21Dec17 Gregale

Large sea waves with an average height of 5.4 metres (very rough) due to Gregale which is a Northeast wind blowing with an average wind speed of 48kmh (Force 6) and gusting up to 80kmh (Force 9) on the day. In this photo, the very rough sea was clearly invading the road below of the capital city. In fact, some wind damage was reported in the Islands due to high gusts. As shown in the surface pressure chart on the fourth thumbnail, this wind was the result of a 1009 hPa low pressure system over Libya contrasting with a strong 1041 hPa Azores anticyclone over the Bay of Biscay generating NE gales from the Balkans as per surface wind field on the fifth thumbnail. The large waves might have been slightly mitigated by the tips of Southern Italy and Sicily reducing sea fetch. The first thumbnail is a smaller wave crashing over the rocks before the waves were observed becoming stronger reaching the roads above with consequences to parked cars. The second thumbnail is a panoramic photo of the Grand Harbour in very windy conditions with the cloudy sky conditions being mostly composed of stratocumulus stratiformis opacus clouds and some cumulus. According to the weather sounding on the third thumbnail, these clouds formed due to instability in the first 3 kilometres of the atmosphere caused by the inflow of a cold airmass over a warmer sea. The upper levels was stable, though very moist, thus not allowing the stratocumulus to become thunderstorm clouds with heavy rainfall though the upper cloud layer produced nimbostratus clouds with some rain. The event ended as it started on the following day with cirrostratus clouds producing solar halos being observed again even after the storm system was leaving the Islands. For the photo and related explanation of the solar halos that preceded this storm, kindly click here.

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