How accurate was the forecast for Twenty-One-E?
Tropical Depression Twenty-One-E · 24 advisories archived · 102 forecasts resolved
We record every National Hurricane Center advisory as it is issued. Once the storm has moved past a forecast’s valid time, that forecast can be checked against where Twenty-One-E actually was. This page is that comparison - 24 advisories archived, 102 forecasts resolved so far.
Every forecast, against what happened
The white line is where Twenty-One-E actually went. Each amber line is one forecast NHC issued, newest brightest - so the spread between them is how much the outlook changed as the storm evolved.
Intensity: predicted against actual
Intensity is the hard part. Track forecasting has improved enormously over thirty years; predicting how strong a storm will get - especially one that intensifies rapidly - has improved far less. For Twenty-One-E, 13 of 102 checked forecasts came in below what the storm actually did.
Track error by lead time
How far the forecast centre was from where the centre actually turned out to be, in nautical miles.
Median error by lead time
Median rather than average: track errors are strongly right-skewed, so an average describes a performance nobody actually had.
| Lead time | Forecasts | Median track error | Median intensity error |
|---|---|---|---|
| 12h | 22 | 19.6 nm | +0 mph |
| 24h | 20 | 31.9 nm | +5 mph |
| 36h | 18 | 38.8 nm | +10 mph |
| 48h | 30 | 43.6 nm | +15 mph |
| 72h | 12 | 65 nm | +20 mph |
Every forecast checked
| Advisory | Lead | Track error | Forecast | Actual |
|---|---|---|---|---|
| 001 | +9h | 55.6 nm | 45 mph | 45 mph+0 mph |
| 002 | +9h | 7.5 nm | 50 mph | 50 mph+0 mph |
| 003 | +9h | 27.7 nm | 60 mph | 55 mph+0 mph |
| 004 | +9h | 3 nm | 70 mph | 65 mph+0 mph |
| 005 | +9h | 3 nm | 70 mph | 65 mph+0 mph |
| 006 | +9h | 25.1 nm | 80 mph | 70 mph+10 mph |
| 007 | +9h | 18.6 nm | 85 mph | 85 mph+0 mph |
| 008 | +9h | 14.6 nm | 90 mph | 90 mph0 mph |
| 009 | +9h | 24 nm | 110 mph | 100 mph+10 mph |
| 010 | +9h | 20.6 nm | 110 mph | 115 mph-5 mph |
| 011 | +9h | 37.1 nm | 120 mph | 120 mph0 mph |
| 012 | +9h | 13.1 nm | 140 mph | 105 mph+35 mph |
| 013 | +9h | 29.4 nm | 130 mph | 90 mph+45 mph |
| 014 | +9h | 10.6 nm | 85 mph | 90 mph-5 mph |
| 015 | +9h | 14.4 nm | 105 mph | 90 mph+10 mph |
| 016 | +9h | 14.7 nm | 105 mph | 95 mph+10 mph |
| 017 | +9h | 32.7 nm | 105 mph | 100 mph+5 mph |
| 018 | +9h | 25.7 nm | 105 mph | 100 mph+0 mph |
| 019 | +9h | 22.7 nm | 105 mph | 105 mph0 mph |
| 020 | +9h | 12 nm | 105 mph | 100 mph+0 mph |
| 021 | +9h | 8.2 nm | 100 mph | 90 mph+10 mph |
| 022 | +9h | 25.6 nm | 90 mph | 70 mph+25 mph |
| 001 | +21h | 26.1 nm | 60 mph | 55 mph+0 mph |
| 002 | +21h | 47.6 nm | 70 mph | 60 mph+10 mph |
| 003 | +21h | 35.8 nm | 75 mph | 65 mph+10 mph |
| 004 | +21h | 24.2 nm | 85 mph | 85 mph+0 mph |
| 005 | +21h | 24.2 nm | 85 mph | 85 mph+0 mph |
| 006 | +21h | 18.9 nm | 90 mph | 90 mph0 mph |
| 007 | +21h | 24.7 nm | 105 mph | 100 mph+5 mph |
| 008 | +21h | 4.2 nm | 115 mph | 115 mph+0 mph |
| 009 | +21h | 18.9 nm | 130 mph | 120 mph+10 mph |
| 010 | +21h | 34.7 nm | 130 mph | 105 mph+30 mph |
| 011 | +21h | 56.8 nm | 140 mph | 90 mph+50 mph |
| 012 | +21h | 31.6 nm | 145 mph | 90 mph+50 mph |
| 013 | +21h | 20 nm | 140 mph | 90 mph+45 mph |
| 014 | +21h | 22 nm | 90 mph | 95 mph-5 mph |
| 015 | +21h | 55.1 nm | 105 mph | 100 mph+5 mph |
| 016 | +21h | 36.2 nm | 105 mph | 100 mph+0 mph |
| 017 | +21h | 50.5 nm | 105 mph | 105 mph0 mph |
| 018 | +21h | 41.2 nm | 100 mph | 100 mph-5 mph |
| 019 | +21h | 32.1 nm | 85 mph | 90 mph-5 mph |
| 020 | +21h | 34.5 nm | 80 mph | 70 mph+10 mph |
| 001 | +33h | 10.8 nm | 75 mph | 65 mph+10 mph |
| 002 | +33h | 64.5 nm | 85 mph | 70 mph+15 mph |
| 003 | +33h | 50.7 nm | 90 mph | 85 mph+10 mph |
| 004 | +33h | 21.5 nm | 105 mph | 100 mph+5 mph |
| 005 | +33h | 21.5 nm | 105 mph | 100 mph+5 mph |
| 006 | +33h | 35.4 nm | 110 mph | 115 mph-5 mph |
| 007 | +33h | 21.2 nm | 115 mph | 120 mph-5 mph |
| 008 | +33h | 29.7 nm | 125 mph | 105 mph+25 mph |
| 009 | +33h | 30 nm | 145 mph | 90 mph+55 mph |
| 010 | +33h | 42.1 nm | 145 mph | 90 mph+50 mph |
| 011 | +33h | 48.1 nm | 145 mph | 90 mph+50 mph |
| 012 | +33h | 21.2 nm | 145 mph | 95 mph+50 mph |
| 013 | +33h | 34.8 nm | 130 mph | 100 mph+35 mph |
| 014 | +33h | 51.6 nm | 90 mph | 100 mph-10 mph |
| 015 | +33h | 71.6 nm | 90 mph | 105 mph-10 mph |
| 016 | +33h | 51.3 nm | 100 mph | 100 mph-5 mph |
| 017 | +33h | 59 nm | 100 mph | 90 mph+10 mph |
| 018 | +33h | 71.8 nm | 80 mph | 70 mph+10 mph |
| 001 | +45h | 16.8 nm | 90 mph | 85 mph+10 mph |
| 002 | +45h | 50 nm | 105 mph | 90 mph+10 mph |
| 003 | +45h | 29.4 nm | 115 mph | 100 mph+15 mph |
| 004 | +45h | 28.8 nm | 120 mph | 120 mph0 mph |
| 005 | +45h | 28.8 nm | 120 mph | 120 mph0 mph |
| 006 | +45h | 31.4 nm | 125 mph | 105 mph+25 mph |
| 007 | +45h | 18 nm | 125 mph | 90 mph+35 mph |
| 008 | +45h | 39.7 nm | 130 mph | 90 mph+40 mph |
| 009 | +45h | 28 nm | 140 mph | 90 mph+45 mph |
| 010 | +45h | 41.5 nm | 140 mph | 95 mph+45 mph |
| 011 | +45h | 45.6 nm | 130 mph | 100 mph+35 mph |
| 012 | +45h | 45.9 nm | 125 mph | 100 mph+25 mph |
| 013 | +45h | 61.9 nm | 115 mph | 105 mph+10 mph |
| 014 | +45h | 61.5 nm | 85 mph | 100 mph-15 mph |
| 015 | +45h | 71.9 nm | 85 mph | 90 mph-5 mph |
| 016 | +45h | 69.9 nm | 80 mph | 70 mph+10 mph |
| 001 | +57h | 18 nm | 115 mph | 100 mph+15 mph |
| 002 | +57h | 61.2 nm | 120 mph | 115 mph+10 mph |
| 003 | +57h | 57.7 nm | 125 mph | 120 mph+5 mph |
| 004 | +57h | 30.6 nm | 130 mph | 90 mph+45 mph |
| 005 | +57h | 30.6 nm | 130 mph | 90 mph+45 mph |
| 006 | +57h | 28.6 nm | 130 mph | 90 mph+40 mph |
| 007 | +57h | 26.6 nm | 130 mph | 90 mph+40 mph |
| 008 | +57h | 21.2 nm | 130 mph | 95 mph+35 mph |
| 009 | +57h | 60.7 nm | 120 mph | 100 mph+25 mph |
| 010 | +57h | 67 nm | 120 mph | 100 mph+20 mph |
| 011 | +57h | 70.2 nm | 115 mph | 105 mph+10 mph |
| 012 | +57h | 68.9 nm | 100 mph | 100 mph-5 mph |
| 013 | +57h | 83.3 nm | 100 mph | 90 mph+10 mph |
| 014 | +57h | 83 nm | 70 mph | 70 mph0 mph |
| 001 | +69h | 15.7 nm | 130 mph | 120 mph+10 mph |
| 002 | +69h | 63.4 nm | 130 mph | 105 mph+30 mph |
| 003 | +69h | 78.1 nm | 130 mph | 90 mph+45 mph |
| 004 | +69h | 39.3 nm | 130 mph | 90 mph+40 mph |
| 005 | +69h | 39.3 nm | 130 mph | 90 mph+40 mph |
| 006 | +69h | 66.6 nm | 125 mph | 95 mph+30 mph |
| 007 | +69h | 61.9 nm | 120 mph | 100 mph+25 mph |
| 008 | +69h | 37.2 nm | 120 mph | 100 mph+20 mph |
| 009 | +69h | 73.1 nm | 105 mph | 105 mph0 mph |
| 010 | +69h | 69.1 nm | 105 mph | 100 mph+0 mph |
| 011 | +69h | 83.3 nm | 100 mph | 90 mph+10 mph |
| 012 | +69h | 90.1 nm | 75 mph | 70 mph+5 mph |
How this is worked out, and what it is not
Each advisory carries the storm’s observed position at the moment it was issued. Strung together, those observations are the storm’s actual path. A forecast made twelve hours earlier for this moment can then be compared against it. Where a forecast is valid between two observations, the position is interpolated linearly; forecasts valid outside the observed window are left unscored rather than extrapolated.
This is not NHC’s official verification. They score against post-season best-track data, reanalysed with aircraft and satellite information that is not available while the storm is running. Our numbers use the operational positions as issued, which is a slightly different - and slightly harsher - question: how well did the forecast match what was known at the time?
Tropical Depression Twenty-One-E is one storm. The point of keeping this archive is that after a season of them, the question stops being anecdote.