We have cruised Alaska ever since 1972 and the last 6 years two cruises a year one at the beginning of the season(late June) and the other in mid August or early September.The cruise lines are arriving much earlier and leaving later than they use to and there are a lot more ships (too many). Morerecently we take two back to back 14 days going from Vancouver to Seward and back.
The weather pattern seems to be warmer than 15 or 20 years ago. Surprisingly there have been very few changes in the cities the ships visit. We have found the weather patterns to be erratic and unpredictable. During a number of seven day cruises going to Seward the weather may be gorgeous, sun shine, a little rain, usually in Juneau.On the return trip to Vancouver it might rain every day, all day. We never have been in Seward where it has not rained all day, so hard sometimes that we did not get off the ship.The rain is unusual. It might soak you or be just a very light rain. In all the times we stopped in Ketchikan, the Rain Capital, we experienced rain only once. Juneau may have light rain in the afternoons.
As for protection it is beneficial to lug along a fold up umbrella although some ships lend you umbrella or sell them. You can buy
almost anything you need for the weather aboard ship or in the stores on the shore. One item you should take long is gloves. It gets awfully cold on the open deck when the ship is moving, creating a wind chill factors. Aboard ship they sell one size fits all knit gloves but they usually run our very early. You can buy gloves on shore.
To illustrate the weather, this week, i.e. May 6-12 in the Juneau area the temperate was in the mid 40's fah. and rain was reported every day for Juneau. You can find out about the weather, on Yahoo or by the Juneau Daily News or Anchorage Daily News in key word.