Dull and damp in the Northeast this weekend, but becoming milder

An atmospheric river of moisture stretched from the Pacific Ocean through Mexico and then northeast over Arkansas and the Ohio Valley. You can see it on the map below. A recent research study (see, for example https://www.researchgate.net/publication/284256679_Atmospheric_rivers_A_mini-review) suggested that 60-70% of middle latitude rainfall can be attributed to them. Although the term only came into use in the 1990s, some aspects of these features have long been studied. It's just that they were not recognized in this way. The bands of moisture were simply associated with various low pressure areas and fronts. This kind of discovery happens in many fields. For example, in medicine, various symptoms and conditions have been observed for decades or even centuries but have only recently been recognized as part of a bigger picture.

Note how the river is not necessarily a continuous feature; in this case, there is a major gap over Mexico. In any case, with southwesterly flow aloft, mild moist air will be aimed at the Northeast. In addition to clouds and episode of rain, areas of fog will form as the moist, mild air passes over the cold ground or water. The following map shows the surface circulation earlier this morning:

Meanwhile, flooding is occurring on much of the Ohio Valley, as shown on this map:

Additional rainfall can aggravate the situation.