wind speed and intensity probabilities...wind speed and intensity probabilities michael j. brennan...

Wind Speed and Intensity Probabilities

Wind Speed and Intensity Probabilities

Michael J. BrennanNational Hurricane Center

L311 Course for Coastal Communities26 March 2012

1

OutlineOutline

• The reality of forecast uncertainty• Quick overview of probability products • Why small probabilities for extreme events matter

• Reading and interpreting the probability products

• Examples

2

Atlantic 5‐Year Mean NHC Forecast ErrorsAtlantic 5‐Year Mean NHC Forecast Errors

3

Track errors increase 40‐50 n mi per day, 5‐day errors approaching 200 n mi

Intensity errors increase quickly to 15 kt by 48 and then level off

48

83

124

167

213

11

1517

1718

How Can You, as Decision Makers, Deal with Forecast Uncertainties?

How Can You, as Decision Makers, Deal with Forecast Uncertainties?

4NHC probability products can help

Verifying 5‐day Position of Ida

TD 11 (later Hurricane Ida)

Advisory Number 1

Issued 10:00 AM EST 4 November 2009

5‐day position error about 600 miles

5‐day NHC Forecast

Overview of Wind Speed Probability Products

Overview of Wind Speed Probability Products

5

How the Wind Speed/Intensity Probabilities are Created

How the Wind Speed/Intensity Probabilities are Created

6

• 1,000 realistic alternative scenarios created using – Official NHC track, intensity

and wind radii forecasts

– Historical NHC track and intensity forecast errors

– Climatology and persistence wind radii model

• Probability of exceeding 34, 50, and 64 kt wind thresholds computed

• Accounts for inland wind decay

Influence of Track Forecast Uncertainty on Probability Products

Influence of Track Forecast Uncertainty on Probability Products

• Different historical NHC track forecast errors are sampled depending on how much spread (disagreement) there is in the track model guidance

• If track model spread is small (good model agreement)

– Probability swath will be narrower with higher probabilities along the official NHC forecast track and lower values along the edges

• If track model spread is large (poor model agreement)

– Probability swath will be wider, with lower values along the NHC official forecast track and a wider area of low probabilities along the edges

7

Gustav (2009)

Fay (2009)

Available Probability ProductsAvailable Probability Products1. Wind Speed Probability

Product• Depicts location‐specific

probabilities for 34‐kt (TS‐force), 50‐kt (58‐mph), and 64‐kt (hurricane‐force) winds

• Text product contains cumulative and individual time period onset probabilities for a fixed set of locations

• Graphic depicts cumulative probabilities for points over a large domain 8

Available Probability ProductsAvailable Probability Products2. Intensity Probability Table• Shows probability of tropical cyclone intensity (maximum wind)

falling in various categories• Tropical depression, tropical storm, hurricane, and Saffir‐Simpson

Hurricane Wind Scale categories 1‐5• Available at the top of the wind speed probability text product

and as a stand‐alone graphic

9

Why do Small Probabilities of Extreme Events Matter?

Why do Small Probabilities of Extreme Events Matter?

10

11

U.S. Hurricane Watch and Warning Statistics(2000‐2008)

U.S. Hurricane Watch and Warning Statistics(2000‐2008)

• Average storm‐total watch length 477 miles• Average storm‐total length w/ hurricane winds 89 miles

for cases when watch issued• Probability of hurricane winds at point under watch 19%

• Average storm‐total warning length 403 miles• Average storm‐total length w/ hurricane winds 99 miles

for cases when warning issued• Probability of hurricane winds at warned point 25%

13

33% 22%<1% <1%<1%<1%

<1%

<1%3%<1%1%<1%3%<1%2%<1%7%<1%

20%<1%

<1%

1%2%<1%5%<1%1%1%1%<1%<1%<1%8%<1%

<1%1%1%<1%3%<1%2%<1%5%<1%1%<1%3%<1%14%<1%

<1%

<1%3%<1%3%<1%1%1%3%<1%2%1%6%

<1%

UConn ButlerUConn1% <1%

14

Golden BeachApalachicola

Intracoastal City

DestinChokoloskee

Suwannee RiverLake OkeechobeeTarpon Springs

Englewood

Aucilla RiverFort PierceMatagorda

San Luis Pass

St. Augustine

Pascagoula

Sabine Pass

Flamingo

Yankeetown

Vero Beach

Ann Maria IslandDry Tortugas

Keaton BeachKey Largo

Port O’ConnorTitusvilleIndian Pass

Ocean Reef

Baffin Bay

New Smyrna Bch

Card Snd Bridge

MS/AL Border

High Island

Cocoa BeachAnclote KeyCraig Key

Sargent

Key West

Steinhatchee R.Florida BayCorpus Christi

Sebastian InletMouth MS River

Jupiter Inlet

Ochlockonee R.

Boca Raton

Cameron

Panama CityBonita BeachBayport

E Cape SableLongboat Key7-Mile Bridge

Freeport

StuartPort AransasFlagler Beach

St. MarksOcean Reef Cstl

Port MansfieldDeerfield Beach

Hallandale Bch

Morgan City

Mouth Pearl R.

Grande Isle

Who is going to get hurricane force winds?

5 day forecast 14

15


Intracoastal City

DestinChokoloskee


Englewood


San Luis Pass

St. Augustine

Pascagoula

Sabine Pass

Flamingo

Yankeetown

Vero Beach




Ocean Reef

Baffin Bay

New Smyrna Bch

Card Snd Bridge

MS/AL Border

High Island


Sargent

Key West



Jupiter Inlet

Ochlockonee R.

Boca Raton

Cameron



Freeport




Hallandale Bch

Morgan City

Mouth Pearl R.

Grande Isle

Who is going to get hurricane force winds?Golden Beach

Intracoastal City

Chokoloskee

Lake Okeechobee

Englewood

Fort Pierce

Jupiter Inlet

New Smyrna Bch

Ocean Reef

Pascagoula

Key Largo

Flamingo

Vero Beach

Titusville

Deerfield Beach

Dry Tortugas

Hallandale Bch

MS/AL Border

Cocoa Beach

Craig Key

Key West

Florida Bay

Sebastian Inlet

Card Snd Bridge

Boca Raton

Panama City

Bonita Beach

Ocean Reef Cstl

Flagler Beach

E Cape Sable

7-Mile Bridge

Stuart

4 day forecast 15

16


Intracoastal City

DestinChokoloskee


Englewood


San Luis Pass

St. Augustine

Pascagoula

Sabine Pass

Flamingo

Yankeetown

Vero Beach




Ocean Reef

Baffin Bay

New Smyrna Bch

Card Snd Bridge

MS/AL Border

High Island


Sargent

Key West



Jupiter Inlet

Ochlockonee R.

Boca Raton

Cameron



Freeport




Hallandale Bch

Morgan City

Mouth Pearl R.

Grande Isle


Golden BeachIntracoastal City

Chokoloskee

Lake OkeechobeeLake Okeechobee

Englewood

Fort PierceFort Pierce

Jupiter InletJupiter Inlet

New Smyrna Bch

Ocean Reef

Ocean ReefPascagoula

Key LargoKey Largo

Flamingo

Flamingo

Vero Beach

Titusville

Deerfield Beach

Dry Tortugas

Deerfield Beach

Hallandale Bch

Hallandale Bch

MS/AL Border

Cocoa Beach

Craig KeyCraig Key

Key West

Florida BayFlorida Bay

Sebastian Inlet

Card Snd BridgeCard Snd Bridge

Boca Raton

Boca RatonPanama City

Bonita Beach

Ocean Reef Cstl

Flagler Beach

E Cape Sable

7-Mile Bridge

StuartStuart

Ocean Reef Cstl

E Cape Sable

3 day forecast 16

17


Intracoastal City

DestinChokoloskee


Englewood


San Luis Pass

St. Augustine

Pascagoula

Sabine Pass

Flamingo

Yankeetown

Vero Beach




Ocean Reef

Baffin Bay

New Smyrna Bch

Card Snd Bridge

MS/AL Border

High Island


Sargent

Key West



Jupiter Inlet

Ochlockonee R.

Boca Raton

Cameron



Freeport




Hallandale Bch

Morgan City

Mouth Pearl R.

Grande Isle


Golden Beach

Golden BeachIntracoastal City

Chokoloskee


Englewood



Jupiter InletNew Smyrna Bch

Ocean Reef


Key LargoKey Largo

Key Largo

Flamingo

Flamingo

Vero Beach

Titusville

Deerfield Beach

Dry Tortugas

Deerfield Beach

Ocean Reef

Hallandale Bch

Hallandale Bch

Hallandale BchMS/AL Border

Cocoa Beach

Craig KeyCraig Key

Key West


Florida BaySebastian Inlet


Boca Raton


Bonita Beach

Ocean Reef Cstl

Flagler Beach

E Cape Sable

7-Mile Bridge

StuartStuart

Ocean Reef Cstl

Ocean Reef Cstl

E Cape Sable

E Cape Sable

2 day forecast 17

18


Intracoastal City

DestinChokoloskee


Englewood


San Luis Pass

St. Augustine

Pascagoula

Sabine Pass

Flamingo

Yankeetown

Vero Beach




Ocean Reef

Baffin Bay

New Smyrna Bch

Card Snd Bridge

MS/AL Border

High Island


Sargent

Key West



Jupiter Inlet

Ochlockonee R.

Boca Raton

Cameron



Freeport




Hallandale Bch

Morgan City

Mouth Pearl R.

Grande Isle


Golden Beach

Golden Beach

Golden Beach

Intracoastal City

Chokoloskee


Englewood



Jupiter InletNew Smyrna Bch

Ocean Reef


Key LargoKey Largo

Key Largo

Key Largo

Flamingo

Flamingo

Vero Beach

Titusville

Deerfield Beach

Dry Tortugas

Deerfield Beach

Ocean Reef

Hallandale Bch

Hallandale Bch

Hallandale BchMS/AL Border

Cocoa Beach

Craig KeyCraig Key

Key West


Florida Bay

Florida Bay

Sebastian Inlet


Boca Raton


Bonita Beach

Ocean Reef Cstl

Flagler Beach

E Cape Sable

7-Mile Bridge

StuartStuart

Ocean Reef Cstl

Ocean Reef Cstl

E Cape Sable

E Cape Sable

Ocean Reef Cstl

1 day forecast 18

Interpreting the Wind Speed Probability Text Product

Interpreting the Wind Speed Probability Text Product

19

20

Wind Speed Probability Text Product

20

21

34 kt probabilities at Charlotte NC

What is the chance that winds of tropical storm force (34 kt or greater) will occur at Charlotte NC during the next five days?

Example Interpretation of Output

21

22



42%


22

23



42%When are these winds most likely to start?


23

24



42%When are these winds most likely to start?

From 18Z Sun to 18Z Mon (18% chance)


24

Case Example Hurricane Katrina (2005)

Case Example Hurricane Katrina (2005)

25

Hurricane Katrina (2005)Hurricane Katrina (2005)• Two examples of how

probabilities evolve1. Landfall of a marginal hurricane

in South Florida– Small probabilities of hurricane

force winds over much of south FL due to uncertainty in track and intensity

2. Landfall of a major hurricane along the Gulf Coast– Initially small probabilities at

locations along the Gulf Coast increase markedly along the track of Katrina as landfall approaches

– Hurricane conditions are almost a certainty somewhere

26

27

Magnitudes of Cumulative Probabilities Vary Greatly But Realistically

Katrina Advisory #5

27

36 hour forecastintensity = 80 mph

category 1 hurricane

10% chance of hurricane-force winds at any individual point along

SE Florida coast

Low probability of hurricane‐force winds due to small size of hurricane force wind field and uncertainty as to whether Katrina will even be a hurricane at landfall

28


Katrina Advisory #9

28

Low probability of hurricane‐force winds due to uncertainty in track and whether Katrina will be a hurricane at landfall

72 hour forecastintensity = 90 mph

category 1 hurricane

Hurricane-force probabilities > 10% at all coastal locations due to track and intensity

forecast uncertainty

29


Katrina Advisory #11

29

72 hour forecastintensity = 100 mphcategory 2 hurricane

Probabilities begin to increase due to increase in intensity forecast even though track uncertainty remains similar – becoming more likely that hurricane‐force winds will occur somewhere

Point locations from the mouth of the Mississippi River to the Big Bend have roughly the same chance

(5-15%) of hurricane-force winds

30



30


Probabilities increase further due to continued increase in intensity forecast and slight decrease in track forecast uncertainty with landfall forecast in about 72 h

Highest probabilities now focused from SE Louisiana to the FL panhandle

31



31


Probabilities along central Gulf coast increase dramatically due to increase in intensity forecast and decrease in track forecast uncertainty with landfall forecast in about 48 h

Chance of hurricane-force winds increasing at locations over southeast Louisiana

32



32


Landfall now within 12 to 24 h, hurricane force winds almost a certainty along forecast track given large size and strength of Katrina along with small short‐range track uncertainty

Hurricane-force winds now likely at locations from southeast Louisiana well

inland into south-central Mississippi

Timing Information About Wind Onset

Timing Information About Wind Onset

33

Onset of 34‐kt WindsKatrina (2005)

• Onset of 34‐kt winds based on deterministic forecast issued with Advisory 16– New Orleans, LA – 8/29 (Mon.) 08Z– Gulfport, MS – 8/29 (Mon.) 11Z

Deterministic wind field from Hurrevac valid 12Z 29 August

34

Wind Speed ProbabilitiesKatrina (2005) Advisory 16

- - - - WIND SPEED PROBABILITIES FOR SELECTED LOCATIONS - - - -FROM FROM FROM FROM FROM FROM FROM

TIME 06Z SAT 18Z SAT 06Z SUN 18Z SUN 06Z MON 06Z TUE 06Z WED PERIODS TO TO TO TO TO TO TO

18Z SAT 06Z SUN 18Z SUN 06Z MON 06Z TUE 06Z WED 06Z THU

FORECAST HOUR (12) (24) (36) (48) (72) (96) (120)

NEW ORLEANS LA 34 X 1( 1) 9(10) 28(38) 34(72) 5(77) X(77)

GULFPORT MS 34 X 1( 1) 8( 9) 23(32) 35(67) 5(72) 1(73)

35





FORECAST HOUR (12) (24) (36) (48) (72) (96) (120)

NEW ORLEANS LA 34 X 1( 1) 9(10) 28(38) 34(72) 5(77) X(77)

GULFPORT MS 34 X 1( 1) 8( 9) 23(32) 35(67) 5(72) 1(73)

Most likely period of onset of 34‐kt winds at New Orleans and Gulfport is between 06Z Monday 8/29 and

06Z Tuesday 8/3036





FORECAST HOUR (12) (24) (36) (48) (72) (96) (120)

NEW ORLEANS LA 34 X 1( 1) 9(10) 28(38) 34(72) 5(77) X(77)

GULFPORT MS 34 X 1( 1) 8( 9) 23(32) 35(67) 5(72) 1(73)

However, the probability that 34‐kt winds will start prior to 06Z Monday 8/29 at both New Orleans and

Gulfport is nearly as large!37

What Actually Happened?• Onset of 34‐kt winds occurred between 00Z and 06Z Monday 8/29 at New Orleans and Gulfport

– At least 3 hours earlier than shown by the official forecast at New Orleans

– At least 5 hours earlier than shown by the official forecast at Gulfport

38

Lesson

• Important information about the onset of wind conditions is contained in the probabilities

• Examine trends from advisory to advisory– How are probabilities of onset changing?– Are chances of onset nearly equal between two consecutive time periods?

39

Case Example Hurricane Earl (2010)

40

Hurricane Earl (2010)• An example of a different type of

uncertainty, in contrast to a hurricane that will almost certainly bring hurricane force winds to some area of the Gulf coast (e.g., Katrina, Ike)

• In this case, tropical‐storm and hurricane‐force winds may or may not affect any locations along the coast of North Carolina depending on the track and size of Earl

• Given the track of Earl parallel to the coast, small changes in the forecast track result in large changes in the wind speed probabilities 41

42

0 0 04

63 4 4

911

14 13 1215 16

2123

36

28

36

13

19

1 0 03

6 7

20

25

2023 23

33

46

56 55 54

62 63

69

79

90 9195

98 99 99 99 99

0

10

20

30

40

50

60

70

80

90

100

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

Cumulative 5‐day Prob

ability (%

)

Advisory Number

Trend of 34‐kt and 64‐kt Cumulative Wind Speed Probabilites at Cape Hatteras, NC for Hurricane Earl (2010)

64‐kt

34‐kt

Day 5 forecast point in official forecast reaches

latitude of Cape Hatteras

Hurricane EarlAdvisory 16 – 5 AM EDT 29 Aug. 2010

Hurricane EarlAdvisory 16 – 5 AM EDT 29 Aug. 2010

43

34-kt probabilities 64-kt probabilities

44

0 0 04

63 4 4

911

14 13 1215 16

2123

36

28

36

13

19

1 0 03

6 7

20

25

2023 23

33

46

56 55 54

62 63

69

79

90 9195

98 99 99 99 99

0

10

20

30

40

50

60

70

80

90

100

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37


ability (%

)

Advisory Number


64‐kt

34‐kt

Official track forecast gradually shifts westward and closest point

of approach shifts toward 72 h

Hurricane Watch Issued

Hurricane EarlAdvisory 23 – 11 PM EDT 30 Aug. 2010

Hurricane EarlAdvisory 23 – 11 PM EDT 30 Aug. 2010

45


46

0 0 04

63 4 4

911

14 13 1215 16

2123

36

28

36

13

19

1 0 03

6 7

20

25

2023 23

33

46

56 55 54

62 63

69

79

90 9195

98 99 99 99 99

0

10

20

30

40

50

60

70

80

90

100

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37


ability (%

)

Advisory Number


64‐kt

34‐kt

Continued westward shift of track and closest point of

approach now around 48 h

Hurricane Warning Issued

Hurricane EarlAdvisory 28 – 5 AM EDT 1 Sep. 2010


47


48

0 0 04

63 4 4

911

14 13 1215 16

2123

36

28

36

13

19

1 0 03

6 7

20

25

2023 23

33

46

56 55 54

62 63

69

79

90 9195

98 99 99 99 99

0

10

20

30

40

50

60

70

80

90

100

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37


ability (%

)

Advisory Number


64‐kt

34‐kt

36-h forecast point about 50 nm east of Cape Hatteras

Hurricane EarlAdvisory 30 – 5 PM EDT 1 Sep. 2010


49


50

0 0 04

63 4 4

911

14 13 1215 16

2123

36

28

36

13

19

1 0 03

6 7

20

25

2023 23

33

46

56 55 54

62 63

69

79

90 9195

98 99 99 99 99

0

10

20

30

40

50

60

70

80

90

100

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37


ability (%

)

Advisory Number


64‐kt

34‐kt

Earl begins to turn more northward and decreasing track uncertainty within 24 h results in

reduction in chances of hurricane-force winds



51


52

0 0 04

63 4 4

911

14 13 1215 16

2123

36

28

36

13

19

1 0 03

6 7

20

25

2023 23

33

46

56 55 54

62 63

69

79

90 9195

98 99 99 99 99

0

10

20

30

40

50

60

70

80

90

100

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37


ability (%

)

Advisory Number


64‐kt

34‐kt

Earl begins to turn east of due north and chance of hurricane-

force winds diminishes to near 0



53


Hurricane Earl – Ground TruthHurricane Earl – Ground Truth• Sustained tropical‐storm‐force were

reported at several locations on the Outer Banks

• Peak sustained wind was 60 kt at KHK Resort on Rodanthe Sound

• Several gusts to hurricane force reported

54

Intensity Probability Table Examples

Intensity Probability Table Examples

55

56

Hurricane Danielle Intensity Probability TableAdvisory 16 – 11 AM EDT 25 Aug. 2010

Hurricane Danielle Intensity Probability TableAdvisory 16 – 11 AM EDT 25 Aug. 2010

• Official NHC 48‐h intensity forecast: 100 MPH (Category 2)

• Verifying intensity: 135 MPH (Category 4)

563% chance of category 4 hurricane in 48 h verifies

Impact of Land Interaction on Intensity Probabilities


• Even if the official track forecast does take the TC over land, many of the 1,000 track realizations can move over land to track uncertainty

• This often occurs in the western part of the Atlantic basin due to numerous islands and large landmasses

• When this occurs, the intensity probabilities will often spread out over a large range of possible intensities, particularly late in the forecast period

57


Hurricane Ike – 11 PM EDT 5 Sep 2008


Hurricane Ike – 11 PM EDT 5 Sep 2008

58

• Official forecast shows Ike avoiding significant land interaction and remaining a major hurricane through 5 days

• Probabilities show the potential for land interaction as intensity probabilities spread out across categories 1‐4 by days 4 and 5



• The intensity probability table CANNOT be used to estimate the intensity at landfall

• Probabilities are valid at a specific TIME – not location – Some realizations will move faster than the official forecast and already be inland at the time of landfall in the official forecast

– These inland realizations will be weaker and contribute to lower intensity probabilities at the forecast time period closest to landfall

• We are working with a new tool that computes the probability of landfall occurring in specific categories

59

Hurricane IkeAdvisory 38 – 10 AM CDT 10 Sep 2008

Hurricane IkeAdvisory 38 – 10 AM CDT 10 Sep 2008

60

• Official Forecast shows landfall around 72 h with forecast intensity of 120 mph (Category 3)

• Many realizations are already well inland by 72 h, and intensity probabilities shows nearly equal or higher probability of category 1 or 2 at 72 h

• Actual landfall intensity: 110 MPH (Category 2)• Intensity at 72 h (12Z 13 September): 100 MPH (Category 2)

SummarySummary• Wind speed probability products help you deal with the uncertainty inherent in forecasting tropical cyclones

• Provide additional information beyond what is available in deterministic forecasts for:– Timing of event onset– Likelihood of various wind speeds occurring at your location

– Likelihood of tropical cyclone intensity• “Low” probabilities of extreme events often warrant action!

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wind speed and intensity probabilities...wind speed and intensity probabilities michael j. brennan...

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