Washington, where it's located

The climate of Washington, D.C. (District of Columbia), the US federal capital, is semi-continental, with cold winters and hot and muggy summers. The city is located on the 38th parallel, in a way, between the cold and the warm part of the United States. In winter, it can be affected by cold air masses coming from Canada, but less frequently than New York; in summer, there are long periods of sunshine and hot weather, but also beneficial thunderstorms in the afternoon or in the evening which are able to relieve the heat of the day.
The following table shows the average temperatures of the Reagan National Airport, a few kilometers (or miles) south of the city center, in Arlington County, Virginia. Here, a mild microclimate is found because of the Potomac River, which widens to the point of becoming almost a lake, so night temperatures are a couple of degrees Celsius (3.5 °F) higher than in the peripheral districts and in the surrounding countryside.
Washington - Average temperatures
Min (°C)-2-13814192221171050
Max (°C)681319242931302620148
Min (°F)283037465766727063504132
Max (°F)434655667584888679685746

Precipitation in Washington is quite abundant, since it amounts to about 1,000 millimeters (40 inches) per year, and it's evenly spread over the seasons. In summer, the rains mainly occur in the form of afternoon or evening thunderstorms, however, some weather fronts may still pass, able to bring some rainy days. Here is the average precipitation.
Washington - Average precipitation

Winter, from December to February, is cold: the average temperature is just above freezing (0 °C or 32 °F). At night, temperatures are generally around freezing or slightly below, while during the day, they typically exceed freezing by a few degrees. The weather is unstable, in fact, there are sunny days followed by sudden cold air outbreaks, with snowfalls and temperatures around freezing, and wind that can exacerbate the feeling of cold. In an average year, about 40 centimeters (15.5 inches) of snow fall, while in Dulles Airport, which is actually located 37 km (23 mi) north-west of the city, in the state of Virginia, snowfall reaches 65 cm (26 in). Big snowstorms, which can block or hinder many human activities, called Snowmageddon, occur on average every 4/5 years. During the most intense cold waves, when the temperature remains below 0 °C (32 °F) even during the day, lows can plunge to -15 °C (5 °F) and sometimes as low as -20 °C (-4 °F).
However, even in winter, there may be some days, or periods lasting a few days, particularly mild, with temperatures reaching 15 °C (59 °F) and sometimes up to 20 °C (68 °F), which confirms the instability of the climate of Washington and in general of the American East Coast.

In spring, from March to May, the weather continues to be unstable: the first warm days alternate with the return of cold weather. In March, it can still snow, and sometimes, this can also happen in early April. But already in April, the temperature can sometimes exceed 25 °C (77 °F).
The National Cherry Blossom Festival lasts three weeks and starts on the last Sunday of March (therefore, it ends in mid-April), when the cherry trees from Japan, donated at the beginning of the 20th century by the mayor of Tokyo, are in bloom.
By May, the average temperatures are pleasant, although there are still cool and rainy days, especially in the first half, while the first heat waves can bring the temperature above 30 °C (86 °F). On sunny days, the first afternoon thunderstorms can erupt as well.

Summer, from June to August, is hot and muggy, especially in July and August. There can be days with pleasant temperatures, around 25/27 °C (77/81 °F), maybe with some rain, but more often, there are hot and sultry days, with highs around 30 °C (86 °F). There may also be heat waves, which can even last a week, with daytime temperatures reaching or exceeding 35 °C (95 °F) and night temperatures around 25/26 °C (77/79 °F). More rarely, as happened in July 2012, the temperature can reach 40 °C (104 °F). The amount of sunshine in summer is good, although in fine weather, thunderstorms can break out in the afternoon, in addition to some weather disturbances, which can bring clouds and rain for a few days.

Autumn, from September to November, is the mildest and the least unstable season of the year, at least in the first part. In the first half of September, the weather is often still characterized by heat and humidity, with sunshine and some afternoon thunderstorms, then it becomes milder and relatively quiet until mid-October, while in the second half of October, it can start to be a bit cold. November is a cold month, and the first snowfalls can occur. There a good number of rainy days also in autumn, but this is still the season when the rains are a little less frequent.

Lincoln Memorial from above

The amount of sunshine in Washington is good in summer, but it is not scarce even in winter because, as mentioned, some sunny days can always occur. On average, there are 9.5 hours of sunshine per day in June, 9 hours in July, and 4/4.5 hours in December and January. Here are the average sunshine hours per day.
Washington - Sunshine

The sea on the Atlantic Coast (see Dewey Beach, Ocean City) is never very warm for swimming, however, it reaches 23 °C (73 °F) in August. Here is the average sea temperature in Atlantic City, a little north of Ocean City.
Atlantic City - Sea temperature
Temp (°C)6446111722232117139
Temp (°F)433939435263727370635548

Along the coast to the east of Washington, in the Chesapeake Bay (see Chesapeake Beach, Annapolis), about an hour's drive from the city, the sea is a bit colder in winter and a bit warmer in summer, in fact, it reaches 23 °C (73 °F) in July and 24 °C (75 °F) in August.

Best Time

The best periods to visit Washington are spring and autumn, to avoid both the winter cold and the summer heat, and in particular, from mid-April to mid-May and from mid-September to mid-October. In early September, there is still often sticky weather, though not as hot as in midsummer. In April, it can sometimes get cold and the weather can be unstable, especially at the beginning of the month, while the second half of the month is generally mild.
In autumn (usually, around late October), you can admire the beautiful colors of the trees losing their leaves (the phenomenon called fall foliage).