You know that dreadful excitement that sets in when the black clouds descend in advance of a big thunderstorm? That must be what it feels like these days at the banks and Wall Street firms that issued residential mortgage-backed securities. They’ve been dealing for more than a year with claims by some of the insurers that backed the fast-eroding bonds, but we seem lately to be looking at a whole new tranche of exposure for the banks.

We told you last week that a group of eight large bond investors, including the New York Federal Reserve Bank, is gearing up to sue Bank of America, demanding buybacks of more than $20 billion of bonds backed by Countrywide-issued residential mortgages that, according to the bondholders’ counsel at Gibbs & Bruns, didn’t meet underwriting standards.