As of this writing, life threatening flash flooding was ongoing throughout the Carolinas. Radar estimates show over a half foot of rain has fallen so far and those amounts could double those areas through Saturday night.
Matthew will continue to weaken because of previous land interaction and now cooler ocean water temperatures. There is still plenty of moisture associated with the storm.
The big weather picture shows a cold front moving east through New York State. That front will trigger showers throughout western New England through Sunday morning. As the cold front continues to move toward the coast, it will begin to tap into some of Matthew’s moisture.
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Look for widespread showers Saturday night and Sunday and more of a soaking rain for the Cape and Islands. The gradient between very little and a lot of rain will be tight. Rain will be measured in tenths of an inch outside of the tropical downpours — while areas that see the steadier rain could see inches of rain.
It all has to do with timing. The slower the cold front moves, the heavier and longer the rain will last. If the cold front moves through quickly, the humid, tropical air will offshore and won’t be a player in our forecast.
Gusty winds will also develop (not from Matthew, but because of a tight pressure gradient) late Sunday and into Monday. Wind gusts may reach 40 MPH at the coast and Gale Watches have been issued for the adjacent waters.