Tywyn • Statistics for December

December: Peak Winter, Biggest Swell

December is Tywyn's biggest and most consistent month. The average swell height ties the annual high at 3.6ft (1.1m), and the average period reaches 8.4s, delivering punchy, powerful surf with occasional longer-period pulses as mature winter storms roll through. Swell from WSW accounts for 35.6 percent, SW for 23.5 percent, W for 11.2 percent and WNW for 7.5 percent, keeping the beach squarely in the productive winter window. It is also the heaviest month of the year: around 22 percent of all observations are 5ft (1.5m) or larger, including a meaningful share of 2-2.5m (6.5-8ft) and even a few waves over 8ft (2.5m) in the heaviest swells. This is when Tywyn can show real size.

Ideal offshore winds hold at 12 percent, thanks to occasional winter high pressure over Scandinavia producing E to NE flow. But the onshore westerly machine is also strongest now, with SW, WSW, W and WNW combining for 38 percent of the wind rose. Expect raw conditions: average air temperature 7.4°C, water temperature 9.3°C, and the highest monthly precipitation at 4.18 mm/day. December is demanding, cold, and at times intimidating, but it is also the peak of the surf season for those willing to suit up.