Even amid the grim news of doomsday forecasts and subway closings, New Yorkers who tuned in to Mayor Bill de Blasio’s televised storm briefings found themselves distracted by the unusual activity on one side of the screen. That would be Jonathan Lamberton, the mayor’s sign-language interpreter, whose arsenal of rapid gesticulations, vigorous frowns and mime-like smiles - a stark contrast to the mayor’s sober mien - raced around social media this week, earning equal parts awe and amusement.