Archives For Reigate

So far this winter Reigate and the SE has been sheltered from much of the weather action, which has focused mainly on the NW and especially Scotland. This coming week might change that somewhat, albeit modestly in comparison with the battering the NW has received.  Charts below show temperatures and pressure are overall on the slide this week while precipitation makes two noticeable spikes.

A moist warm airflow ahead of a slow moving cold front arriving from the north Monday pm could stall over the SE overnight into Tuesday am and bring significant rainfall totals.  Temperatures Monday night could hold up into double figures as a warm moist SW flow funnels up from the subtropics ahead of the sluggish cold front.

slow cold front clearing south

slow cold front clearing south

The wave on the polar front emerges out of the SW through Monday with a characteristic plume of rain and a dip in pressure.  Such waves tend to bring a lot of rain despite hardly showing up on synoptic charts.  A modest kink in the front and isobars (see charts below) is the only hint of potential heavy rain action.  Some wintry precipitation and maybe snow is possible for places on the north side of the front later into Tuesday, in Wales/Midlands for example,  but not for the SE as temperatures remain too high.  The front slips slowly south Mon/Tues bringing some significant rain and then brisk cooler weather from showery westerly winds to follow, some sparse showers could possibly be wintry Tues-Weds but not amounting to much for us, most wintry showers will fall further west.  Temperatures will be cooler at Tmax 5-6 on Tuesday and even cooler on Weds, so cold but frost unlikely as too breezy.

There is a weather warning out for this period especially for the south coast but Reigate totals could reach 20mm+ and exceed 20-30mm for places nearer the south coast in Sussex.  The front is followed by increasingly cool showery westerly winds through Tuesday into Wednesday with the odd wintry shower possible but nothing significant.

There is the possibility of a storm for the UK Wednesday-Thursday bringing gales to our area as a “bomb depression” emerges in the Atlantic this week and probably tracks across Scotland (which would be less windy as a result, for a change).  Details of the storm track remain wobbly at the moment but the strongest winds will be found on the southern flank of the LOW centre and various models suggest that the SE could have some strong to gale force winds, albeit nothing like the scale of Scotland recently.  Models suggest a steep pressure gradient as the low tracks somewhere across the north of the UK.  The precise track will determine the impact on the SE.  Current models suggest the strongest winds for the SE will be associated the warm front approaching Wednesday night / Thursday morning.  Gusts could exceed 50mph inland and more on the coast with some excitable models suggesting more but these usually calm down nearer the time! Nevertheless, wind and rain are likely to be significant and expect updates on weather warnings this week. Colder air follows the storm so showers following cold fronts could turn wintry so watch for updates.

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Our own amateur “Wight-Wash Oscillation (WWO)” is an attempt to quantify the potential impact / severity of storms on the South East of England.  The WWO is simply a crude measure of pressure difference between the Wash and the Isle of Wight ( a mini NAO!).  As a benchmark the St Jude storm (99mph Needles max gust) had a WWO of approx 10mb (976 Wash-986mb Wight) and the modest storm of 12/12/14 was a 9mb on the WWO.

2014-12-11_22-45-11

Current models shown below have a variety of WWO results from 8mb UMET up to 12mb for GFSP.  A difference of 1 or 2mb can make a big difference to the severity of the storm as pressure gradient ultimately drives wind speed. Nevertheless, the WWO is just for fun but interesting to see how it works out in the event. On the basis of our WWO this storm currently looks less severe than St Jude, which is a good thing because that storm caused a reasonable amount of damage.

The unsettled weather this week is being driven by an active jetstream over the Atlantic.  This itself is partly a product of the tremendous temperature gradient over the northern hemisphere and particularly over the North Atlantic with freezing air pouring out of a frigid Canada meeting warm sub-tropical air emerging round the Azores High from the south.  These air masses are set to clash on Monday at the polar front over the Atlantic.

Whatever the storm brings it is looking likely that afterwards a cool northerly wind from the Arctic will bring temperatures further down by the weekend.  Although a long way off, for Reigate and the SE it is unlikely to be snowy and most likely to be the frosty and cool type of cold!  Further ahead some models build a pattern into later January (around 20) that could be conducive to cooler conditions: with possible easterly or NE winds for a time as a low sinks into Europe, though this is not looking either particularly extreme or long-lived.

Meanwhile, the North Atlantic Oscillation is set to remain positive, though decline somewhat to neutral, which means that the LOW over Iceland and HIGH over the Azores synoptic pattern is likely to be maintained, albeit possibly less pronounced and with more anomalous meridional moments of cold incursions possible.  This would mean a continuation of the broadly zonal westerly flow of mild windy and fairly wet unsettled weather for the foreseeable future.  Conflicting signals like this have riddled longer range forecasters this winter: the stratospheric warming was only moderate, much to snow-lovers disappointment, and some supposed good indcators of colder winters (the OPI / early Siberian snow cover in October) have apparently failed to live up to forecasting skill expectations as yet.  A month or so of winter to go and no significant signs of persistent cold … yet!

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Reigate 2014 weather report

2014 weather in Reigate, like the rest of England and Wales, broke some records for rainfall and temperature but in quite different ways.  To compare with 2013 visit here.

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South East England JANUARY rainfall 1873-2014

The annual summary chart (top) shows that Reigate had an exceptionally wet start to the year with January being the wettest on record (MetOffice) with over 180mm of rain (see chart above). The annual total rainfall for Reigate was 931.8mm which is above average for the area and well above the 2013 annual rainfall total of 654mm.  The rainfall record that fell in 2014 was for the highest January monthly precipitation total since accurate UK rainfall records began in 1873, exceeding 180mm in Reigate and the SE.  The January rainfall record fell in a winter that will be remembered for being exceptionally stormy in the SE, beginning with St Jude storm in October 2013 and persisting with numerous Atlantic storms battering the west coast and delivering high rainfall totals and significant flooding events across the SE, including our own River Mole and impacting Gatwick airport.  This blog covered numerous accounts of flooding and storm damage throughout that exceptionally stormy winter.

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UK rainfall anomaly 2014

In SE England and Reigate January and February 2014 were 260% and 274% wetter than average respectively (MetOffice).  Of the remaining months March, April, June, July, September and December were at or below average.  September in Reigate was particularly dry with only 18mm of rain, compared to the average annual SE total nearer 80mm. Nevertheless, despite these dry months for SE England 2014 was the 3rd wettest year on record, mainly due to the exceptional rain totals in the first 2 months.

2014 rainfall and pressure for Reigate

2014 rainfall and pressure for Reigate

From the rain chart below it is notable that for the second year running Friday is the wettest day of the week in Reigate, Fridays delivering 20% of our rain in 2014.  There have been studies showing that this effect of wet ends to the week is due to the build up of aerosol pollution during the commuting week.  This encourages the formation of condensation nucleii and higher rainfall totals on friday…it’s a theory worth pursuing because this is the second year running showing this effect in Reigate.  The other chart shows that southerly winds (SE-SW) brought most rain to Reigate in 2014.  This continues the trend from 2013 where southerly winds from SW to SE similarly accounted for the vast majority of rain arriving in Reigate.

2014 was the warmest year on record according to the Central England Temperature record stretching back to 1772.  The UK mean temperature for 2014 was 9.9C, 1.1C above the 1981-201 average and 1.47C above the longer term CET average.  The Reigate mean temperature for 2014 was 11.5C, with a max 30.2C (23/07) and min -2.9C (31/12).  The temperature record was broken by stealth and not by extreme values.  11 months of 2014 remained above the long term average.   4 months exceeded 2C above average, a further 7 months were above or very near 1C above average and only August dipped below courtesy of extra-tropical storm Bertha.  Extreme heat was mostly absent, as was extreme cold.  There was no snowfall in Reigate during 2014 and a notable absence of frost until some crisp days during December.

2014 warmest year on record

2014 warmest year on record

The annual ring chart below for Reigate also shows how August dipped significantly from the curve.  The rest of 2014 however was much warmer, resuming the upward temperature trend globally after the hiatus or temperature pause over the last 18 years.  Nevertheless, it wasn’t the warmest year for every part of the UK and a useful chart by Ed Hawkins (climate scientist Reading Uni @Ed_Hawkins http://www.met.reading.ac.uk/~ed/home/index.html ) reveals that parts of the UK had a cooler year than others and their warmest annual temperature record, technically, has yet to fall.

The animation below shows Europe bathed in above-average anomalous temperatures for the entire year, except momentarily dipping around August.

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2014 started stormy and windy.  The 3 month mean pressure charts below shows the mean pressure pattern during early 2014 and illustrates the air flow yielding our stormy and wet Jan and Feb: note the strong SW winds that result from the location of the LOW pressure in the Atlantic near Iceland up against the strong Azores high to the south creating a steep pressure gradient and resultant high winds.  This situation yielded a positive North Atlantic Oscillation which correlates with a warm, stormy wet winter. Contrast the winter chart with the rather slack flow of 2014 summer.  These are mean sea level pressure charts for those 3 months seasons.

The charts below show wind regime and gusts in Reigate during the year.  The highest gust of 52mph occured on 25 January.

In August the notable Ex-Hurricane Bertha, despite limited realtime impacts on the UK, delivered a sting in her tail by lowering temperatures across the UK in her wake by introducing a cooler-than-average NW flow for August.

Finally, Reigate had 1560 hours of sunshine in 2014.  2014 was a sunny year though not outstandingly so.  December was notably sunny for the time of year with over 100 hours of sunshine, over 160% more sunny than usual!

refs

http://www.metoffice.gov.uk/hadobs/hadukp/charts/hadukp_daily_plots.html

http://www.metoffice.gov.uk/climate/uk/summaries/2014/annual

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Reigate December 2014 (2013)

  • Tmax 12.7C (11.4)
  • Tmin -2.9C (-1.9)
  • Average temp 5.3C (5.8)
  • Total rainfall 49.4mm (CoCoRaHs) (110mm)
  • Max gust 38 mph (47)
  • Total sunshine 100 hours  (92.8 hours)
  • No snow

For Reigate and the whole of SE England, December 2014 was drier, warmer and sunnier than the long-term average (1961-90), but was it less windy than December 2013?

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In Reigate and for much of Southern England a HIGH pressure moved over to create a cool calm frosty end to the year with more sunshine than usual with a total exceeding 100 hours (92 hours 2013).  Rainfall was also less than usual for December at only 49.4mm (SE Dec average 96mm).  This was despite two low pressure systems this month labelled as “bombs” in the media.  Notably there was no snowfall recorded for Reigate in December and it maintained the warmer-than average theme making 2014 the warmest year on record for England, in the longest running continuous temperature record stretching all the way back to 1772 (more on this later).

2013 was remembered for being extremely stormy for good reason as gales and rain swept the UK through the winter period causing floods and travel chaos.  However, was December 2014 really so much less windy? One storm worthy of note struck this month but the weather “bomb” on the 12 December hardly caused a stir in the SE, with most of the impact being restricted to the NW of the country. The diminutive December max wind gust in Reigate of 38 mph was caused by the non-bomb LOW that sank SE across the UK that introduced the cold final days to 2014, the so-called “arrival of the queen of freezer”!

2015-01-01_15-07-49 december 2013-14 highest av wind speed

The chart above shows the daily highest average (10 minute) wind speed for each day through December 2013 (red line) and 2014 (blue line) as measured in Reigate. The chart shows that December 2013 seems to have started less windy than 2014 but caught up and finished on a more consistently breezy note.  Nevertheless, the difference is perhaps less than might be imagined considering the stormy label given to December 2013, though of course Reigate never experienced the highest gusts nationally which were reserved for coastal areas.  Nevertheless, equally surprising is that there were 14 days when average winds exceeded 10 mph in 2014 while only 11 days exceeded 10mph in the supposedly windy 2013.  The data for 20 mph starts to show the difference between 2013 and 2014: only 1 day exceeded 20 mph in Dec 2014 while 2 days exceeded 20 mph in 2013.

The data with respect to max wind gusts also confounds the idea that Dec 2013 was windier than Dec 2014.  The average maximum wind gust in Dec 2014 was 23 mph whereas the average maximum wind gust in Dec 2013 was 22 mph, no significant difference.  Of course, this all hides the crucial MAX GUST data (peak wind gust in 24 hours): but this doesn’t help either much because there were 18 days with max gusts exceeding 20 mph in 2014 while only 15 days exceeded 20 mph in 2013.  It is left to the absolute value for maximum wind gust to distinguish the two years because it is only in this category that December 2013 markedly exceed values in 2014 with 2013 max gust being 47 mph and 2014 just 38 mph. So it is these maximum gusts that people remember and associate with “storminess” because they do the damage, even if they are only brief moments in more average wind events.  On all the other data December 2014 was windier than 2013!  This also shows, of course, how different weather data can be used to illustrate different angles on a story.

  • Max Gust 2013 47 mph
  • Max Gust 2014 38 mph

It could be concluded that the collective memory of “severe weather” is often down to a few key events that raise public awareness, more than the weather itself.  There might also be thresholds which hit the news and chime with our collective severe weather memory: flooding and snow being the obvious triggers.  December 2013 was as much to do with heavy rain as it was to do with gales, especially for the sheltered inland areas of SE England.  Rainfall this December has certainly been lower than the long term average. The chart below shows the December precipitation average as 75mm for England and Wales and just 56mm for South East England, December 2014 in Reigate is lower still at 50mm, half as much as 2013.

 

So December 2014  has been an unremarkable month for Reigate and SE England although it was nearly as windy as 2013,  but it didn’t quite hit the extremes that make headlines!

More widely December caught the imagination as freezing weather sank south across Europe and the USA where huge snowfalls were recorded in Buffalo NY and in parts of the Alps.  It snowed in Algeria while Iceland experienced balmy December days as warm as 15C.  In the SE we were stuck under a very HIGH pressure (record breaking 1044mb) that gave us the sparkling final days in 2014 with pleasant winter sunshine and some cold temperatures as low as -7C in Redhill airport.  In Reigate our lowest temperature for December of -3C shows how a town location can ameliorate extreme temperatures.  Happy New Year!

Whilst this storm was uneventful for most in the UK the LOW is forecast to bring extreme cold to parts of Europe.  Here’s the story… see below for “in the event” 

Update Fri 8am! MetOffice chart brings LOW further South… this would bring in colder air more quickly for SE on Sat am possibly interacting with the frontal rain and turning it to snow for a short period, check weather warnings before travelling, especially north through the Midlands and N England and Wales tonight.

Quick update on weather prospects for Reigate and SE Boxing Day night and into Saturday 27 Dec.

The low pressure storm arriving Boxing Day afternoon is set to bring chilly conditions, lots of rain and some snow, but for who and when?  The LOW pressure is set to track overnight ESE through Wales, across the Midlands and exit through the Thames Estuary sometime in the early hours on Saturday. Strong winds are likely during this time, initially W/SW winds for the SE and Reigate building Friday pm/ evening and then veering to brisk cold winds NW/N winds by Saturday morning. Winds could reach 30-40 mph in places and more on coasts possible. Wind chill temperatures overnight will be cold, down to -6C in places. There will also be a lot of rain overnight: possibly 10-20mm in places. It is of course the snow that people are interested in.  Whilst snow is likely on the northern side of this low across parts of Wales, Midlands and N England, parts of East Anglia, for us in the SE on the milder south side of the low the snow is initially unlikely and much more marginal and more difficult to forecast.

A bit cheeky but couldn’t resist this apt tweet from WindyWilson in Scotland!

Overnight snow is unlikely for much of the SE because for much of the night we will sit in the warm flow of air to the south of the low centre.  Rain is forecast to arrive sometime mid-late afternoon. Friday will then actually warm up to possibly 6C in the evening in an occluding low as the warm front arrives.  This is called warm air advection and is what drives the lower pressure down as air rises.  It is also likely to keep any precipitation as rain for most of the night.  Overnight this warm air will be forced aloft by the chasing cold polar air sweeping down from the north, this can be seen on upper air charts below.

The charts below show snow fall is only likely on the very back edge of the system as the low pressure slips away across the Channel and drags in the coldest air from the N / NE in its wake. It is only at that point , from early on Saturday am, that the Downs and Reigate and the SE might get some snow but, even then, it is only 50% chance (see skew-t below).

On the atmospheric cross section (skew t) below for Heathrow the warm air can be seen on the 21:00hrs GMT chart as a slight bulge with increased height.  This is an isothermal layer which shows warm air is in the system at this point and likely to melt precipitation starting as snow higher up.  The skew-t diagram on the right is for some hours later at 03:00hrs GMT when temperatures can be seen to have fallen at the surface, winds veered to a cool Northerly direction after the cold front has passed through.  At this time a rough calculation of Dew Point + Temperature yields 3.3, which would give a 50% chance of snow at this location.

 

2014-12-26_20-14-52

Snow might also fall as heavy rain drags colder freezing air from above to the surface. Evaporative cooling, however, is unlikely as a snow making process because it requires less windy conditions.  By Saturday morning the polar Northerly air has arrived and this has dew points low enough for any showers possibly pushing in on the NE breeze to fall as snow at any time during the next 48 hours or so.  So… snow is unlikely for Reigate first thing overnight Boxing Day, there is a 50% chance of snow for the second half of the night, especially over high ground like the North Downs and especially if the rain is heavy enough.  Finally, as dawn breaks on Saturday any rogue showers penetrating our area could fall as snow in the frigid air.

This system is set to bring in a cold weekend and early next week a cool high pressure will keep things dry and frosty.  After New Year things look to be warming up and turning more Atlantic driven on the latest charts as winds bring rain back from west.

 

Why is forecasting snow so tricky?  http://blogs.channel4.com/liam-dutton-on-weather/snow-challenging-forecast-uk/2568

In the event:

Snow fell as forecast by MetOffice across Northern England / Midlands and caused some traffic problems and flight cancellations out of regional airports Manchester and Liverpool.  Snow accumulation up to 10cm was reported and some low wind chill.  The snow zone was a discrete area and to the south the warm sector kept everywhere south of the Midlands free from snow and, as expected, mild throughout (8C) until the polar air arrived behind the LOW. Some sleety showers and possible snow flurries came and went uneventfully across the SE and especially Kent but no accumulations were reported.

The LOW had much more wintry impact into Europe: with Netherlands through to the Alps receiving significant snowfall.  The LOW is forecast to continue SE into SE Europe and drag in some freezing polar continental air into the Balkans, Greece and even reaching as far as the North African coast.  Temperatures as low as -20C are expected across Serbia.  SNOW fell in Algeria.

Update #2 25/12/14: update: cold weather arriving after this LOW, heavy rain overnight Fri-Sat; snow marginal for SE early Sat am, more likely for Midlands and EA, cold weather arrives in lee of this system.  MetOffice warnings updated:

Update #1 25/12/14 latest MetOffice chart lifts pressure and pushes track further south, with low moving SE across our area.  This reduces wind speed, still brings in colder air flow though with risk of snow increased for back northern edge of the system with NE winds. For SE possible sleet/snow on Downs early Sat am. Evaporative cooling could yield more snow for SE if rain sufficiently heavy (drags down cold uppers). Gale risk gone but replaced by some heavy rain, marginal snow risk and retaining the cold easterlies in the aftermath on Saturday with pressure building to dry bright frosty conditions.

2014-12-24_20-10-17

After a pleasantly cool bright and dry Christmas Day, an interesting depression due on Friday and through Saturday is likely to usher in a period of colder weather for the UK and SE in particular. The situation is a little uncertain still but the run of warm mild gloomy temperatures lately this December, already pushed aside gently by a weak cold front passing south through the country today, are likely to be pushed further down into some “proper”cold after the storm passes through by Sunday. This storm, forms in the Atlantic along the polar front and quickly races east towards the UK on Boxing Day Friday.  Storms tend not to deepen much if they move fast, which this one does at first: crossing half the Atlantic in a matter of 24 hours. The storm is mixing some airmasses with contrasting temperatures: cold polar air in the north is about to get up close and personal to mild warm Tropical air from the south west.  They are due to meet in the LOW pressure over the UK soon, so expect some interesting weather!  You can spot the impact of the storm on the upper air temperature chart below but also see the steeper drop to colder conditions thereafter.

GEFS shows cooler days ahead

GEFS shows cooler days ahead

The ECM charts below show upper air temperatures at around 1500m. These “850hPa” charts are commonly used as guides to airmasses because air at 1500m (850hPa pressure level) is not affected by changes day and night or surface characteristics, it is therefore a good guide to true airmass characteristics.  Note the really cold airmass to the north meeting comparatively warm air to the south and SW in this LOW.

2014-12-24_13-50-38

For the South of England the LOW will initially push warmer tropical air ahead with rain arriving for us in the SE on a warm front sometime Friday pm (top diag above Sat 00hrs).  The warm sector is likely to be windy with gusty SW winds and a considerable accumulation of rain, 10-20mm overnight into Saturday.  The warm sector tropical air mass (upper air +5C) could have temperatures near double figures whilst the polar air bearing down from the north is a much more frigid airmass (upper air -6C).  The contrast between these two airmasses could make the frontal rain particularly heavy while the cold front contrast could even have an odd rumble of thunder as cold air undercuts the warm and forces it aloft.  The skew-t diagrams below show the contrast in these two airmasses.

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The LOW centre crosses the North of England and into the North Sea overnight into Saturday when, due to it’s location under the left exit of the jetstream, it is forecast to deepen to possibly around 980mb. quite low especially for a depression located so near the shore.  Deepening occurs as the jetstream aloft encourages air to rise off the surface because air is diverging aloft.  So air is rising off the surface quicker than it can be replaced by air arriving: hence falling surface pressure. This commonly occurs when lows interact with jetstreams on their left hand side, near the exit of a jetstreak.

The classic frontal depression with cold and warm fronts separated by a warm sector only lasts for a matter of hours before the cold front, pushing forward more dynamically than the warm, catches up the warm front and pushes the remaining warm air into the upper atmosphere.  This is an occlusion and signals the end of the development stage of a depression.  The central pressure usually starts to rise after occlusion has occurred.

Whilst the situation is still uncertain, it is likely that Friday afternoon and Saturday will be windy and increasingly cold as the winds veer clockwise from the SW through to North and finally NE and E.  It is the latter NE and E winds that will bring the colder air to the UK and the SE especially.  Continental Europe is currently very cold so any air flowing from this direction will be chilly.  Cold crisp continental air will stay with us for a while as high pressure builds to the west and pushes north over Scotland while the LOW moves over Europe.  This setup allows easterly winds to flow over the UK.  Dry cold is expected as the pressure is likely to rise and stay high.  Expect some frosty nights. The duration of the HIGH varies between models but certainly should keep things cold and crisp through to the New Year.

2014-12-24_10-48-15

cold new year

Further ahead a split in the polar vortex and stratospheric warming are dominating weather chat and these are set to possibly bring colder conditions through January.  On the other hand, Phase 3-4 of the MJO (Madden Julian Oscillation) is usually associated with a positive North Atlantic Oscillation which brings milder westerlies to the UK.  So, it’s interesting times ahead, stay tuned and Happy Christmas!

October 2014 rain and temperature

October 2014 rain and temperature

sunshine hours October 2014

sunshine hours October 2014

Reigate weather summary for October 2014

Tmax 22.6C

Tmin 2.4C

Taverage 13.1C

Total rainfall 112.3mm (CoCoRahs)

Despite memories of big storms like St Jude last year, October 2014 has turned out more than twice as wet with 112.3mm.  For the UK as a whole October 2014 was a warm month and broke records with the warmest “Halloween” on record.  At an average of 13.1C it was warm in Reigate as well, but oddly did not exceed the 2013 average of 14C. So 2014 was wetter but not quite as warm in Reigate as last year.  It was a quieter month with lower wind speeds and less widespread stormy conditions and it looks like Autumn 2014 will continue in this mild wet style but without the threat of big trains of countless Atlantic storms like last year.  Blocking HIGHS over Russia / Scandinavia and to the North look set to slow down the train of Atlantic weather and send it south into the Mediterranean where wet and stormy conditions have already been experienced (“Medicane” Mediterranean hurricane November Malta 2014)

In the UK as a whole October 2014 was the equal-tenth warmest October for the UK in a series since 1910 according to Central England temperatures (CET).  Globally, unconfirmed data suggests that October might be the warmest on record.

In Reigate much of the heaviest rain came in the first half of October in big convective downpours that were hit and miss.  Some tornadoes were reported across the UK and these were quite damaging in places further North.  We have yet to record a tornado in Reigate.  Tornados are extremely tricky to forecast but several agencies try it.. usually the MetOffice do not mention tornadoes in their forecasts because they are so hit and miss, even when potential is there.  Estofex, UKWW and SkywarnUK are agencies that have an eye on tornadic and severe weather conditions in the UK that RGSweather follows carefully.

MetOffice winter prediction 2014-15 as follows: It states “For November-December-January above-average UK-mean temperatures are more likely than below-average” and “Latest predictions for UK-mean precipitation favour near- or above-average rainfall for November-December-January”.  In contrast, other weather agencies and some seasonal model predictions are suggesting the possibility of a colder winter as various early indicators seem to suggest the possibility of incursions of cold air at times.  In particular, early October extensive Siberian snow cover, cooler than average temps over Siberia and a negative OPI index leading to an amplified jetstream (which seems to be occuring now) seem to suggest the possibility cold blasts at times this winter.  In addition, an Easterly phase of the Quasi-Biennial Oscillation (QBO) in the stratosphere favours a weaker flow of upper westerly winds that shut out cold air for the UK and W Europe.  An easterly QBO phase favours colder winters for us, but not always.  Whilst a strong El Nino has not transpired, there are indications that sea surface temperatures in the Pacific (especially central Pacific) are rising, albeit modestly: El Nino tends to link with cooler Euro winters.  These immediate measurable indicators correlating more or less with cold UK winters are interesting but not necessarily enough on their own to drive the atmopshere toward cold conditions.  They might, however, couple with other connections in the Stratosphere to cause stratospheric warming which, some weeks after, can cause downwelling of cold air into the troposphere and further disruption of the westerly flowing jetstream.  This is especially the case if warming is sudden (SST).  IF this all happens and IF surface pressure patterns play ball then the UK could see some cold snowy conditions! Charts below show some of these connections:

Update: confirmed tornadoes today (see foot!)

Potentially interesting, albeit tricky, weather tomorrow for UK, Wednesday. A deep surface LOW sits to the SW of Ireland and is dragging through complex series of fronts associated with various airmasses.  Occluded fronts are tightly wrapped around the LOW, which is due to migrate NE away from the UK during the weekend and pressure to rise.  For the SE tomorrow it’s not so much the fronts but an unstable mass of warm southerly surface air that will be the main cause of any heavy showers tomorrow and some potentially thundery weather, especially when this warm air is forced up by anything… coast, hills or fronts.

Here’s likely scenario for us in Reigate, Surrey SE: Rain is likely, possibly exceeding 10mm, which is fairly wet for SE: higher is possible.  Most of this is likely to be convective rainfall due to unstable and moist air moving in from the south overnight.  Showers, possibly heavy and thundery, are most likely in the morning as the warm surface air moves into our area, causing lapse rates to increase moderately and this encourages lift and cumuliform clouds.  If the sun comes out then surface heating could spark heavy showers and thunderstorms as warm air rises freely through the atmosphere, encouraged by the jetstream overhead that effectively drags air off the ground.  One of the ingredients for thunderstorms, LIFT, is therefore partly in place tomorrow, although it will depend on sunshine for greatest effect.  If it stays overcast, which is possible, then little exciting weather action beyond just rain is likely.

The above charts also show that the southerly / SW airstream is humid because, as it converges on the coast, the model shows rainfall increasing significantly. This increased rainfall on coasts is often caused by convergence which is due to air arriving onto the coast quicker than it is leaving (check the lower wind speeds inland) so the wind effectively PILES UP on the coast and is forced to rise as it has nowhere else to go except UP!  This is called convergence. It is clear from the charts above that any HILLS also encourage lift as South Wales and even the South and North Downs appear to be pushing rainfall totals up locally: this is orographic or relief rainfall.  The charts below shows another feature of the weather tomorrow: the winds are shown to be VEERING with height (left diag) which allows WARM air to advect (move into) into our region (right diag).  Read on for more about how veering winds and WARM AIR ADVECTION can encourage stormy weather.

 

Another ingredient for potentially unstable weather is that winds are VEERING tomorrow, albeit not dramatically, which means they are rotating clockwise directionally with height thus allowing warmer air from the south to move into a location: it is like opening the door to warm air: winds move through a southerly direction and therefore allow warm air to “advect” into our area.  A moderate wind veer is taking place overnight and into tomorrow morning.  Warmer air at the surface is overrun by cooler westerlies aloft that increases lapse rates: steepens the temperature difference between surface and air at altitude.  The air at altitude tends to stay the same temperature and is associated more with direction and origin of airmass than it is with surface heating or advection of warmer air at the surface. An increase in lapse rates adds to instability which encourages air parcels to LIFT off the ground, should surface heating occur if the sun comes out.  The chart above right shows how WARM air is ADVECTED into the South of the UK and migrates NORTH during the day.  This has nothing to do with solar heating… it is an 850hPa chart (1500m) and shows the airmass temperature which is largely independent of surface influences.  It’s a good example of WARM AIR ADVECTION with a moist air stream increasing instability causing showers and possible thunderstorms.

Finally, the warm relatively unstable airmass is being overridden by a NE turning jetstream that will encourages lift and wind shear.  Wind shear is the vertical change of direction and/or speed with height: rotation.  Shear is moderate tomorrow which might also add a twist to rising air that could even produce the odd tornado.  Nice 🙂  After writing this Estofex issued a tornado warning Level 1.

Update 8 Oct: confirmed tornados from today 8 October 2014

http://www.bbc.co.uk/news/uk-england-derbyshire-29542740

 

2014-10-02_20-39-59

September was a warm and unusually dry month.  Central England temperature (CET) came out officially as 15.1C which is 1.5C above the 30 year average (Hadcet) and made September the 4th warmest since 1910. In Reigate our average September temp was 16.5C, not unexpected as we are in the warmer SE of the country.  In 2013 the average for September was 14.4C but the range of temperature was greater: 2013 Tmax 30.9C and Tmin 4.6C were both more “extreme” than the respective Tmax 26.7C and Tmin 5.2C of 2014.  Whilst the remainder of Autumn looks to be cooling down markedly now, 2014 remains on target to be an exceptionally warm year overall.  Despite a cool August all the other months have been above average.

The UK average rainfall for September 2014 was 19.4mm for the month, the driest since 1910.  In Reigate we had 22mm of rain recorded (CoCoRahs) compared to over 47mm in 2013.  The rainfall we got in Reigate this September was restricted to a few heavy thundery showers: many of these moved over London and missed us completely.  However, a notable thunderstorm overnight on 19 Sept delivered some cracking thunder and lightning at about 1am and brought 10mm of rain in less than 20 minutes, and woke quite a few people up!

Overall, however, the month was dry: 16 consecutive days of no rainfall at all came early in the month and mostly dry days with just odd showers characterised the rest of the month.  Sunshine hours totalled 120 hours. Pictures below are a medley of photos from an “extreme” month due to the exceptionally dry conditions, the driest in 100 years in some places.  September in Reigate.

 

September has retained some high daytime temperatures which are set to make it 1.4C or so above Central England Temperature (CET) long term average. New York city also experienced some warm Autumn days with 29C Tmax recently, followed closely by London with near 25C Tmax temps over the last weekend of September.  This month has also been exceptionally dry with Reigate recording just 20mm (tbc). The inevitable happens this coming weekend as Autumn arrives, albeit fashionably late.  The chart below shows upper air temps dipping as cooler polar air arrives from 5 October (note upper air temps are 1500m, so don’t panic about the scale!).

fall in air mass temp this weekend

fall in air mass temp this weekend

The charts below also show a defined change for Reigate and the SE over the first weekend in October and into Monday as high pressure and largely rain-free warm settled conditions this week give way to LOW pressure, frontal rain bands and cooler breezy-er conditions delivered by a lively jetstream from the Atlantic. The Icelandic LOW mentioned in previous posts will, at last, nibble through the anticylcone sat over the UK for so long.  It may not be quite curtains for HIGH pressure and warmth for the SE yet and some recovery is hinted at later, but it looks like the persistent dry and settled conditions will push off this weekend and be replaced by more mobile Atlantic action.

For Reigate, the change afoot starts gradually, with the odd light shower possible Wednesday, Thursday seeing a rise in pressure again and a continuation of warm and dry calm conditions to end the week.  However, a glance to the north west will show an active cold front descending SE during Friday and arriving over SE and Reigate during Saturday morning.  It is likely to push through by afternoon and leave brighter fresher conditions through Sunday, which looks not a bad day at all for Reigate, albeit cooler.

 

The jetstream chart for Thursday and next Monday shows a significant shift south directly over the UK. This is forecast to enhance the trough over Scotland and dig it deeper into the south of the UK during early next week.  Breezy conditions are likely into early next week with Monday having possible country-wide rainfall, especially heavy in the south as the warm air lingers CLOSE by to the south and interacts unfavourably with the colder air mass that could produce a lot of rain. The upper air charts below show how close the warm air lingers to the south of the UK.

RGS weather club produces forecasts for school events on Fridays. Here is the latest for Open Day Saturday 27 Sept and beyond.

Temperatures staying unseasonably warm this weekend and through much of next week: considerably above the September 30 year mean.  Apart from odd light showers early next week it should stay largely dry. No big breakdown yet on the charts, except potential hints later next weekend at the earliest.

Temperatures staying above average and rain below average

Weekend:

Tmax 22C, Tmin 10C+ staying in double figuresovernight, which is well above the seasonal average.

Winds light, no more than 10mph, and mostly from a warm southerly direction.

Cloud will come and go this weekend, odd scattered light shower possible Saturday, but rather unlikely.

Look ahead

After a slight threat of rain early next week from a slackening in pressure and shallow LOW with a weak front to the SW, the HIGH is expected to build back over the UK through mid week and bring in a warm southerly flow with Tmax20C, maintaining the mostly dry, settled, but occasionally cloudy conditions.

High slackens grip slightly early next week, but hangs on

A deep Atlantic storm near Iceland next week will threaten to bust the blocking HIGH pressure over the UK but it looks unlikely to succeed until next weekend or second week of October at the earliest before the inevitable breakdown into “normal” Autumn weather.

September will work out warmer and drier than this August.

Forecasters Tom Tatham and Chris Meredith and SAC

RGS weather club meets every Friday lunchtime in Room 12 to do forecasts for school