U.S. corporate bond yield spreads tightened in spotty trading on Friday as Wall Street labored to get back on its feet after the largest power outage ever in North America .
U.S. stocks rose slightly on feather-light volume on Friday , as Wall Street regrouped after the biggest-ever power outage in North America .
one or more equations that represent the way a system or process works.   these types of models are used to help predict weather.