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NOAA’s Seasonal Hurricane Forecasts: Climate factors influencing the 2006 season and a look ahead for 2007. Eric Blake / Richard Pasch / Chris Landsea(NHC) Gerry Bell / Muthuvel Chelliah (CPC) Stan Goldenberg (HRD) Todd Kimberlain (HPC). Outline. Summary of seasonal forecast methodology

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slide1
NOAA’s Seasonal Hurricane Forecasts: Climate factors influencing the 2006 season and a look ahead for 2007.

Eric Blake / Richard Pasch / Chris Landsea(NHC)

Gerry Bell / Muthuvel Chelliah (CPC)

Stan Goldenberg (HRD)

Todd Kimberlain (HPC)

outline
Outline
  • Summary of seasonal forecast methodology
  • Verification of 2006 Atlantic basin seasonal forecast
  • Climate conditions during the season
  • Reasons for the incorrect forecast
  • Long-term verification
  • Thoughts on 2007
noaa forecast methodology
NOAA Forecast Methodology
  • Assess states of the multi-decadal signal, El Niño, and Atlantic SSTs.
  • Use available CPC/CDC forecasts for El Niño/Atlantic SSTs, incorporate any analog techniques and assume persistence of upper-level conditions.
  • Predict range of overall activity and probabilities of above-, near-, and below-average seasons.
  • Qualitative/Quantitative process.
  • No forecast of hurricane landfalls, just the total seasonal activity for the entire basin.
a measure of seasonal hurricane activity a ccumulated c yclone e nergy or ace
A 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.
  • One ACE unit is equal to one 100 kt system lasting for 6 hours. For comparison, a 50 kt system would have last for 24 hours to earn one ACE unit.
  • Therefore, ACE is maximized for long-lived, major hurricanes, such as Ivan (2004).
slide5

NOAA’s 2006 Atlantic Hurricane Outlooks

Season and May 2006 Aug. 2006 Observed

Activity Type Outlook Outlook Activity Climatology

Chance Above Average 80% 75%33%

Chance Near Average 15% 20% Near Average 33%

Chance Below Average 5% 5% 33%

Tropical Storms 13-16 12-15 10 11

Hurricanes 8-10 7-9 5 6

Major Hurricanes 4-6 3-4 2 2

ACE % of Median 135-205 110-170 90 ~100

slide6

Vertical wind shear generally lower than average

(m/s)

Lower than average

Higher than average

Tropical Storm genesis points during August-October 2006

slide7

August-October sea-surface temperature anomalies in the green box were the second warmest since 1871.

slide8

More clouds and rain showers than average in the Atlantic Ocean.

Less clouds and rain showers than average in the same areas.

In mid-June the atmosphere made a rapid transition out of La Niña conditions

Dry

Wet

2006 aso 200 mb anomalies for velocity potential and divergent wind versus 1995 2005
2006 ASO 200 mb anomalies for Velocity Potential and Divergent Wind versus 1995-2005

Note all of the converging wind vectors in the Green Box.

2006 (average activity

Note the the lack of converging wind vectors in the Green Box.

1995-2005 (above average activity

slide11

Vertical Motion at 300 mb in the Atlantic Basin during August-October 2006

More rising than averageMore sinking than average

Overall, too much sinking air was present in the Atlantic basin during the heart of hurricane season. The sinking process also produces drying and warming in the middle to upper troposphere, which can be detrimental for developing disturbances.

slide12

Stability and dry air over the tropical Atlantic Ocean and Caribbean Sea was greater than average during the hurricane season (figures courtesy CIRA)

Unstable

Dry

Stable

Moist

slide13

Observed

Observed

Available dynamical and statistical ENSO forecast models generally underdid the strength of the event, even in July.

slide14

Large trough during September over the Central and Eastern US prevented any hurricane landfalls.

500 mb anomalies for September 2006

slide17

Current Conditions Atlantic SSTs are warmer than average.

Early March 2007

Warmer than 2006, and nearly as warm as 2005.

Early March 2005

Early March 2006

slide19
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.
slide20

CFS Seasonal Forecasts from March 7

SST

(generallywarm in the deep tropics)

Vertical Shear

(much lower than average in the deep tropics)

conclusions
Conclusions
  • NOAA’s 2006 Atlantic seasonal forecast over-predicted the activity.
  • Rapidly developing El Niño seems to have been the primary cause, forcing sinking air over the Atlantic basin, leading to more stable and drier conditions than usual.
  • Extremely warm waters and favorable atmospheric conditions in the Atlantic (related to multi-decadal oscillation) during September prevented a below average year.
  • Seasonal forecasts are still more skillful than climatology over the long run. The main source of forecast error is the lack of El Niño forecast skill in the summer months.
  • If current conditions and trends persist, 2007 appears active and maybe very active.
  • Official NOAA forecast comes out in mid-May.
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