In late April and early May, it was reported that the Mississippi River was so high that some its tributaries were flowing backwards.
On May 9th, this 988 mb storm off the east coast took on a very interesting appearance in water vapor imagery:
On May 22nd, 159 people were killed when a mile wide EF-5 tornado hit Joplin, MO. Radar showed an incredible strongly curved hook, intense couplet, and a debris ball over the Joplin area:
More radar images here. What I found most interesting about the Joplin tornado was how quickly it grew, which can be seen here.
Just two days after Joplin, a 989 mb low ejected into the southern plains, producing numerous tornadic supercells from Kansas to Texas:
This included an EF-5 which hit El Reno, OK, killing 9. 0Z RUC analysis had warm sector 0-1 km storm relative helicity at 600 M2/S2 over central Oklahoma, where a 45 knot westerly low level jet was directly over southerly surface winds. The 0-1 km SRH was almost as high as the 0-3 km.
On May 25th, the same system produced a squall line with embedded supercells that prompted tornado warnings that went from Effingham, IL to Dyersburg, TN:
On May 29th, this trough with 500 mb winds up to 90 knots produced a 984 mb low in Utah: