The UFDP is on the edge of cliff, and the valley below is 750 m lower and much warmer/drier. For any gridded data set, it is important to use a pixel that is 100% above the cliff.
Here's an explanation from Jim Lutz:
The UFDP lies in part of a PRISM pixel that also includes the very much lower area below the cliff, therefore influencing the local t/p regressions that PRISM uses (especially if you are using the 4k data set). I use the PRISM pixel directly to the north of the one that actually contains the plot (attached). This interpolation ‘issue’ would apply to any gridded data set, so just check that the pixel you are using is 100% on the rim of ’the breaks’ and does not include any of the valley (which is 750 m lower).
PRISM grid cells that I use for UFDP (800m, top and 4k, bottom). The plot is actually in the grid cell immediately south of the highlighted cell.

