On April 18, 2009 at 0152 (Unless otherwise noted, all times in this report are central daylight time based on a 24-hour clock) a Boeing 737-724, N27722 encountered severe turbulence while descending for approach to McAllen Miller International Airport (KMFE). The flight was being operated by Continental Airlines as Flight 511, a Title 14 Code of Federal Regulations (CFR) Part 121 scheduled domestic passenger flight from George Bush Intercontinental/Houston Airport (KIAH), Houston, Texas, to McAllen Miller International Airport (KMFE), McAllen, Texas. The flight was being conducted during night instrument conditions with an active instrument flight rules (IFR) flight plan. A total of 109 people were on board consisting of 2 flight crew, 3 flight attendants, and 104 passengers. One flight attendant sustained minor injuries, 2 passengers sustained minor injuries, and 1 passenger was critically injured. The airplane landed at KMFE at 0216, and arrived at the gate at 0220. In a written report the captain stated that he had briefed all crew members not to perform any services and remain in their jump seats for the duration of the flight due to the possibility of severe weather. The lead flight attendant made a public address announcement to the passengers that due to possible bad weather there would be no service and the seat belt sign would remain on during the entire flight and stressed the need for the passengers to remain in their seats. The flight took off from Houston approximately 3 hours later than scheduled and climbed to 38,000 feet. While en route to KMFE, the pilot had to circumnavigate to avoid thunderstorm cells. The first officer made a similar announcement to the crew and passengers stressing the need to remain seated for their safety. A total of 4 or 5 announcements were made to passengers during the flight to please remain in their seats as several passengers attempted to get up to use the lavatories. The seat belt sign was illuminated for the entire flight. The descent to KMFE began, with the airplane at an airspeed of 280 knots and a descent rate of approximately 1,000 feet per minute. A line of precipitation about 5 to 10 nautical miles wide was indicated on the airplane's weather radar. The depiction was green in color from -2 to +2 degrees. The airplane entered a cloud formation about 20,000 feet mean seal level (MSL) and encountered severe turbulence for about 15 seconds. This included up and down drafts and a roll of about 30 degrees. During the turbulence encounter a passenger, who had gotten up and went into the lavatory, sustained serious injuries. In addition, two passengers and a flight attendant not wearing their seatbelts sustained minor injuries. The descent continued to 10,000 feet MSL as cleared. At that time the lead flight attendant called the captain and notified him of injuries in the aft cabin. The captain continued to the approach and communicating with Air Traffic Control (ATC) the need to land as quickly as possible for a medical emergency. The flight data recorder was removed and secured for further analysis and the airplane was inspected for damage in accordance with Continental and Boeing maintenance manuals. There was no damage noted and the airplane was released for further flight.