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enter image description here Here is a GOES visible image from today, 2019-12-01 about local noon, around Nova Scotia. Note the parallel trains or streaks of clouds, running approximately northwest to southeast.

A few hours ago, the wind was from the NW at 30-40kph (at Halifax); for the past 3 hours it's been from the NNW at about 25kph.

What's the physics behind the streaking? Is it simply cumulus clouds from convection being blown downwind?

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The species is Stratocumuls undulatus Sc un.

They can form for example when cold, dry air moves over a warmer ground layer. A sharp wind shear at the boundary layer blows these waves. (Wikipedia on Sc).

They are not to be confused with Sc radiatus or yet another form, the so-called "cloud streets", which form along the wind direction.

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My wild guess would be windshear at a low level inversion. Cold calm air below, warm windy air above, with clouds forming at the inversion. Could be checked with a look at an atmospheric sounding with wind, temperature and due point curve from the area and the time ... Cartoon for visuals

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