Gert was a 130-knot hurricane that moved across the central north Atlantic
Ocean. It briefly produced hurricane force winds at Bermuda and high waves
along the southeast coast of Newfoundland.
a. Synoptic history
Gert originated from a tropical wave. When it moved from west Africa to the
Atlantic on 10 September, there was already convective banding and some
evidence of a low-level cloud circulation. The developing tropical
cyclone's path was toward the west-northwest from the 10th
to the 16th as it moved to the south of the subtropical high pressure ridge.
The best track starts on the 11th, south of the Cape Verde Islands.
This is when the system is estimated, using satellite imagery, to have
reached tropical depression status. The best track is listed in
Table 1 and is plotted in Figure 1.
Gert reached tropical storm strength on the 12th and became a hurricane
on the 13th. It continued to strengthen to 130 knots by the 16th.
There was a weakness in the ridge downstream ahead of Gert, an area over
which intense Hurricane Floyd had just moved. While intensifying, Gert's
movement responded to the weakness and slowly turned north and then
north-northeastward during 16th through 21st.
Its center remained about 300 nautical miles to the northeast of the Leeward
Islands of the Caribbean during this turn. With some fluctuations in
intensity, winds remained near 115 knots through the 19th, after
which weakening commenced. Gert weakened to 60 knots by the 23rd,
having moved to near southeastern Newfoundland. It then became
extratropical and merged with another extratropical low pressure system.
b. Meteorological statistics
The best track pressure and wind speed time series curves are shown in Figs.
2 and 3, along with plots of the data on which the curves are based. U.S.
Air Force Reserve Unit Hurricane Hunter aircraft monitored Gert on the
16th and 17th, while there was a potential threat to
the Leeward Islands, and again on the 20th and 21st,
while it threatened Bermuda.
Subjective Dvorak satellite wind speed estimates puts the time of Gert's
peak intensity of 130 knots near 0000 UTC on the 16th. About
24 hours of reconnaissance data starting about 0800 UTC on the 16th
indicates that the wind speed weakened to 120 knots and then reached 125
knots based on a GPS dropsonde measurement early on the 17th.
The center of Gert passed about 115 n mi east of Bermuda on the 21st.
The maximum 10-minute wind speed reported from airport at Bermuda was 39
knots with a peak gust to 64 knots. A gust to 76 knots was reported from an
exposed coastal location. There were three instances of one-min mean winds
speeds between 66 and 70 knots from a harbour location during the period
1032 - 1135 UTC on the 21st. The rainfall total for Gert was
Gale conditions were also experienced on the Avalon Peninsula on southeast
Newfoundland as Gert passed close by.
c. Casualties and damages
Bermuda experience some coastal erosion damage along the east and south
sides of the Island.
There were news reports of 27-foot high waves sweeping over the coast near
the southeast tip of Newfoundland. Three persons were swept into the water
while trying to secure their boast. All were rescued. Two persons drowned
on 20 September, when a large and unexpected wave swept them out to sea.
They had been standing at the water's edge at Schoodic Point in Acadia
National Park, Maine. The local Marine Patrol described the wave as a
"rogue wave". This event is believed to be related to large swells
generated by Gert, even though the hurricane was located more than 1000 n mi
south-southeast of Maine at the time.
d. Forecast and warning critique
A hurricane watch and tropical storm warning were issued for Bermuda at 2100
UTC on the 19th and a hurricane warning was issued at 1500 UTC on
the 20th. The hurricane warning was downgraded to a tropical storm warning
at 2100 UTC on the 21th and all warnings were discontinued at
2300 UTC on the 21th.
Average official track forecast errors ranged from 56 n mi (41 cases) at 24
hours to 111 n mi (37 cases at 48 hours to 188 n mi (33 cases) at 72 hours.
These errors are considerably lower than the previous 10-year averages,
which are 89-, 164-, and 242-n mi for 24, 48 and 72 hours and are due to
excellent model guidance from the GFDL, Navy NOGAPS, and UKMET models.
Also, there were synoptic-scale reconnaissance missions that provided data
for initializing guidance model runs on the 15th, 16th,
and 18th of September.
Prior to Gert's wind speed reaching 130 knots, there were some 72-hour
official intensity errors of 35-knot under-forecasts. Later when Gert was
weakening, there were some 35-knot over-forecasts of the wind speed at the
48- and 72-hour forecasts.
Figure 1. Best track for Hurricane Gert, September 1999.
Figure 2. Best track maximum-one-min-sustained-surface-wind-speed
time series for Hurricane Gert, September 1999. Aircraft flight level wind
speeds are adjusted to surface values as follows: 90% of 700-mb wind speed,
80% of 850-mb wind speed, and 85% of 1500-feet wind speed.
Figure 3. Best track minimum-central-surface-pressure vs. time curve
for Hurricane Gert, September 1999.
Table 1. Preliminary Best Track, Hurricane Gert, 11-23 September 1999.
|Lat. (°N)||Lon. (°W)
| 11/1200||12.6||24.2||1006||30||tropical depression
| 1200||14.2||31.9||1001||35||tropical storm
| 23/0000||42.2||55.6||964||60||tropical storm
| 1200||46.6||51.9||972||60||extratropical storm
| 1800||merged with another extratropical system
| 16/0000||17.8||50.8||930||130||minimum pressure