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Tropical cyclone activity and forecasts - June 2008

Tropical cyclone activity

Tropical cyclones ending during the month
  NWP NEP NAT NI SWI AUS TOTAL
Total number ending during the month 2 0 1 0 0 0 3
 
Tropical cyclones active during the month
Name Identifier Basin Start date End date 1-minute MSW 10-minute MSW
Nakri 06W NWP 27 May 03 June 125 105
Arthur 01L NAT 31 May 02 June 35 -
Fengshen 07W NWP 18 June 26 June 95 90

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)
SWI: South-West Indian (west of 90° E)
AUS: Australian (east of 90° E)

1-minute MSW: Maximum sustained wind (knots) averaged over 1 minute (JTWC Hawaii, NHC Miami, CPHC Hawaii)
10-minute MSW: Maximum sustained wind (knots) averaged over 10 minutes (JMA Japan, Southern Hemisphere RSMCs and TCWCs)
Note: Although 1-minute average winds should be stronger than 10-minute average, this may not be the case here since estimates are obtained from different warning centres.

Tropical cyclones still active at the end of the month

Name Identifier Basin Start date
Boris 02E NEP 27 June
Cristina 03E NEP 27 June

Forecast error statistics

The Met Office global Numerical Weather Prediction model produces a 6-day forecast every 12 hours. The performance of the model in forecasting this month's tropical cyclones is assessed below. Mean error statistics for each storm that ended during the month are presented together with a final table of statistics for all storms ending this month. Verification is performed at 24-hour intervals up to forecast time T+120. Some mean error statistics for last year's storms are included for the purposes of a comparison. The present global model has a resolution of 0.5625° x 0.375° x 50 levels. This is equivalent to a horizontal resolution of 62 km x 42 km at the equator.

Individual tropical cyclone summaries

Name: NAKRI Identifier: 06W
Basin: NORTH WEST PACIFIC
Start date: 27.05.2008 End date: 03.06.2008
1-minute MSW: 125 knots 10-minute MSW: 105 knots

Table of mean error statistics
  T+0 T+24 T+48 T+72 T+96 T+120
Possibly verified 141210864
Detection rate (%) 100100100100100100
Mean DX 20-102884171293
Mean DY -8276194401459
Mean AT -3-19171504681054
Mean CT 1392588325904
Track skill (%) ****333922********
Mean DPE 42851602976271398
2007 DPE * 70136201276404517
Intensity skill (%) ****504025670

* DPE for all storms in the 2007 season in the NWP basin

A plot of the storm's observed track and forecasts

The first few forecasts for this typhoon predicted recurvature too early, which resulted in huge errors at longer lead times. However, later forecasts were much better. This resulted in track errors near to or below last season’s average at lead times of 72 hours or shorter. The model showed skill over CLIPER.

 

Name: ARTHUR Identifier: 01L
Basin: NORTH ATLANTIC
Start date: 31.05.2008 End date: 02.06.2008
1-minute MSW: 35 knots 10-minute MSW: - knots

Table of mean error statistics
  T+0 T+24 T+48 T+72 T+96 T+120
Possibly verified 200000
Detection rate (%) 100********************
Mean DX -11********************
Mean DY -89********************
Mean AT 29********************
Mean CT -39********************
Mean DPE 123********************
2007 DPE * 63102151179228265

* DPE for all storms in the 2007 season in the NAT basin

A plot of the storm's observed track and forecasts

Arthur was a short-lived storm and no forecasts were verified.

 

Name: FENGSHEN Identifier: 07W
Basin: NORTH-WEST PACIFIC
Start date: 18.06.2008 End date: 26.06.2008
1-minute MSW: 95 knots 10-minute MSW: 90 knots

Table of mean error statistics
  T+0 T+24 T+48 T+72 T+96 T+120
Possibly verified 141210864
Detection rate (%) 100100100100100100
Mean DX 1364109260476693
Mean DY -21-14-1134631
Mean AT -19-65-96-151-273-238
Mean CT -11940214370648
Track skill (%) ****162224********
Mean DPE 59129197281495695
2007 DPE * 70136201276404517
Intensity skill (%) ****506075330

* DPE for all storms in the 2007 season in the NWP basin

A plot of the storm's observed track and forecasts

Some early forecasts incorrectly predicted that Fengshen would turn northwards before reaching the Philippines. However, having crossed the eastern side of the islands the forecasts were much better. Most predicted landfall near to or just to the east of Hong Kong. Only long lead time track forecast errors were above last season's average. Skill scores were positive.

 

Summary of all tropical cyclones

Mean error statistics for all tropical cyclones (of at least tropical storm strength) which have ended this month are shown below.

Total number of depressions ending this month: 3
Number of storms used in statistics below: 3

Table of mean error statistics
  T+0 T+24 T+48 T+72 T+96 T+120
Possibly verified 30242016128
Detection rate (%) 100100100100100100
Mean DX 152755174447993
Mean DY -8-63791223245
Mean AT -10-42-40-198408
Mean CT 41432151347776
Track skill (%) ****233223********
Mean DPE 551071782895611047
2007 DPE * 66127193262401472
Intensity skill (%) ****505050500

* DPE for all northern hemisphere storms in 2007 season

This month's figures are dominated by the two North-West Pacific typhoons Nakri and Fengshen. Forecasts exhibited similar characteristics for each - initially predicting recurvature too soon, but improving in later runs. Hence, cross-track errors at longer lead times were large. Track errors at shorter lead times were close to or below average. Track skill scores were positive. The overall intensity tendency skill score was 45%.

 

Tropical cyclone activity in the southern hemisphere during 2007-8 season

Tropical cyclone activity
  SWI AUS TOTAL
Tropical depressions  0(3)  5(2)  5(2)
Tropical storms  7(3)  6(8)  13(11)
Hurricanes/typhoons  5(7)  6(5)  11(12)
Total  12(10)  17(15)  29(25)

( ) The number in brackets indicates the figure for the 2006-7 season. Definitions based on RSMC 10-minute average wind speeds are used.

Summary of all southern hemisphere storms in 2007-8

Mean error statistics for all southern hemisphere storms that occurred in the 2007-8 season are shown below. Only storms that reached tropical storm intensity at the time of a forecast are used in the statistics below.

Total number of tropical cyclones which occurred:-     29

Table of mean error statistics
  T+0 T+24 T+48 T+72 T+96 T+120
Possibly verified 201 154 118 87 60 41
Detection rate (%) 100 100 100 100 100 90
Mean DX -2 -34 -106 -164 -268 -316
Mean DY 4 22 -12 -58 -107 -45
Mean AT -4 -26 -22 -28 -64 -149
Mean CT 6 6 -13 -54 -60 -17
Skill (%) **** 33 33 27 **** ****
* 2005-6 sSkill (%) **** 23 16 13 **** ****
Mean DPE 54 122 240 385 549 714
* 2005-6 DPE 62 138 302 498 732 873
Intensity skill (%) **** 38 41 26 13 3

* Real time figures for all southern hemisphere storms in 2006-7.

Southern Hemisphere warning centres recorded slightly higher activity this season than in 2006-7 in terms of verifiable forecasts. However, the numbers of storms and those attaining hurricane strength were similar.

Observed tracks of this season’s TCs.
SWI  Western AUS Eastern AUS

The charts below show forecast positional errors for the last few seasons. Errors were lower than the high values of last season in both basins at all lead times. At shorter lead times, errors were back to levels seen in the few seasons prior to last season.

Skill against CLIPER was higher than last season, but still did not recover to the levels seen in the two seasons prior to that. Cross track errors recovered to fairly small negative values - a slight right-of-track bias. Along track errors showed generally small negative values (slow bias).

After last season's poor verification figures, this season saw a recovery to figures closer to those seen in the previous few seasons.

Intensity tendency skill score averaged for all forecasts was 30% in the 2007-8 Southern Hemisphere season. This compares to 18% and 31% for the previous two seasons.

A revised Southern Hemisphere summary will be issued once best track data has been collected later in the year.

Forecast Positional Errors:
SWI  AUS  Combined southern hemisphere
Southern hemisphere skill
Southern hemisphere cross track errors
Southern hemisphere along track errors

Related internet links

Tropical cyclone FAQ

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