In rare cases, a portable solar panel can hold up in the rain or snow, but it’s not usually recommended. While portable panels won't do as great in pouring rain or blizzards, they can handle some extreme weather conditions.
Many panels use a protective material or coating that makes them water-resistant or even waterproof. If you happen to leave your portable solar panel out in the snow or the rain, it might hold up, depending on the model. However, too much extreme weather could affect the panel's long-term performance or damage the solar cell structure.