This program was started in the mid 1970's as a follow on to
a hurricane damage model developed by the author, for the Texas
Coastal and Marine Council. This agency was supporting a bill
in the Texas Legislature that required homes being built in the
19 coastal counties of Texas meet a new standard of withstanding
140 mph winds. The current standard was 104 mph. It was felt
that homes built to the new standard would suffer 50% less damage.
To build to the new standard would increase the coast of the
building by 0.5 to 1.5 %. Of course, many of the homes on the
coast were built to no standard. In the course of that work,
the author visited the National Hurricane Center to obtain data
for the damage simulation model. There he met with Dr Herb Saffir,
one of the leading experts on hurricanes and wind damage. Dr.
Saffir was coauthor of the Saffir-Simpson scale for hurricanes.
Later, the National Hurricane Center (NHC) provided the author
with a tape that contained historical data on over 700 tropical
storms dating back to 1886. This data was used to develop the
first forecasting models. (Note: The building standard bill did
not pass. The Real Estate lobby was more powerful than the Insurance
Hurricanes are said to be the most powerful and deadly of nature’s phenomena.
Besides winds that can exceed 200 mph, they generate storm surges that can exceed
35 feet. One example of this was Hurricane Camille in 1968. It hit the town of
Port Christian, Mississippi with a 30-foot surge of water. A group of 30 or more
people were having a ‘hurricane’ party on the third floor of the
When the storm surge hit, all
the people were swept out of the building. All but one perished.
She was found in a tree several miles down the coast.
The movement of hurricanes is often unpredictable.
The author, while being interviewed for a newspaper article
in the 1980s commented on this and said, “That’s
why they are named after women.” The lady reporter retorted, “Young
man. Women are NOT unpredictable! They just reserve the right
to change their mind!”
A good example of this is Hurricane BARRY (1983). It was proceeding
north off the east coast of Florida. Just opposite Cape Canaveral
with a space shuttle on the pad, it made a 90-degree left turn.
(Perhaps it wanted to check out the shuttle!) It then proceeded
to cross Florida and enter the Gulf of Mexico. It gathered
more strength and eventually hit Brownville, Texas. (I got
a call from a radio station in Brownsville at 4 AM asking where
the storm was. I suggested politely that they look out their
window. Barry is one of the storms in the HISTORIC HURRICANE
set in the Excel data file historic_hurricane.xls.
If one looks at the comments about these storms, two factors
become apparent. Except of the Hurricane of 1900 that killed
600 people, there are few deaths in the US and many on the
Caribbean Islands. There is little dame on the Islands and
a lot in the US. A possible explanation for the first is that
we have improved our warning capability and the population
has time to evacuate AND they have someplace to go. The poor
people on the Islands are trapped! Likewise there is less infrastructure
on the Islands and much along the US coast. We love to live
on the seashore!
The First Models
The hurricane wind damage model
was presented as a technical paper by the author at a national
conference on hurricanes. One of the speakers from the NHC
announced that one of the primary forecasting models in use
at that time gave results two and three quarter hours after
they obtained a position report on the eye of the hurricane,
instead of three hours and fifteen minutes. (Its hard to remember
that computers in the late 1970 were so slow compared to what
we have today!) A portion of the audience cheered. Later we
found out that forecasters were to give their forecasts three
hours after the position report and then wait another fifteen
minutes to see how wrong they were.
The first models described the movement of hurricanes as a
Markov process – a “random walk” in two
dimensions. During the course of a graduate seminar on ‘Problems
in Operations Research,’ the author described the hurricane
damage model that had been presented and the incident of
the ‘quicker’ forecasts from the NHC. The author
made the boast, “If I couldn’t do better than
that, I’d eat their computer tape of data!” One
of the graduate students suggested that we use a Markov process.
As he said, “ After all, a hurricane is much like a
two-dimensional random walk!” Originally four separate
models were tried. Two were soon abandoned as too cumbersome.
Later the features of the other two were combined into the
basis of the current model.
The first version of the model had the characteristic of being
either very “good” or very “bad” with
no prior indication of what mode it would be in. The criterion
used to evaluate the model were a standard proposed by the
NHC in the 1970’s. Their goal was to have the mean
position error for the 72 hour (3 day forecast) to be less
than 125 nautical miles. At that time, their actual mean
position error for 72 hours was about 375 nm.
For the original model, when it was “good”, the
mean position error was less than 100 nm and when it was “bad” the
error was over 500 nm. After several attempts to improve the
model, Dr Tom Curry, as his PhD dissertation, developed a modified
Markov process model and gave consistent results. Currently
the mean position error for the model is about 175nm for the
72 hr forecast.
The Next Model
The model uses position reports of latitude
and longitude as provided by the National Weather Service.
At the time the model was constructed, these position reports
were given every 6 hours. For a Markov process the probability
of the system being in any state depends only on the previous
state. The process is said to have a memory of one. Translating
this to the movement of hurricanes, the process can be defined
as: “The probability that a hurricane is at any given
position (Latitude and Longitude) at a given time, only depends
on where it was at some previous period of time. Since we are
interested in movement, this implies distance and direction.
To achieve this, we need to use the previous TWO six hour positions
of the hurricane. These give is a measure of the average velocity
it traveled and the direction. The model did not just simply
project this movement in a straight line but used a probability
table for possible locations based on the latest latitude and
As mentioned previously, the initial models were either very
good or very poor. During the research work of Dr. Tom Curry,
we discovered we needed to include a “steering” effect
based on the diurnal movement of storms, i.e. they slowed
down at night and sped up during the day. So the Markov process
was modified to include the position report of the storm
24 hours previous to the latest position. Also, at this time
we found a way to collapse the massive probability transition
matrix of possible latitudes and longitudes based on the
latest position to a set of regression equations simply based
on latitude. These greatly simplified the running of the
The model requires a minimum of five six-hour position reports
of latitude and longitude. The model will then project the
movement of the center of the storm for the next 90 hours,
in six-hour increments. As additional position reports are
obtained they are entered to update the projections.
This version of the model was programmed in the late 1970’s
and updated with features from time to time. As such it is
characteristic of the types of programs written at the time
with a Main menu page with a series of options. There is a
catalog of storm data for past storms. The error analysis option
was not operable on this version since it was used in the early
running of the model to determine the mean position error at
6-hour intervals. Two of the major features added to the model
were the set of forecasting equations cited above and a plot
option that shows the forecasted movement of the storm.
The Current Model
The model was not been updated
in the last fifteen years to incorporate the current mode of “pull-down” menus
or different forecasting intervals. Over the past several years
the NWS has at times issued position reports at 3-hour intervals
and even 2-hour intervals. The current model can only use 6-hour
increments. Where there are 3-hour reports, the ploy we have
used is to run two series of forecast, e.g. Hugo-A and Hugo-B
and run them alternately as the reports come in. As a point
of interest, the error in forecasting the landfall of Hugo
72 hours in advance was less than 30 nm.
In 2004, Dr. Paul Jensen implemented the model using the Visual
Basic for Applications (VBA) macro language for Microsoft
EXCEL. This version incorporates a plotting function using
maps. Also, the error analysis function is made fully operational.
A full description on the use of the model and a file of ‘Historic
Hurricanes’ can be found on this website.
Early in the research into hurricane movement,
two characteristics became apparent. First, storms have a diurnal
cycle. They slow down at night and speed up during daylight
hours due to the energy absorption from the sun. This characteristic
is modeled in part by having the forecast depend on the five-step-back
position that tells the location one day back.
The second characteristic became apparent through the examination
of a sequence of satellite photos. These gave the same effect
as watching a movie film. They showed that the eye of the
hurricane pulses and wobbles. The pulsing was perhaps due
to the diurnal effect. The wobble had the appearance of following
a cycloid path. That is, the path a point on a wheel would
follow as the wheel rolls along. This wobble has the effect
of introducing an error in the forecasts. Since the position
reports are taken every six hours (every three hours as the
storm approaches land), the direction of movement could be
off depending on where the storm was in the cycloid cycle.
If the characteristic of this wobble could be determined
by estimating the parameters of the equation of the cycloid,
then a correction could be made and the accuracy of the forecast
might be improved. This is the direction of current studies.