They are in that the MPEG4 channels (all the HD locals, except the original east and west coast locals) are sent via a different frequency than everything else. (KA frequencies, FYI). This frequency range is VERY susceptible to rain intereference. That is a bit part of why the dish got bigger.
And that is why the dish has a very fine grain adjustment base in comparison to the older bases.
Assuming for the moment that you are not too close to the edge of the spotbeam, for your locals, if your dish was correctly aligned by the installer, then you should be able to handle even a fairly heavy rain or snow to the south.
The problem seems to be very few installers want to take the time to correctly follow the installation and alignment technique that D* prescribes. Mine did not, for instance, but I pushed him to do almost the same thing and I seem to be ok so far.
From the description you give, I'd call D* to have the dish correctly aligned.