June 10 (Reuters) - U.S. energy firms likely added a slightly above-normal 99 billion cubic feet of natural gas into storage last week, according to the average estimate in a Reuters poll released on Wednesday.

The estimate compares with an injection of 110 bcf during the same week a year ago and an average increase for that week of 95 bcf over the past five years (2021-2025).

In the prior week ended May 29, energy firms added 95 bcf of gas into storage. USOILN=ECI

If correct, the forecast for the week ended June 5 would increase stockpiles to 2.677 trillion cubic feet. That figure would be about 0.5% below the same week a year ago and about 5.6% above the five-year average for the week.

The U.S. Energy Information Administration is scheduled to release its weekly storage report at 10:30 a.m. EDT (1430 GMT) on Thursday.

There were 66 total degree days (TDDs) last week, slightly higher than the 30-year normal of 63 TDDs for the period, data from financial firm LSEG showed.

Total degree days measure the number of degrees a day's average temperature is above or below 65 degrees Fahrenheit (18 degrees Celsius) to estimate demand to cool or heat homes and businesses.

Reuters polled 11 analysts, whose estimates ranged from injections of 94 bcf to 102 bcf, with a median increase of 100 bcf.

Early estimates for the week ending June 12 ranged from injections of 68 bcf to 102 bcf, with an average increase of 82 bcf.

Those figures compare with an injection of 97 bcf during the same week last year and a five-year average increase of 73 bcf.

Following is a list of poll participants. All figures are in billion cubic feet: