a ramp 126 and a lid retainer means for keeping the second lid portion in position against a lifting force created by a differential pressure between the inside of the shelter and the outside atmosphere during a storm event the lid retainer means shown in fig5 is comprised of a clip 502 affixed to the second lid portion 110 the clip 502 is received within a hole 504 defined in the first lid portion 112 in the closed and locked position the clip 502 is prevented from lifting up against a lower edge of the fixed first lid portion 112 alternatively the lid retainer means may be comprised of a cooperating chain 408 and receiver 410 as shown in fig4 the lid retainer means may also be a releasable latch mounted on one or more of the side walls cooperating with a stop mounted on the second lid portion 110 or vice versa fig5 also shows a reinforcing structural member 506 affixed to an underside of the first lid portion 112 an alternative lid retainer means is shown in fig7 in this embodiment of the lid retainer means it is comprised of a rotatable latch 702 mounted to one of the walls.