Gas jumped nearly 20 cents a gallon in some cities over the past two weeks because of tight supplies and the rising price of additives, according to the U.S. Department of Energy. Gasoline prices usually climb ahead of the Memorial Day weekend, the start of the summer driving season. This year, however, an atypically large number of refineries have gone off-line for maintenance, which will probably make already tight gasoline supplies more so. U.S. gasoline inventories are at 193 million barrels, the lowest springtime level since 1994, according to the DOE. Adding to the problem has been the rocketing price of MTBE, a natural-gas-based additive that reduces emissions and raises octane in gasoline. Higher natural gas prices over the winter meant the cost of MTBE went up as well. Producers also are also having problems complying with new rules that require them to empty their tanks of winter gas before switching to lower-emissions summer fuel instead of mixing the blends. The combined effect of those events has been to raise the average price of regular gasoline last week to $1.56 a gallon. That's up 11 percent from a month ago and already exceeds the Energy Department's summer price forecast -- even though summer has yet to start. The department predicted prices would peak at $1.52 a gallon.