U.S. auto makers are expected to maintain their recent incentive-induced sales momentum through most of March, but a likely drop in inventory threatens the market’s expansion in early second quarter.

Ward’s is calling for 1.26 million light-vehicle deliveries in March over 27 selling days, a 14.3% jump in the daily sales rate over like-2010, which had 26 selling days.

The forecast also represents a 13.4% rise in DSR over prior-month, which had 24 selling days. This trend is in line with the historical February-to-March change.

It was widely thought February’s sales denoted two shifts in the market, the first being a general hike in baseline demand to a 13 million-plus SAAR. The second was a general increase in incentives that worried analysts, who feared a price war.

Demand should remain solid through March, with forecast volume equating to a 13.3 million-unit SAAR. However, reduced inventory linked to Japan’s earthquake and tsunami likely will bring incentives to a halt and drop the SAAR dramatically in April.

Related document: Ward’s U.S. Lt. Vehicle Sales and Inventory Forecast

The industry began the month with a healthy 60 days’ supply of light vehicles and should be able to meet March demand.

But General Motors has announced a 1-week closure of its plant in Shreveport, LA, due to parts shortages.

Similar production losses are expected around the globe in coming weeks and months.

With reduced model availability, U.S. auto makers almost certainly will put the brakes on April incentives, which likely will drive out the price-conscious buyers who recently returned to the market.

Sales lost to reduced inventory should be made up later in the year, but upward pressure on price would likely delay a sustained expansion of the market for months, as auto makers maintain higher transaction prices.

Ward’s is not yet reducing its 2011 full-year sales forecast of 13 million units, but prolonged parts shortages from Japan might require a more pessimistic outlook.

The Ward’s forecast for March calls for Detroit Three auto makers to improve their collective daily sales 19.6% over year-ago, accounting for 46% of the market.

General Motors could sell more than 250,000 units for a 20.9% market share, while Ford is forecast at just below 200,000 light-vehicle deliveries.

Toyota should grab 14.3% of the market to Honda’s 9.9%, Ward’s forecasts. Nissan could displace Chrysler in the No.5 sales spot, while beating its best-ever February market share (9.3%) with projected sales of 95,000 units.

Hyundai and Kia represent wild cards, as the recent spate of price reductions by competitors trimmed the auto makers’ market shares last month. Ward’s looks for the Korean brands to account for 7.7% of March deliveries, but their shares are expected to rise in the second quarter.