A wild meridional loop in the jet stream on 14-15 Feb injected a cool pool of air over NW Africa that created unsettled conditions, cloud, cold temperatures, rain, snow and dust storms over Morocco and parts of North-Western Sahara. RGSweather went to Morocco to investigate the effects of this African storm and bring you the experience of an unusual African cut-off low first hand! (well, we were going there on an expedition anyway, so why not?!).
A cut-off low is a depression, usually associated with mid or high latitudes when a meridional (bendy) jetstream loops down and leaves a cool pool with a resultant low pressure system “cut-off” as the jetstream returns to a more northerly latitude. Cut-off lows sometimes barely show up on pressure charts but are often persistent features that introduce unsettled weather: often associated with showers as convective activity warms through the atmosphere and gradually “fills” the LOW.
The Moroccan cool pool caused winds to circulate around the low and drag dust and sand over the High Atlas for a period as easterlies blew in across the Sahara as the low moved north and then east.
Snow is common across the Atlas mountains during winter but temperatures fell unusually low and snowfall across the Atlas fell below 1000m.
Gradually, as the cut-off low filled and moved NE, pressure built and skies cleared and, at low altitudes, temperatures rose. Dew points fell as humidity fell and nights remained exceptionally chilly!
The animations below show the cut-off forming and cool-pool with showers and frontal precipitation (snow in mountains) over NW Africa and Morocco. Probable influence of sub-tropical jetstream too but information not available on this at the moment.