Hallo Maggy,
zum Lassen NP kann ich nur soviel sagen, rechnet mit Schnee und mit gesperrten Straßen. Ich hab den Park einmal Ende Mai besucht und da wurde die Straße erst vor ein paar Tagen freigegeben. Wir fuhren durch 10 Meter hohe Schneewände.
Ansonsten ein schöner Park, aber nicht vergleichbar zum Yellowstone NP wenn auch vulkanische Tätigkeiten anzusehen sind.
Anbei noch ein aktueller Auszug von der Hompage des Parks.
Snow Season (late October through mid-June)
Be prepared for winter-like driving conditions and carry tire chains in your vehicle from October through May.
Lassen Volcanic National Park remains open throughout the winter for snow camping, skiing, snowshoeing, and snowplay. The park also offers ranger-led programs and special events. However, the scenic drive through the park is not available until snow season is over.
Local surface roads also may close. Some of the commonly-used roads closed through the snow season are A21 from Chester, Highway 172 connecting Mineral and Mill Creek, and USFS Road 31N17 ("the 17 Road") connecting Mineral and Viola. Call the Park's Information Desk to find out about routes around the Park and local scenic byways that remain open throughout the year.
The Park Road (the main road that connects Hwy 89 through the park) remains open to the Lassen Chalet at the southwest entrance of the Park, and the Loomis Ranger Station 2 miles inside the northwest entrance of the Park. The rest of the Park Road, as well as Butte Lake, Juniper Lake, and Warner Valley Roads, are typically snow-covered and closed to vehicles throughout the snow season.
If weather permits, park road crews begin plowing in April. Park Roads may open earlier or later depending on snow conditions. . Typical opening dates are as follows: Main Park Road in mid-June, Butte Lake Road in late May to early June, Warner Valley Road in late May to early June, and Juniper Lake Road in late June to early July. The earliest opening for the Park Road was April 28 in 1990. The latest recorded opening since 1980 was July 21 in 1995. (It has opened as late as August.)
Use caution when driving in the Lassen area. Roads above 4000 feet tend to be icy in the mornings and evenings. Snow may drift across the road, blocking all or part of the roadway. Watch for snowplows. At the lower elevations, especially near Payne's Creek on Highway 36, deer and other wildlife often cross the roadway.
Gruß
Tom