Dr. Nadia Schadlow, who currently works on strategy on the staff of National Security Adviser H.R. McMaster, is the likely successor to Dina Powell, deputy national security adviser for strategy. Powell announced Friday that she will leave the White House early next year — the first in an expected wave of senior West Wing departures.

Why it matters: Powell was one of the few senior West Wing officials with previous White House experience, and the coming brain drain could leave President Trump with key holes. The administration has always been thinly staffed, and bringing top people in will be hard, in part because of uncertainty surrounding the Mueller investigation.

Why she's leaving: Powell told me she had committed to one year of service, and her family remained in New York.

Defense Secretary James Mattis said in a statement: "With the pending departure of Dina Powell, we are losing an invaluable member of the President's national security team. I personally appreciate Dina's partnership and contributions to the mission of the Department of Defense."

Why she matters: Powell was among the aides who communicated well with Trump, and accompanied him on all his overseas trips. She helped with White House efforts on a restart of Middle East policy.

A departure date has not been set.