A Low-Activity Season Seems Likely - Updated

Updated - 06/30/09
Based on seasonal SST values before a significant El Nino event, 1965 and 1986 are now the best analog years for this seasons expected tropical cyclone activity in the Atlantic basin. Although many factors contribute to a seasonal forecast, historical SST data suggest that 2010 (next year) will see a return to a more normal season of activity in the Atlantic basin (see totals for 1958, 1970, 1973, 1988, 1995, 1998, 2005 and 2007). Exceptions were 1983, 1987 and 1992. It is worth noting that significant El Nino events rarely last more than a year, whereas La Nina events often last two or three years.

Two other years with similar El Nino, SST and atmospheric flow patterns were 1957 and 1972

Original Article
There is now a high probability that a moderate to strong El Nino event will be in place in the Pacific ENSO regions for this hurricane season. The latest 6-Month SST Forecasts continue to trend toward a healthy El Nino event.

First, a look back at some analysis that was posted in 2007:

"Definitions / Number of Events / Percentage:
ENSO Neutral: SST anomaly from -0.5 to +0.5 (33 seasons since 1950) 58%
Moderate El Nino: +0.6 to +0.9 (10 seasons) 18%
Strong El Nino: +1.0 or greater (2 seasons) 3%
Moderate La Nina: -0.6 to -0.9 (8 seasons) 14%
Strong La Nina: -1.0 or greater (4 seasons) 7%
Low activity: 8 named storms or less (20 seasons) 35%
Normal activity: 9 to 13 named storms (27 seasons) 47%
High activity: 14 named storms or more (10 seasons) 18%

The Region 3.4 SST pre-season aggregate average anomaly period: May/June/July. The strongest El Nino pre-season average anomaly for this period was +1.4 in 1997. The strongest La Nina pre-season average anomaly for this period was -1.2 in 1950 and 1988.

Neutral pre-season anomaly vs season totals:
7 low activity seasons / 21% of all Neutral seasons
17 normal activity seasons / 52%
9 high activity seasons / 27%

Moderate pre-season El Nino vs season totals:
9 low activity seasons / 90%
1 normal activity season / 10%
no high activity seasons

Strong pre-season El Nino vs season totals:
2 low activity seasons / 100%
no normal or high activity seasons

Moderate pre-season La Nina vs season totals:
2 low activity seasons / 25%
5 normal activity seasons / 63%
1 high activity season / 12%

Strong pre-season La Nina vs season totals:
no low activity seasons
4 normal activity seasons / 100%
no high activity seasons

When the M/J/J average SST anomaly is +0.6 or greater, 92% of the time the season storm total will be 8 named storms or less. When that average is -0.6 or greater, 92% of the time the season storm total will be 13 named storms or less (and 75% of the time the season storm total will be 9 to 13 named storms, i.e., a normal season. Note that a normal season, because of the increased activity since 1995, is now 11 named storms). A strong pre-season La Nina has never resulted in a high activity storm season. Ninety percent of high activity seasons occur under ENSO neutral pre-season conditions."

The M/J/J SST anomaly for this year will probably be in the +0.7 to +1.1 range - and if this should materialize, a lot of high pre-season storm total predictions are in trouble. With the Atlantic basin likely to remain cooler than normal, the expectation for a below normal season is greater than 90%. Current and anticipated ENSO conditions for a moderate to strong early season El Nino suggest that the best analog years for this season are 1957, 1965, 1982 and 1997 (in no particular order). A prediction of 7/3/1 might be a lot closer to the final total for this season.
Cheers,
ED

Edited by Ed Dunham (Tue Jul 07 2009 03:54 PM)

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