Brian thought it a good use of my time to say:
I read that powerline interference will often times diminish when it rains.
This is presumably due to loose hardware becoming temporarily tightened when
moisture causes the utility pole to swell Well, in my case the noise
substantially worsens and becomes more broadband during periods of rain. Can
anyone explain a possible reason for this? The RFI guy from the local
utility company seems confident that work scheduled for next week in my
neighborhood will correct the problem. I certainly hope this is the case.
Maybe then I can finally hear some dx. Wish me luck.

Well, rain is an attenuator. Perhaps the signal you are looking for is
being attenuated by the rain, and, that attenuation plus the noise generated
by the power lines=lower SNR.

I see it all the time(not in SW bands, I am in Satellite
Communications[8GHz], but the theory has to be the same) Rain attenuates
desired signal, lowering C/Kt