Tropical Storm Alberto (TS)
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
|Lat. (°N)||Lon. (°W)|
|23/1800||15.3||46.1||938||145||Category 4 Hurricane|