cdcollura
EF5
Good day all,
The KEY thing here is the SIZE of the storm. Storm surge vs category has actually been dropped from most literature, even by the NHC, as those "tables" only describe a storm that is a "normal" hurricane size. In other words, a normal hurricane having a 25 nm wide eye, hurricane winds extending out about twice that distance, and surrounded by a gale (TS forced) wind envelope about 150 nm across. In that case, Cat-1 is a 4' surge, cat-2 up to 7', and so on to at least 18' for cat-5. This, also, assumes an average ocean sea-floor bathymetry (slope) off the coast. A 15' surge on the FL east coast means a 25' surge (by the SAME storm) had it hit the FL west coast / Gulf.
Issac was another very large category-1 hurricane (like Irene a year earlier). More impressive, both storms had exceptionally low pressures at landfall, despite being a category 1 (Isaac actually broke a record for having the lowest pressure for a strong tropical storm)!
Above: Surface flow fields of Sandy when off NC coast (wow - That's BIG)!
The sheer size of hurricane (or subtropical gale / storm) Sandy had the effects of strong TS / minimal hurricane forced winds over an extremely large area. This has to do with the "fetch" (distance) the winds blow across the water, and the longer this distance, the larger the generated waves and / or surges. Basically, a storm of category-five strength that is 50 miles across with a 10 mile-wide eye coming ashore in NC could indeed create a storm surge of 15-18 feet. However, a storm like Irene or Sandy coming in can also create a surge of that height, despite being barely a category one. It's all about the distances, or the fetch.
Hurricane Ike in Texas as also a large storm in 2008, coming in as a strong cat-2 near Galveston. The surge approached 20 feet, owing to the storm size and shallow sea floor (Gulf states). 20 feet, using the OLD scale, would yield a strong category 5 storm, but it was only a 2.
Above: Storm surge chart for an AVERAGE sized (normal) hurricane and average sea floor.
There are many examples of large and small hurricanes. Hurricane Charley did make landfall on a coastline with shallow water (Gulf coast) in W Florida on 2004, and as a STRONG category 4 storm. The storm surge barely reached 8 feet. This was because the core of the hurricane (envelope of major winds over 110 MPH) was about 5 miles from the center, about an eye less than 3 miles wide. A very small hurricane wind and gale envelope surrounded Charley, so there was no long fetch to "push" water in. Yet, getting back to hurricane Ike in 2008, the storm eye was about 50 nm wide, with a 900 mile wide gale envelope and hurricane forced winds extending 250+ miles from the center. The storm tides exceeded 10 feet above normal, still under light winds and blues skies, 24 hours before landfall - People were drowning long before they even saw the darkening skies from Ike!
Above: Comparisons of the sizes of two intense hurricanes, Isabel in 2003 and Charley in 2004 ... Showing eye structure and eyewall (max wind) maximas.
In a nutshell ... Storm surge / wave models must take into consideration not only the windspeeds, but the shape and undersea countours near and especially right of landfall, and the size of the storm and how long the fetch (wind distances) are. These factors will greatly determine the threat (just "because it's just a ONE" doesn't mean it can't do more damage than a category 3)!
Also, a larger storm inflicts more damage away from the water than a smaller one of equal (or even stronger) intensity because of wind duration. Having 10 hours of 80-85 MPH winds should cause more damage than 30 minutes of 90-100 MPH winds. This longer duration weakens structures due to fatigue (like bending a metal spoon 2-3 times versus 20 times = breaks).
Considering the size of Sandy, it's tropical storm forced / gale envelope (38 to 73 MPH winds) extended at one point to over 500 nm from the center, with one side over Bermuda and the other ashore in the Carolinas! The hurricane forced winds in Sandy were not even symmetrical, as a matter of fact, on the LEFT side of the storm (south side at NJ landfall and blowing OFFSHORE). The devastation was caused by sustained 50-70 MPH winds blowing across MANY miles of fetch - And forcing this wave / surge right into a "funnel" shaped coast (NJ / NY harbor mouth).
Just be safe and MOVE OUT OF THE WAY when a LARGE system is coming in!
The KEY thing here is the SIZE of the storm. Storm surge vs category has actually been dropped from most literature, even by the NHC, as those "tables" only describe a storm that is a "normal" hurricane size. In other words, a normal hurricane having a 25 nm wide eye, hurricane winds extending out about twice that distance, and surrounded by a gale (TS forced) wind envelope about 150 nm across. In that case, Cat-1 is a 4' surge, cat-2 up to 7', and so on to at least 18' for cat-5. This, also, assumes an average ocean sea-floor bathymetry (slope) off the coast. A 15' surge on the FL east coast means a 25' surge (by the SAME storm) had it hit the FL west coast / Gulf.
Issac was another very large category-1 hurricane (like Irene a year earlier). More impressive, both storms had exceptionally low pressures at landfall, despite being a category 1 (Isaac actually broke a record for having the lowest pressure for a strong tropical storm)!
Above: Surface flow fields of Sandy when off NC coast (wow - That's BIG)!
The sheer size of hurricane (or subtropical gale / storm) Sandy had the effects of strong TS / minimal hurricane forced winds over an extremely large area. This has to do with the "fetch" (distance) the winds blow across the water, and the longer this distance, the larger the generated waves and / or surges. Basically, a storm of category-five strength that is 50 miles across with a 10 mile-wide eye coming ashore in NC could indeed create a storm surge of 15-18 feet. However, a storm like Irene or Sandy coming in can also create a surge of that height, despite being barely a category one. It's all about the distances, or the fetch.
Hurricane Ike in Texas as also a large storm in 2008, coming in as a strong cat-2 near Galveston. The surge approached 20 feet, owing to the storm size and shallow sea floor (Gulf states). 20 feet, using the OLD scale, would yield a strong category 5 storm, but it was only a 2.
Above: Storm surge chart for an AVERAGE sized (normal) hurricane and average sea floor.
There are many examples of large and small hurricanes. Hurricane Charley did make landfall on a coastline with shallow water (Gulf coast) in W Florida on 2004, and as a STRONG category 4 storm. The storm surge barely reached 8 feet. This was because the core of the hurricane (envelope of major winds over 110 MPH) was about 5 miles from the center, about an eye less than 3 miles wide. A very small hurricane wind and gale envelope surrounded Charley, so there was no long fetch to "push" water in. Yet, getting back to hurricane Ike in 2008, the storm eye was about 50 nm wide, with a 900 mile wide gale envelope and hurricane forced winds extending 250+ miles from the center. The storm tides exceeded 10 feet above normal, still under light winds and blues skies, 24 hours before landfall - People were drowning long before they even saw the darkening skies from Ike!
Above: Comparisons of the sizes of two intense hurricanes, Isabel in 2003 and Charley in 2004 ... Showing eye structure and eyewall (max wind) maximas.
In a nutshell ... Storm surge / wave models must take into consideration not only the windspeeds, but the shape and undersea countours near and especially right of landfall, and the size of the storm and how long the fetch (wind distances) are. These factors will greatly determine the threat (just "because it's just a ONE" doesn't mean it can't do more damage than a category 3)!
Also, a larger storm inflicts more damage away from the water than a smaller one of equal (or even stronger) intensity because of wind duration. Having 10 hours of 80-85 MPH winds should cause more damage than 30 minutes of 90-100 MPH winds. This longer duration weakens structures due to fatigue (like bending a metal spoon 2-3 times versus 20 times = breaks).
Considering the size of Sandy, it's tropical storm forced / gale envelope (38 to 73 MPH winds) extended at one point to over 500 nm from the center, with one side over Bermuda and the other ashore in the Carolinas! The hurricane forced winds in Sandy were not even symmetrical, as a matter of fact, on the LEFT side of the storm (south side at NJ landfall and blowing OFFSHORE). The devastation was caused by sustained 50-70 MPH winds blowing across MANY miles of fetch - And forcing this wave / surge right into a "funnel" shaped coast (NJ / NY harbor mouth).
Just be safe and MOVE OUT OF THE WAY when a LARGE system is coming in!
