courtesy
of National
Hurricane Center
FORECAST
ERRORS
Hurricane
forecasters analyze large amounts of data, including
conflicting computer model results, and come up
with their best estimate of a three-day track and
intensity forecast. Just as it is easier to predict
where you will be 12 hours from now as opposed to
72 hours, it is easier to predict where a hurricane
will be right before it makes landfall (although
hurricanes can, and do, unexpectedly alter course).
The figure below shows that, at shorter forecast
time periods, the forecast track error is fairly
small, but when the forecast is farther in the future,
the error increases significantly. Knowing these
estimated errors can help you assess your potential
risk.
AVERAGE
TRACK POSITION FORECAST ERROR
(1 nautical mile = 1.15 statute miles) |
Forecasters
convey track uncertainty through the strike probability
tables and strike probability graphics which show
the areas likely to be affected by the tropical
cyclone. These products are based on the current
best forecast track and the past distribution of
errors. They are created to indicate the statistical
chance that the hurricane center will pass within
65 NM (75 mi) of a location within 3 days of the
initial forecast time.
The
maximum probabilities by forecast period are displayed
in the table below and are based on NHC's average
forecast errors for the various periods. For example,
if a hurricane is expected to make landfall in 48
hours at New Orleans, the highest the probability
can be is 25%. Most coastal locations need to start
evacuations by 48 hours in order to have them completed
before the tropical storm-force winds arrive.
In
most cases, if you wait until the probability is
50% or greater, it will be too late to take effective
actions. Consequently, this table provides critical
information for decision making.
| Forecast
Period |
Maximum
Probability* |
| 72
hours |
10%-15%
|
| 48
hours |
20%-25% |
| 36
hours |
25%-35%
|
| 24
hours |
40%-50%
|
| 12
hours |
75%-85%
|
| *These
probabilities are those which would be computed
if the forecast position (at the given time
period) were directly over a community. A range
of probabilities is given because forecast errors
differ by location. The probability can be 100%
if the center is already close to a location.
In addition, the probabilities can exceed the
maximums listed in the table if the hurricane
center is actually forecast to be at the location
at an earlier time period. |
Hurricane
Intensity Forecasts
Much
like the difficulties in forecasting a hurricane's
track, hurricane specialists are challenged to predict
the exact strength (in terms of wind speed) of a
tropical cyclone. The average errors for intensity
are shown below.
AVERAGE
INTENSITY ERROR BY FORECAST PERIOD
(1 nautical mile = 1.15 statute miles) |
Hurricane
Warning Area
- Size
of the warning area and timing of the warnings
are based on the forecast track, the size of
the storm, and the known uncertainties in the
forecasts.
- Orientation
of the forecast track with respect to the coast
plays a major role in the size of the warning
area.
