Natural gas has a far more violent seasonal cycle than crude. US demand in a cold January can be more than half again what it is in a mild April, while production runs roughly flat all year. Storage is the buffer that reconciles the two: gas is injected into underground reservoirs from April through October and withdrawn from November through March. The US inventory level therefore swings by roughly two thousand billion cubic feet (Bcf) within a single year, a far larger share of the stock than crude inventories ever move.
EIA publishes the Weekly Natural Gas Storage Report on Thursdays at 10:30 ET, for the week ending the previous Friday. It reports working gas, the part that can actually be withdrawn, for the Lower 48 and five regions: East, Midwest, Mountain, Pacific and South Central. South Central is the largest region and the most responsive one, because it holds most of the salt caverns, which can cycle gas in and out many times a year. Depleted reservoirs and aquifers, common in the East and Midwest, fill once in summer and empty once in winter.
As with crude inventories, a level on its own says little. What the market trades is the comparison: this week's net injection against the average injection for the same week over the prior five years, and the total against its 5-year range for the week. A 90 Bcf injection is large in early April and small in June. The surplus or deficit to the 5-year average is the single most quoted gas statistic, because it summarises every weather and supply surprise since the start of the season.
Storage is also why the gas curve has the shape it does. Winter contracts trade above the summer ones because someone has to be paid to hold gas from the injection season into the withdrawal season, and the summer-minus-winter spread on the curve is that payment. When storage runs full early, the spread narrows; when the market fears running short at the end of winter, the March contract rises against April, the spread known as H/J.