Being exposed to predominantly westerly winds, Snowdonia is a notoriously mild, wet region with rainfall greatest in the western half of the area on the highest ground. Snowdon itself is believed to average 200 inches of rainfall a year, comparable to the English Lake District or the western highlands of Scotland. The months from October to January are significantly wetter than those between February and September. Generally speaking Spring in Snowdonia is drier than summer with August being particularly wet.
Warmer weather tends to be found nearer the coast. Hill fog is often encountered when hiking around Snowdonia and the summit of Snowdon is often shrouded in mist - so check forecasts before venturing out and be prepared. It can be extremely cold on the high ground with easterly winds in winter and snow and ice is a common feature inland from the coast.
Obviously under these conditions personal safety is hugely important. There is no substitute for experience when tackling the mountains, so make sure you are with someone who knows the area and/or make sure you have the necessary maps and navigation skills.
With the weather in Snowdonia being so unpredictable and changeable, the Met Office provide a dedicated webpage for weather in Snowdonia. The information includes a hazards list, a brief summary of the weather on the hills, an indication of height and extent of the lower cloud, visibility, wind and temperatures on the hills, freezing level at or below summit level and a general outlook for the next few days. The forecasts are issued daily. The Met Office also provide a 24 hour mountain weather forecast service.
The BBC website not only provides weather forecasts for different regions in Wales including North West Wales, but also a dedicated Mountain Weather Forecast for Snowdonia. There are great links for further information on all things outdoor including walking, caving, climbing, and abseiling. There are also links to webcams, including one for Snowdon so you can check to see if you're likely to be able to see the summit when you're there!
You need to layer up with clothing to cater for any eventuality. This should include warm, windproof and waterproof garments. Cotton shirts are cold when wet with sweat, modern thermal vests are useful all year round. Wear comfortable boots that will protect your feet on all terrain. Take hat, gloves and a spare fleece; it is always colder on the tops. You will need map and compass in case your way is not marked.
Take a reliable watch, whistle, and torch (six blasts or flashes repeated at minute intervals signal an emergency). A survival bag or modern light-weight emergency shelter can also be invaluable. Mobile phones and GPS are useful tools but must not be relied upon entirely as you can often be out of signal around the mountains. A basic First Aid Kit can be useful. For the more adventurous in winter conditions, an ice-axe and crampons may be needed and climbers are urged to wear helmets.
Eat well before you start, cereals & carbohydrates release energy slowly and constantly throughout the day. In addition to sandwiches, take energy food such as chocolate, dried fruit, or glucose which restore energy quickly. Warm drinks do wonders for morale. Streams on mountains are drinkable if fast-running over stony beds. There is safety in numbers, but groups should ensure that party leaders have sufficient and relevant experience.
Do not leave anyone behind. Discuss and agree a contingency plan with everyone. Take special care of anyone lagging behind or less able at all times but particularly in dangerous places. Tell someone your planned route, starting & finishing points and your expected time of return - remember to let them know when you're back. If plans change, let your friends and family know.
Don't press on if conditions are against you - turn back even if it upsets your plans. Send for help as soon as possible, but take time to assess the seriousness of the situation first - you may be able to help yourselves. Give first aid, and keep injured or exhausted people warm until help reaches you. Dial 999 and state "Police for Mountain Rescue"