Seasonal Hurricane Forecasting and What’s New at NHC for 2009Seasonal Hurricane Forecasting and What’s New at NHC for 2009

Eric Blake

Hurricane Specialist

National Hurricane Center

4/2/2009

Eric Blake

Hurricane Specialist

National Hurricane Center

4/2/2009

2009 WeatherBug seminar2009 WeatherBug seminar

Outline

• Verification of NHC Forecasts

• Seasonal Forecasting

• Brief look at 2009 season

• What’s New for 2009?

National Hurricane Center 2008 Forecast VerificationNational Hurricane Center 2008 Forecast Verification

3

2008 Atlantic Verification

VT NT TRACK INT(h) (n mi) (kt)============================ 000 373 5.7 1.8012 346 27.7 7.1 024 318 48.3 10.4036 288 68.6 12.1048 261 88.2 13.6072 221 126.9 14.6096 177 159.8 13.8120 149 191.8 17.2

Values in green exceed all-time records.

* 48 h track error for TS and H only (GPRA goal) was 87.5 n mi, just off last year’s record of 86.2.

Atlantic Track Error Trends

Errors have been cut in half over the past 15 years. 2008 was best year ever.

Atlantic Track Skill Trends

2008 was the most skillful year on record at all time periods.

2008 Track Guidance

Official forecast performance was very close to the consensus models.

Best model was ECMWF, which was so good that it as good or better than the consensus.

BAMD was similar to the poorest of the 3-D models (UKMET).

AEMI excluded due to insufficient availability (less than 67% of the time at 48 or 120 h).

2008 Track Guidance

Examine major dynamical models to increase sample size.

ECMWF best at all time periods (as opposed to last year, when it was mediocre). GFDL also better than last year (and better than HWRF). As we’ve seen before, GFDL skill declines relatively sharply at days 4-5.

NOGAPS and GFNI again performed relatively poorly. GFNI upgrades were delayed.

Guidance Trends

Return to more “traditional” relationships among the models after the very limited sample of 2007.

Guidance Trends

Relative performance at 120 h is more variable, although GFSI has been strong every year except 2005. GFDL is not a good performer at the longer ranges.

Consensus Models

Best consensus model was TVCN, the variable member consensus that includes EMXI. It does not appear that the “correction” process was beneficial.

Atlantic Intensity Error Trends

No progress with intensity.

Atlantic Intensity Skill Trends

Little net change in skill over the past several years.

2008 Intensity Guidance

Split decision between the dynamical vs statistical models. New ICON consensus, introduced this year, was very successful, beating OFCL except at 12 h.

OFCL adds most value over guidance at shorter ranges. Modest high bias in 2008 (2007 was a low bias).

2008 Intensity Guidance

When the complication of timing landfall/track dependence is removed, OFCL performs better relative to the guidance. Dynamical models are relatively poor performers.

Color indicates probability of tropical cyclone formation within 48 hours. Outlined areas denote current position only. Low Medium High < 20 % 20-50% >50%

GraphicalTropical Weather Outlooks

GraphicalTropical Weather Outlooks

2007-08 Genesis Forecast Verification

Summary: Atlantic Track OFCL track errors set records for accuracy at all

time periods. Errors continue their downward trends, skill was also up.

OFCL track forecast skill was very close to that of the consensus models, was beaten by EMXI.

EMXI and GFDL provided best dynamical track guidance. UKMET, which performed well in 2007, did not do so in 2008. NOGAPS lagged again.

HWRF has not quite attained the skill of the GFDL, but is competitive. A combination of the two is better than either alone.

Best consensus model was TVCN (variable consensus with EMXI). Multi-model consensus – good. Single model consensus – not so good. Not a good year for the “corrected consensus” models.

Summary: Atlantic Intensity OFCL errors in 2008 were below the 5-yr

means, but the 2008 Decay-SHIFOR errors were also lower than its 5-yr mean, so no real change in skill.

Still no progress with intensity errors; OFCL errors have remained unchanged over the last 20 years. Skill has been relatively flat over the past 5-6 years.

Split decision between the statistical and dynamical guidance. Simple four-model consensus (DSHP/LGEM/HWRF/GHMI) beat everything else, including the corrected consensus model FSSE.

Seasonal ForecastingSeasonal Forecasting

Dr. Bill GrayColorado State University

“Scientists consider seasonal hurricane prediction to be,

at best, a GRAY area” (Ed Rappaport,

NHC, 1991)

Year after El NiñoEl Niño Years

Composite of tropical cyclone tracks during 14 moderate to strong El Niño years versus

the next year

From Gray 1984

El Niño

• Warming of the equatorial waters in the central and eastern Pacific Ocean

• A natural phenomenon that occurs every 3 to 5 years

• Affects global atmospheric circulation patterns by altering thunderstorm development in the deep tropics

• Generally causes a less active Atlantic hurricane season

El Nino

La Nina

El Niño versus La Niña

Hello there it

El Niño causes extra thunderstorm development over the central and eastern equatorial Pacific. This causes a response in the atmosphere over the Atlantic basin of increased shear and sinking air, causing a drier and more stable atmosphere.

La Niña causes a reduction and westward shift in thunderstorms. This forces the maximum sinking air to be located over the eastern Pacific and allows air to rise more freely over the Atlantic basin, in addition to less shear.

Upper winds

Upper winds

El Niño

La Niña

Vertical Wind Shear

• Tropical cyclones generally require a low vertical wind shear environment to develop, less than about 10-15 mph.

• Vertical shear displaces energy away from the center of a tropical system and slows development.

• By monitoring early season vertical shear (June-July), you can gain knowledge about the peak of hurricane season from August-October (when 90% of all major hurricanes strike).

. ..

Effects of Vertical Wind Shear (V z) on Tropical Cyclones

WEAK SHEAR = FAVORABLE

low clouds

high cloudsSTRONG SHEAR = UNFAVORABLE

EY

E

LOWER-LEVEL WINDS

UPPER-LEVEL WINDS

Neal D

orst/S

tan G

old

enb

ergH

urrican

e Research

Div

ision

AO

ML

/NO

AA

HRD

200 mb (~40,000 ft)

climatology

850 mb (~5,000 ft)

climatology

In general, lots of shear in the basin

200mb zonal wind anomalies (m/s) during June-July of 10 ENSO events.

El Niño

La Niña

Sea-Level Pressure

• Known to influence hurricanes for at least 70 years.

• Pressure is a proxy for multiple qualities in the atmosphere.

• Lower pressures are linked to less sinking, which leads to a more moist atmosphere.

• Higher pressures in the subtropical high linked to stronger winds, and cooler water temperatures through increased upwelling.

Stations used in the Gray 1984b Sea Level Pressure Calculation

Figure courtesy Phillip Klotzbach

Composite map of June-July anomalous surface pressures during 10 active hurricane seasons

Lower than normal

Higher than normal

Pressure isn’t everything!

Sea-Surface Temperatures (SSTs)

• In the Atlantic basin, warmer waters generally mean a more active hurricane season.

• Higher SSTs lead to more instability in the boundary layer of the atmosphere.

• Changes in SST gradients modulates regional circulation patterns.

• Atlantic SSTs also atmospheric proxy.• Cooler waters are linked to higher surface pressures,

which are related to stronger surface winds. These stronger surface winds tend to upwell more cool water, which reinforces the cycle. Stronger (easterly) surface winds also produce more shear in the atmosphere (due to the mean westerly state of the upper troposphere).

J. P. Kossin, 2008 AMS Annual Meeting

Direct local relationship between SST and tropical cyclone intensity

All other things being equal, an increase of underlying SST will lead to an increase in the maximum intensity that a hurricane can achieve. In a large sample, the mean intensity may also increase, but to a much lesser extent.

(Slides on this and the AMM provided by Jim Kossin, UW-CIMSS)

J. P. Kossin, 2008 AMS Annual Meeting

The Atlantic Meridional Mode: SST, wind, and precip anoms

•Amplifies via the wind-evaporation-SST (WES) feedback mechanism

•Strongest signal during the spring, but persists into hurricane season

•Leading mode of basin-wide ocean-atmosphere interaction between SST and low-level winds

J. P. Kossin, 2008 AMS Annual Meeting

Comparative effects of the AMM (local) and ENSO (remote) on vertical wind shear in the Atlantic

Shear regressed onto AMM and N34 indices, and correlations between the indices and storm activity.

units: m/s per standard deviation

J. P. Kossin, 2008 AMS Annual Meeting

The AMM, SST, shear, and Atlantic tropical cyclogenesis

+ genesis point became a major hurricane at some time

Composite map of June-July SST anomalies during 10 active hurricane seasons

Warmer than averageColder than average

A good measure of seasonal hurricane activity: “Accumulated

Cyclone Energy”, or “ACE”

ACE is defined as the sum of the squares of the wind speed every six hours for all tropical storms, subtropical storms and hurricanes.

Therefore, ACE is maximized for long-lived, intense hurricanes, such as Ivan (2004).

Note how the SSTs are closely related to total activity (ACE). SST in the Atlantic alone on a 5 year running mean accounts for over 70% of the variability in ACE.

Atlantic Multidecadal Mode (Ocean Temperature)

Mestas-Nunez and Enfield (1999)

North Atlantic SST Annual Anomaly (50oN-60oN; 50oW-10oW)1

0

-1

- 0.5

0.5

1925

1900

1926 1969

1970 1994

1910 1920 1930 1940 1950 1960 1970 1980 1990

1995-04

2006-2020

2000 2010 2020

Cold Atlantic Warm Atlantic

Florida and U.S. East Coast Major Hurricane Strikes 1903-2005

Composite: 200, 850 strm, windComposite: 200, 850 strm, wind

Anti-cyclonic anomalies

Stronger Tropical Easterly Jet

Reduced easterly trades

Cyclonic anomalies

Inter-hemispheric symmetry of 200-hPa streamfunction anomalies, and Reversal of equatorial zonal wind anomalies.

Reflects global-scale patterns linked to anomalous tropical convection.

NOAA Forecast Methodology

1) Assess states of the multi-decadal signal, El Niño, and Atlantic SSTs.

2) Use available CPC/CDC forecasts for El Niño/Atlantic SSTs, incorporate any analog techniques and assume persistence of upper-level conditions.

3) Predict range of overall activity and probabilities of above-, near-, and below-average seasons.

4) Qualitative/Quantitative process.5) No forecast of hurricane landfalls, just the total

seasonal activity for the entire basin.

Verification of Atlantic Predictions ofVerification of Atlantic Predictions ofNamed Storms, Hurricanes, and Major HurricanesNamed Storms, Hurricanes, and Major Hurricanes

May August

Current Conditions

Mid-April 2006

Mid-April 2007

Early April 2008

A Weakening La Niña

Climate Forecast System (CFS)

– Coupled global model (T62L64)– Integrates for 10 months with 40 ensemble

members using initial conditions.– Objective predictions of 200 mb

streamfunction, vertical wind shear and SSTs.– Shown to have skill comparable with

statistical models in Nino 3.4 SST hindcasts.– Some skill in forecasting year-to-year

changes in important parameters that control hurricane variability.

March 20: CFS forecasts neutral/warm for ASO 2009

CFS Seasonal Forecasts from March 20

SST

(coolish Atlantic,

El Niño?)

Vertical Shear

(near average in the deep tropics)

Latest Nino 3.4 SST guidance

Major 2009 NHC Product ChangesMajor 2009 NHC Product Changes

• Graphical tropical weather outlook Graphical tropical weather outlook (TWO) becomes operational(TWO) becomes operational– Text TWO to include three-tiered Text TWO to include three-tiered

categorical genesis forecastcategorical genesis forecast– ““Special” TWOs to be issued Special” TWOs to be issued

instead of the Special Tropical instead of the Special Tropical Disturbance StatementsDisturbance Statements

• Probabilistic storm surge graphic Probabilistic storm surge graphic becomes operationalbecomes operational

• Development of storm surge Development of storm surge inundation productsinundation products

• Tropical cyclone wind field graphic Tropical cyclone wind field graphic becomes operationalbecomes operational

Genesis Bins for 2009ATLANTIC

Range (%) % Expected % Verified # Forecasts

0-20 (Low) 9 6 916

30-50 (Med) 37 34 246

60-100 (High) 70 62 108

EASTERN NORTH PACIFIC

Range (%) % Expected % Verified # Forecasts

0-20 (Low) 11 19 540

30-50 (Med) 38 52 166

60-100 (High) 69 79 63

NHC will issue operational public quantitative/categorical genesis forecasts in 2009 and include categorical forecasts in the text Tropical Weather Outlook.

Minor 2009 NHC Product ChangesMinor 2009 NHC Product Changes• ““Repeat” section of Public Advisory will change Repeat” section of Public Advisory will change

to a more parsable formatto a more parsable format

• NHC Monthly Tropical Weather Summaries will NHC Monthly Tropical Weather Summaries will be shortened to a tabular summary of cyclones be shortened to a tabular summary of cyclones during the month and a short narrative of during the month and a short narrative of records of interestrecords of interest

• ““Z” time will always be referenced as “UTC” in Z” time will always be referenced as “UTC” in all advisory productsall advisory products

• Time zone referenced in the tropical cyclone Time zone referenced in the tropical cyclone discussion (TC) will now be the same time zone discussion (TC) will now be the same time zone that is used in the public advisory (TCP)that is used in the public advisory (TCP)

...SUMMARY OF 1100 PM AST INFORMATION…...SUMMARY OF 1100 PM AST INFORMATION…LOCATION...23.7N 72.2WLOCATION...23.7N 72.2WMAXIMUM WINDS...40 MPHMAXIMUM WINDS...40 MPHPRESENT MOVEMENT...WEST OR 275 DEGREES AT 8 MPHPRESENT MOVEMENT...WEST OR 275 DEGREES AT 8 MPHMINIMUM CENTRAL PRESSURE...1001 MBMINIMUM CENTRAL PRESSURE...1001 MB

Recent Additions to NHC WebRecent Additions to NHC Web

• Graphical tropical weather Graphical tropical weather outlook (TWO) RSS/XML outlook (TWO) RSS/XML feedfeed

• Improved version of Improved version of website for PDAs and website for PDAs and smart phonessmart phones

• Experimental PODCAST Experimental PODCAST available when NHC media available when NHC media pool activated (usually pool activated (usually when Hurricane warning in when Hurricane warning in effect for U.S.) effect for U.S.)

http://www.nhc.noaa.gov/audio/index.shtmlhttp://www.nhc.noaa.gov/audio/index.shtml