U.S. stock markets plunged over 300 points today, in tandem with a steep slide in oil prices that briefly brought the price of a barrel of West Texas Intermediate (WTI) crude oil below $ 50 per barrel.

The Dow Jones Industrial Average ended down 331.34 points, or 1.86 percent, even as WTI crude, a U.S. benchmark, closed at $ 50.04 per barrel -- the lowest closing price since April of 2009.

The decline in oil prices, from their late June 2014 high, is simply mind-boggling. Prices at that time were more than double the current level.

Credit: NASDAQ.
Credit: NASDAQ.

So when does the oil price decline actually stop?

Brian Youngberg, a senior analyst at Edward Jones who focuses on the energy sector, says he's expecting an oil price bottom in the first half of 2015 -- but sees more possible downside pressures in the interim.

The reason is simple: supply, supply, supply.

"Russia is producing at the highest levels since the Soviet Era, and Iraq is producing at its highest levels since the 80s," says Youngberg. "There’s just plenty of oil to go around right now, and not enough places to put it."

Indeed, U.S. oil production, buoyed by the shale oil boom, continued to tick up throughout most of 2014, according to data provided by the U.S. Energy Information Administration, exceeding 9 million barrels per day in October of 2014. Five years earlier, in October of 2010, it was only at 5.6 million barrels per day.

And of course the original cause of the oil price decline is the decision by OPEC, whose members produce about 40 percent of global crude oil, not to cut production levels back in November. In sum, oil prices continue to plunge because of a persistent supply glut in the global market.

Gasoline hasn't been this cheap since 2009. But why now, and how long will prices stay low? Here's what you need to know, in two minutes. (Gillian Brockell/The Washington Post)

The oil price drop on Monday may have stung investors, but U.S. consumers are more than happy to put the resulting cheap gas in their tanks. The national average gas price for regular gasoline on Monday stood at $ 2.20, a plunge of well over a dollar from a year ago prices, according to AAA.

The auto club said last week that declining gas prices saved Americans $14 billion at the pump in 2014, with the bulk of the savings coming at the end of the year.

The gas price plunge has now gone on for over 102 days straight, says AAA -- since September 25 of 2014. That streak is the longest on record, and longer even than during the Great Recession, when prices fell for 86 days straight. During the current period of steep declines, prices have plunged by $1.15 nationwide.

"Drivers can find at least one station selling gas for less than $2.00 per gallon in 40 states," notes AAA spokesman Michael Green. "About 35 percent of all U.S. stations are selling gas for less than $2.00 per gallon today. The number of stations selling gas below $2.00 will continue to grow as prices drop even further this week."