Midweek Rainfall UncertaintyAdded : Sunday at 12:30 There is growing confidence that Tuesday night into Wednesday will bring a spell of rain south-eastwards across much of the British Isles as a frontal system moves in from the Atlantic, eventually introducing much cooler temperatures to southern parts of the British Isles.
What remains rather less certain is just how active that system becomes as it crosses the country, with the main numerical models showing some fairly significant differences in the intensity and organisation of the rainfall.
By Wednesday morning the ECMWF ► and ARPEGE are particularly keen on developing a more organised feature across southern Britain, with an area of low pressure forming along the frontal boundary and some potentially heavy rain developing across parts of England and Wales :-
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The ECMWF ► is especially lively, showing some intense bursts embedded within the broader area of rain as the system pushes eastwards. ARPEGE has a broadly similar idea, although there are differences in the precise position of the heaviest precipitation. The GFS ► is somewhat less aggressive, maintaining the frontal rain but generally producing a less intense and less organised system as it crosses the country :-
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The difference appears to hinge on how much development takes place along the frontal boundary as it encounters the warm and relatively moist air sitting across southern Britain. A more pronounced wave or secondary low would slow the eastward progression of the rain and allow rainfall totals to build, whereas a weaker feature would allow the front to continue moving through more readily. This is therefore one of those situations where relatively small differences in the evolution of the surface low could have quite a large impact on both rainfall intensity and duration.
Behind the system, a noticeably cooler and fresher airmass will spread in from the northwest, finally bringing an end to the unusually warm conditions of recent days and giving the second half of the week a much more autumnal feel :-
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METEOROLOGIST : BARBER |