Tropical Cyclone Report

Hurricane Frances

Frances was a Cape Verde-type hurricane that reached a peak intensity of category 4 on the Saffir-Simpson Hurricane Scale. It affected the Bahamas as a category 3 hurricane and the Florida east coast as a category 2 hurricane.

a. Synoptic History

Frances developed from a vigorous tropical wave that moved westward from the coast of Africa on 21 August. Convection associated with the wave gradually became better organized, and the first Dvorak satellite intensity estimates were made on 24 August. A tropical depression formed from the wave near 0000 UTC 25 August about 655 n mi west-southwest of the southern Cape Verde Islands. The "best track" chart of the tropical cyclone's path is given in Figure 1, with the wind and pressure histories shown in Figure 2 and Figure 3, respectively. The best track positions and intensities are listed in Table 1.

The depression moved westward on the south side of the Bermuda-Azores high and intensified, becoming a tropical storm later on 24 August. Frances turned west-northwestward on 26 August as it became a hurricane, and that heading and the intensification continued until 28 August, when Frances reached a first peak intensity estimated at 115 kt. The hurricane turned westward late on 29 August while it slowly weakened during a concentric eyewall cycle. Re-intensification began on 30 August, and Frances reached a second peak intensity of 125 kt (category 4) late on 31 August as it passed north of the Leeward and Virgin Islands. The hurricane moved west-northwestward on 1-2 September, which brought the center just north of the Turks and Caicos Islands and the southeastern Bahama Islands. The maximum winds remained 120-125 kt during this time while the hurricane underwent a series of concentric eyewall cycles. Moderate westerly vertical shear developed later on 2 September, and Frances weakened notably during the next two days. It was a category 3 hurricane, with winds of 100-110 kt, over the central Bahama Islands on 2-3 September and a category 2 hurricane, with winds of 85-90 kt, over the northwestern Bahamas on 3-4 September.

Steering currents weakened as Frances reached the northwestern Bahamas due to a high pressure ridge building west of the cyclone. This caused storm moved slowly westward across the Gulf Stream on 4 September. The shear weakened, which allowed slight re-intensification over the Gulf Stream, followed by slight weakening just before Frances made landfall over the southern end of Hutchinson Island, Florida near 0430 UTC 5 September as a Category 2 hurricane. Frances gradually weakened as it moved slowly west-northwestward across the Florida Peninsula, and became a tropical storm just before emerging into the northeastern Gulf of Mexico near New Port Richey early on 6 September.

Frances did not strengthen over the Gulf, with maximum sustained winds remaining 50-55 kt with a pressure near 982 mb. It moved northwestward and made a final landfall near the mouth of the Aucilla River in the Florida Big Bend region about 1800 UTC 6 September. The northwestward motion continued until 7 September, when Frances re-curved northeastward into the westerlies over eastern Alabama and western Georgia. Frances weakened to a tropical depression early on 7 September and then became extratropical over West Virginia early on 9 September. As an extratropical cyclone, Frances briefly had gale-force winds as it accelerated northeastward across New York later on 9 September. The cyclone turned eastward across northern New England and southeastern Canada, dissipating over the Gulf of St. Lawrence late on 10 September.

b. Meteorological Statistics

Observations in Frances (Figure 2 and Figure 3) include satellite-based Dvorak technique intensity estimates from the Tropical Analysis and Forecast Branch (TAFB), the Satellite Analysis Branch (SAB) and the U. S. Air Force Weather Agency (AFWA), as well as flight-level and dropwindsonde observations from flights of the 53^rd Weather Reconnaissance Squadron of the U. S. Air Force Reserve Command. Microwave satellite imagery from NOAA polar-orbiting satellites, the NASA Tropical Rainfall Measuring Mission (TRMM), the NASA QuikSCAT, the NASA Aqua, and Defense Meteorological Satellite Program (DMSP) satellites were also useful in tracking Frances. Additionally, WSR-88D Doppler radars in Puerto Rico and the southeastern United States provided detailed data on Frances, as did a U. S. Department of Defense radar located in the Bahamas.

The Air Force Reserve and NOAA Hurricane Hunters flew 34 operational missions for Frances, including standard reconnaissance and synoptic surveillance. The aircraft made a total of 59 formal fixes of Frances - 56 from the Air Force and 3 from NOAA. A NOAA aircraft reported winds of 144 kt at a flight level of 8000 ft at 1726 UTC 31 August, while Air Force aircraft measured winds of 138 kt at 700 mb at 1114 UTC 31 August and again at 0543 UTC 2 September. A dropwindsonde in the northern eyewall reported winds peak of 166 kt at a level of 904 mb near 0539 UTC 2 September. The lowest aircraft-measured pressure was 935 mb at 0712 UTC 1 September, with a 936 mb pressure reported at 0718 UTC 2 September. Several research flights also occurred, with one such mission deploying a series of drifting buoys that reported useful pressure and sea surface temperature data from Frances as well as the subsequent Hurricane Jeanne.

Frances brought hurricane conditions to much of the central and northwestern Bahamas, southeastern Florida, and the adjacent Atlantic. Ship reports of winds of tropical storm force associated with Frances are given in Table 2, and selected surface observations from land stations and data buoys are given in Table 3. The maximum sustained wind reported from a land station was 87 kt at North Eleuthera in the northwestern Bahamas at 1000 UTC 3 September. The Coastal Marine Automated Network (C-MAN) station at Settlement Point on Grand Bahama Island reported a 10-minute mean wind of 73 kt at 2320 UTC 4 September and a peak gust of 96 kt. San Salvador in the Central Bahamas reported a peak gust of 104 kt at 1900 UTC 2 September. In Florida, an Army Corps of Engineers (USACE) station at Port Mayaca reported sustained winds of 74 kt at 0500 UTC 5 September, while a portable instrumented tower run by the Florida Coastal Monitoring Program (FCMP) at Ft. Pierce reported 70 kt sustained winds at 0402 UTC 5 September along with a peak gust of 94 kt. Unofficial reports include a sustained wind of 70 kt from the Jupiter police department and a gust to 94 kt in Martin County. It should be noted that a gust of 108 kt at Port Canaveral that was reported operationally was found to be unreliable in post-analysis.

Shipping mostly avoided Frances as it crossed the Atlantic. The highest winds reported by a ship were 43 kt from a ship with the call sign C6FV9 at 1200 UTC 5 September. Two drifting buoys reported sustained winds of 78 kt, although the reliability of these reports is uncertain.

The lowest reported pressure from a land station was 948.1 mb at San Salvador in the Bahamas at 2000 UTC 2 September. North Eleuthera reported a minimum pressure of 958.6 mb at 1400 UTC 3 September, while Settlement Point reported a minimum pressure of 963.2 mb at 1600 UTC 4 September. In Florida, the USACE station at St. Lucie lock reported a minimum pressure of 962.1 mb at 0600 UTC 5 September, while the FCMP tower at Port Salerno reported a minimum pressure of 962.8 mb at 0550 UTC that day. A storm chaser on southern Hutchinson Island reported an unofficial pressure of 959.0 mb at 0525 UTC that day, while a second storm chaser at Sewall's Point reported a pressure of 962.0 mb at 0345 UTC that day. Additionally, a drifting buoy measured a pressure of 955.2 mb at 0307 UTC 2 September.

Frances produced notable storm surges along both the Atlantic and Gulf coasts of Florida. The highest measured storm surge was 5.89 ft above mean sea level on the Florida east coast at the St. Lucie Lock. The National Weather Service Melbourne Weather Forecast Office (WFO) estimated storm surge at 8 ft near Vero Beach and 6 ft around Cocoa Beach. Lesser storm surge values were estimated elsewhere along the Florida east coast, while storm surges of 1-2 ft above normal tide levels were reported as far north as the Georgia coast. Along the Gulf coast, a storm tide of 6 ft was estimated in Pinellas County, FL, while storm tides of 3-5 ft were estimated in the Florida Big Bend area.

Frances also produced significant storm surge on several of the Bahama Islands, which the inundated the airports at Freeport, Grand Bahama, and Marsh Harbor, Abaco. However, exact surge values are not available.

Frances caused widespread heavy rains and associated freshwater flooding over much of the eastern United States (Figure 4). The maximum reported rainfall was 18.07 in at Linville Falls, North Carolina (Table 4). This was part of a swath of rains in excess of 10 in along the Appalachian Mountains in western North Carolina and northeastern Georgia. Rainfalls in excess of 10 in also occurred over large portions of the central and northern Florida Peninsula and southeastern Georgia, with storm totals of 15.84 in at High Springs, FL, 15.81 in near Chassahowitzka, FL, and an unofficial total of 15.08 in at Port Orange, FL (Table 3). Storm-total rainfalls of 5-10 in were common elsewhere along Frances' track as a tropical cyclone, with reports of 3-6 in totals along the extratropical portion of the track.

A total of 101 tornadoes have been reported in association with Frances - 23 in Florida, 7 in Georgia, 45 in South Carolina, 11 in North Carolina, and 15 in Virginia. Many of the tornadoes occurred in an outbreak over South and North Carolina on 7 September.

c. Casualty and Damage Statistics

Frances is directly responsible for seven deaths - five in Florida, one in the Bahamas, and one in Ohio. Three deaths were caused by wind, two by storm surge, one by freshwater flooding, and one by lightning. The hurricane is indirectly responsible for 42 deaths - 32 in Florida, 8 in Georgia, 1 in the Bahamas, and 1 in Ohio.

The American Insurances Service Group reports that Frances causes $4.43 billion in damage to insured property in the United States, with $4.11 billion occurring in Florida. Applying a two-to-one ratio to this figure to account for damage to uninsured property yields a damage estimate of $8.86 billion. Additionally, space and military facilities in the Cape Canaveral area reported over $100 million dollars in property damage. Therefore, the best estimate of the total property damage from Frances is $9 billion, which in terms of unadjusted damage makes Frances the fourth most costly hurricane in United States history behind Andrew of 1992, and Charley and Ivan of 2004. This total does not include agricultural or economic losses. Widespread property damage also occurred in the central and northwestern Bahama Islands. However, no monetary damage figures are available at this time.

d. Forecast and Warning Critique

The overall track forecast errors for Frances were exceptionally good. Average official track errors (with the number of cases in parentheses) were 20 (53), 36 (53), 52 (53), 66 (52), 80 (48), 100 (44), and 128 (40) n mi for the 12, 24, 36, 48, 72, 96, and 120 h forecasts, respectively. These errors are 50-70% lower than the average official track errors for the 10-yr period 1994-2003^[1] (44, 78, 112, 146, 217, 248, and 319 n mi, respectively), (Table 5). Not only are the average track forecast errors outstanding, but only five of the individual track forecasts at any time exceeded the long-term average errors - two 12-h forecasts, one 36-h forecast, and two 96-h forecasts. As good as the official track forecasts were, they were bettered by some of the numerical guidance. The lowest track forecast errors amongst the guidance were from the Florida State University Superensemble (FSSE), which on average outperformed the official forecast at all times.

Some of the low track forecast errors can be attributed to Frances' long track south of the subtropical ridge and the occasionally slow motion, both of which favor low track forecast errors. However, the forecast errors stayed relatively low through the time where Frances slowed near the Bahamas and turned northwestward across Florida - which were tougher forecast scenarios.

The average intensity forecast errors were also smaller than normal. Average official intensity errors were 2, 7, 10, 13, 15, 14, 14, and 16 kt for the 12, 24, 36, 48, 72, 96, and 120 h forecasts, respectively. For comparison, the average official intensity errors over the 10-yr period 1994-2003 are 6, 10, 12, 15, 19, 20, and 21 kt, respectively. While the average errors were low, there were two periods of rather large forecast errors. The first was early in Frances' life when the amount of intensification was underforecast. The second was as Frances approached the Bahamas when the impact of the vertical shear was underestimated, resulting in a series of forecast intensities that were too strong.

Table 6 shows the watches and warnings issued for Frances. A hurricane watch was issued for the southeastern Bahamas and the Turks and Caicos Islands at 0900 UTC 31 August, with a hurricane warning issued 6 h later. These were 33 and 27 h respectively before the closest approach of the center to the Turks and Caicos. A hurricane watch was issued for the central Bahamas at 1500 UTC 31 August, while a hurricane warning was issued at 1500 UTC 1 September. These were 52 and 28 h respectively before the arrival of the center at San Salvador Island. A hurricane watch was issued for the northwestern Bahamas at 1500 UTC 1 September, with a hurricane warning issued at 0000 UTC 2 September. These were 43 and 34 h respectively before the arrival of the center at Eleuthera Island.

In the United States, a hurricane watch was issued for the east coast of Florida at 0300 UTC 2 September, with a hurricane warning following at 1500 UTC that day. These were 73 and 61 h respectively before the center reached Hutchinson Island. Along the Gulf coast, a tropical storm watch was issued for the eventual landfall area at 1500 UTC 4 September, with a tropical storm warning following 6 h later. These were 51 and 45 h respectively before the center reached the Aucilla River.

A series of watches and warnings were issued for the islands of the northeastern Caribbean in anticipation that Frances would come close enough to produce tropical storm conditions. Also, a hurricane warning was issued for portions of the Florida Gulf coast in anticipation that Frances would maintain hurricane strength all the way across the Florida Peninsula, or re-intensify over the Gulf of Mexico. These watches and warnings did not verify.

Acknowledgements

Much of the data for this report was supplied by the National Weather Service WFOs in Key West, Miami, Tampa, Melbourne, Jacksonville, and Tallahassee FL, as well as by the South Florida Water Management District, the St. John's River Water Management District, and the Florida Automated Weather Network (FAWN). NOAA buoy and C-MAN data were provided by the National Data Buoy Center. NOS data were provided by the NOAA National Ocean Service. Remote Automated Weather Stations (RAWS) data were provided by the National Interagency Fire Center. United States Geological Survey (USGS) data were provided by the NWISWeb web site. The rainfall map and much of the supplementary rainfall data was provided by David Roth of the Hydrometeorological Prediction Center. Much of the tornado data was provided by the Storm Prediction Center. Several of the unofficial observations were obtained from the Weather Underground web site.

Figure 1: Best track positions for Hurricane Frances, 25 August - 8 September 2004.

Figure 2: Selected wind observations and best track maximum sustained surface wind speed curve for Hurricane Frances, 25 August - 8 September 2004. Aircraft observations have been adjusted for elevation using 90% and 80% reduction factors for observations from 700 mb and 850 mb respectively. Dropwindsonde observations include actual 10 m winds (sfc), as well as surface estimates derived from the mean wind over the lowest 150 m of the wind sounding (LLM), and from the sounding boundary layer mean (MBL). Objective Dvorak estimates represent linear averages over a three-hour period centered on the nominal observation time. Vertical black lines indicate times of landfalls.

Figure 3: Selected pressure observations and best track minimum central pressure curve for Hurricane Frances, 25 August - 8 September 2004. Objective Dvorak estimates represent linear averages over a three-hour period centered on the nominal observation time. Vertical black lines indicate times of landfalls.

Figure 4: Storm-total rainfalls for Hurricane Frances, 25 August - 8 September 2004. Figure courtesy of David Roth of the Hydrometeorological Prediction Center.