Preliminary Report
Hurricane Isidore
04 - 17 September 1990

1990 PRELIMINARY REPORTS

Tropical Storm Arthur (TS)
Hurricane Bertha (1)
Tropical Storm Cesar (TS)
Hurricane Diana (2)
Tropical Storm Edouard (TS)
Tropical Storm Fran (TS)
Hurricane Gustav (3)
Tropical Storm Hortense (TS)
Hurricane Isidore (2)
Hurricane Josephine (1)
Hurricane Klaus (1)
Hurricane Lili (1)
Tropical Storm Marco (TS)
Hurricane Nana (1)

a. Synoptic History

Isidore developed from a vigorous tropical wave which moved off the African coast on 3 September. Due to the time of year and the location it appeared for a while that the system was going to be a typical Cape Verde hurricane. However, a persistent mid-level trough in the central Atlantic caused the storm to move in a general north-northwest course over open waters.

Satellite classifications began on 4 August when METEOSAT images clearly indicated a large and well defined low level circulation with deep convection. On 0000 UTC 5 September the system was a strong depression and by 1200 UTC on the same day it was a tropical storm. Isidore began strengthening rapidly and based on satellite wind estimates became a hurricane at 0000 UTC 7 September. The hurricane reached its maximum intensity on that morning when objective T-numbers oscillated around 5.0 on the Dvorak scale. On the same day, an unnamed vessel reported winds of 75 mph from the northeast with gusts to 92 mph and a surface pressure of 997 mb. There were some uncertainties with this ship report because its given location was about 130 nautical miles to the west of the center.

As the hurricane moved in a general north-northwest course it encountered strong upper-level winds induced by the same trough which was steering the hurricane northward. Due to the strong shearing Isidore was downgraded to tropical storm status at 1800 UTC 8 September. However, an upper low developed along that trough and moved southward producing a better environment for Isidore to re-intensify. As the shear relaxed, an eye developed within a circular mass of convection and once again the system was upgraded to a hurricane until it moved over cooler waters and weakened several days later.

Isidore always moved in a general north-northwest motion but several short waves raced northeastward in advance of the hurricane producing various meanders in Isidore's track. Isidore was absorbed by a large extratropical low on 17 September north of 50°N.

b. Meteorological Statistics

Based of ship reports, the area of gale force winds expanded to more than 400 nautical miles in the northern semicircle when the hurricane began to interact with the mid-latitude system.

c. Casualty and Damage Statistics

There were no reports of casualties or damage associated with Isidore.