The quick answer is that WWHM is not going to be much help if you are not in Washington state.
WWHM, WWHM2, & WWHM3 are WA state specific GUI front ends for the HSPF computational engine. I haven't tried WWHM3 but read about it. I hear that there may be a version of "WWHM3" that's been developed for part of California. These programs were developed (prior to WinHSPF) in response to WA state adopting a stormwater flow control standard that can only be assessed through continuous model simulations. The state was not comfortable that us dumb engineers would be able to figure out HSPF so they created WWHM as a more limited yet more user friendly (than DOS-HSPF) program with consistent output reporting that agency staff could review to assess if a project complied with the flow-duration standard.
With WWHM2, I don't recall an easy way to import or manipulate WDM files, which would be key to changing the geographic region. WWHM2 was set up entirely around WA state code and would not easily analyze projects that used a different criteria than WA state flow duration criteria, and was not usable to track water levels in ponds as is needed in cases. I found WWHM2 to be limited to the specifics of designing a flow control system to meet WA standard for flow duration control. Maybe WWHM3 is somehow more flexible (but I doubt it really addresses these issues, WA state is not paying their consultant to develop a nationwide tool!)
For nationwide continuous modeling tool IMO the preferred freeware/gov't sanctioned option would be EPA BASINS4.0, which includes WinHSPF, in part because EPA provides a WDM database (rainfall records, ET records, etc to use for input), or SWMM5. But really, who can afford to set up continous models for a project if the local code does not *require* it like here in WA.... until then CN based methods will likely continue to be the most cost-effective modeling approach.
