Cold Air
Because of relatively mild and snow free conditions in the first half of January, many stories appeared in the media about the lack of winter and mild temperatures. In a classic case of Beware What you Wish For, cold winter temperatures have struck back with a vengeance. Dangerously cold weather covered eastern North America this morning, spilling out over the Atlantic Ocean. Under clear skies overnight, temperatures in eastern Canada plunged below -30 (F) at many locations. The infrared cloud image from 12:45 UTC this morning bears testament to the cold air, with brightness temperatures colder than 240 K (-33 C, or -27 F) in the clear air. These cold temperatures match those of high clouds associated with a storm moving towards the north Atlantic at the eastern edge of the image.
A common feature observed in images recorded when Arctic air spills out over the ocean is the development of a band of stratocumulus clouds. The edge of the cloudbank roughly parallels the coast, and the distance between the coast and the cloud edge tells you something about the air-sea temperature difference and the initial moisture content of the air. As dry, cold air leaves the continent, heat and moisture are added to the boundary layer from the ocean surface. The warming and moistening will reduce the stability of the lowest part of the atmosphere, allowing the development of shallow convection. At some point from shore, sufficient moisture is added that the convection becomes visible. That is, clouds develop. Note in the image how air flow down Delaware Bay and across Chesapeake Bay is likely adding moisture to the offshore flow. Clouds develop downstream of these features much closer to shore than over adjacent waters where the airflow emerges unmoistened from the Continent.
The color-enhanced version of the grey-scale image highlights several features. Note that the coldest air (dark blue) over central New Hampshire and southern Maine is just upstream of a region where clouds develop very close to shore. This is a small region of mid-level cloudiness, with a surface echo in marginally higher dewpoints (-12 F at Portland vs. -15 to -17 at points north and south alont the coast) In adjacent, drier, areas, cloud development occurs farther offshore. Clouds extending to Cape Cod in this image were producing road-slickening snow showers. The enchancement also highlights the relative warmth of rivers, such as the ice-free Susquehanna (and its North Branch) in Pennsylvania and the Hudson in New York, and the ridge and valley geography of south-Central Pennsylvania.