I've always been uncomfortable attributing these pre-tropical-storm surges to the tropical storm, although I recognize it is a relatively convenient thing to do. In reality, the tropical storm is embedded in a relatively humid large-scale circulation that would have likely led to a surge of moisture into the southwest U.S. even in without the presence of the tropical storm. Perhaps the public doesn't care about such semantics.
The greatest moisture and coverage of precipitation will likely affect the four corners region over the next two days due to the pre-Rosa moisture surge and the remnants of Rosa eventually tracking through the region. Downscaled forecasts produced from the SREF show the greatest probability of two inches or greater of precipitation occuring over portions of Arizona and western Colorado. Some potential for such heavy precipitation also exists in portions of Utah, but is more localized.
For the Salt Lake Valley, the numbers being spit out by our downscalled SREF show accumulations through 1200 UTC (0600 MDT) Wednesday 3 October of anywhere from about 0.05 inches to 0.90 inches.
Basically, much is going to depend on the chaotic nature of this event and monsoon precipitation systems. I wouldn't pick or choose any specific outcome for any given location at this stage in our part of the world as it really is a crap shoot how this will all come together. Further, the potential for heavy precipitation in burn scar areas remains a very serious concern, as highlighted in the National Weather Service graphic below.
|Source: NWS (Downloaded 8:43 MDT 1 October 2018)|