Ed Dunham
Former Meteorologist & CFHC Forum Moderator (Ed Passed Away on May 14, 2017)
Reged:
Posts: 2565
Loc: Melbourne, FL
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Re: La Nina Still Expected
Fri Jun 15 2007 12:51 PM
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I'll start with your last question first - there were really no other clues in the ENSO analysis of pre-season conditions that would indicate which way a season would ultimately go. Too many other variables influence the final result, e.g., windshear, dust and upper level lows (or the lack thereof). Of course post season analysis can be helpful with this - but that offers no help in determining the expected activity for a season (or even the next season). However, it can be of some help in the initial identification of analog years if you are able to make a reasonable expectation of what the ENSO conditions are likely to be during the season.
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 around -0.5 - but if it comes in at -0.6, a lot of high pre-season storm total predictions are in trouble.
ADDED: Current and anticipated ENSO conditions for a moderate La Nina along with similar upper air patterns suggest that 1964 might be the best analog for this season. The 1964 season had 12 named storms and 6 hurricanes - and quite interesting that all of the hurricanes that season were major ones.
Cheers,
ED
Edited by Ed Dunham (Sat Jun 16 2007 12:32 AM)
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