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The Infamous "Omega Block"

The images below are forecasts of the 500 millibar geopotential heights and temperatures for Thursday morning and early Saturday afternoon. The 500 millibar level is a standard meteorological level aloft. It is roughly 18,000 feet above the ground, but it's height varies due to changing pressure. The geopotential heights are lines of constant height (how high you have to go to reach the 500 mb level). The wind pretty much flows parallel to these lines. The first image for Thursday morning shows a 500 mb trough (the axis of this trough is depicted by a black, dotted line) over our region, which is associated with cooler temperatures. Once this trough moves to the east, the temperature will moderate. Highs Thursday will be mostly in the 50's - maybe 60 across some interior, low elevations. Friday's highs should average a degree or two higher. There will be a few clouds developing during tonight, in response to the trough, however it will become sunny Thursday and we can expect a ton of sun for Friday & the weekend!

The second image shows the 500 mb forecast early Saturday afternoon. It shows a blocking pattern known as an "Omega Block". You can see where it gets its name as the flow around this set-up roughly traces out the Greek letter Omega (drawn in white on the image). The air in the center of this pattern (around the high pressure center) is sinking and warming, so some nice warm weather will be found under it. The core of this warm air will remain to our west, however enough of it should be able to mix in Sunday to allow much of the region to get up around 70 degrees, although some shore points may stay cooler or turn cooler as local on-shore wind flows develop. Saturday's highs will be more like 60 or so, except cooler in coastal areas - especially those open to a northeast wind like eastern Long Island and southeastern MA.

It's still too early to plant outdoors as low temperatures in many spots will be near or below freezing during the next couple of nights. The video loop below shows the forecast surface pressure, wind, and temperature profile over the next few days. Notice the northeast wind flow coming around the high pressure system that positions itself to our north. That will be a cool wind. It looks to be more of a factor Saturday (especially in coastal and southeast portions of southern New England and parts of Long Island. By Sunday the high should be centered close enough to us to allow for light & variable wind, except perhaps some shore points which may sea-breeze.

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