National Hurricane Center Forecast Verification
Updated 3 April 2013
- Forecast verification procedures
- Annual NHC verification
- Official five-year mean
errors and distributions
- Official error trends
- Model error trends
- NHC official forecast
- Performance measures and
2. Forecast verification procedures
For all operationally
designated tropical cyclones in the Atlantic and eastern North
Pacific basins, the NHC issues official forecasts of the
cyclone's center position and maximum 1-min surface wind speed.
These forecasts are issued every 6 hours (at 0300, 0900, 1500, and
2100 UTC), and each contains projections valid 12, 24, 36, 48, 72,
96, and 120 h after the forecast's nominal initial time (0000,
0600, 1200, and 1800 UTC, respectively). These forecasts can be
found in two of NHC's public products, the Tropical Cyclone
Forecast/Advisory and the Tropical Cyclone Discussion. Over the
years there have been numerous changes to the NHC forecast
products and procedures; a partial chronology is given in Table 1. Periods of record in the digital
databases used to generate the verifications in this report are
given in Table 2 and Table
At the conclusion of
each season, official forecasts are evaluated by comparison with
the cyclone's "best track" database. Best track data represent
NHC's analysis of a tropical cyclone, and consist of 6-hourly
representative estimates of the cyclone's center location and
maximum sustained wind as well as other parameters, determined by
a post-storm analysis of all available storm data. Track forecast
error is defined as the great-circle distance between a cyclone's
forecast position and the best track position at the forecast
verification time. Forecast intensity error is defined as the
absolute value of the difference between the forecast and best
track intensity at the forecast verifying time. Current practice
is to include a forecast in the verification only if the system
was a tropical or subtropical cyclone at both the forecast and the
verifying time; all other stages of development (e.g.,
extratropical, tropical wave, remnant low) are excluded.
When forecasts from both regular and special advisories are
available for the same synoptic time, current practice is to
verify the regular forecast.
comparisons between official forecasts from different storms or
different years, a simultaneous (homogeneous) verification of a
skill-baseline model such as CLIPER (Neumann 1972, Aberson 1998)
or SHIFOR (Jarvinen and Neumann 1979, Knaff et al. 2003) is
usually done. The skill baseline models can be run off of
operational parameters, or they can be run post-storm using best
track data. It is generally preferable to use operational
parameters to assess the skill of operational forecasts; however
only in recent years are the required parameters available. Very
small differences in sample size can arise depending on whether an
operational or best track baseline is used. Operational skill
baselines were used for the verifications in Sections 3, 4,
6, and 8
below; while best-track baselines were required for the longer
period data presented in Section 5
and Section 7. It should also be
noted that the 5-day CLIPER model was updated in 2005, and is now
based on dependant data sets covering the periods 1931-2004 for
the Atlantic basin and 1949-2004 for the Eastern North Pacific
basin. In 2006, an enhancement was made to the SHIFOR model
that includes a weakening of the cyclone over land; this version
of the model is known as "Decay-SHIFOR5".
addition to forecasting the location and intensity of a tropical
cyclone, NHC also forecasts the extent of 34 and 50 kt winds in
four quadrants surrounding the cyclone out to 72 h from the
time, and the extent of 64 kt winds out to 36 h from the initial
These "wind radii" forecasts are available in the Tropical Cyclone
Forecast/Advisory. Only since 2004 have wind radii been
included in the post-storm best track database, but even so, these
estimates are not considered reliable enough to warrant a formal
verification of the wind radii forecasts.
Chronology of changes in NHC forecast procedures.
||First recorded 24 h forecasts.
||50 kt wind radii forecasts introduced (out to
||First recorded 48 h forecasts.
||Forecasts extended to 72 h. 12 h forecast
||Forecast projections adjusted to be relative
to synoptic time (the beginning of the forecast cycle),
rather than relative to the forecast release time. Prior to
1967, a 24 h forecast based on data from 12Z, but issued at
16Z, would be valid at 16Z the following day. Current
practice is that the forecast is valid at 12Z the following
||34 kt wind radii forecasts introduced (out to
||NHC assumes forecast responsibility for
eastern North Pacific basin.
||36 h forecasts introduced; 34 and 50 kt wind
radii forecasts extended to 36 h.
||Forecast release time moved 1 h earlier, from
4 h to 3 h after synoptic time.
||50 kt wind radii forecasts extended to 72 h;
64 kt wind radii forecasts introduced (out to 36 h).
||Forecasts extended to 96 and 120 h (no public
||34 kt wind radii forecasts extended to 72 h.
||Public distribution of 96 and 120 h forecasts
First year of availability of information in NHC digital best
track database (the "b-decks" of the Automated Tropical Cyclone
Forecast System, ATCF).
||Atlantic storms and hurricanes.
||Eastern North Pacific storms and hurricanes.
||Atlantic non-developing depressions.
||Eastern North Pacific non-developing
inclusion of "low" stage prior to the first tropical point.
pre-TC low stage is included as far backward as the low
First year of availability of information in NHC digital
forecast database (the "a-decks" of the Automated Tropical
Cyclone Forecast System, ATCF).
||24 h forecasts.
||48 h forecasts.
||72 h forecasts.
||Operational initial position (0 h) and 12 h
||36 h forecasts.
||96 and 120 h forecasts. Wind radii forecasts.
||Revised CLIPER5 model to reflect new
dependent data sets: 1931-2004 (Atlantic), 1949-2004
(eastern North Pacific).
||Forecasts associated with special advisories no longer
overwrite original advisory. Original advisory
forecast is now retained in a-deck as “OFCO”.
||Decay-SHIFOR forecasts. Available as "OCD5" using
operational input, and as "BCD5" using best track input.
Forecast track used to evaluate the decay component
was taken from operational or best track CLIPER5 model,
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