Summer Synoptic Setups

[Thundery wintry showers]

This is the follow-up to the Winter Synoptic Setups thread, detailing the sort of setups that occur during the summer.

Low Pressure Scenarios

The Standard Summer Synoptic Setup

This is traditionally the prevailing weather setup over the British Isles in summer. The Azores anticyclone sits to our south-west, while pressure is low to our north, resulting in "unsettled" low pressure weather, particularly in the north. Depressions typically bring bands of rain from west to east, with drier brighter interludes in between. Southern areas, with the Azores High close by, see the warmest and sunniest weather as the Azores High extends its influence northwards. Recent examples of this kind of synoptic setup occurred in summer 1998 generally, and June/July 2002.

Summer weather under low pressure conditions

When low pressure prevails in summer, it is common for forecasters to say it will be "unsettled", or "sunshine and showers", but in reality, different low pressure scenarios can give rise to different weather types, many of which couldn't be accurately described as "sunshine and showers". Here is a summary of what to expect in general from different low-pressure scenarios.

If you see a chart like this, with frontal systems moving west-east across Britain at intervals, you will usually see areas of cloud and rain move over from time to time, with the cloudy wet weather lasting for upwards of a few hours at a time. In between frontal systems, the general rule is that if the last front to pass over was a warm front, then it will be cool and drizzly near windward coasts and hills, and warm and muggy elsewhere. If a frontal system stalls over an area of the British Isles, expect it to be dull and wet for a prolonged period.

However, after a cold front has passed through, the specifics of the weather type depend on various factors:

If fronts are draped around the low pressure system, especially if they are warm or occluded fronts, it won't follow that the weather behind a cold front will be bright and showery. Rather, it will be cloudy with some bits and pieces of rain for most places. If winds are very strong, then clouds will tend to spread outwards rather than rising upwards, so cloudy scenarios, rather than "sunshine and showers", will tend to result in this case too.

Conversely, if the area behind a cold front is devoid of frontal activity, and particularly if winds are light or moderate, then the result will be "sunshine and showers". Strong surface heating from the sun during the day causes convective clouds to rise during the morning, leading to a mix of clear intervals and heavy showers during the afternoon; if winds are light then showers will be slow moving, and sometimes thundery. If you see thin black lines on a Met Office FAX chart which resemble fronts but don't have triangles or circles, expect sunshine and showers also, because such features denote lines of showers rather than persistent frontal rainfall.

Summer High Pressure Setups

Most interest during the summer on these forums centres on high-pressure setups. Here is a synopsis of what to expect from various high pressure setups:

The Azores High can ridge into southern areas during the summer, and this tends to bring very warm and sunny conditions to southern Britain- sometimes temperatures can reach 30C. However, with moist westerlies in the north, northern areas tend to be cloudy. A good example of this pattern occurred in early June 2004.

This synoptic setup, with high pressure right on top of the British Isles, tends to bring hot dry sunny weather to all parts. High pressure was sat over the British Isles for much of the time during the famous hot dry sunny summers of 1976, 1989 and 1995. Sunshine is more likely to be guaranteed during the summer with high pressure than during the winter half-year as the strong sun tends to burn off any low cloud. Watch out for fronts draped within the highs though; if fronts hang around Britain while high pressure is on top of us, then the weather may be dry but cloudy instead.

If you like hot thundery weather, then this is the most ideal setup for it. High pressure over the Continent draws up hot southerly winds from southern France, Spain and/or the Mediterranean, bringing temperatures into the mid to high 20s C, sometimes the low to mid 30s in the south. Thundery showers may develop over the Continent and spread northwards, while homegrown thunderstorms can develop across the British Isles also. The cold front on the map marks the battleground between cool Atlantic air and the hot continental air, and if this frontal battleground is located over the British Isles during an afternoon, severe thunderstorms often result.

In general, if the winds are southerly or south-westerly, the thunderiest region is south-east England, exposed to the storms moving up from the continent, which then transfer northwards up the eastern side of England. If, however, the southerlies have an easterly component, then thunderstorms often move up the western side of Britain.

This setup, equivalent to the kind of synoptic setup that many snow lovers in southern and eastern Britain look out for during the winter, has high pressure to the north and northeast, and low pressure typically to the south and south-west. The continental airmasses are warm during the summer, and so the easterly winds bring plenty of warm sunshine to much of central and western Britain. In the south, the low pressure may pump some thunderstorms up from the near Continent, which sometimes transfer up the western side of Britain also. However, eastern Scotland and north-east England are often cold and cloudy in this setup with the easterly winds bringing low cloud, fog and drizzle off the North Sea.

When pressure is high to the west of Britain it enables cold northerly winds to affect eastern and central parts of Britain. Western areas are usually dry and sunny with cool nights. For eastern areas, if fronts are embedded in the northerly airstream it tends to be cold, damp and cloudy, while if the airstream is free from frontal activity, the weather will be cold and bright, with the surface heating from the sun and low pressure to the east combining to give heavy afternoon showers, sometimes with hail and thunder. As a general rule sunshine tends to be rather below average in the east. The first half of June 1995 was dominated by this setup, before the high moved east to bring the famous hot dry sunny summer.