Coastal weather events come in all shapes and sizes, as
does the impact they have on the coast.
Naturally, the two are correlated – though not always in
the way you might think.
Size Matters
A tightly wrapped, intense storm can do a lot of damage
in a small area. But a broader, less windy storm that is slower moving can be
even more destructive, by impacting a broader area with still punishing winds
and a more significant storm surge and spiraling off storms bands that can
include very heavy rainfall and even tornadoes. Then when it heads inland, the
threat of flooding from a disintegrating storm puts the final touches on its power.
So Does Duration
A fast-moving storm hits and moves on. A slow-moving one
just grinds away, piling on rainfall and erosive waves for hours and even days.
A great example of the power of persistence was Tropical
Storms Debby and Isaac earlier this year. Tropical Storm
Debby never made it to hurricane strength but, by spinning away out in the Gulf
of Mexico for more thantwo days, it scoured west Florida beaches
in a way not seen in decades… while its rainfall-driven
flooding ended up causing just one less fatality than a far more intense storm
(with twice the wind speed at peak) that sped through the same general
area.
Hurricane Isaac was a slow moving storm that sent
torrential rains all through southern Louisiana. The impact of Isaac is still
being tallied even at the time of this article.
“Category” Is Just A Number, Not Necessarily An
Indication Of Impact
Hurricane Isaac barely crossed the 74 mph threshold
before making landfall south of New Orleans, but it carried with it the storm
surge of a much stronger storm. Why? It was large (if not well organized) and
it had a couple of days to push water out ahead of it – resulting in a surge
that rivaled Hurricane Katrina
in many places even if the wind was nowhere near as
strong. It reminded the weather pros that focusing on a storm’s category can
lead some to underestimate its true strength.
What The Wind Can’t Do, The Water Does
Again, looking at Hurricane Isaac, nominal
hurricane-force winds weren’t the issue here. Rather, it was the significant
storm surge, the inundating rainfall and the very slow forward motion of the
storm that made this system significant in terms of
damage and destruction. Folks along the coast got sloshed with surge, while
those inland had to cope with swollen rivers and overwhelmed dams. On
wind alone, Isaac wasn’t much… add in the compounding
catastrophe of deluge and duration, and it became a major storm event.
The Storm Is Past, But The Damage Is Just Beginning
We’ve seen this phenomena a couple of times recently:
Middling tropical storms bring maximum destruction when they drive far inland.
Either their weakened winds are still potent enough to wreak
havoc on a landscape unused to any such strength or
abundant rainfall pushes rivers and streams into disastrous torrents. Irene
devastated towns in the northeast that hadn’t seen storm rainfalls of this
magnitude for decades. Debby combined flash floods with
tornadoes to cut a swath through the Southeast. Isaac ripped into the Gulf
Coast with a watery combination of surge and downpours, put thousands underwater
or in the dark… and, ultimately, brought much needed rainfall to the parched
midwest.
One Final Thought
As surprising as these storms’ strength may have been,
one thing that was no surprise was that pre-storm preparations and plans worked
and made the storms more survivable. Coastal areas with wide
beaches could withstand hours and days of battering waves
– because the beaches took the hit, not the upland properties and
infrastructure.
The billions of dollars spent to protect New Orleans
post-Katrina withstood its first real test, and passed with flying colors. And
in coastal areas threatened with serious inundation, the people who heeded the
calls to evacuate were safe, while those who did not often put themselves – and
the first responders who had to go out in the midst of the storm to rescue them
– at real risk.
For more information, go to www.asbpa.org, Facebook or
www.twitter.com/asbpa.
Island Sun (September 28, 2012)
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