Tropical cyclone verification northern hemisphere 2021
A summary of tropical cyclone activity in the Northern Hemisphere for the 2021 season together with an assessment of the performance of the Met Office global model in predicting the track and intensity of these tropical cyclones.
1. Introduction
A summary of tropical cyclone activity in the Northern Hemisphere for the 2021 season is presented below together with an assessment of the performance of the Met Office global model in predicting the track and intensity of these tropical cyclones.
Tropical cyclones are experienced in the North Pacific, North Atlantic and North Indian Oceans and nearby tropical seas with occasional tropical or subtropical storms in the Mediterranean Sea. For the purpose of tropical cyclone verification the northern hemisphere is divided in to five basins; the North-West Pacific (west of 180° E to the Malay Peninsula), the North-East Pacific (east of 180° E), the North Atlantic, the North Indian Ocean (west of the Malay Peninsula) and the Mediterranean Sea. Mean error statistics for the four main basins are presented together with a table of statistics for the whole Northern Hemisphere. Verification is performed at 12-hour intervals up to forecast time T+168, although statistics are only presented at 24-hour intervals in this report. The global model resolution in operation was 0.140625° x 0.09375° x 70 levels. This is equivalent to a horizontal resolution of 16 km × 10 km at the equator.
Tropical cyclone forecast verification - measures of error
Advisory positions received in real time from RSMC Tokyo, JTWC Hawaii, NHC Miami and CPHC Honolulu are used as verifying observations of storm location. Best track data from these centres will be obtained once they become available. Past experience shows that use of best track rather than real time data usually only makes minor differences to seasonal error statistics. Some mean error statistics for last season are also included for the purposes of a comparison. Forecast tracks are only verified when a depression reaches tropical storm status.
2. Tropical cyclone activity
| NWP | NEP | NAT | NI | MED | TOTAL | |
|---|---|---|---|---|---|---|
| Tropical depressions (<34 knots) | 6 (3) | 0 (4) | 0 (1) | 0 (0) | 0 (0) | 6 (8) |
| Tropical storms (34-63 knots) | 13 (11) | 11 (13) | 14 (17) | 3 (1) | 2 (3) | 43 (45) |
| Hurricanes/typhoons (>63 knots) | 10 (12) | 8 (4) | 7 (13) | 3 (4) | 0 (0) | 28 (33) |
| Total | 29 (26) | 19 (21) | 21 (31) | 6 (5) | 2 (0) | 77 (83) |
Categories assigned based on 1-minute averaged winds
Basin name abbreviations:
NWP : North-west Pacific (west of 180° E)
NEP : North-east Pacific (east of 180° E)
NAT : North Atlantic
NI : North Indian (west of Malay Peninsula)
MED : Mediterranean Sea
The number in brackets indicates the figure for the 2020 season.
3. Summary of northern hemisphere storms
3.1 North-west Pacific basin storms
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | T+168 | |
| Possibly verified | 234 | 188 | 147 | 111 | 79 | 56 | 39 | 28 |
| Detection rate (%) | 100 | 98 | 98 | 95 | 94 | 93 | 95 | 89 |
| AT error (km) | 0.6 | -17.9 | -55.5 | -99.0 | -145.2 | -87.3 | -5.9 | 106.9 |
| CT error (km) | -2.4 | -10.4 | -23.6 | -40.9 | -92.8 | -171.3 | -188.8 | -155.6 |
| Track skill (%) | ***** | 63.1 | 67.6 | 65.7 | ***** | ***** | ***** | ***** |
| * 2016-20 skill (%) | ***** | 61.8 | 69.7 | 70.4 | ***** | ***** | ***** | ***** |
| DPE (km) | 38.6 | 74.3 | 144.5 | 229.2 | 362.8 | 401.0 | 474.6 | 584.0 |
| * 2016-20 DPE (km) | 33.7 | 69.7 | 124.3 | 195.0 | 276.3 | 376.8 | 519.0 | 733.7 |
| Central pressure bias (mb) | 0.4 | 3.1 | 2.8 | -0.3 | -1.0 | -4.2 | -10.3 | -10.0 |
* 5-year running mean DPE and skill for all north-west Pacific storms 2016-20
Plot of the observed tracks of all storms in the north-west Pacific basin
Track forecast errors in the north-west Pacific basin
Track forecast skill in the north-west Pacific basin
The total number of storms was similar to 2020, but they were longer lived so there were more verifiable forecasts. Track forecast errors were above the 5-year running mean at all, but the longest lead times. The central pressure bias became negative at longer lead times.
3.2 North-east Pacific basin storms
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | T+168 | |
| Possibly verified | 117 | 79 | 53 | 34 | 23 | 14 | 9 | 6 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 |
| AT error (km) | 2.2 | 11.4 | 27.0 | 91.4 | 165.2 | 198.4 | 294.8 | 473.0 |
| CT error (km) | -5.0 | -26.0 | -53.9 | -69.7 | -34.1 | 47.5 | 32.1 | -46.2 |
| Track skill (%) | ***** | 54.8 | 56.6 | 55.9 | ***** | ***** | ***** | ***** |
| * 2016-20 skill (%) | ***** | 54.1 | 67.6 | 67.5 | ***** | ***** | ***** | ***** |
| DPE (km) | 28.5 | 68.5 | 131.7 | 211.9 | 257.6 | 254.0 | 410.7 | 510.8 |
| * 2016-20 DPE (km) | 35.4 | 71.6 | 116.0 | 168.5 | 231.2 | 309.8 | 372.8 | 424.6 |
| Central pressure bias (mb) | -0.6 | 2.4 | 4.5 | 6.8 | 8.6 | 9.9 | 12.6 | 13.8 |
* 5-year running mean DPE and skill for all north-east Pacific storms for 2016-20
Plot of the observed tracks of all storms in the north-east Pacific basin
Track forecast errors in the north-east Pacific basin
Track forecast skill in the north-east Pacific basin
Activity was similar to 2020, although a greater proportion of storms reached hurricane strength. Track forecast errors were mostly a little higher than the 5-year running mean. There was a growing weak bias with forecast lead time.
3.3 North Atlantic basin storms
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | T+168 | |
| Possibly verified | 155 | 109 | 85 | 69 | 53 | 43 | 31 | 20 |
| Detection rate (%) | 100 | 98 | 96 | 94 | 91 | 84 | 84 | 85 |
| AT error (km) | -0.1 | -2.4 | 2.4 | -3.4 | -7.7 | 36.8 | 21.3 | 53.9 |
| CT error (km) | 1.0 | -11.1 | -28.5 | -40.6 | -47.5 | -23.3 | -81.1 | 12.4 |
| Track skill (%) | ***** | 68.6 | 76.0 | 75.0 | ***** | ***** | ***** | ***** |
| * 2016-20 skill (%) | ***** | 68.4 | 73.4 | 72.6 | ***** | ***** | ***** | ***** |
| DPE (km) | 28.1 | 60.0 | 95.9 | 145.8 | 175.7 | 251.8 | 419.1 | 574.1 |
| * 2016-20 DPE (km) | 28.6 | 63.9 | 119.1 | 191.8 | 279.0 | 360.1 | 472.2 | 590.0 |
| Central pressure bias (mb) | 2.7 | 3.4 | 1.7 | -1.2 | -2.4 | -3.5 | -5.3 | -2.5 |
* 5-year running mean DPE and skill for all North Atlantic storms for 2016-20
Plot of the observed tracks of all storms in the North Atlantic basin
Track forecast errors in the North Atlantic basin
Track forecast skill in the North Atlantic basin
2021 was another active season, although not as active as the exceptional 2020 season. Track forecast errors were lower than 2020 and the 5-year running mean. There was a slight overdeepening of central pressure at longer lead times, mostly caused by forecasts for Hurricane Larry.
3.4 North Indian Basin Storms
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | T+168 | |
| Possibly verified | 30 | 20 | 11 | 4 | 1 | 0 | 0 | 0 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 100 | ***** | ***** | ***** |
| AT error (km) | -9.8 | -15.8 | -29.9 | -112.0 | -439.0 | ***** | ***** | ***** |
| CT error (km) | -2.9 | 18.7 | 0.3 | -82.8 | -322.0 | ***** | ***** | ***** |
| Track skill (%) | ***** | 76.6 | 81.2 | 66.2 | ***** | ***** | ***** | ***** |
| * 2016-20 skill (%) | ***** | 46.9 | 55.1 | 62.4 | ***** | ***** | ***** | ***** |
| DPE (km) | 49.8 | 54.8 | 76.9 | 174.2 | 545.0 | ***** | ***** | ***** |
| * 2016-20 DPE (km) | 44.7 | 78.4 | 134.6 | 181.2 | 235.2 | ***** | ***** | ***** |
| Central pressure bias (mb) | -2.2 | -4.9 | -3.9 | -12.8 | -63.0 | ***** | ***** | ***** |
* 5-year running mean DPE and skill for all North Indian storms for 2016-20
Plot of the observed tracks of all storms in the North Indian basin
Track forecast errors in the North Indian basin
Track forecast skill in the North Indian basin
Activity was similar to that in 2020. Track forecast errors up to 48 hours were low. Beyond that few forecasts were verified.
3.5 Combined statistics for whole northern hemisphere
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | T+168 | |
| Possibly verified | 546 | 402 | 300 | 220 | 156 | 113 | 79 | 54 |
| Detection rate (%) | 100 | 99 | 98 | 95 | 94 | 90 | 91 | 89 |
| AT error (km) | 0.0 | -8.2 | -26.2 | -40.1 | -53.1 | -4.3 | 41.5 | 133.9 |
| CT error (km) | -1.9 | -11.9 | -29.5 | -46.5 | -70.2 | -89.0 | -122.3 | -82.4 |
| Track skill (%) | ***** | 64.0 | 69.0 | 67.6 | ***** | ***** | ***** | ***** |
| * 2016-20 skill (%) | ***** | 61.7 | 70.0 | 70.3 | ***** | ***** | ***** | ***** |
| DPE (km) | 33.9 | 68.3 | 127.7 | 200.2 | 285.9 | 328.2 | 446.5 | 571.3 |
| * 2016-20 DPE (km) | 33.4 | 69.1 | 121.0 | 185.9 | 262.7 | 351.0 | 458.8 | 582.7 |
| Central pressure bias (mb) | 0.7 | 2.6 | 2.5 | 0.3 | -0.4 | -2.0 | -5.6 | -4.4 |
* 5-year running mean DPE and skill for all northern hemisphere storms for 2016-20
2021 saw activity close to that in 2020, but with a smaller number of tropical cyclones reaching hurricane strength. The active Atlantic was counterbalanced again by the quiet Pacific.
Track forecast errors were lower than last season and generally near to the 5-year running mean . Cross track errors were negative (left-of-track bias), whilst the along track (forward speed) bias was mixed. Detection percentages were mostly high, but dropped off at longer lead times.
Track forecast errors for the whole northern hemisphere
Track forecast skill for the whole northern hemisphere
Cross-track errors for the whole northern hemisphere
Along-track errors for the whole northern hemisphere
The 5-year running mean of track forecast errors has been flat-lining for a few years since the step improvement seen as a result of a model change in 2014 dropped out of the statistics.
5-year running mean track forecast errors for the whole northern hemisphere
The intensity bias as measured by central pressure was slightly positive (weak bias) at short lead times and slightly negative (strong bias) at longer lead times. 10m winds had a weak bias which reduced with lead time.
Central pressure forecast bias for the whole northern hemisphere
Peak 10m wind forecast bias for the whole northern hemisphere
4. Further tropical cyclone information
The Met Office tropical cyclone web pages contain information on tropical cyclone forecasting at the Met Office. Monthly updates of tropical cyclone activity and forecasts are made, together with observed and forecast information of recent storms, track and intensity prediction error statistics, lists of names and real time tropical cyclone forecast guidance.
For further information on tropical cyclone forecasting email the Met Office.
