# Presentation: 462042

## 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)

**Notes:**
Gerry Bell at CPC is the head of the NOAA seasonal forecast effort and he takes counsel from the other 3 agencies.

## 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

**Notes:**
I’ll give a brief overview of how the seasonal forecast is made. I’ll talk about the 2006 forecast, what happened during the season and some of the reasons for the less than satisfactory forecast.  I’ll show a couple of slides on longer-term verification and close with a glimpse of 2007

## 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.

**Notes:**
What goes into the forecast?  Every one of the members brings a different element... we use the CPC forecast for the ENSO element, TPC generally monitors what has been going on.  Mix of qualitative/quantitative ideas... whether it be a nino forecast.  In the years that I have been on the team, the weakest element is the El Nino forecast. People talk about the spring predictability barrier, recently it has been more like the summer predictability barrier.  

## 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).

**Notes:**
Similar to Gray’s Net Tropical Cyclone parameter (NTC).  

## NOAA’s 2006 Atlantic Hurricane Outlooks

**    **_**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**

**NOAA’s 2006 Atlantic Hurricane Outlooks**

**Notes:**
117% of the median is generally above average, while 

## Vertical wind shear generally lower than average

**Vertical wind shear generally lower than average**

**Lower than average**

**Higher than average**

- Tropical Storm genesis points during August-October 2006

**(m/s)**

**Notes:**
Usually a busy season.  Generally unfavorable conditions in the Caribbean and Gulf.  Normal shear conditions not favorable for TC genesis in the Atlantic.  After we saw this, thought it was in error.  Shear was way too long  for an average season.

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

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

**Notes:**
Can’t blame the SSTs in this case.  Multi-decadal signal can be seen in the time series.

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

**In mid-June the atmosphere made a rapid transition out of La Ni****ña conditions**

**Dry**

**Wet**

**Notes:**
This is a hovmoeller diagram of OLR anomalies near the equator spanning the whole globe, blues correspond to more clouds and oranges correspond to less clouds. Typical La Nina effects continued from late 2005 to aboutt mid-June.  Lack of thunderstorms over the dateline, and more convection than average in the Atlantic basin.  Big change to the atmospheric circulation in mid-June.  Convection increases just west of the dateline, atlantic gets drier and the walker circulation is shifted eastward.  At the time, couldn’t tell whether it was associated with MJO variability or a much more significant change.

## Rapid El Niño warming after early August forecast  

**Rapid El Ni****ñ****o warming after early August forecast  **

**Notes:**
In this case, atmosphere changed before the SSTs warmed.  Rather dramatic changes, especially for so late in the season.  

## 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 (average activity

- 1995-2005 (above average activity

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

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

**Notes:**
Complicated figure.  The area to focus on is in the Caribbean Sea.  Red areas are large scale converging air while blue is generally diverging air.  The deep reds imply strong convergence, which in the upper atmosphere leads to strong sinking air, leading to warmer drier conditions aloft that do not favor the development of thunderstorms.

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

- More rising than average           More 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.

**Notes:**
Western part of the area had strong sinking, while closer to average in the eastern Atlantic.

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

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

**Stable**

**Unstable**

**Moist**

**Dry**

**Notes:**
Sinking affects two parameters in particular.  Sinking produces warming aloft, which causes less instability over the tropics.  Sinking is also a warming and drying effect, neither of which is favorable in the large scale for hurricane formation.

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

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

**Notes:**
What happened?  Well, models generally didn’t anticipate the onset of the event.  Furthermore, ASO anomalies only about 0.7C above average.  2004 had similar values, in fact, a little higher for Nino 3.4.  Obviously, water temperature weren’t the predominant factor, and the atmosphere was dictating the changes.

## 500 mb anomalies for September 2006

**500 mb anomalies for September 2006**

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

**Notes:**
The hurricanes that we did have were steered far out to sea, a dramatic change in steering from the past 3 seasons.

## Slide 15

**Notes:**
50% of the time it verifies in the ranges

## Slide 16

**Notes:**
About 2/3rd of the time in the August fcst.

## Current Conditions Atlantic SSTs are warmer than average.

_**Current Conditions **_Atlantic SSTs are warmer than average.

-  Early March 2006

- Warmer than 2006, and nearly as warm as 2005.

-  Early March 2007

-  Early March 2005

## The demise of the 2006 El Niño

**Notes:**
The el nino peaked rather early, and has dissipated.  In the last couple of weeks the ssts are cooling fairly quickly in the eastern equatorial pacific.  Depending on the atmosphere... the atmosphere appears to be ready to support la nina development.

##           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.

- 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.

**Notes:**
The CFS assists making forecasts of upper-level conditions... SST... shear.... All parameters that with a three month lead time.  If we were really dreaming there might even be a mean 500 map for the season which could lead to areas of higher and lower risk for landfalls. 

## CFS Seasonal Forecasts from March 7

- Vertical Shear
- (much lower than average in the deep tropics)

- SST
- (generallywarm in the deep tropics)

**Notes:**
Forecasts for ASO this year... generally a neutral pattern in the pac and warm in the atlc in the red box.  However there isn’t much skill at this time frame but we should have a better idea in late April.  Last year the CFS too quickly cooled the Atlc waters but was very good in the EPAC.

## La Niña conditions possible!

**La Ni****ñ****a conditions possible!**

**Notes:**
The CFS... which has hindcast skill comparable to ENSO CLIPER and other statistical enso forecasts.. Suggests a return to la nina... with a rather large spread in ensemble members though by aug!  In fact if this ensemble mean forecast verifies... it qualifies as a moderate to strong la nina.

## February Nino 3.4 SST guidance

- JF

**Notes:**
This figure is already outdated as the waters are quickly cooling.  A lot of the models, even only 1 month old, didn’t anticipate the quick demise of the Nino.  Note the magenta dot is the Jan-Feb average, suggesting a lot of the models have some catching up to do.  One of the models that did catch the rapid demise was the CFS.

## February 2007 SST anomalies

- °C

**Notes:**
Here is the last month’s SSTS... warm TATL and a cooling pac.  If you look at how feb SSTs best correlate with ace in the next year...

## Correlations of Feb SSTs to next season’s ACE 

- Correlations of Feb SSTs to next season’s ACE

**Notes:**
Nino temperatures, atlantic temperatures and pacific pdo type signal all resemble an active year.

## Vertical Wind Shear anomalies (m/s) 

- Vertical Wind Shear anomalies (m/s)

**25 kt!**

- Higher

- Lower

**Notes:**
Since the middle of January.. The shear has dramatically dropped in the Atlantic. A quick plot of the last 30 days of shear shows much below average already, and could be a little foreboding for 2007.

## 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.

## Dry air in the middle to upper levels over the tropical Atlantic Ocean and Caribbean Sea was more pronounced than average during the hurricane season (figures courtesy CIRA)

**Dry air in the middle to upper levels over the tropical Atlantic Ocean and Caribbean Sea was more pronounced than average during the hurricane season (figures courtesy CIRA)**

**Dry**

**Dry**

**Moist**

**Moist**

**Notes:**
Sinking also dries the atmosphere, with the area seeing more dry periods than moist.