Archives For May 2013

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.

weather balloon

Weather information is captured from satellites, surface weather stations, ships, planes and also weather balloons which are launched regularly from sites all over the world.  Weather balloons have instruments which continuously capture data during their ascent through the atmosphere on several weather parameters including temperature, wind speed, humidity, pressure etc. The map shows the track of a balloon launched today, that ascended to 40,000 m over Swindon. The car icon shows the chase vehicle used to collect the instruments when the balloon eventually falls to earth.  You can track more weather balloons live at

Heads up, Reigate!  Or rather… wrap up, stay in and keep warm: SE weather for the end of this week looks awful and, whilst nothing approaching the US Mid-West, it could be described as potentially severe for the SE.

Pick words from this list and repeat and then remind yourself it’s nearly June! “COLD, heavy persistent rain, windy, wet snow, sleet, hail, heavy thundery showers, freezing wind chill, 35 mph northerly gusts… at the end of May!

Thursday: bright start but with cold northerly winds +20mph and heavy showers building in the afternoon.  Feeling chilly at only 10ºC max.  Overnight temperatures fall to 3ºC and any showers could even fall as icy sleet or even wet snow on the Downs. 

Friday: things actually look worse than earlier this week: COLDER northerly winds will drag temperatures right down to 3 or 4ºC in heavy showers and a maximum of  8 – 10°C with temperatures falling lowest in showers.  The centre of a nasty deepening LOW pressure will track directly across the SE on Friday and we can expect persistent rain, some heavy at times and accompanied by really cold winds (for the time of year).  Later in the day heavy thundery showers have the potential to bring hail and sleet.  Significant instability is forecast (*CAPE >350 j/kg; before the Oklahoma tornado CAPE measured 5000 j/kg), currently the highest this year in the SE, which means there is a fair chance of severe weather events in the form of thunderstorms.  Watch out for mountainous cumulonimbus cloud formations extending to the height of Mount Everest and possible funnel or mamatus clouds under dark bases.
Freezing levels will be down to 600m and during any heavy showers evaporative cooling could potentially cause heavy rain to turn to sleet as snow forms above 1000m and heavy rain draws down colder air.  Evaporative cooling happens when enough of the heavy rain evaporates on the descent to take heat out of the air and freeze supercooled droplets or sleet into proper wet snow.  Strong winds gusting 35mph will make it feel truly nasty.  This all sounds ridiculous for May… but the potential is there..let’s hope it’s not quite this bad!

Friday night will be a chilly and wet experience outside.  Temperatures will fall to 4ºC but feel near freezing in northerly winds. Not good for camping.

The weekend recovers reasonably well with a weak high pressure ridge bringing bright and dry spell Saturday behind the Friday storm. The fine weather  could hang on through Sunday but showers could threaten as pressure falls away as a deep LOW approaches from the NW to threaten Monday with rain.  The rest of half term looks unsettled but a bit warmer and drier at the end and best in the NW of the UK where HIGH pressure is likely to ridge across.

*CAPE = convective available potential energy: how buoyant a parcel of air is and this determines speed of vertical extent of convective cloud formation.

In the end: Max 8.4C; Min 4.3C; Rainfall 8mm; gust 24mph NW; So, in the end, pretty foul and unpleasant but fortunately no observed wet snow or sleet anywhere.  Timings of light rain onset were good; heaviest rain pm and cold temps made it feel more like January. No thunderstorms heard but lightning was recorded in SE on ATD lightning detector; CAPES fell to just over 100j/kg in the last run of GFS in the morning, which is a little disappointing for extreme weather enthusiasts!

The “something and nothing” weather in the South East of the past week and the uncertainties in the forecast are set to continue for a while. Some met-people call these conditions “unforecast-able”. Models seem to be unreliable beyond a few days and even hours. Rainfall has been especially hit and miss to forecast in the SE: predictions have been varying wildly for specific days between torrential, heavy, some and then no rain arrives at all! The reason is possibly the lack of the usual “zonal flow” in the jet stream: i.e. west to east flowing jet.  The jetstream is meandering north-south and weather systems are more or less STATIC: the UK has been stuck in a low pressure trough for over a week.  The normal procession of low pressure systems (depressions) and brief sunny HIGH pressure ridges seems a distant memory: it simply hasn’t been a feature of our weather for ages. Forecast models seem to struggle with this.

meridional flow jetstreamThe overall synoptic weather situation remains the same. That is: a big blocking HIGH over the Atlantic and very weak westerlies with the jetstream in a North-South pattern (meridional) bringing down cool northerly winds direct from the Arctic which “pool-up” across Northern France and Southern UK creating a LOW pressure trough.  LOW pressure in Spring with a stronger sun can mean pleasant warm sunny spells but showers: it is the showers which, fortunately, have barely troubled Reigate.  However, (and this does look more certain!), a significant little LOW is set to spiral down the N Sea Thursday – Friday, deepen along the way and strengthen Northerly winds and bring rain, especially to the SE: some frontal rain and showers are predicted to accompany this LOW but again – it could be rather hit and miss depending on how close the LOW gets to the SE England and the strength of accompanying fronts.

storm risk thursThere is a 30% chance of thunderstorms over Reigate area through Thurs and Friday afternoons (as the sun heats the surface which creates bubbles of warm air through the day which convect upwards through the cool Polar airmass creating tall cumulonimbus clouds).  With upper air temperatures at 5000 feet as low as -12ºC later this week any vigorous showers may fall as hail.  Frontal rain attached to the LOW will certainly feel chilly in the wind. Night time temperatures could fall as low as 4ºC and any wind will make it feel distinctly cool.  A weak ridge over Sat and Sun may bring pleasantly warmer and drier weather but some models are showing a return to LOW pressure in a storm arriving from the NW on Monday bank 

Meanwhile, sincere sympathy and thoughts to those caught by the terrible EF5 tornado which caused such terrible damage in Moore, Oklahoma yesterday.  Following events on Twitter and news reports on destroyed schools was very sad and upsetting indeed.  The tension between following exciting weather and the potential for witnessing terrible disasters unfold in front of their eyes was palpable for the storm chasers and met-enthusiasts involved.  Unfortunately, the weather in MidWest continues to threaten areas with tornadic conditions: lately in New York state too.  Take care out there.  Our UK weather is usually mercifully benign in comparison.  

Devastating tornadoes hit Oklahoma suburbs schools and towns today.

News at

supercell thunderstorm

Info @RGsweather

Reigate weekend in brief: Saturday dry (but v wet North England: low risk of it extending our way); Sunday wet; Monday showers NE wind.

Update: forecast for weekend proved v tricky: Model rainfall forecasts struggled with the easterly cut off low set-up.  Saturday was pleasant and Sunday turned out dry and warm. Note: GFS rainfall seems not to cope well with meridional jet stream with cut-off low over UK scenario!

Wondering if the weather is a bit odd recently? Well, you may have it right… the weather is literally upside-down from what we would normally expect in the UK. We would normally expect LOW pressure systems to whizz over the country from west to east. Temperature normally decreases further north and we usually expect warm air to arrive from the south. This weekend turns this normal expectation upside-down! A WARM air mass is pressing down from the NE, having arrived on a long journey from the Mediterranean and Eastern Europe (where there is a heatwave). This warm air is due to meet the COOL polar air hovering over the UK and cause torrential rain over Saturday up North. Reigate will see none of this and will stay mostly dry on Saturday.  On Sunday, however, cooler air will push up from the South and cause heavy rain for the SE and Reigate. Warm air from the North?  Cool air from the South? Slightly unusual. 

warm frontWhere air masses of different temperatures meet the warmer air is forced to rise up and over the cool: a front.  The greater the difference in temperature between the air masses and the steeper the temperature gradient, the greater the uplift.  This causes condensation, clouds and rain.  Fronts usually approach the UK from the West.  Currently a huge HIGH pressure is blocking all westerly winds.  A warm front moving south from across the North Sea is most unusual!

The cause of all this is the development of a cut-off LOW over Europe. A very looping jetsream (meridional) has been drawing polar air down across the UK this week and this cold air has pooled up over Western Europe, hence all the convection and showers. Meanwhile, the jet stream anticsnorthward moving limb of the jetstream has been dragging warm air up over Eastern Europe (which has a heatwave).  This warm air is due to meet the cool air over the UK this weekend.
The good news is that Reigate is likely to have a reasonable Saturday out of all this. Sunday could be pretty wet though.
The cut-off LOW will hang around for early next week and the unsettled theme continues for us in Reigate until the end of next week when things could improve for Bank Holiday as HIGH pressure builds in from the Atlantic. This means the west and north are likely to see better weather than the SE for Bank Holiday, but that’s a long way off to be certain. Keep tuned!

The UK is trapped in a “cold washing cycle” with no end in sight this week.  The meandering north-south flow of the jetstream mentioned earlier is partly to blame for fixing LOW pressure over the UK which simply is not budging. Our LOW is sandwiched between a HIGH to the west (Atlantic) and the east (Baltic) and is going nowhere for a while. Hence the heavy showers are set to continue and there is worse to come… starting tomorrow!
A deepening wave depression is set to form on the polar front and spin out from the Atlantic across SW, Central and Southern UK during Tuesday and Wednesday. Heavy rain, gusty winds, cool temperatures and even wet snow are predicted. The worst of this will be in the south west, Wales and parts of central southern England but Reigate and the SE will get continuous rain for 24 hours peaking in intensity overnight Tuesday into Wednesday. Once the fronts move away during Wednesday morning cool unstable north westerly winds and warming surface temperatures will create unstable conditions with a risk of heavy showers and thunderstorms through Wednesday and Thursday pm.
Rain is likely to arrive Reigate Tuesday during the morning and it won’t stop until sometime Wednesday am, to be replaced by heavy showers and a risk thunderstorms. Reigate could see >30mm of rain before the end of the week.
The weekend looks utterly dreadful, especially for the SE: LOW pressure swings close to the SE from the continent and this could bring heavy thundery showers close to Reigate for the weekend.

Throughout the convective weather later this week: points to watch out for are tremendous cumulonimbus clouds and even tornadic conditions with mamatus clouds and perhaps the odd funnel cloud?
Some models show a ridge of HIGH pressure building from the west next week – this is forecast to bring a decent end to May.

Classic short wave depression: the following synoptic charts show how the wave depression forms out of the polar front jet stream.  An innocent kink in the front is the first indication of lower pressure.  Thereafter the fronts become more pronounced, central pressure falls and the whole circulation moves across the UK. Note that, throughout this episode, the “mother low” to the north of Scotland barely moves.

p.s. “cold washing cycle” is not a meteorological term!

Last week our total rainfall in Reigate was 14mm, some models are predicting 40mm+ this week! Much of this will fall in heavy frontal rain mid-week and in thundery showers Weds/Thurs thereafter.  The jetstream is developing a meridional flow at the moment, which means it is meandering wildly and developing slow moving weather systems which sit around for long periods; the more usual zonal flow is when the jetstream flows west to east and drags weather systems briskly across the UK and keeps weather moving. Meridional flow gives rise to interesting weather: LOW pressure with heavy showers is the basic scenario this week: great for growing awesome cumulus clouds!

LOW pressure sits right over the UK all this week bringing sunshine and showers but with an added twist mid-week when a deepening short-wave trough is predicted to cross southern regions bringing potentially heavy frontal rain across the whole of the south, including Reigate. Thereafter, on Wednesday heavy thundery showers with hail could develop as winds slacken off.  Watch out for tall cumulonimbus clouds and rainbows! Any showers could build into single cell thunderstorms which will be slow moving in the slack winds and persistent and cause localised flooding. Around Reigate these showers will probably be quite isolated and some places will miss them completely but if you get caught in one you’ll know it! Temperatures remain uninspiring at 15C, although climbing slightly through the week. The end of the week remains very uncertain but could develop more heavy rain for us in the SE as the cut-off LOW over Europe feeds a miserably cool NE wind our way.
As always, this is early days and the end of the week looks especially variable and interesting!
Please check @RGSweather for updates!

The jetstream meanders like a river of air in the high atmosphere. Much of the time the jetstream blasts pretty straight west to east, at other times it loops wildly north to south. This week the jetstream is due to loop wildly, like a meandering river and threatens to form a special weather feature called a “cut-off low”.

How do cut-off lows form and why are they special?

1# A loop of the jetstream descending over the UK is forecast to form a TROUGH of low pressure over Europe this week. The southward limb of the jet directs cool polar air towards Europe and LOW pressure.

2# The loop becomes so sinuous (bendy) that, like a meandering river, the neck is cut-off as the jetstream re-forms to the north.  The LOW to the south becomes “cut-off” as HIGH pressure builds to the north.

3# The cut-off LOW over Europe is left as an “ox-bow” of cool unsettled weather, especially near the centre of the low pressure.

4# The surface warms and CONVECTION occurs through the cool air: air rises forming big convective storms.  These are forecast to migrate from the continent and effect especially the South and SE of England.  Higher pressure should keep the north more settled. 

5# Cut-offs fill gradually as warm air convects aloft, reducing instability.

6# The rest of May looks to have more rain and cool unsettled conditions as LOW pressure dominates.

(pics courtesy of netweather)

Thursday morning will see winds in excess of 100mph for Reigate… fortunately well above the town at 20,000 feet or so! This is the jetstream. These winds are pivotal in producing strong surface winds which will “touch-down” later in the day in Reigate. Surface gusts of over 40mph are possible with +30mph average winds. These winds are due to a deepening area of LOW pressure crossing the north of the UK over the next few days. The Irish Sea, English Channel, south coast and hilly areas like the South and North Downs will see the highest gusts. With trees in leaf there may be the odd branch falling. Batten down the hatches and secure loose bins! (Video: you can see the LOW crossing Northern England.  Reds 60mph; yellows 40mph; greens 30mph).

Quick update for NEXT week: LOW pressure dominates as a cyclone sinks down from the NW and an upper TROUGH sits over the UK while HIGH pressure builds to the NORTH: this will bring a distinctly AUTUMNAL feel to Reigate next week: temperatures struggling to get into the mid-teens, with periods of showery and breezy weather: so, NOT hot at all: cool, unsettled and rather wet for the time being.  More details later! Check @RGSweather for regular updates.