Last week, Photo Gallery user Buz uploaded a series of photos of bizarre-looking clouds rising and twisting off the North Carolina coast at Wrightsville Beach. I have taken the liberty of combining the photos into a panorama,* which, although not perfect (beach panoramas are tricky because of the waves), gives a general idea of a wide-eye view of what Buz was seeing:
Based on a discussion of the photos over on the AccuWeather.com Forums (feel free to put in your two cents there), I thought at first that Buz had caught ocean-effect snow, or at least the clouds that precede it, in the act of forming. (Ocean-effect snow is produced in the same fashion as lake-effect, with cold water blowing over warm water, causing clouds to rise and precipitation to fall).
But then I looked at the radar animation** for that day (the pictures were taken at sunset is around 22 Z time, according to the Navy sunrise/sunset site), and realized it wasn't quite that simple.
CLICK TO ANIMATE
From the radar animation, however, it's clear that the cell, which I've pointed out with the arrow, did not form over the ocean, but in fact came from the land. Snow or rain showers arising from instability (defined as cold air over the warm earth) tend to form in a cellular fashion, and probably what the panorama shows is the little green spot shown indicated on the radar to the east of Wrightsville Beach (which is just below Wilmington), with the moisture (from the much warmer ocean) streaming upwards into the clouds.
So it was ocean-effect in a way, but not spawning the showers, just adding moisture to them. So why don't we see photos like this all the time? It's a story of extreme temperature contrast between the ocean and the air above it. The early-season cold spell brought unusually low temperatures racing over the land (according to AccuWeather.com, the average temperature on the 18th in Wilmington was 39, compared to a normal average of 55), while the ocean (which cools more slowly) was still quite warm (around 70 according to SSEC). The cold air blowing over this extremely warm water caused "steam" to form rapidly and visibly (you'd never see this type of difference between land and air on the Great Lakes, for example, which cool to the 50's before the earliest lake-effect outbreaks).
*Arcsoft Panorama Maker 4 Pro has allowed me to make many panoramas on the Photo Gallery based simply on photos taken with my regular camera. As long as you are careful to 1.) Take the photos quickly (before things change) and 2.) With enough overlap between each photo in the sequence, you can get pretty good results. Beach panoramas are particularly tricky because the waves change in each piece of the final image.
**Courtesy the Plymouth State NEXRAD archive.
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