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The first of two hurricane seasons that fall under the US’s forecast responsibility begins on May 15th. I could have written the usual:

         1.    Be prepared.
         2.    This year will feature this many storms, and here’s the science of why
         3.    We’re all doomed!

But, I’m not. Instead we have to focus on something specific. Often overlooked, it is the number one cause of death in a tropical cyclone. It has over the last century taken the lives of perhaps a million people worldwide. I’m talking about storm surge. It’s often not the wind that kills, it’s the water.

This year the National Hurricane Center will be issuing site specific storm surge forecasts for the first time ever. This is a vital and needed change.  The vast majority of Hurricane Katrina’s victims drowned in that storm’s 30-foot surge. In Texas, 8,000 of Galveston’s 38,000 people drowned during the deadliest disaster to ever hit the United States. Hundreds of thousands drowned when Cyclone Nargis scythed across the Irrawady Delta in Burma.

A powerful tropical cyclone can create a surge that rivals the worst tsunami. Typhoon Haiyan bought remarkable surges to its landfall zone. High water marks 46 feet above mean sea level were found on Samar Island.

Category 5 Super Typhoon Haiyan, with satellite-estimated winds of 190 - 195 mph at landfall on November 8, 2013, pushed a massive storm surge of up to 23 feet (7 meters) into Tacloban, Philippines, newly-published storm surge survey results reveal. A team of researchers led by Yoshimitsu Tajima of the Department of Civil Engineering at the University of Tokyo found that at Haiyan's initial landfall point on the east coast of Samar Island, massive waves on top of the storm surge crashed against the coast, creating high water marks an astonishing 46 feet (14.1 meters) above mean sea level--some of the highest high-water marks ever recorded from a tropical cyclone. The world record is 13 - 14.6 meters (43 - 48 feet) from Australia's March 5, 1899 Bathurst Bay Cyclone. The greatest storm surge and high water mark recorded in an Atlantic hurricane are from Hurricane Katrina of 2005, which had a peak storm surge in Pass Christian, Mississippi of 27.8 feet (8.46 meters). The sea bottom was very flat in this region, so the waves on top of the surge were relatively small, and the highest high water mark from Katrina was just a few inches higher, at 28 feet (8.53 meters.) When deep water lies just offshore, as is the case for the east coast of the Philippines' Samar Island, huge waves will develop when the eyewall of an intense tropical cyclone moves over. These huge waves broke very close to shore during Haiyan, and were able to run-up the steep hillsides to incredible heights.
Here’s the thing about storm surge. It’s not just your average sea-side flooding. It comes quicker than you can react.

Almost every surviving witness along the Mississippi Gulf Coast said, in interviews later, that Katrina’s surge came in an instant. The waters rose to roof level and higher in towns like Waveland and Pass Christian and Gulfport. The sea erased entire neighborhoods. Very little of Waveland remained standing. Home sites were swept completely clean and hundreds died. The only reason the survivors lived is they happened to wander pass a window to see it coming and react quickly. A good many people drowned in their homes. Accounts from the Great Galveston Hurricane of 1900 are the same. In a single instant, during the storm’s worst, the waters rose some six feet and then higher. This destroyed a significant portion of the city. Thousands drowned. Now Houston is a major city, and Galveston is not. The hurricane's storm surge is why.

There is footage from Tacloban City in the Philippines of the same thing. In a span of 10 minutes, the waters in the streets rose several meters. This was broadcast live across the Philippines that morning. Millions watched that on television in horror. It is believed that the surge was some 7 to 9 meters high in the area of Tacloban City.

We saw it with Hurricane Sandy. The waters came like a wall. People who chose to hunker down in coastal neighborhoods on Long Island, on Staten Island, on the Jersey coast had little or no warning. You would not have been able to hear it coming over the wind.

The storm surge is a dome of water, uplifted by the low central pressure of the hurricane. It’s not just a wave, or a tide. This is why it arrives in an instant along with the center of the hurricane. Storm surges are worse to the right of the track of the eye.  In a large hurricane, the surge can be significant, destructive, and deadly along hundreds of miles of coast. The shape of the coast can magnify the size of the storm surge and some cities are especially prone. The right angle formed by Long Island and New Jersey funnels surges into New York Bay, if the hurricane strikes New Jersey, moving northwest into that state. That is why relatively weak hurricanes, like Sandy, can create immense surges in and around New York City. Combine this with an astronomical high tide, like with Sandy, and you get tens of billions of dollars in damage, and dozens dead.

In some areas, and this is largely controlled by the hurricane’s size, power, and track and the lay of the seabed beneath it, a significant and deadly storm surge can arrive some 12 to 24 hours prior to the core of the hurricane. This is called a forerunner surge. Only the upper Gulf Coast and parts of the west coast of Florida appear prone to this phenomenon. Hurricane Ike’s surge is the most famous and recent example. The link is a PDF so rightclick and save.

Recent and historical hurricanes have shown large forerunner surge (early increase in coastal water level) in advance of landfalling tropical cyclones. One a few occasions these have exceeded 3m on the day before landfall while the storm is hundreds of km from land, but it is still not well understood why some storms and locations have large and dangerous forerunners while other storms show a negligible rise.
Because forerunner surge occurs at times when winds are both relatively weak and shore-parallel, it is not linked to the cross-shore wind forcing that drives surge at landfall. Instead, it is becoming increasingly clear that Coriolis effects on the wind-driven alongshore current form a primary forerunner generation mechanism. It would be very worthwhile to investigate aspects of generation and dissipation, including:

         1.    Effect of bottom stress on forerunner growth, limiting amplitude, time scales, and decay (Because of the vastly increased surface stresses and corresponding decreased time scales, bottom stresses may play a proportionately much larger role in forerunners than in milder conditions),

         2.    Increased inundation in narrow-entranced coastal bays and wetlands caused by slow forerunner time scale (The slower forerunner time scales may pre-fill bays in advance of the main surge, and make moot the high frictional properties of wetlands),

         3.    The division between forced and free forerunners, and properties of the generated coastally trapped waves (Hurricane Ike (2008) in particular generated the largest coastally trapped wave ever measured, which was 300km in advance of the storm track at the time of landfall),

        4.    Forerunner scaling, in particular compared to cross-shore surge (This will help to identify situations likely to have significant/insignificant forerunners).
These topics could be addressed through the analysis of historical data, through analytical models and scaling arguments, through simple toy models, and using high resolution depth-integrated and three-dimensional computations.

The Hurricane Center will offer a product titled Potential Storm Surge Flooding starting this year.
Beginning with the 2014 Atlantic hurricane season, NHC will issue the Potential Storm
Surge Flooding Map for those areas along the Gulf and Atlantic coasts of the United
States at risk of storm surge from a tropical cyclone. This experimental product provides valuable new information on the storm surge hazard associated with tropical cyclones, by highlighting geographical areas where inundation from storm surge could occur and the height above ground that the water could reach. The map depicts inundation levels that have a 10 percent chance of being exceeded, and can be thought of as representing a reasonable worst-case scenario for any individual location.

The first map will usually be issued at the same time as the initial hurricane watch or, in
some cases, with a tropical storm watch. The map is based on the latest forecast track
and intensity for the tropical cyclone, and takes into account likely forecast errors. The
map is subject to change every six hours in association with each new NHC full advisory
package. Due to the processing time required to produce the map, it will not be available until about 45 to 60 minutes following the advisory release.

The Hurricane Center writes:
Here are some things to know about this map:
• The first map will usually be issued at the same time as the initial hurricane watch or, in some cases, with a tropical storm watch. The map is based on the latest forecast track and intensity for the tropical cyclone, and takes into account likely forecast errors.

• The map shows inundation levels that have a 10 percent chance of being exceeded, and can therefore be thought of as representing a reasonable worst-case scenario for any given location.

• The map is subject to change every six hours in association with every new NHC full advisory package. Due to the processing time required to produce the map, it will not be available until about 45 to 60 minutes following the advisory release.

I have no idea what type of hurricane season we’re going to have. Most forecasts indicate it will be mild due to a growing El Nino in the western Pacific and typically, this correlates with fewer storms in the western Atlantic. There is still not much skill in forecasting the number of tropical cyclones in the Atlantic before the fact and there's no skill at all at forecasting who those cyclones might hit.  The US has had a streak of no major landfalling hurricanes that will be nine years long this November, but we’ve seen how destructive Category one and two storms can be. It only takes one storm to be a complete catastrophe.

Hurricane Season in the Atlantic begins on June 1st. I see nothing to indicate we'll be getting an early start either. Despite a warm Atlantic, the atmosphere appears to be hostile to the formation of tropical storms, for now.

For the Eastern Pacific, this could be a doozy of a year, with the increasing probability that we will have a major El Nino event on our hands by late summer. Eastern Pacific Hurricane Season begins on May 15. Both seasons end November 30th.

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