Preliminary Report
Hurricane Helene
19 - 30 September 1988

1988 PRELIMINARY REPORTS

Tropical Storm Alberto (TS)
Tropical Storm Beryl (TS)
Tropical Storm Chris (TS)
Hurricane Debby (1)
Tropical Storm Ernesto (TS)
Hurricane Florence (1)
Hurricane Gilbert (5)
Hurricane Helene (4)
Tropical Storm Isaac (TS)
Hurricane Joan (4)
Tropical Storm Keith

The forty second tropical wave of 1988 hurricane season that moved off the African Coast on 15 September became Tropical Depression Number Fourteen on 19 September and strengthened to Hurricane Helene by 21 September. Thereafter, the hurricane turned toward the northwest and developed to category four strength on the Saffir/Simpson Hurricane Scale by the 23rd. Helene the turned toward the north and began to weaken gradually.

 

a. Synoptic History

Deeply organized convection accompanied the tropical wave, that was to become Helene, as it moved off the African coast on 15 September. Showers and gusty winds of 23 to 35 mph were observed over the Cape Verde Islands as the wave passed through the area on 17 September. Strong ridging over the eastern Atlantic forced the developing system on a westerly track. Based upon satellite imagery, the wave became a tropical depression on 19 September and strengthened to Tropical Storm Helene the following day. A major trough in the mid Atlantic turned the storm toward the northwest on the 21st, and Helene immediately began began to deepen at a moderate rate.

The storm became Hurricane Helene on 21 September and, based solely on Dvorak technique satellite estimates, deepened to a minimum central pressure of 938 millibars at 1800 UTC on the 23rd. A secondary trough reinforced the previously mentioned mid Atlantic trough and turned Helene toward the north by the evening of the 24th. The hurricane gradually lost strength as it moved northward at a forward speed of 14 mph for the next several days. On 29 September Helene's forward motion accelerated to 35 mph as it approached the strong southwesterly jet stream. Helene's forward speed increased to 58 mph on 30 September as the hurricane became an extratropical storm and headed toward Iceland.

b. Meteorological Statistics

The minimum central pressure of Hurricane Helene occurred near 1800 UTC on 23 Septemberand was estimated to be 936 millibars by satellite meteorologists at the National Hurricane Center. However, Air Force Global Weather Center (AFGWC) meteorologists estimated a central pressure of 948 millibars at the same time. The official low pressure of 938 millibars for Helene is a compromise of these two estimates. Figure 2 provides an excellent example of the uncertainty involved in using satellite imagery to estimate the central pressure of tropical cyclones. Take note, at 0600 UTC NHC satellite meteorologists estimated the central pressure of the hurricane at 948 millibars, while AFGWC estimated a central pressure of 987 millibars. Thus, we have a difference in the satellite estimated central presuure in Helene of 39 millibars which translates to a difference of 58 mph.

Figure 2 also indicates the hurricane increased in strength on 28 September after several days of weakening. This strengthening occured during the time Helene was approaching the southern boundary of the very strong jet stream. It is surmised that strong difluence aloft as the hurricane approached the jet stream created a favourable outflow for Helene and resulted in the short term increased in the strength. Once the hurricane came under the strong southwesterly flow aloft, it began to lose strength due to shearing and accelerated to the northeast in excess of 35 mph. The interaction that occurs between tropical cyclones and jet streams is not well understood but is not only mentioned as a possible explantion for the short term increase in the hurricane's strength and organization.

c. Casualty and Damage Statistics

Since Helene formed over the open waters of the Atlantic and never came close to land, and no ships within 300 nautical miles of the cyclone reported winds of 58 mph of greater, it is assumed there were no casualties or damage produced by the hurricane.