There is nothing unpredictable about Uruguay climate, due to its location within the temperate zone. Certainly, no country is extent of uncommon weather patterns, like anywhere else, but it's fairly safe to say that Uruguay is a year-round destination weather ways.
It has a reasonable stable weather nationwide. Although it does have pronounced seasonal variations, extremes in temperature are rare. so it never gets too hot or too cold.
In general, the coldest temperatures hover around freezing, with the occasional mark in the range of -4 or -5 °C. during winter time.
The coldest month is July, with an average low of 8ºC. Montevideo for instance, has winter average highs and lows of 14 and 6 °C (57.2 and 42.8 °F) respectively, buth high humidity makes the temperatures feel colder.
A disagreeable characteristic of Uruguay weather is high winds - particularly in spring and winter - due to the country's absence of mountains that act as natural barriers.
The Pampero - blows from the south, from the Argentinian Pampas and it can make temperatures feel a lot lower than they actually are. Conversely, summer may see the warm Zonda winds coming from the north.
It doesn't get unbearable hot in summer either. The warmest month of January and February may see a jump in temperature up to 40 °C in some parts of the country, Montevideo included, which has average highs and lows in summer 28 and 17 °C (82.4 and 62.6 °F), respectively.
Northwestern Uruguay, however, is farther from large bodies of water and therefore has warmer summers and milder and drier winters than the rest of the country.
As for rainfall, is evenly distributed throughout the year, with Montevideo averaging 1,235 millimeters (48.6 in) for the twelve month period.
Due to the number of beaches and seaside resorts the country has in store, the absolute best time to visit Uruguay is during summer month - December to March.
Being peak season, prices are also higher, so you could opt for the off peak season of April-May instead and still enjoy a lovely Uruguay weather wherever you travel to.
For more info regarding Uruguay climate and weather forecasts for the whole country, please visit the Dirección Nacional de Meteorología - Site in Spanish.