The Extremely Active 1995 Atlantic Hurricane Season: Environmental Conditions and Verification of Seasonal Forecasts

The 1995 Atlantic hurricane season was a year of near-record hurricane activity with a total of 19 named storms (average is 9.3 for the base period 1950–90) and 11 hurricanes (average is 5.8), which persisted for a total of 121 named storm days (average is 46.6) and 60 hurricane days (average is 23.9), respectively. There were five intense (or major) Saffir–Simpson category 3, 4, or 5 hurricanes (average is 2.3 intense hurricanes) with 11.75 intense hurricane days (average is 4.7). The net tropical cyclone activity, based upon the combined values of named storms, hurricanes, intense hurricanes, and their days present, was 229% of the average. Additionally, 1995 saw the return of hurricane activity to the deep tropical latitudes: seven hurricanes developed south of 258N (excluding all of the Gulf of Mexico) compared with just one during all of 1991–94. Interestingly, all seven storms that formed south of 208N in August and September recurved to the northeast without making landfall in the United States. The sharply increased hurricane activity during 1995 is attributed to the juxtaposition of virtually all of the large-scale features over the tropical North Atlantic that favor tropical cyclogenesis and development. These include extremely low vertical wind shear, below-normal sea level pressure, abnormally warm ocean waters, higher than average amounts of total precipitable water, and a strong west phase of the stratospheric quasibiennial oscillation. These various environmental factors were in strong contrast to those of the very unfavorable conditions that accompanied the extremely quiet 1994 hurricane season. The favorable conditions for the 1995 hurricane season began to develop as far back as late in the previous winter. Their onset well ahead of the start of the hurricane season indicates that they are a cause of the increased hurricane activity, and not an effect. The extreme duration of the atmospheric circulation anomalies over the tropical North Atlantic is partly attributed to a transition in the equatorial Pacific from warm episode conditions (El Niño) to cold episode conditions (La Niña) prior to the onset of the hurricane season. Though the season as a whole was extremely active, 1995’s Atlantic tropical cyclogenesis showed a strong intraseasonal variability with above-normal storm frequency during August and October and below normal for September. This variability is likely attributed to changes in the upper-tropospheric circulation across the tropical North Atlantic, which resulted in a return to near-normal vertical shear during September. Another contributing factor to the reduction in tropical cyclogenesis during September may have been a temporary return to nearnormal SSTs across the tropical and subtropical North Atlantic, caused by the enhanced tropical cyclone activity during August. Seasonal hurricane forecasts for 1995 issued at Colorado State University on 30 November 1994, 5 June 1995, and 4 August 1995 correctly anticipated an above-average season, but underforecast the extent of the extreme hurricane activity. Corresponding author address: Dr. Christopher W. Landsea, NOAA/AOML/Hurricane Research Division, 4301 Rickenbacker Causeway, Miami, FL 33149. E-mail: [email protected] MAY 1998 1175 L A N D S E A E T A L . TABLE 1. Summary of information on named tropical cyclones occurring during the 1995 Atlantic season. Information on tropical storms (TS—1-min surface winds of 18–32 m s21), minor hurricanes (MH—33–49 m s21), and intense hurricanes (IH—$50 m s21) with highest Saffir–Simpson (Simpson 1974) category is shown. Dates indicate the days in which the storm was at least tropical storm force in intensity. Individual totals of named storm days (NSD—days in which the storm has at least 18 m s21 winds), hurricane days (HD—days of winds at least 33 m s21), intense hurricane days (IHD—days of winds at least 50 m s21), and hurricane destruction potential (HDP—sum of winds squared every 6 h that the storm is of hurricane force in units of 104 kt2) are also provided. Named storm Maximum category Dates of named storm stage Maximum sustained surface winds (m s21) NSD HD IHD HDP 1. Allison 2. Barry 3. Chantal 4. Dean 5. Erin 6. Felix 7. Gabrielle 8. Humberto 9. Iris 10. Jerry MH-1 TS TS TS MH-2 IH-4 TS MH-2 MH-2 TS 3–5 Jun 7–10 Jul 14–20 Jul 30–31 Jul 31 Jul–4 Aug 8–22 Aug 9–11 Aug 22 Aug–1 Sep 22 Aug–4 Sep 23–24 Aug 33 31 31 21 44 62 31 49 49 23 2.50 3.25 6.75 0.50 4.50 14.00 1.50 10.00 12.75 1.25 0.75 0 0 0 2.50 9.25 0 8.50 7.50 0 0 0 0 0 0 1.75 0 0 0 0 1.3 0 0 0 5.3 24.2 0 21.0 17.6 0 11. Karen 12. Luis 13. Marilyn 14. Noel 15. Opal 16. Pablo 17. Roxanne 18. Sebastien 19. Tanya TS IH-4 IH-3 MH-1 IH-4 TS IH-3 TS MH-1 28 Aug–2 Sep 29 Aug–11 Sep 13–22 Sep 27 Sep–7 Oct 30 Sep–5 Oct 5–8 Oct 9–18 Oct 21–23 Oct 27 Oct–1 Nov 23 62 52 33 67 26 52 28 39 5.25 12.75 9.25 9.75 5.00 3.00 10.00 3.00 5.50 0 10.50 7.75 2.75 2.75 0 5.00 0 3.00 0 8.25 0.50 0 1.00 0 0.25 0 0 0 52.3 19.9 4.6 9.0 0 11.2 0 6.1