Forecasts have been consistent in the past days for a storm of historic proportions over parts of southern New England. What conditions that are present now argue for the development of a strong winter storm? The image above is the GOES Sounder Land Surface Temperature; cold air is present over southern Canada, with surface temperatures near -30 C, associated with a surface high pressure system. The high pressure will act to reinforce the cold air at the surface, preventing or delaying any changeover to liquid or mixed precipitation. (A MODIS Land Surface Temperature plot at 1500 UTC on 26 January similarly shows cold air banked over southern Canada).
Winds over southern New England early on the 26th continued out of the north and northwest, maintaining cold air at the surface. The ASCAT (from METOP-A) imagery above shows brisk northwesterly winds south of southern New England just before 0100 UTC, with southwesterlies east of Georgia and South Carolina just before 0300 UTC. Those southwesterlies are helping moisten the atmosphere, and heavy snows require abundant moisture. MIMIC Total Precipitation (below) testifies to the moistening that is occurring off the southeast coast as this system develops.
[Added: The 1540 UTC ASCAT winds show the surface circulation east of Hatteras and the mouth of the Chesapeake! Winds south of New England have shifted to northeasterly. The location of the circulation well off the coast suggests cold air be maintained over land.]
Given that moisture and cold air are present, what features argue for the development of a strong storm? The GOES-13 water vapor images, below, with lightning superimposed, shows the potent system approaching the coast and blossoming over the Gulf Stream. Strong sinking behind the system is indicated by the development of warm water vapor channel brightness temperatures (Yellow in the enhancement), and strong rising ahead of the system helps to generate strong convection. Convection also occurred over the Deep South late on 25 January in response to solar heating. The system depicted in the Water Vapor imagery is vigorous.
Column Ozone is frequently used as a proxy of tropopause folding; tropopause folds accompany very strong storm development and the vertical circulation associated with the potential vorticity maximum associated with the folding draws ozone down into the troposphere. GOES Sounder Total Column Ozone figures, below, at 0000 UTC and 1200 UTC show relative high values over the upper midwest at 0000 UTC and over South Carolina at 1200 UTC. These two impulses will merge as the forecast nor’easter develops over the Atlantic.