Storm Surge Questions

[Delancey]

So when a hurricane is spinning around out in the ocean, it drags along with it a bubble of water that is the product of the storm's intense low pressure lifting the water up above mean sea level combined with wind driven waves rushing in to the center. When the hurricane makes landfall, dragging this bubble of water with it, the bubble becomes a storm surge which can cause a considerable amount of damage.

When a hurricane is at sea is the shape of this bubble symmetrical? Or is it biased by the forward velocity of the storm?

Also, what happens when a hurricane path runs parallel to the coast, as could be the case with Matthew along east coast of Florida, Georgia, and the Carolinas?

If the path was parallel to the coast but the eye were to stay offshore, would it actually be sucking water out of the rivers and inlets rather than pushing water into them? Sort of an anti-surge?

[boatman61]

Never having experienced a hurricane I can't answer specifically however I have experienced storm surges caused by combinations of Spring tides and high sustained winds.. the combined forces teamed up to mount the Cuxhaven breakwater and wreck the pontoons that we and the lifeboat were tied to.. these were big steel pilings with pontoons secured by chain.. and winds only around 60kts. I can only suppose that the further N Matthew tracks the greater the surges will be during flood tides.
As for the second assumption.. in my time on the Neuse whenever there was a prolonged W wind the levels would drop by a few feet.. I once spent a week sat on the bottom alongside Orientals Town Dock (much to the annoyance of the towns lawman) waiting for the wind to turn and restore water levels.

[Delancey]

Recent forecast path.

[Delancey]

Low pressure.

[Delancey]

Wind driven waves towards center.

[GordMay]

Storm surge is caused primarily by the strong winds in a hurricane or tropical storm. The low pressure of the storm has minimal contribution.
The wind circulation around the eye of a hurricane blows on the ocean surface and produces a vertical circulation in the ocean. In deep water, there is nothing to disturb this circulation and there is very little indication of storm surge. As with a tsunami, you won't even notice the "bubble" offshore.
Once the hurricane reaches shallower waters near the coast, the vertical circulation in the ocean becomes disrupted by the ocean bottom. The water can no longer go down, so it has nowhere else to go but up and inland.

[Delancey]

I have seen a couple references to the low pressure only accounting for 5% of the height of the surge. So what about the surge when the path is parallel to the coast. Any thoughts? How far out does the surge extend from the eye?

It looks like winds will be easterly as the storm advances up the coast of Florida suggesting it will pile it on the beach. However, as it reaches Georgia and South Carolina it looks like winds will be more northeasterly and parallel to or coming from the shore so maybe there is less concern there?

[Shrew]

Originally Posted by Delancey

I have seen a couple references to the low pressure only accounting for 5% of the height of the surge. So what about the surge when the path is parallel to the coast. Any thoughts? How far out does the surge extend from the eye?

This would be like noticing high tide vs low tide in deep water, away from shore. It would be impossible to discern. Even in an anchorage, you need to look at the waterline on shore to gauge where in the tide cycle you are. (without a tide chart or sounding for reference)

The deck is always the same distance above the water and the difference between keel and seabed is variable.

[barnakiel]

Originally Posted by Delancey

(...)

When a hurricane is at sea is the shape of this bubble symmetrical? Or is it biased by the forward velocity of the storm?

Also, what happens when a hurricane path runs parallel to the coast, as could be the case with Matthew along east coast of Florida, Georgia, and the Carolinas?

If the path was parallel to the coast but the eye were to stay offshore, would it actually be sucking water out of the rivers and inlets rather than pushing water into them? Sort of an anti-surge?

It is non-symmetrical. Like a ring on the water - highest ahead of the system.

I asked a similar question from my NOAA friend who works in their hurricane monitoring center.

Apparently, surge is caused by the water pushed ahead of the system. It is mostly a result of wave action less so by pressure differences. Sea level difference caused by the suction in the eye is very small compared to the surge effect from waves. You can also imagine this like a compression ahead of the system, similar to isobar compression ahead of a moving low.

As he summarised this: imagine the storm being a heavy displacement boat. Now simply remember how surge from such a boat affects the dock or a shore of a river. A simplification, but one that works.

For a system traveling along the coast there will be surge ahead of the system and as the system passes by, followed by a very rapid ebb once the system passes you.

Let me know if these theories work out in your scenario.

I am most interested in your observations!!!

Cheers,
b.

[barnakiel]

Originally Posted by Delancey

Low pressure.

Nay. This is not what causes the surge. Read Gord's post. I heard the same story from another and equally reliant source.

PLS Let me know what you find. I am most interested in all observations!

Cheers,
b.

[Delancey]

Doesn't make sense to me that the surge is being "pushed" when the low pressure and spiraling winds are being drawn into the center of the storm. No? Imagine a shop vac being dragged along just above the surface of the water. Is it "pushing"? Or is it "pulling"?

Obviously the diagrams I have posted are graphic exaggerations. I get it that at sea the bulge is imperceptible. I also get it that the low pressure contribution is small but that doesn't mean it is nonexistent.

[jimbunyard]

As would be expected in such a dynamic system, there is no easy 'correct' answer.

The 5% number seems reasonable, but 5% of 28 ft (experienced in Katrina at places on Bay St. Louis) is about a foot and a half. The water at the place I was staying during that storm started rising (2 hours after the eye passed), and stopped 4 inches from coming into the house. It went down almost as quickly, about a foot an hour.

Much depends on the forward speed of the storm, the diameter, and the orientation of the coastline to the direction of the storm. Several years ago I made a trip to Clearwater, FL., as a storm or hurricane (I think it was Cindy) was crossing the upper peninsula towards the Gulf. I've been across the Mobile Bay bridge countless times, but that was the only time I've ever seen the bottom of the bay under the bridge for virtually the entire length of the causeway. So yes, there is a potential for 'anti-surge'.

With Matthew being still a fairly compact storm (and this could certainly change), it would seem the main concerns at the time would be how close it comes to the coast, and how fast it proceeds up the coast. Faster and further off shore is better, where the wind will be more or less sideshore for most of the time. The way it looks now though 'prepare for the worse, hope for the best'...

[barnakiel]

Yep.

The same drawing.

http://www.hurricanescience.org/imag...ulge_COMET.jpg

Noticed the pressure factor only 5% of the surge? And the position of the surge relative to the eye.

This is what my NOAA friend talked about.

For a storm passing along the coast I believe expect a quick flood/ebb sequence.

b.

[barnakiel]

Originally Posted by Delancey

Doesn't make sense to me that the surge is being "pushed" when the low pressure and spiraling winds are being drawn into the center of the storm. No? Imagine a shop vac being dragged along just above the surface of the water. Is it "pushing"? Or is it "pulling"?(...)

Forget the vacuum. It is wrong imagery.

Take a round basin and mark a line round it maybe 1 inch from the bottom. Now fill water up top the mark. Now take a hand batter the style we use for whipping whites. Turn it on, dip it in the basin and move towards the side of the basin. Where do you see the surge now?

Stop thinking about the wind and pressure. Think about what the water is doing.

If waves traveled as wind does, surfers in Nazare could not use the waves whipped up in North Atlantic storms, could they?

b.

[Delancey]

This morning's maps.