Costco (COST) could be a solid addition to your portfolio given its recent upgrade to a Zacks Rank #2 (Buy). This upgrade primarily reflects an upward trend in earnings estimates, which is one of the most powerful forces impacting stock prices.The Zacks rating relies solely on a company's changing earnings picture. It tracks EPS estimates for the current and following years from the sell-side analysts covering the stock through a consensus measure -- the Zacks Consensus Estimate.Individual investors often f ...