Despite the gloomy stratocumulus which didn’t shift much this weekend, it has been warmer than usual. Saturday night was especially balmy with Tmin of 16c and today saw Tmax a shade off 19c. This week warm Tropical upper air will continue to waft up from the South being generated by a deepening TROUGH of LOW pressure spinning up just NE of the Azores. This will continue to pump a warmer than usual plume of upper air at +13c 850hPa (5000ft, the height air mass characteristics are usually measured) which flirts off and on with the south of the UK almost all week. The result is warmer than average September temps by some +4 ot 5c. It will, however, gradually get edge away as the week progresses.
A weakening HIGH pressure over the UK will gradually melt away and drift over the continent and allow cooler air into Northern Britain. Reigate will remain in the warmer air most of the week by the looks of things.
Most models agree that the Azores LOW will fill gradually and drift NE over Spain and France during the latter half of the week and bring a nasty rash of showers to those areas. These might encroach on Southern England by the end of the week and weekend bringing potential for thundery outbreaks but this is too far off to be certain.
The distribution of rainfall is somewhat unusual, with a large accumulation of rainfall accumulating in a great spiral to the west of Spain associated with the Azores “Low” and lots of rain in Eastern Europe associated with the large Russian LOW. Note that the SE of England and Reigate in particular might just be one of the driest places in Europe by the end of next weekend as we seem to be sandwiched between these two great weather systems… well, models permitting! *one of driest places in Europe, probably! The GFS shows no rain at all for Reigate this week, but that’s quite a long shot!
Another observation to match the LOW over the Azores is the negative forecast for the NAO: the North Atlantic Oscillation measure. The NAO shows the pressure difference between Iceland and the Azores: when NAO is positive there is a big difference / pressure gradient and a strong jetstream and unsettled, fast moving weather across the Atlantic. When negative, like this week, there is a meridional jetstream, a weak flow west to east and weather systems can get stuck… so not much change is expected. This matches the forecast this week… not much change and no fast moving weather.
The ensemble model below shows that there will be a gradual decline in temperature during the course of next week and certainly beyond. More unsettled weather is also likely to arrive sometime next weekend but models still look rather uncertain so… keep checking back!