You should always take the weather into consideration before setting out on any hike. If you're hiking in a mountainous area, be aware that weather in the mountains is generally colder and more severe than in the valleys—and the weather can change quickly. Often in higher elevations, especially above treeline, rain, snow and fog are possible at any time of the year.
Before setting out on a hike, be sure to check the most up-to-date forecast for the area you'll be hiking. If you're hiking to mountain summits, be sure you know what weather expected there, not just in the valley below.
Always check the forecast again just before you start your hike as the weather may have changed between the time you left home and arrived at the trailhead.
LIGHTNING & RAIN:
Pay attention to weather forecasts and avoid being caught on exposed ridges or above treeline during electrical storms.
Know an escape route in case fast-moving storms catch you off-guard.
If a storm hits, spread your group out and keep track of each other so if one person is hit, others can help.
If you don't have time to get to a lower elevation, squat down to reduce your height and minimize your contact with the earth. Take off your pack if it has a metal frame. If you have a foam pad, put it under you.
Rain can also make leaves on the ground and rocks extremely slippery; and after heavy rains and in spring, streams can change from trickles to torrents, making crossings difficult.
If you ford a stream, be extremely careful.
Keeping your boots on will give you better footing and prevent your feet from going numb from the cold water.