With Atlantic water temperatures near or just above 50 degrees in a number of locations, the differential between the warming land and the cooler water has allowed for that rite of Spring known as the Marine Layer to rear its ugly and foggy head across the region. Marine Madness is our March and early April phenomena -- low clouds, fog, drizzle on occasion as an easterly flow sets up shop and blows cooler, stable air inland to compensate for a warmer land surface.
It's a nastier and more widespread version of the localized sea breezes that the Shore gets each summer, with fog and sometimes drizzle the added byproduct of the east breeze. This marine layer setup is best served when high pressure is situated to our north or northeast and winds on the clockwise rotation around the high pressure center come in from the east and southeast.
Tonight...and probably tomorrow night...will feature two more go-rounds with the marine layer, which is something that several days out can be difficult to forecast but becomes a bit easier in the shorter term thanks to the advances in computer modeling. Below are three graphic shots from the higher resolution NAM run by Penn State -- it's a pretty decent forecasting tool to look at for localized trends in temperature, wind, and cloud cover. It's not perfect and like all of the other computer models struggled mightily with yesterday's clearing trend and temperatures. However, it has performed very well at other times (Thursday's cooler temperatures were correctly called for on the NAM).
As the morning wears on, the low clouds and fog burn off from northwest to southeast. This is caused by the sun's rays being able to work through the deck of clouds, warming the ground just enough to cause the atmosphere to mix and help burn off the fog and low overcast. The layer of stable air farther inland is not as thick as it is near the coast as it is farther removed from the cooler water...and as a result it can warm up a bit more quickly...and burn off the clouds quickly. Depending on how fast that burning off of the overcast takes place, sunshine typically returns by mid to late morning through the region.
We typically rinse and repeat this cycle several times through this particular time of the year...thankfully, not as often as San Francisco!