Tesoro Shares Plunged: What You Need to Know

Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of Tesoro (NYS: TSO) are plunging today, down by as much as 10%, after the company updated its fourth-quarter guidance.

So what: The company expects to report a net loss between $0.55 and $0.80 per share, compared to the $0.65 profit analysts are looking for. Tesoro cited an extremely weak margin environment in California and the collapse of the West Texas Intermediate (WTI) to Brent crude oil spread.

Now what: The Tesoro Index for the California area was a negative $0.05 per barrel, down over $6 compared to the sequential and year-over-year prices. The WTI to Brent spread fell from $26 per barrel in September to just $8 per barrel in December. The decline reduces benchmark margins at Tesoro's refineries and the margin on long-haul foreign crude oil barrels indexed to WTI. When you're in the oil business, volatility is simply the name of the game.

Interested in more info on Tesoro? Add it to your watchlist byclicking here.

At the time thisarticle was published Fool contributorEvan Niuholds no position in any company mentioned.Click hereto see his holdings and a short bio. Try any of our Foolish newsletter servicesfree for 30 days. We Fools may not all hold the same opinions, but we all believe thatconsidering a diverse range of insightsmakes us better investors. The Motley Fool has adisclosure policy.

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