We're witnessing an amazing intensification of the storm center marching up the Interstate 95 corridor. The first evidence of this intensification came in water vapor satellite imagery, in which the colors here show purple and green for deep moisture, orange for dry air. Note the tremendous intrusion of dry air moving north over the Carolinas, a sign of intense sinking air on the backside of the northward moving, intense, upper level disturbance. Ahead of the energetic upper level disturbance, note the fan shape of moisture, indicative of upper level divergence - air moving away from itself. This results in rising air from lower levels of the atmosphere to fill the void aloft, and rising air is the key to cloud growth, lowering surface barometric pressure, and storm development.
Of course, the barometer is one of the oldest and most reliable instruments in weather observation, so monitoring the fluctuations in the air pressure can be an important indicator of what's to come. Lowering atmospheric pressure indicates a strengthening storm, and monitoring the center of fastest falling pressure often indicates the next position of the storm. Note the two barometric pressure charts, below - the first shows current pressure, illustrating where the storm center is, and the second is a pressure tendency chart, showing where the storm may move next.
Surface barometric pressure, showing the storm center shifting rapidly from Virginia toward Chesapeake Bay:
Pressure Tendency, showing rapid pressure fall/rise couplet, indicating the likelihood of a developing swath of strong to damaging wind:
All of this adds up to a rapidly strengthening storm, lending credence to the forecast of heavy rain - and heavy snow - for the Northeast corridor. At the time of this writing, snowfall amounts of 9"-10" have been reported in some locales of West Virginia and Maryland, moving northeast. Additionally, several wind damage reports are coming in from Virginia, where the combination of pronounced drying on the water vapor imagery, and a strong pressure fall/rise couplet, support the damaging wind event. These strong winds will sweep through Long Island overnight, and New England early Thursday morning.