Preliminary Report
Hurricane Grace
25 - 29 October 1991

1991 PRELIMINARY REPORTS

Tropical Storm Ana (TS)
Hurricane Bob (3)
Hurricane Claudette (3)
Tropical Storm Danny (TS)
Tropical Storm Erika (TS)
Tropical Storm Fabian (TS)
Hurricane Grace (2)

a. Synoptic History

Analyses of upper-air data show that the incipient circulation of Grace formed primarily from a mid-level low between Bermuda and the Dominican Republic on 23 October. The circulation slowly developed downward. A 12 mile west (270°) wind report from the German Ship "Holtsencarrier" indicated that the low reached the surface a little to the north of the ship by 1800 UTC on the 25th.

Grace was initially subtropical in character. Several observations support this classification. First, the surface low center was cloud-free when it formed and remained generally without deep convection until the 27th. Second, when cloudiness near the center began to increase, it consisted of several low to mid-level cloud swirls, indicating a broad circulation with light winds near the center. This was confirmed by a series of 3-hourly observations from a ship (tentatively identified as the Walter Jacob from Germany). The ship encountered fairly light winds near the center. In contrast, observations from the ship show that 35 mile winds blew at large (100-300 nautical miles) radii, both east and west of the center. Wind speeds of similar strength occurred well-north of the center, and increased to 39 miles at Bermuda and nearby ships by the 26th. The best track reflects these reports by showing that the system intensified from a subtropical depression to a subtropical storm at 0600 UTC on the 26th. Finally, all satellite classifications during this period by the NHC Tropical Satellite Analysis and Forcast Unit, the Air Force Global Weather Center (AFGWC), and the NESDIS Satellite Analysis Branch (TSAF, USAF and SAB, respectively in figs 2 and 3) were subtropical.

Cloudiness near Bermuda, which originated a few days earlier near the U.S. southeast coast, became increasingly convective and banded as it was enveloped by the expanding circulation. The clouds gradually wrapped inward toward the circulation center from the west between 25 and 27 October. Convection began to persist near the center at about 1800 UTC on the 27th. The first reconnaissance flight into the system, by an Air Force Reserve Unit aircraft, also occured then and encountered winds which had become strongest near the circulation center. These characteristics are consistent with the pressure of a tropical system and (although AFGWC continued making subtropical classifications throughout the system's lifetime) are used as an indication that the subtropical storm had transformed into Tropical Storm Grace.

The organization and concentration of convection continued to increase near the center late on the 27th. Based on satellite intensity estimates and a reconnaissance report estimating surface wind speeds of 75 miles, Grace was upgraded to a hurricane on the best track at 0000 UTC 28 October, although flight-level winds did not reach that threshold for about another 36 hours.

Grace initially followed a sinuous course toward the northwest at about 12 miles. As it did so, an extratropical cyclone formed well to the north, off the New England coast. The extratropical cyclone rapidly became powerful and the strong, mostly westerly deep-layer mean flow around its southern flank quickly came to dominate the steering of Grace. The hurricane slowed and made a hairpin turntoward the east on 28 Ocober. Grace then accelerated markedly, from about 3 miles at midday on the 28th to around 46 miles only 24 hours laters.

Reconnaissance reports of near 81 miles surface and flight-level winds just west of the center imply that Grace still had a storm-relative circulation of about 81 miles on the morning of the 29th. However, the rapid eastward motion, superimposed on this circulation, produced a very asymmetrical surface wind distribution. Relatively light winds occurred to the north of the center. Winds of only about 23 miles were observed at Bermuda as the hurricane center passed by about 45 nautical miles to the south near 0800 UTC. On the other hand, very strong winds were present to the south of the center. When the reconnaissance aircraft exited the hurricane center toward the south near 1400 UTC, it encountered wind speeds at 850 mb that exceeded 115 miles along about a 15 nautical mile segment of the flight track centered about 30 nautical miles south of the center. The maximum flight-level wind speed was 128 miles. Surface wind estimates were near 115 miles. Using this data, the maximum intensity of Grace is estimated to have been 104 miles at 1400 UTC 29 October.

Grace turned toward the east-northeast during the afternoon of 29 October and was overtaken by a vigorous cold front associated with the extratropical cyclone. The merger occurred near 1800 UTC 29 October, when the rapidly-moving cold front undercut and quickly destroyed Grace's low-level circulation. Animation of satellite imagery showed that the remnant mid and upper-level moisture from Grace became caught up in the outer part of the extratropical cyclone circulation, far from the extratropical storm center. The remnants became indistinguishable within a day of the merger.

b. Meteorological Statistics

Bermuda reported tropical (or subtropical) storm force winds on 3 hourly and 1 immediate observations, all more than 15 hours prior to the closest approach of the hurricane, and before passage of the intense cold front at about 0810 UTC 29 October. The highest of those winds was 41 miles at 0300 UTC on the 27th. The peak gust of 61 miles occurred during the previous hour.

c. Casualty and Damage Statistics

There were no reports of damage or casualties on Bermuda due to Grace.

Grace was a relatively large system, owing to its early subtropical nature. It generated large swells, from about 15 feet at buoy 41001 offshore from North Carolina to about 10 feet near the Florida coast. Because these swells were noted near the U.S. southeast coast prior to the passage of the cold front they are attributed directly to Grace. Those waves, in conbination with spring tides, caused isolated minor beach erosion, but no reported significant damage or casualties from North Carolina southward through Florida.

As the extratropical cyclone intensified, Grace became only a secondary contributor to subsequent extroadinary sea conditions reported over much of the western North Atlantic during the last week of October and first few days of November. Treacherous swells, surf, and associated coastal flooding occurred behind the cold front along portions of the Atlantic shoreline extending from Puerto Rico and the Dominican Republic, to the Bahamas, the United States, Canada and Bermuda. While irrefuctable evidence is not available, there were probably no casualties or damage then which could be attributed directly to Grace. Instead, it is beleived that the much larger extratropical cyclone was the primary phenomenon at blame. Gale force winds with gusts to 69 miles covered a huge area behind the cold front. Along Bermuda's north shore they destroyed 4 or 5 boats and caused minor damage to vegetation. Sustained winds of 92 miles and open-water heights to 101 feet (Canadian buoy 44137) were reported over the northwestern Atlantic, well to the north of the relatively more local influence of Grace.