Paul Kocin spoke at Millersville AMS pre-2011. He said that you should never sleep on an intensifying surface low coming off the Appalachian Mts. They can give you tornadoes in the summer and a surprise snowfall in the winter.
80,000 power outages in 5 hours in Montgomery County, MD. Trees were coming down on police and fire apparatus while they were responding to calls. Roads could not be pre-treated because a small amount of rain fell during the day. That is my benchmark for what a "high impact" storm can deliver in the east coast megalopolis.
Jan 26, 2011 is in my Top 5 weather events of all time. I was in Lancaster at the time and remember how powerful the front end thump was impressive. Then we just snizzled all day before the afternoon thump came in hard right at sunset. Looking back at the after action reports in our work library, apparently OPM issued an early departure mandate, but it wasn't enforced my mid level managers. Everyone in DC proper was looking out their window and seeing drizzle thinking it would be a bust. It wasn't until the spouses started calling from home and relayed conditions in the suburbs getting so bad that everyone bailed. By then it was too late. That's why you see language now on PM early departure notices that say "managers must dismiss staff no later than X time".