrast, areas with deep bathymetry generally experience smaller storm surges but higher waves. Storm surges and waves differ in that a storm surge is a dome of water that rises across an extended area, whereas a wave is thin pulse of water that pushes towards the coast. We see waves every time we go to the beach, but large storm surge events are much more rare.

The graphics below depict the differences in storm surge potential when tropical cyclones strike shallow deltas, like the northern Bay of Bengal, compared to locations with deeper water, like mid-oceanic volcanic islands.

Profile of coastal flooding near a shallow delta. Note the flat topography both onshore and offshore, the large storm surge that can travel inland for 30 miles, and the small waves. Graphic: Hal Needham

Profile of coastal flooding near mid-oceanic volcanic islands. Note the deep water, steep topography, small storm surge and large waves. Graphic: Hal Needham

The effects of coastline shape and water depth are so important for storm surge generation that weaker storms striking shallow deltas can produce higher storm surges than stronger storms hitting deep coastlines. For example, Hurricane Isaac (2012), generated a storm surge exceeding 12 feet (3.66 meters) in South Louisiana, even though the storm was stalled out as a category-1 hurricane with maximum sustained winds of 80 mph. By contrast, Hurricane Iniki (1992) struck the Hawaiian Islands as a category-4 hurricane with maximum sustained winds of 145 mph, but only generated a 6-foot (1.83-meter) storm surge (U.S. Department of Commerce 1993). The water along the Hawaiian coast is considerably deeper than Louisiana.

Hurricane Isaac (2012) generated a substantial storm surge, even though the storm was classified as a category-1 hurricane. Image Link

The shape and water depth of the Bay of Bengal are so efficient at generating storm surge that the area has broken many weather records. The highest credible storm surge event in history occurred in Bangladesh in 1876, as the storm tide reached 45 feet (13.7 meters) (Dube et al. 1997). The deadliest storm surge on record happened in this same region in 1970, when Cyclone Bhola killed approximately 300,000 people (Frank and Husain 1971).

While Tropical Cyclone Amphan is not forecast to inflict such horrific losses, we should always pay attention when tropical cyclones visit this part of the world.

Reason #2: Strong Pre-Landfall Winds and Large Wind Field

Amphan’s strong pre-landfall winds have already displaced tremendous amounts of water towards shore. This water starts piling up along the coastline long before the more intense winds arrive.

I found that storm surge heights correlate best with the wind speed 18 hours before landfall for basins with shallow water in a journal article in which I was the lead author in 2014. Although this study used wind and surge data from more than 100 tropical cyclones that struck the U.S. Gulf Coast, the physics for the Bay of Bengal has similarities and we should pay close attention to pre-landfall cyclonic winds in that basin as well.

Correlation between storm surge levels and maximum sustained wind speeds at three hour increments preceding landfall. Source: Earth Interactions.

The importance of pre-landfall winds is evident when considering that Hurricane Katrina (2005) generated a higher storm surge than Hurricane Camille (1969), although both storms made landfall along the Mississippi Coast in the United States, and Camille was classified as a category-5 hurricane at landfall, whereas Katrina was a category-3 storm. While Katrina’s large wind field certainly contributed to its large and extensive storm surge, the storm was considerably more powerful before landfall, generating maximum sustained winds of 152 knots (175 mph) 18 hours before landfall in Mississippi. These winds were within 11 knots (13) mph of Camille’s winds at the same time interval, and Camille’s wind field was geographically smaller, thereby enabling Katrina to generate a larger storm surge.

Satellite image of Hurricane Katrina on Sunday, August 28, 2005. This image shows the impressive size and profile of the storm the day before landfall. At this time, winds easily exceeded the category-5 threshold. Image: NOAA

The Joint Typhoon Warning Center (JTWC) tracking map for Tropical Cyclone Amphan estimates that maximum sustained winds reached 100 knots (115 mph) around May 19, 2020 at 1800 UTC, approximately 18 hours before landfall. This is equivalent to a category-3 hurricane, which would be classified as a major hurricane in the U.S.

Prior to that time the winds had exceeded 140 knots (160 mph), reaching the equivalent to category-5 in the U.S. Although the JTWC forecasts Amphan’s intensity to drop to 85 knots (97 mph) before landfall, the stronger pre-landfall winds, displacing massive amounts of water into a smallow sub-basin of water, will generate substantial storm surge.

Tracking map from the Joint Typhoon Warning Center provides estimates of storm intensity, position and the geographic size of various wind fields. This map shows a forecast for Amphan’s winds to weaken prior to landfall. Source: https://www.metoc.navy.mil/jtwc/jtwc.html

The JTWC map also shows the geographic area of various wind fields, revealing that Amphan has a broad wind field capable of pushing much water. For example, on Tuesday morning, the radius of Amphan’s 50-knot (57-mph) winds extended at least 120 nautical miles from the center of circulation. This broad wind field will enhance storm surge levels.

Reason #3: This Region has Experienced Many Changes

The third reason Tropical Cyclone Amphan could generate a high-impact storm surge along the coast of India and Bangladesh is that the region has observed many changes that exacerbate impacts of coastal flooding.

A surprising change is a loss of memory about the horrific cyclone history in this part of the world. When I built the first global storm surge database, I noticed that the period of hyperactivity in the Bay of Bengal between 1960 and 1970 was the most active on record anywhere in the world. Consider this: In this 11-year period, Bangladesh observed seven storm surge events exceeding 8.8 meters (28.9 feet), meaning they had seven storm surges higher than Hurricane Katrina in 11 years. Since then, the region has been relatively tranquil, except for isolated cyclones, like the 1999 cyclone that hit Orissa, India.

The table below shows the 10 highest storm surge events for the Northern Indian Ocean basin, taken from Needham et al. (2015). Original data are from Dube et al. (1997). Water levels are listed in meters.

Rank	Height (m)	Year	Maximum Surge Location	Country
1	13.70a	1876	Precise Location Unknown	Bangladesh
2	12.00b	1737	Sunderbans	India
2	12.00b	1864	Calcutta and Surroundings	India
4	9.60a	1966	Precise Location Unknown	Bangladesh
5	9.10a	1960	Precise Location Unknown	Bangladesh
5	9.10a	1963	Precise Location Unknown	Bangladesh
5	9.10a	1970	North of Chittagong	Bangladesh
8	8.80a	1961	Precise Location Unknown	Bangladesh
8	8.80a	1961	Precise Location Unknown	Bangladesh
8	8.80a	1967	Precise Location Unknown	Bangladesh

Water heights: a= storm tide; b = storm surge

Since the period of tropical cyclone hyperactivity ended in the 1970s, this region has experienced several changes that make it quite vulnerable to coastal flood impacts. Population growth places a tremendous number of people in harm’s way in Bangladesh, the most densely populated mega-country on the planet, where the density is 2,600 per square mile (Streatfield and Karar 2008). By contrast, the population density would only be 1,740 per square mile if everyone in the world was placed in the U.S.

Bangladesh is a low-lying, densely populated country vulnerable to coastal flooding. Image Link

Combining the dense population with rapidly rising sea levels, produces a dangerous combination of factors for flood impacts even when a powerful cyclone is not bearing down on the coastline. We can only hope that the many people along the coast of India and Bangladesh heed local warnings and do everything within their power to protect themselves from the severe coastal flooding that is likely from Tropical Cyclone Amphan.

The latest advisory from the India Meteorological Department forecasts storm surge levels to reach as high as 4-5 meters (13.2 – 16.5 feet) in the Parganas region and 3-4 meters (10-13.2 feet) in areas of West Bengal. Given the geographic size, pre-landfall wind speeds, coastal shape and shallow water of the region, those water levels seem probable.

[See “housekeeping note” below the references about how to contact me]

Non-Linked References

Dube, S.K., A.D. Rao, P.C. Sinha, T.S. Murty, N. Bahulayan, 1997: Storm surge in the Bay of Bengal and Arabian Sea: The problem and its prediction. Mausam, 48, 283-304.

Frank, N.L., and S.A. Husain, 1971: Deadliest tropical cyclone in history. Bulletin of the American Meteorological Society, 52, 438-&.

Streatfield, P.K., and Z.A. Karar, 2008: Population Challenges for Bangladesh in the Coming Decades. Journal of Health, Population and Nutrition, 26, 261-272.

U.S. Department of Commerce, 1993: Natural Disaster Survey Report, Hurricane Iniki, September 6-13, 1992. Report available online at: http://www.nws.noaa.gov/om/assessments/iniki/iniki1.pdf.

“Housekeeping Note”

Thank you for taking time to read my first blog post on my new site. I am in the process of launching a new blog site for 2020 Atlantic Hurricane Season. Within the next few weeks I hope to have the site better developed.

In the mean time, if you would like to reach out to me to discuss flood impacts from Tropical Cyclone Amphan, please send me an email to: hal@marineweatherandclimate.com. I am available for phone and video interviews.

You can also check out these websites for my partners and affiliated projects:

The U-Surge Project (www.u-surge.net)

U-Surge is an international storm surge data project, which provides the first data-driven storm surge analyses across seven global water basins.

CNC Catastrophe and National Claims (https://adjustingexpectations.com/)

CNC provides more than 30 years of expertise in the flood insurance and adjusting market. I work with CNC to innovate cutting-edge technologies that improve flood risk analysis and coastal resiliency.

Marine Weather and Climate (www.marineweatherandclimate.com)

Marine Weather and Climate has specialized in data-driven science communication to help improve resiliency for flood-prone coastal communities for more than a decade. The “Science Communication” tab provides extensive links to print and video media projects.

Welcome to Hurricane Hal’s Coastal Hazards Blog!

Hey everyone- welcome to Hurricane Hal’s Coastal Hazards Blog. I am launching this site at the beginning of the 2020 Atlantic Hurricane Season.

As I launch this blog, I am retiring Hurricane Hal’s Storm Surge Blog, a blog I ran for 10 years…from 2010 through 2019. I started that blog as a way to communicate storm surge risk during the 2010 Atlantic Hurricane. At the time, I didn’t imagine it would live on.

I uploaded 220 posts on my old blog, for an average of 22 posts per year, or almost two posts per month.

The coastal flood events I remember the best during that time were Hurricane Isaac (2012), Superstorm Sandy (2012), Super Typhoon Haiyan- Philippines (2013), Hurricane Harvey (2017), Hurricane Irma (2017), Hurricane Florence (2018), Hurricane Michael (2018) and Hurricane Dorian (2019).

Here are some selected images from some of those storms:

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I did a lot of international collaboration for Haiyan (2013), which led to my work on the NOVA/ PBS documentary Killer Typhoon (2014). This work was timely, as I just partnered with PAGASA, the Philippines agency responsible for forecasting and responding to severe weather events, the summer before Haiyan. They shared historic documents with me, as I built the first storm surge database for East Asia.

I started going into hurricanes to document storms and collect data during 2018. I traveled to hurricanes Florence, Michael and Dorian, where coverage generally switched from my blog to social media. I’m excited for this new blog to be a place that connects well with social media platforms, but provides a consistent landing page for frequent posts about coastal hazards.