# Presentation: 259891

***INTRA VS. INTERSEASONAL FACTORS IN SEASONAL HURRICANE FORECASTS***

**Steve Letro**
**Email: Steve.letro@noaa.gov**
**Meteorologist in Charge**
**National Weather Service, Jacksonville**

## First... A Bit of History...For At Least The Last 3 Decades, The Quest For Reliable Seasonal Hurricane Forecasts In Terms Of Number, Intensity and Most Importantly, Likely Landfall Areas Has Been One Of the “Holy Grails” Of Tropical Meteorology

**  **

## Dr. Bill Gray Headed Up The Quest With His Original Work On Seasonal Forecasts Back In The Early 1980’s

**He Did This By Identifying Correlations Between Large Scale Patterns Preceding The Hurricane Season With Past Activity**

## By Now, We Are All Familiar With Some Of The More Common Predictors He Used...

**  ****ENSO Phase**
**  ****Atlantic SST Anomalies**
**  ****Antecedent Pressure Patterns**
**  ****QBO Phase**
**  ****And so on...**

**Of The Above, Strong ENSO Phases And Warm SST’s Are Most Commonly Identified As Having The Best Correlations**

## The Technique Demonstrated Skill... But Beginning In 1995 Atlantic Basin Activity... Including Landfalls... Demonstrated An Alarming Upward Trend That Was Not Explicitly Accounted For In The Forecast Scheme

**  **

## This Led To The Hypothesis Of The Atlantic Multi Decadal Oscillation As A Key Component In The Hyperactive Hurricane Period

**  **

## This Correlation Was Based On The Premise That The AMO Was A Product of A More Active Phase Of the Atlantic Thermohaline Circulation... Leading To Warmer Sea Surface Temperatures and A More Favorable Background Environment for Tropical Cyclone Development

**  **

## This Correlation Was Based On The Premise That The AMO Was A Product of A More Active Phase Of the Atlantic Thermohaline Circulation... Leading To Warmer Sea Surface Temperatures and A More Favorable Background Environment for Tropical Cyclone Development

**  **

## Just As The Original Forecast Scheme Could Not Account For Everything, The AMO Alone Also Does Not Dictate Development... The Records Show Several Years In The Cool Phase With Significant Activity (1969, 1990, etc)... And Several In The Warm Phase With Far Less Activity (1946, 1957)

**  **

## Still, By Refining The Technique Over Time and Accounting For The AMO, The Fact Remained That Overall The Method Enjoyed A Good Degree of Success!

**  **

- But, As We Have Seen, Individual Seasons Can Still Demonstrate Very Annoying “Errors” (see 2006)!

## These “Errors” Became A Major Issue In 1996...Based On the 1994 & 1995 Seasons, The Early Season Forecast For ‘96 Of Another Hyperactive Season With Well Above Normal Landfall Probabilities Created A Nearly Unprecedented Level of Angst and Apprehension 

**  **

## The Early Season 2006 Forecast Looked Grim...

**NAMED STORMS:****	****                 17**
**HURRICANES:****	****	****	****	****  9**
**MAJOR HURRICANES:****	****  ****	****  5**
**U.S. STRIKES****	****	****	****	****  ?**
**U.S. HURRICANE STRIKES:****	****  ?**
**MAJOR U.S. HRCN STRIKES****	****  ?**

## The 2006 Season Final Tally...

**NAMED STORMS:****	****                9**
**HURRICANES:****	****	****	****	****5**
**MAJOR HURRICANES:****	****  ****	****2**
**U.S. STRIKES****	****	****	****	****2**
**U.S. HURRICANE STRIKES:****	****0**
**MAJOR U.S. HRCN STRIKES****	****0**
**Only 2 weak tropical storm landfalls!**

## So What Happened in 2006???

**Most Of The Blame (or credit) For The Quieter Season Has Gone To The Unexpectedly Rapid Development of an El Nino Event In The Pacific.**

***El Nino Usually Creates Wind Shear Which Inhibits Tropical Cyclone Development***

## OVERALL... EL NINO TENDS TO PRODUCE:

**FEWER OVERALL STORMS**
**FEWER INTENSE STORMS**
**FEWER MAJOR STORMS                                             AFFECTING THE UNITED STATES**

***BUT...***

## Some Facts To Consider...

**This El Nino Was Mainly In A Weak, Early Developmental Stage During much of the 2006 Hurricane Season.**
**The Best Correlations Between El Nino and Decreased Hurricane Activity Usually Occur Once The Event Is Fully Established...*****Which It Wasn’t!***

## In Fact... September Was Actually  More Active Than Normal In Terms of Numbers!

- 5 Named Storms On
-    the Charts
- Hurricanes
- 2    Major Hrcns

- Thankfully, The One Storm (Ernesto) That DID Affect The U.S.  Was Weak!

## In Fact, Based On The Nino 3.4 Indices, It Could Reasonably Be Argued That The Only Significant Effect On The 2006 Season From El Nino Was It’s Early Demise As The Event Became Better Established In The Fall 

**  **

- (+0.4 in JAS To +0.9 in SON)

## If El Nino Was Not To Blame For The Quiet Season, Then What Happened?

**The Answer May Well Lie In A Fundamental  Understanding Of How A Hurricane Actually Works**

## Nature’s Great Heat Engine... The Hurricane

**Outflow (“Exhaust”)**

**Ocean (“Fuel****”)**

**Energy Release**
**(“Cylinders”)**

** **

**In Simplistic Terms, A Hurricane Ingests Fuel (warm, moist air), Processes It Into Energy (Wind) in The “Cylinders”, i.e, the Rainbands, and Expels The**** ****moisture –depleted air (The Exhaust) Through The Outflow At The Top Of The System**

**Nature’s Great Heat Engine... The Hurricane**

## The Three Main Parts Of The Engine...1.   Availability of Fuel 2.   Ability To Process The Fuel Into       Energy3.   Ability to Exhaust The WasteIf Any Of These Factors Are Interfered With... The Engine Cannot Function Efficiently 

## What Affects These Parameters?

**Availability of Fuel**
**     ****Oceanic Heat Content:  While the AMO Provides a **_***Background***_** of Warmer Than Normal Values, Intraseasonal Variables Such as ENSO, Upwelling, and Reduced Insolation (Strong Saharan Air Layer) Can Reduce These Values In Different Parts Of The Basin In Any Given Season **

***Let’s Look At SST’s Over The Last Few Years...***

## August-October 2004

***August-October 2004***

## August – October 2005

***August – October 2005***

## August – October 2006

***August – October 2006***

## African Dust Blanketing Much of the Atlantic “Main Development Region”, or MDR. This continued through much of 2006 and 2007

**African Dust Blanketing Much of the Atlantic “Main Development Region”, or MDR. This continued through much of 2006 and 2007**

**This Saharan Air Layer (SAL) Is Indicative Of Stable, Sinking Air... A Very Unfavorable Environment For Efficiently “Processing” The Tropical Cyclone Fuel Into Energy!**

## August – September 2007... Notice The Dramatic Decrease In The Eastern Atlantic Warm Anomaly 

***August – September 2007... Notice The Dramatic Decrease In The Eastern Atlantic Warm Anomaly ***

## What Affects These Parameters?

**2.  Ability To Process Fuel Into Energy**
**     ****Atmospheric Stability:  The Ability of the Lower Atmosphere To Develop & Sustain Convection In Seedling Disturbances. This is Determined To A Large Extent By The Positioning Of Synoptic Scale Features In The Tropics... Also Subject To Significant Intraseasonal Variation!**

## Tropical Atlantic Instability vs. Development 

**Tropical Atlantic Instability vs. Development **

- Ingrid

- Gabrielle

- Jerry, Karen, Melissa

- Unstable

- Stable

## Gulf Development vs. Instability 

**Gulf Development vs. Instability **

- Erin

- Humberto

- Lorenzo

- Unstable

- Stable

## Dean & Felix:  Instability In W. Caribbean At Time Of Max Intensity

- Dean

- Felix

**Dean & Felix:  Instability In W. Caribbean At Time Of Max Intensity**

- Unstable

- Stable

## What The Preceding Demonstrates Is That Atmospheric Stability Varies Greatly Both Within The Season and In Differing Portions Of The BasinThe Problem Is That These Are Intra Seasonal Variations That Are Not At All Well Handled By Purely Inter Seasonal Considerations

## What Else Affects These Parameters?

**2.  Ability To Process Fuel Into Energy**
**      ****Vertical Shear:   High Shear Values Adversely Affect The Ability of Convection To Efficiently Organize Around The Center**

## While A Positive ENSO Event (El Nino) Certainly Correlates With Above Normal Shear... A Quick Look At The 2007 Shear... With La Nina Conditions in Place... Shows That Shear Over The Tropical Atlantic Is Clearly Dependent On More Than Just ENSO Phase!

- Note The Anomalously High Shear Present Until Early September!

## While A Positive ENSO Event (El Nino) Certainly Correlates With Above Normal Shear... A Quick Look At The 2007 Shear... With La Nina Conditions in Place... Shows That Shear Over The Tropical Atlantic Is Clearly Dependent On More Than Just ENSO Phase!

- Note The Anomalously High Shear Present Until Early September!

**Also Note The Dramatic Decrease In Shear Beginning In September... At Precisely The Same Time The La Nina Was Strengthening!**

**Meanwhile, Shear Across The Caribbean Was Much  More Consistent With La Nina Conditions... Yet Despite The Overall Lighter Shear...There Was Little...If Any... Initial Development In Those Regions **
**(Though Dean & Felix Almost Certainly Benefitted From It!)**

***Even Though The Background Of A Particular ENSO Phase Indicates An Overall Correlation With Shear... There Are Clearly Other Intra Seasonal Changes Taking Place That Significantly Impact Shear At Any Specific Time Or Place***

## What About The Last Component Of The “Engine”?

**Divergent Pattern Aloft To Enhance Outflow**
**      ****Anomalously Strong Convergence Across The MDR Inhibits Storms From Sustaining Strong Convection By Limiting The Efficiency Of The “Exhaust” In The Upper Levels. This Pattern Was Evident Throughout Both 2006 & 2007.**
**     **

## 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**

- 2006

- 1995-2005

## 2006

**Very High Values = Exhaust Is Choked Off = Less Potential for Development**

***UPPER LEVEL “EXHAUST” POTENTIAL***

## Note That The Area That Was Least Favorable For Development in 2006 Is The Same Area That Caused So Much Trouble In 2005!

**   **

## While The 2007 Anomalies Are Not As Great... They Still Indicate A Less Than Favorable Environment

**While The 2007 Anomalies Are Not As Great... They Still Indicate A Less Than Favorable Environment**

- 2007

- 1995-2005

## So What Is The Message?

***Despite The Inter Seasonal Predictability Of Such Key Large Scale Parameters As ENSO Phase, AMO, etc... There Is Still A Great Deal Of Intra Seasonal Variation In These Parameters Between Various Sections Of The Basin***

**The Result Is That Seasonal Activity Is Ultimately Determined By The Phasing Of These Parameters In Time And Space... Which Is Currently Beyond The State Of The Science On An InterSeasonal Scale **

## It’s Important To Note That The Same Principle Applies To Specific Interseasonal Track Forecasts

**While Basic Probabilities Can Be Inferred Based On The Assumption Of Increased Or Decreased Activity... Where Storms Go Is Dependent On The Evolution Of The Large Scale Features In Place At The Time During The Life Of Any Particular Storm... Which Also Currently Has Not Been Shown To Have Any Significant Interseasonal Predictability**

## Why Is This Important?

***There Is The Danger That The Ability To Predict Tropical Cyclone Activity Based On InterSeasonal Factors Has Been Oversold And Expectations ...And Assumptions...  About Individual Upcoming Seasons Have Been Raised To Unrealistic Levels***

## This Creates An Environment Where Our Credibility Will Only Be Maintained By Increasing The Emphasis On ALL The Factors Involved In Tropical Cyclone Frequency And Motion... Including The Relative Degrees Of Predictability On BOTH The Interseasonal AND Intraseasonal Scale

## The End!

***The End!***
***www.srh.noaa.gov/jax***