Do you need to know, when is the best time to go to Dominica? Then you should take a look here, what weather you can expect there during the year.
The climate of Dominica is tropical, the hot weather is moderated by northeast winds. There are heavy rainfalls sometimes. Some areas of Dominica are bordering with a tropical monsoon climate. Sometimes there are hurricanes during the summer.
When speaking about the usual weather in this state, the information is relevant for example for these popular places: Morne Trois Pitons National Park, Portsmouth, Roseau, Scott's Head.
Let's look now what weather you can expect in the area around the capital and in many other cities. The warmest month here is August, the average maximum temperature is 31.3 ℃ (88.34 ℉). The average minimum temperature in this month is 25.2 ℃ (77.36 ℉). The coldest month here is March, the average maximum temperature is 28.3 ℃ (82.94 ℉). The average minimum temperature in this month is 22.3 ℃ (72.14 ℉). 
When travelling to some places in Dominica, always consider the altitude of the the place as well. The highest point here is Morne Diablotins (1447 m) and the lowest point is Caribbean Sea (0 m). So the differences in altitudes in this country are not negligible and you should expect the weather to be different in places in different altitudes. Keep in mind, that the temperature usually decreases by 1.2 - 1.9 ℃ (2.2 - 3.5 ℉) per 200 m (656 ft), depending on the other conditions. (And we should add, that in the very special condition named thermal inversion even the temperature may be higher in higher altitudes.)
Typical (usual) weather in Dominica in January is this: The average highest temperature in Dominica in January is 29.2 ℃ (84.56 ℉). The average lowest temperature is 22.2 ℃ (71.96 ℉). At the beginning of January you can expect lower temperatures, the average highest temperature is around 28.85 ℃ (83.93 ℉). At the end of January you can expect lower temperatures, the average highest temperature is around 28.95 ℃ (84.11 ℉). The average rainfall is 161.6 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:13 (hours and minutes), in the middle of the month 11:18 and at the end of the month 11:26.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Dominica in February is this: The average highest temperature in Dominica in February is 28.7 ℃ (83.66 ℉). The average lowest temperature is 22 ℃ (71.6 ℉). At the beginning of February you can expect higher temperatures, the average highest temperature is around 28.95 ℃ (84.11 ℉). At the end of February you can expect lower temperatures, the average highest temperature is around 28.5 ℃ (83.3 ℉). The average rainfall is 107.1 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:26 (hours and minutes), in the middle of the month 11:37 and at the end of the month 11:50.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Dominica in March is this: The average highest temperature in Dominica in March is 28.3 ℃ (82.94 ℉). The average lowest temperature is 22.3 ℃ (72.14 ℉). At the beginning of March you can expect higher temperatures, the average highest temperature is around 28.5 ℃ (83.3 ℉). At the end of March you can expect higher temperatures, the average highest temperature is around 29 ℃ (84.2 ℉). The average rainfall is 132 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:50 (hours and minutes), in the middle of the month 12:03 and at the end of the month 12:16.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Dominica in April is this: The average highest temperature in Dominica in April is 29.7 ℃ (85.46 ℉). The average lowest temperature is 23.1 ℃ (73.58 ℉). At the beginning of April you can expect lower temperatures, the average highest temperature is around 29 ℃ (84.2 ℉). At the end of April you can expect higher temperatures, the average highest temperature is around 30.2 ℃ (86.36 ℉). The average rainfall is 121.8 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:16 (hours and minutes), in the middle of the month 12:29 and at the end of the month 12:40.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Dominica in May is this: The average highest temperature in Dominica in May is 30.7 ℃ (87.26 ℉). The average lowest temperature is 24.5 ℃ (76.1 ℉). At the beginning of May you can expect lower temperatures, the average highest temperature is around 30.2 ℃ (86.36 ℉). At the end of May you can expect higher temperatures, the average highest temperature is around 30.75 ℃ (87.35 ℉). The average rainfall is 218.6 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:40 (hours and minutes), in the middle of the month 12:51 and at the end of the month 12:58.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Dominica in June is this: The average highest temperature in Dominica in June is 30.8 ℃ (87.44 ℉). The average lowest temperature is 24.6 ℃ (76.28 ℉). At the beginning of June you can expect lower temperatures, the average highest temperature is around 30.75 ℃ (87.35 ℉). At the end of June you can expect lower temperatures, the average highest temperature is around 30.8 ℃ (87.44 ℉). The average rainfall is 158.4 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:58 (hours and minutes), in the middle of the month 13:02 and at the end of the month 13:02.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Dominica in July is this: The average highest temperature in Dominica in July is 30.8 ℃ (87.44 ℉). The average lowest temperature is 24.7 ℃ (76.46 ℉). At the beginning of July you can expect lower temperatures, the average highest temperature is around 30.8 ℃ (87.44 ℉). At the end of July you can expect higher temperatures, the average highest temperature is around 31.05 ℃ (87.89 ℉). The average rainfall is 181.5 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 13:02 (hours and minutes), in the middle of the month 12:58 and at the end of the month 12:50.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Dominica in August is this: The average highest temperature in Dominica in August is 31.3 ℃ (88.34 ℉). The average lowest temperature is 25.2 ℃ (77.36 ℉). At the beginning of August you can expect lower temperatures, the average highest temperature is around 31.05 ℃ (87.89 ℉). At the end of August you can expect lower temperatures, the average highest temperature is around 30.95 ℃ (87.71 ℉). The average rainfall is 242.1 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:50 (hours and minutes), in the middle of the month 12:40 and at the end of the month 12:28.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Dominica in September is this: The average highest temperature in Dominica in September is 30.6 ℃ (87.08 ℉). The average lowest temperature is 24.1 ℃ (75.38 ℉). At the beginning of September you can expect higher temperatures, the average highest temperature is around 30.95 ℃ (87.71 ℉). At the end of September you can expect lower temperatures, the average highest temperature is around 29.8 ℃ (85.64 ℉). The average rainfall is 298.1 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:28 (hours and minutes), in the middle of the month 12:16 and at the end of the month 12:03.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Dominica in October is this: The average highest temperature in Dominica in October is 29 ℃ (84.2 ℉). The average lowest temperature is 23.9 ℃ (75.02 ℉). At the beginning of October you can expect higher temperatures, the average highest temperature is around 29.8 ℃ (85.64 ℉). At the end of October you can expect higher temperatures, the average highest temperature is around 29.45 ℃ (85.01 ℉). The average rainfall is 341.5 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:03 (hours and minutes), in the middle of the month 11:50 and at the end of the month 11:38.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Dominica in November is this: The average highest temperature in Dominica in November is 29.9 ℃ (85.82 ℉). The average lowest temperature is 23 ℃ (73.4 ℉). At the beginning of November you can expect lower temperatures, the average highest temperature is around 29.45 ℃ (85.01 ℉). At the end of November you can expect lower temperatures, the average highest temperature is around 29.2 ℃ (84.56 ℉). The average rainfall is 383.4 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:38 (hours and minutes), in the middle of the month 11:27 and at the end of the month 11:18.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Dominica in December is this: The average highest temperature in Dominica in December is 28.5 ℃ (83.3 ℉). The average lowest temperature is 22.9 ℃ (73.22 ℉). At the beginning of December you can expect higher temperatures, the average highest temperature is around 29.2 ℃ (84.56 ℉). At the end of December you can expect higher temperatures, the average highest temperature is around 28.85 ℃ (83.93 ℉). The average rainfall is 245.2 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:18 (hours and minutes), in the middle of the month 11:13 and at the end of the month 11:12.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.