B737-700 flight crew reported encountering severe turbulence at FL400.

Attributes
ACN	1307387
Time
Date	201511
Local Time Of Day	1801-2400
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 222 Flight Crew Type 8000
Person 2
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 240
Events
Anomaly	Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Speed All Types Deviation - Track / Heading All Types Inflight Event / Encounter Weather / Turbulence

Narrative:

While at cruise altitude FL400; aircraft encountered severe turbulence. This caused the aircraft to momentarily exceed our mach max operating speed along with aircraft flight path deviations. The aircraft then encountered a rapid decay in indicated airspeed while still encountering severe turbulence. We [advised ATC] and exercised captain's emergency authority and began a descent to regain critical airspeed and give the aircraft a larger margin of safety in which to operate. We descended 4000 feet in order to regain our loss of airspeed; once the aircraft was no longer in severe turbulence and completely controllable; we coordinated a new cruise altitude with air traffic control and cancelled our emergency status.we had our weather radar operating with a tilt below our flight path. Our radar indicated no weather on our flight path or within 50 miles of our flight track. Access to our internet weather program would have possibly given us a clearer picture of weather that was not being depicted on our aircraft weather radar.

Original NASA ASRS Text

Title: B737-700 flight crew reported encountering severe turbulence at FL400.

Narrative: While at cruise altitude FL400; aircraft encountered severe turbulence. This caused the aircraft to momentarily exceed our Mach Max Operating speed along with aircraft flight path deviations. The aircraft then encountered a rapid decay in indicated airspeed while still encountering severe turbulence. We [advised ATC] and exercised Captain's emergency authority and began a descent to regain critical airspeed and give the aircraft a larger margin of safety in which to operate. We descended 4000 feet in order to regain our loss of airspeed; once the aircraft was no longer in severe turbulence and completely controllable; we coordinated a new cruise altitude with Air Traffic Control and cancelled our emergency status.We had our weather radar operating with a tilt below our flight path. Our radar indicated no weather on our flight path or within 50 miles of our flight track. Access to our internet weather program would have possibly given us a clearer picture of weather that was not being depicted on our aircraft weather radar.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.