Jump to content
Thunder?
Local
Radar
Pollen
IGNORED

Winter Synoptic Setups Not Associated With Snow


Recommended Posts

Posted
  • Location: Lincoln, Lincolnshire
  • Weather Preferences: Sunshine, convective precipitation, snow, thunderstorms, "episodic" months.
  • Location: Lincoln, Lincolnshire

    Here is an overview of the setups we often see during the winter that do not generally bring snow, and that can persist for prolonged periods, together with the weather we can usually expect from them.

    Atlantic depressions, westerly and south-westerly winds

    atlantic.jpg

    This is probably Britain's most common winter weather setup. Low pressure systems move from west to east to the north of the British Isles at intervals, bringing fronts with them- fronts are associated with cloud and persistent rain, with steady light rain at the warm front, and heavier and often shorter lived bursts of rain at the cold front. Pressure is consistently low over and to the west of Iceland, and high over the Azores, encouraging the westerly and south-westerly winds to funnel in across Britain.

    Following a warm front, tropical maritime air (denoted by the green arrows, originating to our southwest) affects the British Isles and brings very mild and damp weather, although the cloud can sometimes break up to bring sunshine to the east of high ground.

    Following a cold front, returning polar maritime air (denoted by the light blue/green arrows, originating in polar regions) will dominate, bringing broken cloud, sunny intervals and showers. The returning polar maritime air is not particularly cold because the low pressure over and to the west of Iceland forces the ex-Arctic air to take a very long track over the ocean, which raises the temperature of the air such that by the time it reaches us, temperatures are near or a little above the long-term average.

    Generally speaking in these situations, if "warm sectors" and tropical maritime air dominate, the weather will be very mild, very dull and wet in the west but relatively dry and sunny in the east. If returning polar maritime air dominates, the weather will tend to be wet (especially in the west) but also sunny (especially in the east) and with temperatures near or a little above the long-term average.

    Watch out for the term "mild zonality", which is commonly used to describe this particular setup. It is so-called because "zonality" indicates free west to east movement of depressions and/or persistent westerly winds, while the temperature is generally mild.

    On the other hand, "cold zonality" is usually a reference either to setups with lows frequently moving west to east and Britain remaining in cold polar air, or to the returning polar maritime snow setup described in the "Snow Setups" thread with predominantly westerly and north-westerly winds bringing snow events. However, for westerly weather to be cold, pressure must not be low to the west of Iceland, so that returning polar maritime airmasses have a shorter track over the ocean.

    Persistent high pressure over Britain

    highpressure.jpg

    In Britain we have been taught that high pressure usually brings good weather. In reality, it depends on how you define 'good', and also on the characteristics of the high.

    When high pressure is right on top of the British Isles in winter, the weather tends to be rather cold and dry. Sunshine amounts are dependent on how much cloud is trapped within the 'high'. A high with a lot of cloud trapped within it will give much 'anticyclonic gloom'- days, maybe even weeks, of dull dry weather with layers of featureless grey cloud. On the other hand, a high associated with clear skies will tend to bring cold crisp sunny weather and night frosts, although freezing fog can often form in these situations and be slow to clear during daylight hours.

    The anticyclonic gloom scenario is particularly likely in areas where the winds are off the sea- so with the high positioned over Scotland, eastern England is often dull, while a high centred over southern England will mean dull weather over western Scotland, Ireland and north-west England.

    Because of the variable nature of sunshine within winter high pressure areas, high-pressure dominated months range from the exceptionally sunny (e.g. January & December 2001, February 2003) to the exceptionally dull (e.g. February 1993) and can also contain wide local and regional variation in sunshine totals.

    The Bartlett/Euro High

    bartlett.jpg

    This is a setup often dreaded by snow lovers. High pressure becomes established over Europe, forcing low pressure systems to be deflected north-eastwards once they have passed Iceland. This keeps Britain in a mild, moist south-westerly airflow in association with tropical maritime air and maintains a 'warm sector' over Britain, with any colder returning polar maritime airmasses kept away to the north. Once the Bartlett high establishes over Europe, it can persist for weeks, even months, on end, and prove very difficult to shift.

    The weather in southern and eastern Britain is usually very dry and very mild. Sunshine is dependent on how much cloud is circulating around the high, as some highs have cloud trapped within them while others are associated with clear skies. February 1998 produced a classic example of a 'sunny' Bartlett High with temperatures reaching 16-19C around the middle of the month with largely unbroken sunshine.

    However, in the north and west, under the influence of the south-westerly winds bringing a lot of moisture off the Atlantic, the weather under the Bartlett High setup is nearly always dull and damp, with very high rainfall totals in western Scotland. Correspondingly, months dominated by the Bartlett High often have big contrasts between the dry sunny southeast, and the dull wet northwest, and tend to be particularly unpopular with people living in the northwest.

    The mid-Atlantic high

    midatlantic.jpg

    The mid-Atlantic high brings mild damp weather up from the tropics and around from the north-west, as shown by the green arrows. Hence, even though the winds are typically from the north-west, they bring above average temperatures. The large amounts of moisture picked up over the ocean produce large amounts of cloud and some rain for north-west Scotland, and this cloudy damp weather usually transfers around the high through northern and eastern parts of Britain. Areas in the shelter of high ground may see a little sunshine break through at times.

    Cold polar air is directed into eastern Europe, where temperatures tend to be somewhat below average.

    For southern and western areas, where the high is particularly dominant, sunshine amounts may again depend on how much cloud is trapped within the high. In the case of a 'cloudy' high it will be dull almost nationwide, while in the case of a 'sunny' high, western and southern areas in particular may see large amounts of sunshine and some frosty nights. Contrasting examples include November 2004 (which was dominated by a 'cloudy' high) and January 2000 (which was dominated by a relatively 'sunny' high)

    Months dominated by the mid-Atlantic high often see the high retreat southwards on occasion (bringing mild damp westerly winds to all parts). Snow events can occur when the high moves north-westwards towards Greenland allowing cold polar air to attack from the north, but more often than not in these situations, the high is subsequently 'toppled' by Atlantic weather systems and the northerly winds amount to a 36-hour blast with wintry showers confined to northern Scotland and the east coast.

    Persistent southerly winds

    southerly.jpg

    When pressure is consistently low to our west and high to our east, southerly winds will prevail over the British Isles. The characteristics of the weather will depend on how dominant the 'low' and the 'high' are, relative to each other.

    If the low pressure is the more dominant, the weather will be very mild, dull and damp, with some rain for western areas in particular. If high pressure is dominant, the weather will be dry, with sunshine amounts again determined by how much cloud there is around the 'high'. It may be mild when the air originates in the Mediterranean, but on the cool side if they are originating somewhere in continental Europe. Despite the lack of moist ocean influence into southerly winds, however, such highs tend to be cloudy more often than not.

    Snow is almost unheard of in this setup, although it is not unusual for this setup to be followed by the high moving over to Scandinavia and starting to drag cold continental air westwards towards Britain, whereupon snow becomes more of a realistic possibility.

    Link to comment
    Share on other sites

    • Replies 0
    • Created
    • Last Reply

    Archived

    This topic is now archived and is closed to further replies.

    ×
    ×
    • Create New...