The House’s top budget writer is warning his colleagues so-called sequestration budget cuts to the defense budget could be greater than previously estimated and would imperil weapons programs.

Rep. Norm Dicks (D-Wash.), ranking member of the House Appropriations Committee (HAC) and its defense subcommittee, says his staff concluded under sequestration the defense budget would be cut by $60.6 billion next year–not the $54.7 billion cited in a Sept. 14 White House report that uses a slightly different definition of defense spending. In a “dear colleague” letter to House members, Dicks also details the harm he said would come to the military if Democrats and Republicans cannot agree to stop the sequestration cuts from starting next year.

“This letter will examine the impact of sequestration on the whole range of federal responsibilities and, I hope, help make the case for Congress to act responsibly by agreeing to a more sensible approach to deficit reduction,” writes Dicks, who is retiring this year.

The $500 billion in decade-long sequestration reductions to planned defense spending will start in January if congressional Democrats and Republicans cannot agree on a plan to replace them with other government savings. Many lawmakers and President Barack Obama oppose sequestration–which would total $1.2 trillion and also slash non-defense spending–but cannot agree on an alternate plan.

Dicks’ 15-page letter shows his HAC staffers don’t agree with calculations in a Sept. 14 report from the White House’s Office of Management and Budget (OMB) for how much discretionary defense spending would be cut as part of the $1.2 trillion in reductions. Sequestration was created by the Budget Control Act of 2011 (BCA) and triggered by the failure of a Select Committee on Deficit Reduction to agree last year on $1.2 trillion in savings.

OMB’s report says discretionary defense spending accounts–except military-personnel funding exempted by the White House–would indiscriminately be cut 9.4 percent under sequestration. That OMB report was required by the Sequestration Transparency Act (STA).

Yet Dicks says that “OMB, in accordance with the STA, looked at only one aspect of sequestration.”

“As another motivation to act, the BCA also set up a second, separate sequestration to enforce the firewall between security and non-security appropriations,” Dicks writes. “Because the Joint Committee failed, a new and lower defense firewall goes into effect, requiring an additional cut in defense spending. Based on levels in the agreed upon continuing resolution for FY 2013, we estimate…an additional reduction of 1.9 percent in 2013 only for discretionary defense (function 050) accounts.”

The Pentagon and rest of the government is running on a temporary budget, or continuing resolution, for the next six months.

Dick’s letter concludes that the “sum of the two sequestrations on defense appropriations (the largest subset of defense function 050) equals $60.6 billion, including $50.5 billion associated with the 9.4 percent cut and an additional $10.1 billion from the smaller, firewall enforcement sequestration.”

OMB and HAC-D Democrats, it is worth noting, are working off of different definitions of defense spending, which explains the different calculations for the amount of the base 9.4 percent cut.

He notes that the sequestration cuts would apply to both the Pentagon’s base budget and its separate Overseas Contingency Operations account for war funding. Pentagon officials have talked about reprogramming funding, if sequestration kicks in, to protect some war funding and large weapons programs.

Dicks cites multiple concerns in his letter about sequestration’s impact on the military, including major weapon systems.

The cuts to procurement accounts would translate to eight fewer UH-60 Black Hawk helicopters and five fewer CH-47 Chinooks, slowing the Army’s plans for modernizing its utility and heavy-lift helicopters.

“The CH-47 Chinook has proven especially valuable in Afghanistan because of its effectiveness at high altitudes compared to other utility aircraft,” Dicks writes.

“Sequestration would take up to 11 Stryker vehicles out of the program, hurting the Army’s ability to keep Stryker brigades fully outfitted,” he adds. “Two fewer F-18G (Growler) aircraft would be built, impairing the fielding of electronic warfare capabilities. Sequestration will make it more difficult to avoid a carrier-based strike fighter shortfall by building three less F/A-18E/F aircraft. And one less P-8A would slow the Navy effort to field new surveillance aircraft.”

Dicks cites further concerns about sequestration cutting $1.7 billion from the Navy shipbuilding account.

Under sequestration, research funding for the F-35 Joint Strike Fighter would dip by $1 billion, thus cutting four aircraft and reducing advance procurement monies, Dicks says. Sequestration would trim the Air Force aerial-refueling tanker program by $99.5 million “and potentially slow” the contract with Boeing [BA], he says. The Air Force’s next-generation bomber would face a $33.7 million cut under sequestration, he says.

Dicks’ letter offers little overall commentary on the potential sequestration cuts to defense. Still, he reminds his colleagues that Defense Secretary Leon Panetta has warned  sequestration “could pose a significant risk to national security” and would “literally undercut our ability to put together the kind of strong national defense we have today.”