NOAA developing new storm surge model

Starting this year, the National
Hurricane Center in Miami will begin using a new product called the Saffir-Simpson
Hurricane Wind Scale that doesn’t include storm surge forecasts.

Although the new scale uses the
same wind speed levels to categorise hurricanes, it will no longer tie specific
storm surge ranges to each hurricane category.

The change to the scale, which was
developed in 1971 by engineer Herbert Saffir and former National Hurricane
Center Director Robert Simpson, was made after surge levels of several
hurricanes in recent years did not match the levels called for by the scale.

Hurricane Katrina in 2005 and
Hurricane Ike in 2008, which made US landfalls as Category 3 and Category 2
storms respectively, caused much larger than expected storm surges as called
for by the old Saffir-Simpson scale. Conversely, Hurricane Charley caused much
less than expected storm surge when it made landfall in western Florida as a
Category 4 hurricane in 2004.

The Weather Channel Meteorologist
Steve Lyons said at the National Hurricane Conference last week that he stopped
referring to the storm surge element of the Saffir-Simpson scale long ago
because he realised it was erroneous.

Lyons pointed to a number of issues
with the surge scale, starting with the fact that storm surge from a tropical
cyclone in any particular area would depend on when the storm made landfall and
the tide level at the time.

“The timing of the landfall is
critical to get the water rise above ground,” he said, noting that wave action
on coastlines – which was not a component of the old Saffir-Simpson scale –
also causes water to rise.

“Where ever the storm surge threat
is low, the wave action threat is high,” he said.

This phenomenon is caused by
differences in local bathymetry. Shallow waters, like those found the Gulf of
Mexico, can cause higher storm surge along coastlines, while deeper waters, like
those around the Cayman Islands, can inhibit surge, but cause larger wave
action, as was experienced during Hurricane Ivan in 2004.

Lyons noted that wave action could
also cause water level rises well in advance of the arrival of the centre of a
hurricane.

“The waves usually do outrun the
hurricane unless it’s moving more than 20 miles per hour,” he said.

In addition, Lyons said large waves
can occur far afield from the centre of a hurricane, even outside the cone of
track probability. He said that during Hurricane Ike, large waves affected
South Padre Island, more than 300 miles away from where the hurricane made
landfall.

“It was perfect conditions outside,
with only light winds at 10 to 15 knots, and yet there were huge 25-foot waves
that looked like the north shore of Hawaii,” he said.

New storm surge model

NOAA will continue to issue storm
surge forecasts in its hurricane advisories, but expressed in terms of height
above ground level – or inundation – to give the public a better understanding
of the potential for flooding in various locations.

A better forecast for storm surge
is vital because surges have been responsible for some of the largest losses of
life associated with tropical cyclones along the coastlines. 

Emergency management personnel also
need accurate surge forecasts, which include time elements, to make evacuation
plans.  With Hurricane Ike, some
residents along the Texas coastline found their area inundated with water well
in advance of landfall of the hurricane, blocking their escape.

The National Hurricane Center’s
plan is to develop a new inundation model that takes into account all the
variables, said Jamie Rhome, a storm surge specialist with the National Hurricane
Center.

Rhome said the new model would
forecast total water level, which takes into account storm surge, tides, freshwater
flow and background water anomalies.

Accounting for all of the variables
in a scientific model requires incredibly complicated technology, Rhome said.

“It’s more complicated than you can
imagine.”

To get it right, the model will
have to be coupled with bathymetry and topography information supplied by the
geographic information system.  There
will also have to be close coordination between the National Hurricane Center
and the many local weather forecast offices along coastlines.

But it will still be impossible to
predict surge at any precise location.

“Surge is an inherently local
event,” Rhome said. “There are rivers, streams, roads and such that the
National Hurricane Center could never include in a storm surge forecast.”

Rhome pointed out the incongruence
of how the National Hurricane Center has told people for years not to
concentrate on the thin black line of a forecast path of a hurricane, and yet
the storm surge models of the past have been concentrated on the forecast path.

“We have to move away from a
deterministic approach to storm surge forecasting and move toward an ensemble
approach,” he said.

Eventually, it is envisioned the
Hurricane Center will be able to issue storm surge watches or warnings that
could even extend past areas under tropical storm or hurricane watches or
warnings.

The project to create a new storm
surge model is a three-phase, 10-year plan. 
The first three-year phase began last year.  It is hoped the new model can become fully
implemented by 2019.

Rhome said in the end, the National
Hurricane Center wants the model to yield street level inundation, not only
along the coastlines, but as far inland as necessary.  He said that some people believe the
Hurricane Center is aiming too high in its goals, but he disagrees.

“If you’re going to
do this, you have to swing for what you want.

TOPscaleSTORY

The Weather Channel Meteorologist Steve Lyons, shown here centre flanked by Chris Pope, left, and Derek Haines of Dart Realty at the National Hurricane Conference in Orlando, was never a big fan of the storm surge element of the old Saffir-Simpson hurricane scale.
Photo: Alan Markoff
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