降雪 and 積雪: snowfall preparation and accumulated depth
Compare 降雪 covering a road or prompting preparation with 積雪 as snow cover about ten centimeters deep and snow checked on a roof. Separate preparation, accumulated state, and checking.
Snowfall and preparation for it
降雪（こうせつ） means snowfall. 「夜間の降雪で道が白く覆われた。」（Overnight snowfall covered the road in white.） 夜間の locates the snowfall overnight, and 道が白く覆われた reports the road being covered as a result.
「降雪に備えて玄関に雪かき用のスコップを置いた。」（We kept a snow shovel at the entrance in preparation for snowfall.） 備えて gives the purpose of preparing for snowfall. 置いた describes placing the shovel at the entrance. Keeping the shovel there does not establish that snowfall had begun or snow had already been removed.
Accumulated depth and what was checked
積雪（せきせつ） refers to accumulated snow or snow cover. 「朝の積雪は十センチほどだった。」（The snow cover that morning was about ten centimeters deep.） ほど marks an approximate depth. About ten centimeters describes the accumulation, rather than a duration or rate of snowfall.
「屋根の積雪を安全な場所から確認した。」（We checked the snow accumulated on the roof from a safe place.） The object checked is snow on the roof, and 安全な場所から identifies the location of checking. 確認した reports a check, not the completed removal of the snow.
Separating phenomenon, preparation, and state
The 降雪 examples give a road covered by falling snow and a shovel kept in preparation. The 積雪 examples give an approximate accumulated depth and a check of snow on a roof. The purpose of preparing differs from the state of snow already accumulated.
朝, 十センチほど, and 屋根 make the accumulation examples specific. Preserve their time, approximation, and object without turning them into an exact fixed depth, a current snowfall rate, or completed snow removal.
What action and snow information do 「降雪に備えて玄関に雪かき用のスコップを置いた。」 and 「朝の積雪は十センチほどだった。」 give?
Show the answer and why
The first describes preparing for snowfall by keeping a shovel at the entrance. The second gives accumulated snow about ten centimeters deep that morning. Neither adds snowfall beginning, completed removal, or an exact fixed depth.