Cb arcus

Cumulonimbus clouds are heavy, dense clouds with considerable vertical extent usually dark grey or almost black. This photo taken at around 07:45 CET is a panoramic view of a cumulonimbus arcus cloud, also known as shelf cloud. This shelf cloud developed at the leading edge of an approaching severe thunderstorm due to downdraughts of precipitation cooled air probably respresenting the edge of the cold front itself that approached the Islands as per actual surface pressure chart on the third thumbnail combined with a cut-off low over the Western Mediterranean. Actually, the weather forecast as per forecast surface pressure  chart on the seventh thumbnail called for the triple point of a Wintry low pressure system to pass over the Maltese Islands whilst instead it passed to the north of Sicily making for difficulty in forecasting exactly where the strongest cumulonimbus clouds could be viewed. The air temperature before the storm was 15C before dropping to 10C just after the storm passed. It moved relatively quickly and despite the line of heavy rain on the radar image of the fourth thumbnail, the squall line itself only dropped an average of around 5mm in few minutes and an official maximum wind speed of 65kmh (Force 8) though very localized higher wind gusts of around 100kmh were reported. The weather sounding on the second thumbnail showed very moist and unstable mid and high levels at altitudes above 2800 metres. The wind profile just before the passage of the cold front was sufficient for severe storm organization, though vertical wind shear (10-15 m/s) and storm-relative helicity (100-200 m^2/^2) were mostly concentrated across the lowest 1000 metres owing to a Southerly low-level jet that hardly increased at higher levels anymore.

Estofex explanation: The cut-off low over the W Mediterranean is accompanied by a surface cyclone near Sicily. The SW-erly flow at its forward flank advects an elevated mixed layer (EML) from Libya atop a moist maritime airmass over the Ionian Sea.

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