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Tropical Cyclone Wind Speed Probability Text Products


Wind speed probability products are designed to answer these questions:

  1. What are the chances of experiencing these winds?
  2. When is the event most likely to start?
  • Cumulative probabilities (0-12, 0-24, 0-36 etc) through 120 hours (5 days) indicate the overall likelihood that the event will occur sometime in the given time period.These can be stepped through incrementally or animated on the web pages.
  • Specific sustained wind speed thresholds available:
    • 34 knots (39 mph)Tropical Storm strength
    • 50 knots (58 mph)
    • 64 knots (74 mph)Hurricane strength

What the numbers mean

  • Low probability numbers for extreme events are not insignificant.
  • 30% probability of experiencing 64 knot winds means a roughly 1 in 3 chance of hurricane conditions
  • The chance of having 34 knot winds (tropical storm conditions) will be much higher for the same storm

We welcome any and all feedback on these products. Please e-mail your comments to W-HFO.Webmaster@noaa.gov.

How to read the Wind Probability Text Product

These tables do not directly indicate what the wind speeds will be at any location at any time, nor do they indicate when the winds will cease.An example product is included below.

The top section

These are values for onset and cumulative probabilities for select land and marine locations across Hawaii and the Central Pacific.

Taking Honolulu for example, the probabilities listed are:
HONOLULU       34 41  43(84)   7(91)   1(92)   X(92)   X(92)   X(92)
HONOLULU       50  3  41(44)  16(60)   1(61)   X(61)   X(61)   X(61)
HONOLULU       64  X  26(26)  17(43)   1(44)   X(44)   X(44)   X(44)

The first row are the probabilities of tropical storm force winds of 34 knots (39 mph). The values in parentheses are the cumulative probability of experiencing those winds at anytime up to the time period given in the column heading.So over entire 120 hour (5 day) period, there is a 92% chance of experiencing 34 knots of wind at some time. The values not in parenthesis indicate the probability of that level of winds beginning during the time period in the column heading.The time most likely to see the winds begin is the second period (06z to18z Monday) with a 43% chance.

The second row indicates the probabilities of experiencing 50 knots (58 mph) or greater wind speeds. The probability is 61% that such a level of strong winds will occur sometime in the 120 hour forecast period. The most likely time for the winds to begin is again the second period, with a 41% chance.

The last row indicates the probabilities of experiencing hurricane force (64 knots or 74 mph) winds or greater. Honolulu has a 44% chance of experiencing such winds during the 120 forecast period, with the second period being the most likely time for those winds to begin with a 26% chance.

The bottom section

Provides similar probability values as above, except using the forecast location of the center of the storm. As you look farther down the forecast in time, overall probabilities are decreasing. For example, note the section indicating the probabilities for the forecast location point the storm at 48 hours.

48 245N 1611W 34X4( 4)24(28)33(61)8(69)X(69)X(69)
48 245N 1611W 50XX( X)6( 6)20(26)5(31)X(31)X(31)
48 245N 1611W 64XX( X)2( 2)12(14)2(16)X(16)X(16)

The storm is forecast to be a 90 knot hurricane at this time, yet there is only an overall 16% chance that the actual location we are forecasting the storm to be will have greater than 64 knots of wind and a 69% chance that winds will be tropical storm force or higher. Although this may seem contradictory at first, it is evidence of the forecast errors in location and intensity that accounts for this. The reality with hurricane forecasts is that they are not perfect. There is a chance that the storm may end up 150 miles from where we forecast it to be 2 days in advance. There is also a chance that the storm may be exactly where we forecast it to be, but have diminished to a tropical depression with winds less than 34 knots. In either case, the location would not experience strong winds, despite the forecast from 48 hours earlier. The probability forecasts take these possibilities, and 998 others into account, therefore you will never see a 99% or higher value after the first period.

As a result, do not be tricked into thinking low probabilities farther out in time really mean nothing is going to happen. Monitor each successive forecast and view how the probabilities are changing.

Example of Tropical Cyclone Wind Speed Probabilities

HURRICANE NOTREAL PROBABILITIES NUMBER   2
NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI
2100Z SUN JUL 10 2005

...THIS IS AN EXPERIMENTAL PRODUCT FOR 2005...

AT 2100Z THE CENTER OF HURRICANE NOTREAL WAS LOCATED
NEAR LATITUDE 18.4 NORTH...LONGITUDE 154.3 WEST WITH
MAXIMUM SUSTAINED WINDS NEAR 115 KTS...135 MPH.

CHANCES OF EXPERIENCING 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 IP(CP) WHERE
    IP  IS THE PROBABILITY OF THE EVENT BEGINNING DURING
        AN INdivIDUAL TIME PERIOD (INdivIDUAL 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 0.5 PERCENT
LOCATIONS SHOWN WHEN THEIR TOTAL CUMULATED 5-DAY
   PROBABILITY IS AT LEAST 2.5 PERCENT

Z INDICATES UNIVERSAL COORDINATED TIME (GREENWICH)


  - - - - 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

BUOY 51004     34 99   X(99)   X(99)   X(99)   X(99)   X(99)   X(99)
BUOY 51004     50 99   X(99)   X(99)   X(99)   X(99)   X(99)   X(99)

HILO           34 99   X(99)   X(99)   X(99)   X(99)   X(99)   X(99)
HILO           50 69   3(72)   X(72)   X(72)   X(72)   X(72)   X(72)
HILO           64 32   2(34)   X(34)   X(34)   X(34)   X(34)   X(34)

SOUTH POINT    34 99   X(99)   X(99)   X(99)   X(99)   X(99)   X(99)
SOUTH POINT    50 89   3(92)   X(92)   X(92)   X(92)   X(92)   X(92)
SOUTH POINT    64 77   4(81)   X(81)   X(81)   X(81)   X(81)   X(81)

KAILUA-KONA    34 99   X(99)   X(99)   X(99)   X(99)   X(99)   X(99)
KAILUA-KONA    50 72  15(87)   X(87)   X(87)   X(87)   X(87)   X(87)
KAILUA-KONA    64 50  22(72)   1(73)   X(73)   X(73)   X(73)   X(73)

KAHULUI        34 99   X(99)   X(99)   X(99)   X(99)   X(99)   X(99)
KAHULUI        50 12  27(39)   1(40)   X(40)   X(40)   X(40)   X(40)
KAHULUI        64  3  16(19)   1(20)   X(20)   X(20)   X(20)   X(20)

LANAI CITY     34 99   X(99)   X(99)   X(99)   X(99)   X(99)   X(99)
LANAI CITY     50 10  47(57)   2(59)   1(60)   X(60)   X(60)   X(60)
LANAI CITY     64  3  32(35)   2(37)   X(37)   X(37)   X(37)   X(37)

KAUNAKAKAI     34 59  30(89)   4(93)   X(93)   X(93)   X(93)   X(93)
KAUNAKAKAI     50  5  41(46)   7(53)   X(53)   X(53)   X(53)   X(53)
KAUNAKAKAI     64  1  27(28)   4(32)   X(32)   X(32)   X(32)   X(32)

BUOY 51002     34 99   X(99)   X(99)   X(99)   X(99)   X(99)   X(99)

HONOLULU       34 41  43(84)   7(91)   1(92)   X(92)   X(92)   X(92)
HONOLULU       50  3  41(44)  16(60)   1(61)   X(61)   X(61)   X(61)
HONOLULU       64  X  26(26)  17(43)   1(44)   X(44)   X(44)   X(44)

LIHUE          34 11  40(51)  35(86)   4(90)   X(90)   X(90)   X(90)
LIHUE          50  X  15(15)  40(55)   7(62)   1(63)   X(63)   X(63)
LIHUE          64  X   7( 7)  31(38)   5(43)   1(44)   X(44)   X(44)

NIIHAU         34  6  30(36)  40(76)   8(84)   1(85)   X(85)   X(85)
NIIHAU         50  X   6( 6)  33(39)  10(49)   1(50)   X(50)   X(50)
NIIHAU         64  X   2( 2)  21(23)   7(30)   1(31)   X(31)   X(31)

20N 160W       34 11  36(47)  15(62)   2(64)   X(64)   X(64)   X(64)
20N 160W       50  X   8( 8)   9(17)   X(17)   X(17)   X(17)   X(17)
20N 160W       64  X   2( 2)   5( 7)   X( 7)   X( 7)   X( 7)   X( 7)

25N 160W       34  X   4( 4)  20(24)  27(51)   6(57)   1(58)   X(58)
25N 160W       50  X   X( X)   4( 4)  13(17)   3(20)   X(20)   X(20)
25N 160W       64  X   X( X)   1( 1)   6( 7)   2( 9)   X( 9)   X( 9)

30N 160W       34  X   X( X)   X( X)   1( 1)   5( 6)   1( 7)   X( 7)

BUOY 51003     34  6  13(19)   7(26)   1(27)   X(27)   X(27)   X(27)
BUOY 51003     50  X   2( 2)   1( 3)   X( 3)   X( 3)   X( 3)   X( 3)

BUOY 51001     34  X   5( 5)  20(25)  24(49)   6(55)   X(55)   X(55)
BUOY 51001     50  X   X( X)   4( 4)  10(14)   2(16)   X(16)   X(16)
BUOY 51001     64  X   X( X)   1( 1)   5( 6)   1( 7)   X( 7)   X( 7)

FR FRIG SHOALS 34  X   X( X)   X( X)   3( 3)   1( 4)   1( 5)   X( 5)

$$

FORECASTER TEST

List of Locations that may appear in the top section of the Probabilistic Wind forecast product.
LocationLat/Lon
Niihau21.9°N 160.1°W
Lihue21.9°N 159.3°W
Honolulu21.3°N 157.9°W
Kaunakakai21.1°N 157.2°W
Kahului20.9°N 156.4°W
Lanai City20.8°N 156.9°W
Kailua-Kona19.7°N 156.0°W
Hilo19.7°N 155.0°W
South Point18.9°N 155.7°W
Buoy 5100123.3°N 162.2°W
Buoy 5100217.1°N 157.8°W
Buoy 5100319.1°N 160.8°W
Buoy 5100417.4°N 152.5°W
Johnston Island16.7°N 169.5°W
Midway28.3°N 177.6°W
French Frigate Shoals23.8°N 166.3°W
Palmyra5.5°N 162.0°W
10N 145W10.0°N 145.0°W
10N 160W10.0°N 160.0°W
10N 175W10.0°N 175.0°W
15N 145W15.0°N 145.0°W
15N 160W15.0°N 160.0°W
15N 175W15.0°N 175.0°W
20N 145W20.0°N 145.0°W
20N 175W20.0°N 175.0°W
25N 145W25.0°N 145.0°W
25N 160W25.0°N 160.0°W
25N 175W25.0°N 175.0°W
30N 145W20.0°N 145.0°W
30N 160W30.0°N 160.0°W
30N 175W30.0°N 175.0°W
35N 145W35.0°N 145.0°W
35N 160W35.0°N 160.0°W
35N 175W35.0°N 175.0°W