I couldn't let today pass without sharing what a strange set of circumstances came with the center of high pressure today. Skies were clear most of the night, then a thin band of clouds developed pre-dawn from the Cape to New Haven, CT. I acknowledged these clouds on the air in my broadcasts and mentioned they'd likely be stubborn at the South Coast, but most other areas would remain cloud-free. That was at just after 5 AM. By 6 AM it was obvious the clouds were spreading north, and that required a quick analysis to figure out what was going on. After a series of real-time and model analyses, I was able to determine a few things...
1) An inversion, which was expected, had developed around 950 mb.
2) Very low relative humidity was projected by the guidance through nearly all levels of the atmosphere. The NMM did an excellent job, however, at forecasting a thin layer of 90%+ RH values below 950 mb at the South Coast.
3) Further analysis showed that in this 950 mb zone - at the top of the boundary layer and just below the inversion - a slight shift in the wind was forecasted to occur between 09Z and 13Z - winds were progged to blow from the southeast very lightly at this level, and this coincided perfectly with the onset and expansion of clouds.
4) During this time of SE winds between 2000 and 2500 feet (just below the inversion), warm advection occurred at that altitude, further strengthening the inversion.
I assumed the clouds were the result of this warm advection in a very thin layer, and cloud bases lined up well with this. It appeared this wind shift would make it no farther north than the Massachusetts Turnpike, so by my 6:21 AM broadcast I had ammended my forecast appropriately, and forecasted clouds to clear from north to south from 10 AM to 2 PM, Turnpike to South Coast, as the winds shifted. Much to my surprise, the clouds actually would end up clearing from SOUTH to NORTH, very suddenly and quite pronounced. The best I can figure is that the warm advection continued to the point of stabilization, squashing any cloud production when coupled with subsidence, though I can't be certain and am open to ideas. Click here to see a loop of the visible satellite imagery.
As if this weren't enough strangeness for one day, it wasn't long after that I got the following email, from an observer driving from Grantham, NH, to Hanover, NH: "