Monday 9 a.m.
A low pressure area from Kentucky will jump the near the New Jersey coast tonight, then strengthen as it heads toward Maine (and joins forces with another low pressure area heading east from Minnesota). This storm system has pockets of heavy precipitation but large gaps in the precipitation pattern. In the I-95 corridor from Philadelphia to New York City, snow this afternoon will melt on the main roads, but if there is a fair amount of sleet, roads will become slick because the pellets of ice don't melt anywhere near as quickly as snowflakes. Overnight, enough mild air should come in to change all the precipitation to rain. Farther inland, however, it may stay near or just below freezing much of the night, increasing the likelihood of travel troubles.
From Providence to Boston, initial snow could accumulate a few inches before the changeover, but by the time you get out to a line from about Uxbridge to Waltham, 6 inches could accumulate with more than a foot north toward the mountains. This video has more, and also shows why cold weather will hang around for a while. It's the opposite of what was happening at this time last year.
When the flow is from the east, cold air in the low levels of atmosphere gets trapped east of the Appalachians. Mild air flowing in the from the southwest simply sails by overhead, unable to displace the cold dense air below. You can see the effect on this pressure analysis. On the west side of the mountains, where it is warm, the pressure drops quickly. The cold air in the lowest few thousand feet blunts the pressure falls east of the mountains. Over the warmer waters offshore, the pressure can drop quickly again, and that's what often shows up as a secondary low pressure area off the coast. The numbers on the map are temperatures at 9 a.m. EDT. The arrows approximate the ground level air flow direction.
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Find out how the storm will affect you
An extensive lake-effect snow outbreak will occur with the coming cold.
Street flooding in some of the heavier rain