Tropical Cyclone Report

Tropical Storm Arthur

Arthur was a tropical storm, with wind speeds up to 50 kt, that moved east-northeastward across the western North Atlantic Ocean.

a. Synoptic History

The origin of Arthur was a weak low-level circulation first detected in the eastern Gulf of Mexico on July 9^th. This system was likely associated with a decaying frontal zone that had persisted in the area for several days. The circulation and associated low pressure meandered for a few days, then accelerated northeastward across the southeastern U.S. on the 13^th. This was in response to a mid-level trough amplifying southward along the U.S. east coast.

The circulation moved along the coast of South and North Carolina on the 14^th. By 1800 UTC of the 14^th, the circulation and its associated convection were well enough organized to become Tropical Depression One, centered about 40 n mi west-southwest of Hatteras, North Carolina. The "best track" of the tropical cyclone's path begins on the 14^th and is plotted in Figure 1. Figure 2 and Figure 3 are graphs of the wind and pressure histories as a function of time. Best track positions and intensities, every six hours, are listed in Table 1.

A mid-level low cut off from the westerlies and deepened as it dropped southward over the Canadian Maritimes. The depression responded by accelerating east-northeastward and slowly strengthening. It became a tropical storm on the 15^th when its winds increased to 35 kt. By the time Arthur strengthened to its estimated peak winds of 50 kt on the 16^th, it was centered about 350 n mi south of Nova Scotia and its forward speed was increasing to 35 kt. Arthur turned northward late on the 16^th around the above-mentioned low and became extratropical as it moved northward over eastern Newfoundland. Thereafter, the extratropical storm was tracked until the 19^th as it slowed and became nearly stationary between Newfoundland and Greenland and weakened below gale strength.

b. Meteorological Statistics

The best-track values of maximum 1-min, 10-m wind speeds, every six hours, are plotted in Table 2, along with satellite-based Dvorak technique wind speed estimates from the Tropical Analysis and Forecast Branch (TAFB), the Satellite Analysis Branch (SAB) and the U. S. Air Force Weather Agency (AFWA). Best-track minimum central pressures are plotted in Table 3, along with Dvorak technique pressure estimates. Selected ship and data buoy observations are also plotted in Figure 2 and Figure 3. In addition, several non-rain-contaminated ocean surface wind speeds in the 45- to 48-kt range were observed within Arthur's circulation by the QuikSCAT scatterometer on the 15^th and 16^th. A Canadian data buoy 44141 observed a pressure of 997.5 mb as the center passed about 40 n mi to its south and also reported an 8-min wind speed of 39 kt with a gust to 52 kt as the center passed. The ship WESTON reported 44 kt on the 16^th while located about 140 n mi south-southeast of the center of Arthur. Ship reports of wind speeds of 34 knots or greater are listed in Table 2.

Before Arthur became a tropical cyclone, its originating low pressure system spread heavy rain across portions of north Florida, Georgia and South Carolina on the 13^th. Later, Arthur's extratropical stage produced rainfall over Newfoundland on the 17^th ranging up to about one inch. Wind speeds along Newfoundland's east coast reached 34 kt at Bonavista on the 17^th.

c. Casualty and Damage Statistics

There were no casualties or damage reported in association with Arthur.

d. Forecast and Warning Critique

Arthur was a tropical storm for less than 48 hours. Average official track forecast errors (number of cases in parentheses) for Arthur are 80 (5), 138 (3), and 184 (1) nautical miles for the 12-, 24-, and 36-hour forecast periods, respectively. These errors are considerably larger than the average official track errors for the 10-year period 1992-2001 of 43, 80, and 115 nautical miles. This is due primarily to the large increase in forward speed which was not correctly anticipated by the primary track guidance models nor by the official forecast.

Average official intensity forecast errors were 4, 7, and 5 knots for the 12-, 24-, and 36-hour forecasts, respectively. For comparison, the average official intensity forecast errors for the 10-year period 1992-2001 were 7, 11, and 14 knots.

Figure 1: Best track positions for Tropical Storm Arthur, 14-16 July 2002. Arthur's extratropical stage track positions for 16-19 July are also plotted.

Figure 2: Selected wind speed observations and estimates and best-track maximum sustained surface wind speed curve for Tropical Storm Arthur, 14-16 July 2002. Arthur's extratropical stage best-track wind speeds for 16-19 July are also plotted.

Figure 3: Selected pressure observations and estimates and best track minimum central pressure curve for Tropical Storm Arthur, 14-16 July 2002. Arthur's extratropical stage pressures for 16-19 July are also plotted.