GOES-R IFR Probabilities, hourly from 0200 through 1015 UTC on 18 May 2015 (click to enlarge)

Easterly winds south of a High over the Canadian Maritimes ushered in cool, moist air over the Northeastern United States early on Monday. IFR Probabilities, above, show the progress of the low ceilings and reduced visibilities that accompanied the change in air mass. The low clouds eventually penetrated to the Delaware River, as shown in the GOES-14 SRSO-R animation here (from this blog post).

In the animation above, observations of ceilings and visibilities over southern New England approach IFR conditions quickly as the IFR Probability ‘front’ passes through.