Tuesday 7 a.m.
A low pressure area drifting slowly northeastward from North Carolina is causing bands of rain, some heavy with embedded thunderstorms, to its north. This rain will slowly spread to Pennsylvania and New Jersey later today and tonight, then farther north tomorrow and tomorrow night. Anytime there is a banded structure to the precipitation and highly varying rainfall rates, chances are there will be quite a lot of variation in the rainfall amounts received from one place to another.
As the low pressure area departs, there is no strong high pressure system coming in to replace it. Some moisture will linger, and this can lead to varying sky conditions and even a few showers.
A strong cold front will cross the Great Lakes Saturday and reach the East Coast on Sunday. This front can trigger one or more lines of showers and thunderstorms. After that front goes through, a strong northwesterly flow of much cooler air will take over. Downwind from the Great Lakes, some showers can break out, and it'll be so chilly some residents will think it's going to snow (and over the highest terrain it could!). Our concern with the new air mass will be the potential for a frost or freeze. I suspect that city gardens from Chicago to New York City will be okay, but if you live at a normally colder location, you might want to resist the rush to plant annuals.
Today's Northeast video
This video concentrates on Chicago but could also be useful if you're in Detroit, Milwaukee or any other place around the Great Lakes:
The Middle Atlantic storm has a history of causing thunderstorms. This map shows the lightning strokes from the middle of last night through early this morning.
One concern: the chance of cold frontal snow squalls that could move all the way to the East Coast tomorrow night. Sudden snow squalls have been implicated in chain reaction collisions that turn deadly and damaging.
These two maps show the change from the very, very cold flow likely this Saturday to the much milder Pacific-origin westerly flow later next week.
When we look more closely, we see a variety of disturbances embedded in the main current, each capable of temporarily increasing or cutting off the chance of snow. This map shows the setup:
This map shows the circulation around the offshore storm and a larger but less intense storm moving into the Great Lakes. With this sprawling storm likely to be in the region for several days, the weather can vary widely.
...speculation about a snowstorm Monday or Tuesday, and one is still possible. However, timing and placement remain elusive. This map shows the GFS ensemble mean "solution" for Tuesday morning showing snow just off the New England coast. Watch this story evolve on accuweather.com all weekend.