Presidential succession, the ability of Congress to legislate (including vital emergency appropriations) and the Supreme Court's capacity to achieve the required quorum to function are areas of extreme concern to those knowledgeable of continuity of government issues. Here are some excellent resources, including more chilling scenarios.

If DC were hit on a weekday when Congress and the Supreme Court were in session and when POTUS and VPOTUS were in town, then it would be chaos for quite awhile -- just when the nation most needed its government to make momentous decisions.

http://www.continuityofgovernment.org/

http://www.continuityofgovernment.org/SecondReport.pdf

...recommended a constitutional amendment that would provide for filling mass vacancies or mass incapacitations in the U.S. House and Senate through temporary appointments until
special elections could be held to fill the vacancies. without such an amendment, an attack might cause Congress to fall below the quorum requirement for conducting business and potentially disable Congress for many months until the numerous vacancies could be filled.

...the current system would be inadequate in the face of a catastrophic attack that would kill or incapacitate multiple individuals in the line of succession. The current system must be corrected to ensure continuity in the
executive branch.

http://www.au.af.mil/au/awc/awcgate/crs/rs21089.pdf

The Legislative Branch. The Constitution provides that in the event of vacancies in the representation from any state, the governor of the affected state shall issue writs of election to fill such vacancies or, in the case of a Senate vacancy, may, if so empowered by state law, make a temporary appointment until an election may be held, in accordance
with state law.

The Judicial Branch. The Constitution establishes the Supreme Court of the United States and prescribes the statutory creation of inferior federal courts, but is silent regarding the continued functioning of the federal judiciary during or after an incapacitating catastrophe. In locales of the United
States where federal courts could not function due to an emergency, the President might temporarily declare martial law and vest minimal trial court authority in military tribunals convened by commanding officers in the field dispatched to enforce federal law.

http://www.aei.org/article/29045

September 11, 2001, should have been a wake-up call. If United 93 had not been delayed and left on time, its passengers would not have known they were on a suicide mission, and the plane would likely have hit the Capitol Dome that morning, wreaking havoc and perhaps destroying Congress for many months, leaving the country with a form of martial law.

The fact is, the Presidential Succession Act of 1947 is woefully inadequate to the modern threats we face, and Congress needs urgently to reconsider it. At the same time, we could have very serious questions of legality and struggles among important actors, in the fog of war and the aftermath of a devastating attack, over who is or should be acting president, and whether a rump group could actually serve as a legitimate Congress, including perhaps choosing a Speaker who would become acting president. Here, we need a functioning Supreme Court to make key judgments of legitimacy, and there is no plan to reconstitute the court if it falls below its statutory quorum of six justices.