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Tropical Cyclone Report

Tropical Storm Ana

20 - 24 April 2003

Jack Beven
National Hurricane Center
19 December 2003

Ana was the first Atlantic tropical storm of record to form in April. It moved generally eastward across the central Atlantic.

a. Synoptic History

A non-tropical low formed about 210 n mi south-southwest of Bermuda on 18 April when an upper-level trough interacted with a surface frontal system. Moving generally northward, the low produced sporadic bursts of central convection starting early on 19 April. After turning northwestward, the low looped back toward the southeast early on 20 April. The central convection became better organized and the low separated from the frontal system. It is estimated the low became a subtropical storm at 0600 UTC 20 April about 215 n mi west of Bermuda. The "best track" chart of Ana'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.

Additional development occurred during its first day, and based on satellite microwave data showing a warm core it is estimated that Ana became a tropical storm near 0000 UTC 21 April with winds of 50 kt. This would be the peak intensity of the system. Ana turned east-southeastward on 21 April and eastward the next day while maintaining 45-50 kt winds. The system turned east-northeastward on 23 April while gradually weakening, and it merged with a cold front the next day about 810 n mi east of Bermuda.

Ana continued eastward as a 30-35 kt extratropical low through 25 April. It turned east-northeastward with some intensification on 26 April, then it was absorbed by a frontal system between the Azores and Portugal the next day.

Ana is the first tropical storm of record in April in the north Atlantic basin. The only other tropical or subtropical cyclone known in April is a subtropical storm between Puerto Rico and Bermuda from 21-24 April 1992.

b. Meteorological Statistics

Observations in Ana (Figure 2 and Figure 3) include satellite-based Dvorak and Hebert-Poteat 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). Microwave satellite imagery from the NOAA polar-orbiting satellites, the NASA Tropical Rainfall Measuring Mission (TRMM), the NASA QuikSCAT, and the Defense Meteorological Satellite Program (DMSP) satellites were also useful in tracking Ana.

Two ships reported tropical-storm force winds during Ana. The Atlantic Forest reported 44 kt winds and a pressure of 998.0 mb at 0900 UTC 22 April. The Rosa Delmas reported 41 kt winds at 0300 UTC 23 April.

Several aspects of Ana's best track are problematic. The first is the timing of the cyclone's transition from an extratropical to a subtropical cyclone. Satellite microwave wind data showed that a tight inner wind core - a step in that transition - formed early on 19 April. However, conventional satellite imagery showed that the cyclone was still attached to the frontal system at that time. The cyclone separated from the front near 0600 UTC 20 April and therefore is estimated to have become subtropical at that time.

A second issue is the time Ana transitioned to a tropical cyclone. Data from the Advanced Microwave Sounder Unit on the NOAA polar-orbiting satellites indicated that an upper-level warm core was present by late on 20 April. Based on this, Ana is estimated to have become tropical by 0000 UTC 21 April. However, experimental cyclone phase diagrams (Hart 2003) suggest that the transition could have occurred 6-18 h earlier.

A final issue regards the maximum intensity. Estimates from satellite imagery using the Dvorak and Hebert-Poteat techniques suggest maximum winds of 45 kt. However, data from the Seawinds scatterometer on the QuikSCAT satellite showed 55 kt winds west of the center at 2311 UTC 20 April. While the accuracy of this measurement is uncertain due to the possibility of rain contamination, it appears at least somewhat reliable when compared to other nearby Seawinds data. The maximum intensity is set to 50 kt in a compromise between the scatterometer winds and the intensity estimates from the Hebert-Poteat technique. There is a possibility that Ana could have been stronger than 50 kt at the time of that Seawinds overpass.

c. Casualty and Damage Statistics

Ana did not significantly affect any land areas, and there were no casualties or damages from its winds and rains. However, swells generated by Ana caused a boat to capsize at Jupiter Inlet, Florida on 20 April. Two people onboard drowned.

d. Forecast and Warning Critique

Average official track errors (with the number of cases in parentheses) for Ana were 37 (11), 46 (9), 79 (7), 123 (5), and 98 (1) n mi for the 12, 24, 36, 48, and 72 h forecasts, respectively1. These errors are lower than the average official track errors for the 10-yr period 1993-2002 (45, 81, 116, 150, and 225 n mi, respectively). These errors are also lower than those for the Climatology-Persistence method (58, 140, 269, 429, and 621 n mi for the 12, 24, 36, 48, and 72 h forecasts, respectively), and thus the forecasts have skill relative to that measure.

Average official intensity errors were 9, 9, 9, 9, and 10 kt for the 12, 24, 36, 48, and 72 h forecasts, respectively. For comparison, the average official intensity errors over the 10-yr period 1993-2002 are 6, 10, 13, 15, and 19 kt respectively.

No watches or warnings were issued for Ana.

References:

Hart, R.E., 2003: A cyclone phase space derived from thermal wind and thermal asymmetry, Monthly Weather Review, 131, 585-616.

1All forecast verifications in this report include the depression stage of the cyclone. National Hurricane Center verifications presented in these reports prior to 2003 did not include the depression stage.



Table 1: Best track for Tropical Storm Ana, 20-24 April 2003.
Date/Time
(UTC)
PositionPressure
(mb)
Wind Speed
(kt)
Stage
Lat.
(°N)
Lon.
(°W)
 18 / 0000 29.1 66.2 1008 30 extratropical
 18 / 0600 29.9 66.0 1008 30 "
 18 / 1200 30.8 65.8 1008 30 "
 18 / 1800 31.9 66.3 1007 30 "
 19 / 0000 33.0 66.9 1007 30 "
 19 / 0600 33.8 67.6 1006 35 "
 19 / 1200 34.5 68.6 1006 35 "
 19 / 1800 34.3 69.1 1006 35 "
 20 / 0000 33.6 69.0 1005 40 "
 20 / 0600 32.7 68.9 1003 40 subtropical storm
 20 / 1200 32.0 68.2 1000 45 "
 20 / 1800 31.7 67.3 998 45 "
 21 / 0000 31.4 66.4 995 50 tropical storm
 21 / 0600 30.6 65.8 995 50 "
 21 / 1200 30.0 64.8 995 50 "
 21 / 1800 29.4 63.1 994 50 "
 22 / 0000 29.4 61.5 994 50 "
 22 / 0600 29.5 60.1 994 50 "
 22 / 1200 29.6 58.5 995 45 "
 22 / 1800 29.7 56.6 996 45 "
 23 / 0000 30.0 54.7 998 40 "
 23 / 0600 30.6 53.0 1000 40 "
 23 / 1200 31.4 51.5 1000 40 "
 23 / 1800 31.9 50.6 1001 35 "
 24 / 0000 32.1 49.6 1003 35 "
 24 / 0600 32.0 48.7 1004 35 extratropical
 24 / 1200 31.8 47.8 1005 35 "
 24 / 1800 31.5 46.6 1005 35 "
 25 / 0000 31.5 45.4 1005 35 "
 25 / 0600 31.6 44.2 1004 35 "
 25 / 1200 31.7 43.1 1003 30 "
 25 / 1800 31.8 42.1 1004 30 "
 26 / 0000 31.9 40.8 1005 30 "
 26 / 0600 32.3 38.4 1002 35 "
 26 / 1200 32.9 35.6 1002 35 "
 26 / 1800 33.6 32.8 1002 40 "
 27 / 0000 34.4 29.2 1002 40 "
 27 / 0600 35.3 25.8 999 40 "
 27 / 1200 36.1 22.4 1000 40 "
 27 / 1800     absorbed by frontal system
 21 / 1800 29.4 63.1 994 50 minimum pressure
 22 / 0000 29.4 61.5 994 50 "
 22 / 0600 29.5 60.1 994 50 "

Best track positions for Tropical Storm Ana

Figure 1: Best track positions for Tropical Storm Ana, 20-24 April 2003.

Selected wind observations and best track maximum sustained surface wind speed curve for Tropical Storm Ana

Figure 2: Selected wind observations and best track maximum sustained surface wind speed curve for Tropical Storm Ana, 20-24 April 2003.Solid black bars denote wind ranges for the Hebert-Poteat subtropical cyclone technique. X's include surface winds from ships and the QuikSCAT scatterometer.

Selected pressure observations and best track minimum central pressure curve for Tropical Storm Ana

Figure 3: Selected pressure observations and best track minimum central pressure curve for Tropical Storm Ana, 20-24 April 2003.



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Page last modified: Wednesday, 26-Apr-2006 17:19:32 UTC