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Tropical cyclone forecast verification - northern hemisphere 2016

1. Introduction

A summary of tropical cyclone activity in the Northern Hemisphere for the 2016 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. For the purpose of tropical cyclone verification the northern hemisphere is divided in to four basins; the North-West Pacific (west of 180° E to the Malay Peninsula), the North-East Pacific (east of 180° E), the North Atlantic and the North Indian Ocean (west of the Malay Peninsula). Mean error statistics for each basin 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 0.234375° x 0.15625° x 70 levels. This is equivalent to a horizontal resolution of 26 km × 17 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

Tropical cyclone activity
NWP NEP NAT NI TOTAL
Tropical depressions (<34 knots) 7 (3) 1 (5) 1 (1) 0 (0) 9 (9)
Tropical storms (34-63 knots) 11 (7) 8 (9) 8 (7) 4 (3) 31 (26)
Hurricanes/typhoons (>63 knots) 13 (19) 13 (17) 7 (4) 1 (2) 34 (42)
Total 31 (29) 22 (31) 16 (12) 5 (5) 74 (77)

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)

The number in brackets indicates the figure for the 2015 season.

3. Summary of all northern hemisphere storms

3.1 North-west Pacific basin storms

Mean Error Statistics
T+0 T+24 T+48 T+72 T+96 T+120 T+144 T+168
Possibly verified 221 174 132 94 62 39 21 15
Detection rate (%) 100 100 100 100 100 100 100 93
AT error (km) -3 -12 -27 -73 -71 -102 -275 -488
CT error (km) -6 -8 -23 -30 20 54 84 76
Track skill (%) ***** 65 71 69 ***** ***** ***** *****
* 2015 skill (%) ***** 56 64 66 ***** ***** ***** *****
DPE (km) 41 72 133 222 296 422 568 841
* 2015 DPE (km) 38 77 125 183 246 326 422 564
Central pressure bias (mb) 14.4 16.9 17.5 15.5 10.1 4.5 6.1 4.9

* DPE and skill for all north-west Pacific storms in 2015 season

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

2016 saw a similar number of tropical cyclones to 2015, although fewer at the higher end of the intensity scale. Track forecast errors were mostly a little higher than last season's values, particularly at longer lead times. However, skill against CLIPER was at its highest values ever achieved. There was a weak bias at short lead times which reduced with increasing lead time.

3.2 North-east Pacific basin storms

Table of mean error statistics
T+0 T+24 T+48 T+72 T+96 T+120 T+144 T+168
Possibly verified 216 174 135 103 77 54 37 24
Detection rate (%) 100 99 100 100 97 96 92 96
AT error (km) 9 18 23 26 35 19 52 6
CT error (km) -4 -9 -19 -25 -13 7 -17 -39
Track skill (%) ***** 66 75 66 ***** ***** ***** *****
* 2015 skill (%) ***** 47 60 62 ***** ***** ***** *****
DPE (km) 35 75 115 163 213 281 313 414
* 2015 DPE (km) 39 80 134 206 244 301 383 444
Central pressure bias (mb) 6.7 9.0 10.8 12.0 12.1 11.5 11.5 11.6

* DPE and skill for all north-east Pacific storms in 2015 season

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

Despite being an active season, 2016 was quieter than the exceptionally active 2015. Track forecast errors were at their lowest ever recorded at nearly all lead times. Skill scores against CLIPER were higher than last season. There was a  weak bias at all lead times.

3.3 North Atlantic basin storms

Table of mean error statistics
T+0 T+24 T+48 T+72 T+96 T+120 T+144 T+168
Possibly verified 160 129 103 81 64 53 46 36
Detection rate (%) 100 98 96 96 98 100 93 94
AT error (km) -2 -7 -6 -25 -53 -39 -105 -39
CT error (km) 1 1 4 18 12 5 -36 -83
Track skill (%) ***** 66 76 76 ***** ***** ***** *****
* 2015 skill (%) ***** 65 71 69 ***** ***** ***** *****
DPE (km) 29 70 105 152 239 353 438 536
* 2015 DPE (km) 30 71 133 208 386 673 921 1548
Central pressure bias (mb) 4.6 6.0 6.8 3.9 2.0 0.8 -3.5 -5.7

* DPE and skill for all North Atlantic storms in 2015 season

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

2016 saw higher levels of activity than 2015. Track forecast errors were at their lowest ever at short lead times and at longer lead times were much lower than last year's large values. Skill scores against CLIPER were higher than last season's values. There was a weak bias at short lead times which reversed to a strong bias at the longest lead times. The model was particularly skilful at predicting the most noteworthy tropical cyclone of the season, Hurricane Matthew.

3.4 North Indian Basin Storms

Table of mean error statistics
T+0 T+24 T+48 T+72 T+96 T+120 T+144 T+168
Possibly verified 29 19 10 6 3 1 0 0
Detection rate (%) 100 95 100 100 100 100 ***** *****
AT error (km) -11 -23 -20 -65 -46 -270 ***** *****
CT error (km) -3 27 78 148 194 136 ***** *****
Track skill (%) ***** 58 69 59 ***** ***** ***** *****
* 2015 skill (%) ***** 35 61 63 ***** ***** ***** *****
DPE (km) 41 87 125 184 202 303 ***** *****
* 2015 DPE (km) 42 77 114 165 218 138 ***** *****
Central pressure bias (mb) -2.1 -2.8 -2.7 -1.5 12.0 9.0 ***** *****

* DPE and skill for all North Indian storms in 2015 season

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

Cyclone activity was similar to 2015. Track forecast errors were mostly a little higher than last season, although skill scores against CLIPER were mostly higher. At short lead times there was a small negative (strong) bias in forecast central pressure.

3.5 Combined statistics for whole northern hemisphere

Table of mean error statistics
T+0 T+24 T+48 T+72 T+96 T+120 T+144 T+168
Possibly verified 626 496 380 284 206 147 104 75
Detection rate (%) 100 99 99 99 99 99 94 95
AT error (km) 1 -1 -4 -23 -26 -37 -87 -113
CT error (km) -4 -5 -12 -11 8 20 -4 -38
Track skill (%) ***** 65 74 70 ***** ***** ***** *****
* 2015 skill (%) ***** 54 63 65 ***** ***** ***** *****
DPE (km) 36 73 119 180 246 346 422 557
* 2015 DPE (km) 38 77 129 192 254 334 427 545
Central pressure bias (mb) 6.8 9.2 10.7 9.7 7.5 5.5 3.4 1.6

* DPE and skill for all northern hemisphere storms in 2015 season

Northern hemisphere activity was a little lower than last year with fewer intense tropical cyclones in particular.

Track forecast errors were the lowest ever achieved up to 96 hours and close to the values of the last two seasons at longer lead times. Skill scores against CLIPER were the highest ever achieved. Hence, the large improvement in performance seen since the 2014 has continued and has been built upon. Cross track errors were mostly small, although larger than last season. There was a slight slow bias at longer lead times.  Detection percentages were high.

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

With this season's track forecast errors at similar or lower values to the last two seasons, the five-year running mean of track forecast errors shows a continued acceleration of the long term downwards trend.

Five-year running mean track forecast errors for the whole northern hemisphere

There were slight increases in the the mean bias in central pressure and 10m wind compared to 2015. However, the biases were still mostly lower than those seen in earlier years.

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.

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