Weekend Update

There's a series of weather systems to track on the surface maps Saturday morning. A cold front dropping through the Great Lakes and Ohio Valley marks the leading edge to arctic air. Behind the front, temperatures drop off considerably, with temperatures in the 10s and 20s in the Ohio Valley but below zero in Chicago and well below zero across the Plains and Upper Midwest. Meanwhile, across the Eastern Seaboard, moisture is gathering ahead of a developing low pressure area in the Gulf of Mexico. Rain is changing to snow across the South and a significant snowstorm is in the works (by southern standards) for some cities in Alabama, Georgia northeastward to the Carolinas.

It's the interaction of these two weather systems thats been the main point of concern for the weekend forecast for the last several days. The big question for most of the week has been just how close to the coast that surface low pressure would track based on how sharp the upper level trough would be that is carrying the arctic air eastward. In the end, the flatter, further east low track is the winner this go around as the upper level winds will carry the moisture and surface low pressure area well offshore. There's a narrow window between 10PM tonight and 8AM on Sunday that some light precipitation could sneak onto the Cape in the form of some flurries or a brief period of light snow, but it shouldn't amount to much.

Very cold air remains in the forecast for Sunday and Monday as a chunk of modified arctic air passes through New England. While the core of the cold will pass a bit further north than expected early this week, it's still going to be awfully chilly and temperatures will struggle to 20°F on Monday.

Our next shot of precipitation comes late Monday night and into the day on Tuesday as a piece of energy embedded in the jet stream cuts through New England. Out ahead of this small disturbance southwest winds will tug warmer air northward and try and moderate our temperatures. However, the advancing milder air will be forced up and over the existing dome of cold air here at the surface, meaning a shield of clouds and precipitation will develop to our west and cross the region. This action alone should be enough to produce a period of steady precipitation for a few hours Tuesday, which should be in the form of wet snow. However, there's two extra things to watch:

1. Will the southwest wind allow enough warming to produce a mix with rain? Temperature forecasts from computer guidance indicates yes, but I imagine that the surface temperatures will be lower than expected meaning more in the way of snow than mix or rain.

2. Before things warm up, could the southwest wind produce bands of ocean effect clouds and snow off of Buzzards Bay and Nantucket Sound? For several hours late Monday night into Tuesday morning, we will have a period where several necessary conditions are met before the low levels of the atmosphere warm.

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