A ridge of high pressure is set to send moisture and hot air toward Yosemite National Park and Lake Tahoe this weekend, raising unseasonable heat across one of the largest snowpacks in recent memory, increasing flood risks, while unstable air churns up thunderstorms.  The combination of warm and unsettled weather will likely lead to yet another round of snowmelt-related floods and river rises across the Sierra Nevada. … Large portions of the snowpack are likely to melt into a slushy mess along some of the hot spots, including streams, rivers and lakes along the foothills of the Sierra. This includes the Walker, West Fork and Merced rivers.

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