Its been so dry this summer that I saw my 1st Asian Tiger Mosquito last night. Last year I suspect they were out by May. These things are much faster than normal mosquitoes and they are sneaky too. They seem to intentionally stay out of your line of sight.
The Maryland page about these mosquitoes does not mention Montgomery County but I have seen them for at least 2 years now.
http://www.mda.state.md.us/plants-pests/mosquito_control/_asian_tiger_mosquito_md.php
It doesn't mention Montgomery County specifically, but at the top of the page it does say this: "...the tiger mosquito has extended its known range to all Maryland counties except Allegany and Garrett." I think that covers Montgomery. :)