a. Synoptic History
Erika became a category three hurricane
on the Saffir/Simpson Hurricane Scale, was the only named
tropical cyclone of 1997 to form from a
tropical wave at low latitudes, and just missed the
islands of the northeastern Caribbean Sea.
Erika was first tracked as a tropical wave and large area of disturbed weather moving westward
from Africa to the eastern tropical Atlantic Ocean on 31 August. The system immediately showed
evidence of a large-scale low-level cyclonic turning. But it was not until 3 September, when located
about 1000 nautical miles east of the Lesser Antilles, that a low-level
center was defined well enough for it to be upgraded to a
tropical depression. The best track
begins at this time as shown in Fig. 1 (52K GIF) and as listed in
The depression quickly strengthened to tropical storm
Erika on the 3rd and to a hurricane on the 4th as it moved mostly west-northwestward at 15 knots or so
under the steering control of a well-established subtropical high pressure ridge. There was a hint of an
eye as infrared satellite imagery
showed a warm spot embedded in the deep convection over the center early on the 4th, but visible
satellite imagery later showed a partially exposed low level center. The strengthening of Erika to
a hurricane, based on drifting buoy data east of the Lesser Antilles, occurred under what appears to
be an unfavorable shearing situation. However, deep convection soon reappeared over the center
and strengthening continued, while Erika moved toward the west-northwest.
On the 5th through the 8th, the forward motion gradually decreased as the center of the
hurricane came within about 75 nautical miles to the northeast of the northeastern-most Lesser Antilles...just
far enough away for hurricane conditions to miss these islands. By the 8th, Erika had turned toward
the north with a movement of only five knots as an amplifying trough over the western north Atlantic
eroded the subtropical ridge and weakened the nearby steering currents.
Erika reached its peak intensity of 110 knots at 1800 UTC on the
8th and retained this wind speed for a period of about 24 hours, while it was located 300 nautical miles
north of the Caribbean islands and started to accelerate northward. Reconnaissance aircraft and satellite
imagery indicated an eye diameter of about 30 nautical miles during this time and the hurricane's radius of
tropical storm force wind speeds expanded to 250 nautical miles.
The hurricane passed about 300 nautical miles east of Bermuda on the 10th and became
embedded in westerly steering currents which caused a turn toward the east-northeast on the 11th
and 12th. By this time, weakening had commenced due to a combination of cool sea surface
temperatures and westerly winds aloft. Winds dropped below hurricane force on the 12th. However,
Erika periodically retained deep convection near its center for another four days along with wind
speeds between 45 and 60 knots while it moved mostly eastward across the
north Atlantic. The center passed very near the western-most Azores on the 15th and tropical storm
conditions were experienced in these islands. Erika then lost most of its deep convection and became
by the 16th. It continued moving northeastward for several more days, followed by dissipation on
the 20th while located about 200 nautical miles southwest of Ireland.
b. Meteorological Statistics
Figures 2 (22K GIF) and
3 (25K GIF)
show curves of minimum sea-level pressure and maximum one-minute surface
wind speed, respectively, as a function of time. Satellite data plotted in these
figures are based on the Dvorak satellite intensity estimating
technique as applied at the Tropical Analysis and Forecast Branch
(TAFB), the Synoptic Analysis Branch
(SAB) and the U.S. Air Force
Global Weather Center (AFGWC).
Ship reports of wind speeds of 34 knots or higher caused by the
hurricane are listed in Table 2. The large number of such
reports in northern latitudes is the result of Erika's path through the
north Atlantic shipping lanes.
The NOAA Gulfstream high-altitude jet flew missions which
resulted in data available for the 0000 UTC NCEP
model runs on the 4th and 5th. This was when Erika was threatening the Caribbean
islands and several days in advance of the recurvature across the
north Atlantic. This data set provides an opportunity to evaluate
the impact of synoptic-scale high-altitude dropsonde missions.
A NOAA drifting data buoy
reported a 60-knot wind speed at 1600
UTC on the 4th, when Erika was located some 500 nautical miles east
of the Lesser Antilles. The best track takes Erika to a hurricane
at 1800 UTC based on this report, although there is considerable
uncertainty about the accuracy of drifting buoy wind measurements
as well as the method used to adjust the wind speed to the 10 meter
There were no reports of tropical storm force or higher
sustained winds from the islands of the northeastern Caribbean as
Erika passed nearby. The highest report received was 32 knots
sustained wind speed with gusts to 41 knots from
St. Thomas in the U.S. Virgin Islands on the 7th, when Erika was centered 125
nautical miles north-northeast of this location. There were,
undoubtedly, stronger winds over the higher terrain of the islands
from the Virgin Islands east and southward through Antigua and
Montserrat. The largest rainfall total reported from the islands
is 3.28 inches from St. Thomas.
NOAA research aircraft
dropped GPS dropsondes into the eye
wall on the 7th and 8th, as Erika was strengthening to its maximum
intensity. While maximum 700-millibar flight level winds were near
110 knots late on the 8th, the vertical wind profile obtained near
the eye wall showed that wind speeds between 150 and 1500 meters
reached nearly 50 percent higher. The wind speed from this
dropsonde nearest to the surface (approx. 15 meters) was 117 knots
and the best track surface wind speed of 110 knots is based on this data.
The highest sustained surface wind report seen from the Azores was
26 knots with a gust to 39 knots at Lajes Air Base at 1900 UTC on
the 15th as Erika's center was passing 180 nautical miles to the northwest.
A report from Flores at 2300 UTC on the 15th gave a gust to
76 knots. A report from Lajes showed a gust to
91 knots from a 200-foot tower. A storm rainfall total of
2.35 inches was also reported from Flores.
c. Casualty and Damage Statistics
The only effects to Puerto Rico were from the large waves and
swells generated by the hurricane. Two surfers died in the northern and eastern
waters due to the high wave action. Most of the islands of the northeastern Caribbean suffered
minor damage from wave action and there was likely minor wind damage at higher elevations. The
general mood as expressed in the media was one of relief that a dangerous hurricane had turned north
and missed the islands.
The passage of the hurricane caused the lower-tropospheric winds to blow from the southwest
and advect a cloud of falling ash over Antigua from the active volcano in
d. Forecast and Warning Critique
There were 49 official forecasts issued while Erika was a
tropical storm or hurricane. The average track errors were 12, 46,
78, 99, 121, and 191 nautical miles at 0, 12, 24, 36, 48, and 72
hours. These numbers are small compared to the previous official
ten-year-average errors. The interpolated
GFDL model errors were
approximately 20 percent smaller than the official errors at 48 and
72 hours for about 30 simultaneous forecast cases. The official
track forecasts also had a left bias for the few days when Erika
was threatening the islands of the northeastern Caribbean Sea,
which was prior to the recurvature toward the north.
The official wind speed forecasts had a negative bias just
before Erika began strengthening toward its 110-knot maximum winds.
Negative errors of up to 45 knots for the 72-hour forecast issued
early on the 5th were the result of strengthening occurring under
what appeared to be a strong vertical wind shear environment.
A list of the various watches and warnings issued for this
hurricane is given in Table 3.
Hurricane warnings were issued for
a number of the island countries of the northeastern Caribbean Sea.
Although hurricane conditions did not occur in the warning area,
the storm passed sufficiently close to the islands to justify the
warnings. Although tropical storm warnings
were not issued for the Azores, National Hurricane Center advisories contained the
equivalent statement that tropical storm conditions were expected
to occur there.
Watch and warning summary, Hurricane Erika, September 1997.
||tropical storm watch issued
||Antigua, Montserrat, Barbuda, Nevis, St. Kitts, and Anguilla|
|tropical storm warning issued||Dutch St. Maarten|
|05/0300||tropical storm watch issued
||Guadeloupe, St. Martin, and St. Barthelemy|
||tropical storm warning issued
||Antigua, Montserrat, Barbuda, Nevis, St. Kitts, Anguilla, and Dominica|
|tropical storm watch issued
||U.S. and British Virgin Islands|
|05/1200||tropical storm warning issued
||Guadeloupe, St. Martin, and St. Barthelemy|
||hurricane warning issued
||Antigua, Montserrat, Barbuda, Nevis, St. Kitts, Anguilla, and Dominica; St. Martin and
St. Barthelemy; Dutch St. Maarten|
|hurricane watch issued
||U.S. and British Virgin Islands and Puerto Rico|
|06/1200||discontinue tropical storm warning
||Guadeloupe and Dominica|
|06/1800||discontinue hurricane warning
||Barbuda, Antigua, Nevis, St. Kitts, Montserrat, and Dominica|
|06/2100||discontinue hurricane warning
||St. Barthelemy, St. Martin, Dutch St. Maarten, and Anguilla|
|07/2100||discontinue hurricane warning
||U.S. and British Virgin Islands and Puerto Rico|