Archives For anticyclone

HIGH sits over UK

HIGH sits over UK

High pressure continues to dominate September weather in the UK. In Reigate it hasn’t rained for 8 days and temperatures during these sunny days with broken cloud have regularly reached the low 20’s and even Tmax 23c on Tuesday.  The HIGH is edging to the NE and this will encourage light easterly winds over the SE which often brings more cloud off the North Sea and a risk of more dreary anticyclonic gloom at times perhaps building up by this weekend.  Cloud at night will maintain temps easily into double figures for Reigate but depress them somewhat into high teens in the day.  Nevertheless, it will remain dry into the weekend and any sunny spells will feel delightfully warm for late summer.

 The blocking anticyclone is called an omega block which is named as it looks like the Greek letter.  Traditionally these are persistent blocks which can last for a long time maintaining dry settled conditions, as it is now.  The jetstream remains far to the north taking stormy weather over Greenland and Iceland (where the Holuhraun fissure eruption is taking place and the larger Bardarbunga volcano threatens to blow as the caldera sinks by a remarkable 20 metres).  Winds over Iceland are generally blowing away from Europe at the moment and taking the SO2 eruption cloud over East Iceland and Norway.  Last weekend there was a brief spike of SO2 over England and Wales which was brought down by a cold front that carried some sulphur dioxide from Iceland over Ireland and into Scotland, Wales and England.  The spike short lived and unlikely to cause any health impacts. 

The models (GFS and ECM) have been predicting the end of this HIGH pressure for a while but it has persisted and bucked the computer models.  Currently, models see the HP (high pressure) drifting slowly further to the NE next week and pressure will fall first across southern UK creating conditions for showers to drift up on a slack Southerly flow: some models bring the first light showers as early as Sunday and Monday, however this is probably too early. 

2014-09-10_21-54-15

Another factor that is worth watching is a developing tropical disturbance off Cape Verde in the Atlantic which model runs are predicting will arrive in the Mid-Atlantic in the middle or end of next week.  The track is not certain yet, of course, but it could have an impact on how this high pressure eventually either falls apart or even builds back in again after a possible unsettled spell from mid-week next week. 

2014-09-08_21-40-07

worst case scenario is only one possible outcome of many… direct hit of extra-trop on UK

High pressure persists this week over the UK as shown by the chart and the amazingly clear satellite pic showing great swathes of clear skies over a remarkably snow-free Europe today 11 March. It’s worth remembering that on this day last year we saw the lowest wind chill temps of the year in Reigate at -11.9c early on 12 March with -10c at 9:30am on 11 March during an episode of extremely cold NE winds which brought a blizzard to the Channel Islands.

Back to this week… the high pressure has brought delightfully sunny days and Tmax temps to Reigate over 19c last weekend , especially warm and spring like on Sunday. Since then a cooler NE/E breeze around the edge of the high as it slipped north and east has kept us cooler and occasionally more gloomy with anticyclone gloom and stratocumulus blanket on some days.

The outlook this week remains settled but with a distinct drop-off of temps over the weekend and especially into next week.  This is due to the HIGH slipping west and the jetstream dragging down cooler NW winds fresh from Greenland.  A LOW moving across N Scotland and over to Scandinavia this weekend will start the process of dragging down NW winds that will make next week cooler, especially further east where it looks like the N Sea will get a full blast of chilly air at times.

Nevertheless, it is unlikely to become terribly unsettled down here in the SE as the longer term picture looks like high pressure hangs on in the south keeping us mostly dry.  The north of the UK might see more frontal action as LOWS bring fronts across more northerly parts.  Models seem to be keeping the south relatively dry for another week to 10 days.

After the wettest winter on record, groundwater levels in the chalk aquifers are the highest EVER in the SE and the Mole Valley particularly.  There are still 50 flood warnings in the SE and locally flood water continues to pour across the A25 between Reigate and Dorking near Brockham.  This groundwater flooding, though of course nothing like as damaging as the fluvial river flooding during the winter along the R Mole and the Thames, could last for some months yet as the winter rain continues to percolate into the chalk water table and flows out along geological boundaries at inconvenient locations such as normally dry river beds and bournes and across impermeable surfaces such as roads. The map showing soil moisture below shows how wet the soil remains after this extremely wet winter.

Confidence now high for the HEAT WAVE starting weekend then continuing thru early next week and possibly beyond.  Updates later.

Early days still, but there is some indication of July warming up considerably from next weekend after a breezy mid-week. Models are coming together showing the Azores High building right across the UK from next weekend (7 July) bringing anticyclonic sunny and warm weather and a warm upper air flow. Some charts show temperatures in the south east exceeding 25°C and some even show 30°C! Whilst the UK is set to be much warmer than normal (positive anomaly), much of Europe is cooler than usual, still stuck in the trough of cool air.

warm UK 7 July

 It is worth remembering that the sun is very strong this time of year and, although a weather advisory seems tedious, it is worth slapping on the sun cream and adding a hat: slip,slap, slop!  Of course, our own heat wave will be nothing compared to the temperatures the SW USA is experiencing right now.  Record breaking heat is pushing temps up to the hottest ever recorded 54ºC.  That beats previously discredited records in Libya. ttp://www.nypost.com/p/news/national/western_issues_vegas_could_treated_Z7IWo9K7JwgXdDcqNOJ7jO

UV levels

Good news! HIGH pressure is building across the UK for early June and should last well into next week. The “Azores High” is moving our way and this will bring settled, calmer, warmer, drier, brighter and occasionally sunny weather to Reigate and much of the UK. Some rain is possible as fronts brush across the west and a trough over Europe brings a low risk of thundery showers later next week for the SE but this is not certain at this stage. The weather is looking mostly good for early June with temps reaching Tmax 20ºC.  

SSTs late May 2013Despite this the SE will remain average or cooler than average for the time of year due to being on the cool side of the HIGH: winds will be from a cool east or NE crossing the North Sea (still cool 8-9C) as the HIGH migrates to the North and East of the country.  

high and low pressure diagram

High pressure is where air sinks over a wide area. Sinking air is caused by cooling either from Tropical air chilling over comparatively cool oceans, like the “Azores High”, or air cooling as heat is lost from frigid continents in winter, like the continental HIGH over Asia./ Siberia.  Subsiding air inhibits convective updrafts of air (thermals) which create cloud and rain, so HIGHS are usually dry. HIGH pressure systems, also called anticyclones, also BLOCK the passage of low pressure cells pushing frontal systems away to the edges.  So, HIGHS are more or less the “opposite” of LOW pressure which has dominated our weather for most of May. In LOW pressure systems (cyclones or depressions) air rises, cools and condenses to form clouds and rain.  

air flows from high to low!Air flows from HIGH to LOW: that is, winds blow from areas of HIGH pressure to LOW pressure.  Due to the rotation of the Earth wind is deflected to the right in the Northern Hemisphere.  This is called the coriolis force and creates the familiar pattern of spiraling winds circulating anti-clockwise into LOWS and clockwise out of HIGHS in the Northern Hemisphere (it’s the opposite in the Southern Hemisphere).

 
fohn effect #1

fohn effect #1

The HIGH is forecast to move to the NE of the UK and this will bring in cooler NE and E winds to the SE: but light winds always. Remarkably, on Tuesday the relative location of HIGH pressure to the East and LOW to the SW of Iceland will mean Northern Iceland could be warmer than Reigate! The reason is an unusual weather phenomenon called the Föhn Effect.  Iceland has big ice sheets and mountains in the south and central region,including Vatnajokull, which is the biggest ice cap in Europe and has the highest mountains in Iceland.  The southerly / SW wind building over Iceland, created by the movement of the HIGH to the east and movement of a LOW to the South, will roll up and over these mountains and ice sheets and create LOTS of rain and snow!  On the windward slopes, in the teeth of these SW winds, saturated air cools as it is forced to rise over mountains.  However, as it it cools there is a release of latent heat due to condensation.  On the leeward side of the mountains and ice sheets the comparatively dry air sinks and warms up at a faster rate than it cooled. This is because the descending drier air warms more rapidly than the ascending cool air cooled (due to that latent heat being released)! (Still with me?!)  The result is a warm, dry wind streaming down from the mountains of central Iceland and bathing Northern Iceland in temperatures over 18ºC. It is rather like an “extreme” rainshadow effect experienced to the east of mountains in the UK, for example.  Föhn winds are associated with most high mountain ranges which experience stable saturated airmasses being forced to rise over them, such as the Alps and the Rockies.  In the Rocky Mountains, the Fohn wind is called the Chinook or “snow-eater” which melts snow rapidly across the plains at this time of year.