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The QPF plumes (above from 06 UTC last night) are interpolated from 0.5 degree grid point forecasts to the airport location. However, this could be done using just the four closest grid points, or for more nearby points using a distance-weighting function of some sort. I could not find an answer regarding the exact procedure employed.
Based on the statement above about the GEFS procedures, I assume that both of these plume products (above - total snow accumulation and below - probability of snow) are derived from the forecast for the nearest 0.5 degree grid point (i.e., the crux of Pat's question), which could be located anywhere within about a 25 km radius of the airport. No big deal for the flatlands but very significant here. Where is the nearest grid point wrt TUS? Very important question and, again, I could find no precise answer online.
So, if any reader knows the answer, please let us know.