When relatively high dewpoints move over the cold waters of the Gulf of Maine in Spring, advection fog can form. Sometimes this happens underneath clear skies, sometimes it happens underneath high clouds. In both cases, the IFR Probability field should give a reasonable answer — but how can that prediction be validated? In the example above, the apparent fog/low cloud bank propagates off to the east, and when it is fully ashore near Yarmouth, NS, visibilities drop from near-IFR to IFR conditions. The IFR probability field has an appearance that suggests plenty of high clouds are overlaying the visibility-restricting lower clouds, yet a consistent signal of higher probabilities of IFR conditions is maintained in/around the Gulf of Maine northward into Maine.