I'd also suggest your firm join the major Chambers of Commerce in those two counties, and do a very targeted mailing to their members ( perhaps as an add-in mailing to their regular newsletters??) Don't just do a flyer (you want to stand out, right?). Instead send an oversized postcard that is professionally designed and written, perhaps with a flap that opens.
Share the news with all your current clients, as you never know who they know. Send them a formal announcement ( again professionally designed) announcing your expansion.
Best of luck with everything!