The Saint Lawrence valley to the northwest is close to sea level and majority of the ridges in the White Mountains are between 4000 and 5000 ft. In winter there's usually a more stable layer beginning somewhere 5000 and 10,000 feet that acts as a lid on the air flow. So the flow of air originating over the Saint Lawrence valley speeds up as it gets constricted in the vertical to pass over a 4000-5000 ft. range. Now add to this the fact that Mount Washington itself juts up 1500 feet higher than all the surrounding ridges of the White Mountains. The orographic funneling effect is due to the range as a whole, but Mount Washington sticks up even higher, right into the core of the orographic jet.