The jetstream meanders like a river of air in the high atmosphere. Much of the time the jetstream blasts pretty straight west to east, at other times it loops wildly north to south. This week the jetstream is due to loop wildly, like a meandering river and threatens to form a special weather feature called a “cut-off low”.
How do cut-off lows form and why are they special?
1# A loop of the jetstream descending over the UK is forecast to form a TROUGH of low pressure over Europe this week. The southward limb of the jet directs cool polar air towards Europe and LOW pressure.
2# The loop becomes so sinuous (bendy) that, like a meandering river, the neck is cut-off as the jetstream re-forms to the north. The LOW to the south becomes “cut-off” as HIGH pressure builds to the north.
3# The cut-off LOW over Europe is left as an “ox-bow” of cool unsettled weather, especially near the centre of the low pressure.
4# The surface warms and CONVECTION occurs through the cool air: air rises forming big convective storms. These are forecast to migrate from the continent and effect especially the South and SE of England. Higher pressure should keep the north more settled.
5# Cut-offs fill gradually as warm air convects aloft, reducing instability.
6# The rest of May looks to have more rain and cool unsettled conditions as LOW pressure dominates.
(pics courtesy of netweather)