Changes to the Wind Speed Probability Product for 2014

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Changes to the Wind Speed Probability Product for 2014

Service Change Notice
28 February 2014

There will be two changes to the Wind Speed Probability Product in 2014:

1: The name of one of the parameters in the product will change from "Individual Probability" (IP) to "Onset Probability" (OP). This parameter is defined as the probability of the event (e.g., hurricane-force winds) beginning at a particular location during an individual time period (e.g., between 24 and 36 hours). The name "onset probability" better highlights the difference between this parameter, which provides information on when the event is likely to begin, and the "Cumulative Probability", which describes the likelihood that the event will occur at all.

2: Removal of the table that gives the likelihood that a tropical cyclone will be at any of several different intensity categories (e.g., depression, storm, category 1 hurricane, etc.) at fixed points in time (e.g., 12 hours, 24 hours, etc.). This table was originally intended to help users assess the risk of different storm intensities at particular times. However, NHC has learned that many users consult this table to estimate the chances that a tropical cyclone will make landfall at one or more of the various intensity categories. Unfortunately, the current methodology provides a highly misleading estimate of landfall intensity. Because of the likelihood of misuse for land-threatening storms, NHC and CPHC are discontinuing this table until a better procedure to estimate intensity risk can be developed.

To understand the problem, consider a storm that is forecast to be just off the coast at 72 hours. The probability that the cyclone will be at any particular intensity category at 72 hours is estimated by creating a large number of potential track and intensity scenarios, based on the latest official forecast and a climatological error distribution. If the 72-hour forecast is for the storm to be very close to land, a sizable fraction of the potential scenarios will have already encountered land by that time. For these scenarios, the expected 72-hour intensity will be much lower than what would be expected for a storm that had not yet made landfall. So even though the table accurately assesses what the intensity is likely to be at 72 hours, it grossly underestimates the likely intensity for the moment when the storm comes ashore.

Example of Wind Speed Probability Product for 2014

ZCZC MIAPWSAT4 ALL                                                  
TTAA00 KNHC DDHHMM                                                  

TROPICAL STORM ISAAC WIND SPEED PROBABILITIES NUMBER  23            
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092012               
2100 UTC SUN AUG 26 2012                                            
                                                                    
AT 2100Z THE CENTER OF TROPICAL STORM ISAAC WAS LOCATED NEAR        
LATITUDE 24.2 NORTH...LONGITUDE 82.3 WEST WITH MAXIMUM SUSTAINED    
WINDS NEAR 50 KTS...60 MPH...95 KM/H.                               
                                                                    
Z INDICATES COORDINATED UNIVERSAL TIME (GREENWICH)                  
   ATLANTIC STANDARD TIME (AST)...SUBTRACT 4 HOURS FROM Z TIME      
   EASTERN  DAYLIGHT TIME (EDT)...SUBTRACT 4 HOURS FROM Z TIME      
   CENTRAL  DAYLIGHT TIME (CDT)...SUBTRACT 5 HOURS FROM Z TIME 

WIND SPEED PROBABILITY TABLE FOR SPECIFIC LOCATIONS             
                                                                    
CHANCES OF SUSTAINED (1-MINUTE AVERAGE) WIND SPEEDS OF AT LEAST     
   ...34 KT (39 MPH... 63 KPH)...                                   
   ...50 KT (58 MPH... 93 KPH)...                                   
   ...64 KT (74 MPH...119 KPH)...                                   
FOR LOCATIONS AND TIME PERIODS DURING THE NEXT 5 DAYS               
                                                                    
PROBABILITIES FOR LOCATIONS ARE GIVEN AS OP(CP) WHERE               
    OP  IS THE PROBABILITY OF THE EVENT BEGINNING DURING            
        AN INDIVIDUAL TIME PERIOD (ONSET PROBABILITY)          
   (CP) IS THE PROBABILITY OF THE EVENT OCCURRING BETWEEN           
        18Z SUN AND THE FORECAST HOUR (CUMULATIVE PROBABILITY)      
                                                                    
PROBABILITIES ARE GIVEN IN PERCENT                                  
X INDICATES PROBABILITIES LESS THAN 1 PERCENT                       
PROBABILITIES FOR 34 KT AND 50 KT ARE SHOWN AT A GIVEN LOCATION WHEN
THE 5-DAY CUMULATIVE PROBABILITY IS AT LEAST 3 PERCENT.             
PROBABILITIES FOR 64 KT ARE SHOWN WHEN THE 5-DAY CUMULATIVE         
PROBABILITY IS AT LEAST 1 PERCENT.    
                                                                 
                                                                    
  - - - - WIND SPEED PROBABILITIES FOR SELECTED LOCATIONS - - - -  
                                                                    
               FROM    FROM    FROM    FROM    FROM    FROM    FROM 
  TIME       18Z SUN 06Z MON 18Z MON 06Z TUE 18Z TUE 18Z WED 18Z THU
PERIODS         TO      TO      TO      TO      TO      TO      TO  
             06Z MON 18Z MON 06Z TUE 18Z TUE 18Z WED 18Z THU 18Z FRI
                                                                    
FORECAST HOUR    (12)   (24)    (36)    (48)    (72)    (96)   (120)
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
LOCATION       KT                                                   
                                                                    
FT PIERCE FL   34  9   2(11)   X(11)   X(11)   X(11)   X(11)   X(11)
 
W PALM BEACH   34 14   2(16)   X(16)   X(16)   X(16)   X(16)   X(16)
 
MIAMI FL       34 99   X(99)   X(99)   X(99)   X(99)   X(99)   X(99)
 
MARATHON FL    34 99   X(99)   X(99)   X(99)   X(99)   X(99)   X(99)
MARATHON FL    50 14   X(14)   X(14)   X(14)   X(14)   X(14)   X(14)
 
KEY WEST FL    34 99   X(99)   X(99)   X(99)   X(99)   X(99)   X(99)
KEY WEST FL    50 99   X(99)   X(99)   X(99)   X(99)   X(99)   X(99)
 
MARCO ISLAND   34 99   X(99)   X(99)   X(99)   X(99)   X(99)   X(99)
 
FT MYERS FL    34 48   1(49)   2(51)   X(51)   X(51)   X(51)   X(51)
 
VENICE FL      34 37   5(42)   2(44)   1(45)   X(45)   1(46)   X(46) 
 
TAMPA FL       34 18   8(26)   3(29)   2(31)   X(31)   1(32)   X(32)
 
TALLAHASSEE FL 34  X   7( 7)  10(17)   6(23)   6(29)   1(30)   X(30)
 
ST MARKS FL    34  1   9(10)   9(19)   6(25)   5(30)   1(31)   1(32) 
 
APALACHICOLA   34  3  11(14)  16(30)   9(39)   7(46)   1(47)   X(47)
APALACHICOLA   50  X   X( X)   2( 2)   2( 4)   1( 5)   1( 6)   X( 6)
APALACHICOLA   64  X   X( X)   X( X)   1( 1)   X( 1)   X( 1)   X( 1)
 
PANAMA CITY FL 34  1  11(12)  20(32)  13(45)   7(52)   1(53)   1(54)
PANAMA CITY FL 50  X   X( X)   3( 3)   4( 7)   3(10)   1(11)   X(11)
PANAMA CITY FL 64  X   X( X)   X( X)   1( 1)   1( 2)   X( 2)   X( 2)
 
COLUMBUS GA    34  X   X( X)   3( 3)   6( 9)  11(20)   2(22)   1(23)
 
MONTGOMERY AL  34  X   X( X)   7( 7)  10(17)  18(35)   3(38)   1(39) 
MONTGOMERY AL  50  X   X( X)   X( X)   X( X)   5( 5)   2( 7)   X( 7)
MONTGOMERY AL  64  X   X( X)   X( X)   X( X)   1( 1)   1( 2)   X( 2)
 
PENSACOLA FL   34  X   6( 6)  24(30)  25(55)  14(69)   2(71)   X(71)
PENSACOLA FL   50  X   X( X)   2( 2)  14(16)  12(28)   1(29)   1(30)
PENSACOLA FL   64  X   X( X)   X( X)   4( 4)   5( 9)   2(11)   X(11)
 
MOBILE AL      34  X   3( 3)  22(25)  31(56)  20(76)   2(78)   X(78)
MOBILE AL      50  X   X( X)   2( 2)  15(17)  21(38)   2(40)   X(40)
MOBILE AL      64  X   X( X)   X( X)   3( 3)  12(15)   1(16)   X(16)
 
GULFPORT MS    34  X   3( 3)  22(25)  33(58)  21(79)   2(81)   X(81)
GULFPORT MS    50  X   X( X)   2( 2)  19(21)  22(43)   2(45)   X(45) 
GULFPORT MS    64  X   X( X)   X( X)   5( 5)  13(18)   2(20)   X(20)
 
STENNIS SC     34  X   2( 2)  19(21)  32(53)  23(76)   3(79)   1(80)
STENNIS SC     50  X   X( X)   1( 1)  15(16)  22(38)   2(40)   X(40)
STENNIS SC     64  X   X( X)   X( X)   4( 4)  12(16)   1(17)   X(17)
 
BURAS LA       34  X   5( 5)  29(34)  33(67)  14(81)   2(83)   1(84)
BURAS LA       50  X   X( X)   5( 5)  25(30)  15(45)   2(47)   X(47)
BURAS LA       64  X   X( X)   1( 1)   8( 9)  11(20)   1(21)   X(21)
 
JACKSON MS     34  X   X( X)   3( 3)  11(14)  33(47)   6(53)   1(54)
JACKSON MS     50  X   X( X)   X( X)   X( X)  12(12)   4(16)   X(16)
JACKSON MS     64  X   X( X)   X( X)   X( X)   3( 3)   2( 5)   X( 5)
 
NEW ORLEANS LA 34  X   1( 1)  16(17)  29(46)  23(69)   3(72)   1(73)
NEW ORLEANS LA 50  X   X( X)   1( 1)  10(11)  18(29)   3(32)   1(33)
NEW ORLEANS LA 64  X   X( X)   X( X)   1( 1)   9(10)   1(11)   X(11)
 
BATON ROUGE LA 34  X   X( X)   9( 9)  18(27)  24(51)   6(57)   X(57)
BATON ROUGE LA 50  X   X( X)   X( X)   2( 2)  14(16)   3(19)   X(19)
BATON ROUGE LA 64  X   X( X)   X( X)   X( X)   5( 5)   2( 7)   X( 7)
 
NEW IBERIA LA  34  X   X( X)   7( 7)  12(19)  20(39)   7(46)   X(46)
NEW IBERIA LA  50  X   X( X)   X( X)   1( 1)   9(10)   2(12)   1(13)
NEW IBERIA LA  64  X   X( X)   X( X)   X( X)   3( 3)   2( 5)   X( 5)

SHREVEPORT LA  34  X   X( X)   X( X)   1( 1)   9(10)   6(16)   1(17)
 
PORT ARTHUR TX 34  X   X( X)   X( X)   3( 3)  10(13)   5(18)   X(18)
PORT ARTHUR TX 50  X   X( X)   X( X)   X( X)   1( 1)   2( 3)   1( 4)
PORT ARTHUR TX 64  X   X( X)   X( X)   X( X)   X( X)   1( 1)   X( 1)
 
$$                                                                  
FORECASTER PASCH                                                    
NNNN