Storm Surge Vs Hurricane Intensity Trends

Good day all,

Just have been noticing something very interesting now since we are in an incredible hurricane up-cycle and there are many storms to "study". We all know how storm surge is dictated by hurricane intensity, but nothing is mentioned anywhere about TRENDS in intensity (making landfall while weakening / strengthening).

The storm surge is a big destructive element in a hurricane (or typhoon) and has loon since been characterized by a chart. Similar to the one below...

Category-1: 74 to 95 MPH winds - Storm surge of 3-5 feet.
Category-2: 96 to 110 MPH winds - Storm surge of 5-8 feet.
Category-3: 11 to 130 MPH winds - Storm surge of 8-12 feet.
Category-4: 131 to 155 MPH winds - Storm surge of 12-18 feet.
Category-5: 156 MPH or more - Storm surge of over 18 feet.

Obviously, such a chart emphasizes only AVERAGES. It does not consider MANY important factors such as wind duration, sea floor bathymetry, moon phases, co-incidences with astronmoical tides, and shape of the coastline.

For example, the same storm that can produce a 15 foot surge on the Florida east coast can produce a 25 surge on the west coast of that state, because the west coast of Florida has very shallow bathymetry offshore. If you go 1 mile offshore of Palm Beach, FL the water can be 200 feet deep. You go 20 miles offshore of Tampa, the water is only 20 feet deep!

In places like Hawaii, storm surge from a hurricane is minimal since the water is so deep a short distance offshore. A category 4 hurricane in Hawaii can produce a surge of only 5 feet since it is an island and is very deep offshore! Now in a place like Bangladesh, the same storm can produce a 40 foot surge due to a "concave" coast and shallow shelf offshore. Again, the seafloor and coastline shape makes all the difference.

A storm surge coming ashore interects with the coastline geography and undersea bathymetry in many complicated ways. In addition to this, there may also be "wind setup", where water can be "piled" up by strong winds, especially into areas like bays and rivers opening into a larger body of water. It can even happen in closed bodies of water, such as lakes.

With this said, I have never heard any mention into what happens when a surge is created and the storm quickly weakens. I am stating this because hurricane Katrina came in at category 3 or 4, but had a storm surge over 30 feet ... Was this surge created when it was category 5, and never had enough time to dissapate somewhat?

I am thinking this way since a storm surge is essentially a WAVE ... But one with a very long period much like a tsunami, but more "dome" shaped. Have you ever seen winds blowing hard on shore at a beach creating large waves, then these strong winds either shift or suddenly get light? What happens to the waves? Ofcourse, the waves do not suddenly "stop" when the wind stops, but gradually subside (a RESIDUAL wind swell). The sea had stored energy as harmonic motion (waves) and will remain in motion until this energy is expended.

Now, back to the storm surge. In an intense hurricane, winds and low pressure produce a "dome" of seawater a couple of feet high (in very deep water only). The water at the sea surface of this dome is also moving in a strong surface current called "mass transport", and this current may extend to depths of 200 or 300 feet in strong hurricanes!

If this is going on, and the wind suddenly "stopped", the dome of water will not necessarily go away immediately, and neither would the tons and tons of water in motion atop that dome shaped wave suddenly "stop" either! Does your car "stop" when you are moving and turn off the engine - ofcourse not ... Newton's law - It COASTS to a stop.

If you are going 60 MPH and stop the engine, a second or two after you will still be going like 59 MPH or so until "drag" brings you to a halt. If you have water set in motion, and the wind stops suddenly - same thing - until drag comes into play!

For a storm surge, this can take hours. Lets say we have a hurricane, and its 100 miles south of New Orleans. It is category-5 with 175-MPH winds. As one can imagine, the dome of seawater and mass-transport atop it would be immense. Now, a few hours go by, and the same hurricane weakens (shear, dry air, cool water, etc) to 130-MPH (category-3) as it makes landfall.

The surge will NOT be indicative of a category-3 storm. The WINDS would be, bit NOT the surge. Since all that water is still in motion and only slowed down slightly as the winds above it dropped, there is a good possibility the storm surge will be much more that what you would find in a category-3 hurricane.

Many examples support this theory, each former category-4 or 5 storms that weakened at landfall...

Hurricane Ivan in 2004: Storm weakened to 130-MPH or less at landfall. Storm surge right of the storm was over 20 feet.

Hurricane Rita in 2005: Storm made landfall at 120-MPH, yet had storm surge of at least 20 feet in some areas.

Hurricane Katrina in 2005: Storm made landfall at about 140-MPH, yet had the largest storm surge ever recorded at over 30 feet right of its center!

Should a study be applied to forecasting storm surges on weakening hurricanes?

Should regular inertia / wave theory be applied to surge height too?

How about intensifying storms, does the converse apply? Maybe a category 4 storm making landfall that was category 2 just hours prior would have a smaller surge than expected?

For example on this part, hurricane Charley in 2004. Came in as a strong category-4 storm only 4 hours after being category-2. The storm surge was no more than 10 feet high - on the FL west coast!!

If this is the case, you cannot wipe your head and say "swew" just because a category-5 storm suddenly weakened to a category-2 and you live on the beach expecting a category-2 surge!

Same for Katrina in south FL ... It was a tropical storm at 45 MPH in the morning and made landfall near Fort Laderdale as an 80 MPH hurricane. The storm surge was only 3 feet, like that of a tropical storm.

Same with Andrew in 1992. It came in as a rapidly intensifying category 5 at 165-MPH sustained. Surge in the Biscayne bay should have been 20-25 feet, but it was only 17 feet at the Burger King headquarters.

This will also mean that a rapidly strengthening system should have a smaller than expected surge, and a weakening storm have a higher than expected surge?

I have just wanted to put this post up and see what you guys think of it. I feel a study should be done on this ASAP to improve storm surge forecasting.
 
That was a very interesting read, thanks for posting.

Also, what about the effect of wind "piling" water into bays? This is a little different from storm surge. During Rita New Orleans was never really under the storm surge dome (I think). Instead the constant east-southeast winds just kept forcing water into Lake Ponchatrain. It doesn't take much wind to do this either, 10-20mph for a couple days can do the job.
 
GREAT topic and I would love to know if there are any studies on this. I believe there is something to be said for the storm surge and weakening hurricanes. There obviously is reason to believe that the surge doesn't just go away once a hurricane weakens.

Perhaps someone has done a study on this already? Any NHC posters reading this?
 
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