Tropical cyclone forecast verification - southern hemisphere 2018-19
A summary of tropical cyclone activity in the southern hemisphere for the 2018-19 season together with an assessment of the performance of the Met Office global model in predicting the tracks of these tropical cyclones.
1. Introduction
A summary of tropical cyclone activity in the Southern Hemisphere for the 2018-19 season is presented below together with an assessment of the performance of the Met Office global model in predicting the tracks of these tropical cyclones.
Tropical cyclones are experienced in the South Indian Ocean, tropical seas to the west, north and east of Australia and the western and central South Pacific Ocean. There are also occasional tropical or subtropical storms in the South Atlantic Ocean. For the purpose of tropical cyclone verification the Southern Hemisphere is divided in to three basins; the South-West Indian (west of 90° E), Australian (east of 90° E) and South Atlantic. Mean error statistics for each basin are presented together with a table of statistics for the whole Southern Hemisphere. The global model produces a seven-day forecast every 12 hours. 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 during the season 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 from RSMCs La Réunion and Nadi, Fiji, Bureau of Meteorology, Australia and JTWC Hawaii 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 track 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
| SWI | AUS | SAT | TOTAL | |
|---|---|---|---|---|
| Tropical depressions (<35 knots) | 1 (0) | 0 (0) | 0 (0) | 1 (0) |
| Tropical storms (35-63 knots) | 3 (3) | 7 (9) | 2 (1) | 12 (13) |
| Hurricanes/typhoons (>63 knots) | 9 (4) | 8 (6) | 0 (0) | 17 (10) |
| Total | 13 (7) | 15 (15) | 2 (1) | 30 (23) |
Basin name abbreviations:
SWI : South-West Indian (west of 90° E)
AUS : Australian (east of 90° E)
SAT : South Atlantic
The number in brackets indicates the figure for the 2017-18 season.
N.B. 10-minute averaged wind speeds from RSMCs used when available. Data from JTWC (1-minute averaged) is only used when other data is unavailable and maximum wind speeds are scaled to make them equivalent to the RSMC 10-minute averages.
3. Summary of all southern hemisphere storms
3.1 South-west Indian basin storms
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | T+168 | |
|---|---|---|---|---|---|---|---|---|
| Cases verified | 150 | 122 | 94 | 72 | 54 | 37 | 21 | 11 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 |
| AT error (km) | 0 | -8 | -011 | 2 | 43 | 45 | 73 | 228 |
| CT error (km) | -1 | 6 | 24 | 50 | 48 | 45 | 5 | -265 |
| Track skill (%) | ***** | 59 | 65 | 65 | ***** | ***** | ***** | ***** |
| * 2017-18 skill (%) | ***** |
66 |
71 | 74 | ***** | ***** | ***** | ***** |
| DPE (km) | 38 | 71 | 136 | 218 | 279 | 343 | 462 | 730 |
| * 2017-18 DPE (km) | 35 | 61 | 107 | 156 | 232 | 386 | 610 | 733 |
| Central pressure bias (mb) | -1.3 | -1.3 | -1.9 | -7.3 | -10.8 | -14.9 | -20.7 | -28.6 |
* DPE and skill for all south-west Indian storms in 2017-18 season
Plot of the observed tracks of all storms in the south-west Indian basin
Forecast positional errors in the south-west Indian basin
Forecast skill in the south-west Indian basin
There was much greater activity in this region this season than in the previous season with a large number of intense cyclones. Track forecast errors were mostly a little higher than last season's record low values. Skill scores against CLIPER were slightly down on last season. Track biases were mostly small. The central pressure bias was negative at T+48 onwards indicating model predictions were too intense by this measure.
3.2 Australian basin storms
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | T+168 | |
|---|---|---|---|---|---|---|---|---|
| Cases verified | 145 | 113 | 86 | 67 | 47 | 31 | 19 | 12 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 |
| AT error (km) | -2 | -8 | 15 | -8 | -58 | 42 | 319 | 326 |
| CT error (km) | 3 | 4 | -14 | -5 | 30 | 15 | 35 | 61 |
| Track skill (%) | ***** | 73 | 76 | 73 | ***** | ***** | ***** | ***** |
| * 2017-18 skill (%) | ***** | 68 | 67 | 73 | ***** | ***** | ***** | ***** |
| DPE (km) | 35 | 81 | 118 | 169 | 224 | 325 | 514 | 481 |
| * 2017-18 DPE (km) | 38 | 88 | 144 | 190 | 220 | 215 | 226 | 325 |
| Central pressure bias (mb) | 2.1 | 0.3 | -3.1 | -6.0 | -10.7 | -17.3 | -18.3 | -17.0 |
* DPE and skill for all Australian storms in 2017-18 season
Plot of the observed tracks of all storms in the eastern Australian basin
Plot of the observed tracks of all storms in the western Australian basin
Forecast positional errors in the Australian basin
Forecast skill in the Australian basin
This basin was more active this season than last. Track forecast errors were lower than last season at all short times and track forecast skill against CLIPER was mostly higher than last season. There was a fast bias at long lead times. The central pressure bias was negative at most lead times.
3.3 South Atlantic basin storms
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | T+168 | |
|---|---|---|---|---|---|---|---|---|
| Cases verified | 8 | 4 | 1 | ***** | ***** | ***** | ***** |
***** |
| Detection rate (%) | 100 | 100 | 100 | ***** | ***** | ***** | ***** |
***** |
| AT error (km) | -38 | -119 | -141 | ***** | ***** | ***** | ***** |
***** |
| CT error (km) | -7 | -32 | -41 | ***** | ***** | ***** | ***** |
***** |
| DPE (km) | 109 | 133 | 148 | ***** | ***** | ***** | ***** |
***** |
| * 2016-17 DPE (km) | 53 | 92 | ***** | ***** | ***** | ***** | ***** |
***** |
| Central pressure bias (mb) | 2.2 | 1.0 | 0.0 | ***** | ***** | ***** | ***** |
***** |
* DPE and skill for all south Atlantic storms in 2016-17 season
Plot of the observed tracks of all storms in the south Atlantic basin
There were two named storms in this region this season. Tracks and forecast errors as shown.
3.4 Combined Statistics for whole Southern Hemisphere
| T+0 | T+24 | T+48 | T+72 | T+96 | T+120 | T+144 | T+168 | |
|---|---|---|---|---|---|---|---|---|
| Cases verified | 303 | 239 | 181 | 139 | 101 | 68 | 40 | 23 |
| Detection rate (%) | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 |
| AT error (km) | -2 | -10 | 0 | -3 | -4 | 44 | 190 | 280 |
| CT error (km) | 1 | 4 | 6 | 24 | 40 | 32 | 19 | -95 |
| Track skill (%) | ***** | 67 | 70 | 69 | ***** | ***** | ***** | ***** |
| * 2017-18 skill (%) | ***** | 67 | 69 | 74 | ***** | ***** | ***** | ***** |
| DPE (km) | 38 | 77 | 128 | 194 | 254 | 335 | 487 | 600 |
| * 2017-18 DPE (km) | 37 | 75 | 125 | 171 | 227 | 315 | 429 | 521 |
| Central pressure bias (mb) | 0.4 | -0.5 | -2.5 | -6.7 | -10.7 | -16.0 | -19.6 | -22.6 |
* DPE and skill for all southern hemisphere storms in 2017-18 season
For the second season in a row the southern hemisphere had much increased activity compared to last season. In particular there was a high proportion of intense cyclones.
In general track forecast errors were slightly higher than last season's record low values. Skill scores against CLIPER were mostly near to last season's values. Track forecast biases were low at most lead times.
Forecast positional errors for the whole southern hemisphere
Forecast skill for the whole southern hemisphere
Along-track errors for the whole southern hemisphere
Cross-track errors for the whole southern hemisphere
There was a continued downwards trend in the five-year running mean of track forecast errors.
Five-year running mean track forecast errors for the whole southern hemisphere
The central pressure bias was similar to last season, but became strongly negative at long lead times due to overdeepening of strong cyclones. Despite the longstanding weak bias in 10m winds, the bias actually became positive (winds too strong) at long lead times. Mean absolute errors for central pressure grew with lead time, but for 10m winds remained fairly constant at all lead times.
Central pressure forecast bias for the whole southern hemisphere
Peak 10m wind forecast bias for the whole southern hemisphere
Central pressure forecast mean abosolute error for the whole southern hemisphere
Peak 10m wind forecast mean absolute error for the whole southern hemisphere
Further tropical cyclone information
The Met Office tropical cyclone web pages contain information on tropical cyclone forecasting at the Met Office. Updates of tropical cyclone activity and forecasts are made monthly, together with observed and forecast track 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 at the Met Office please email us.
