Archives For March 2014

There is some interesting weather potential this week, but indications that the SE will see least of any drama which will be mostly further west. The well-established warm plume of S/SE wind from Africa/ Mediterranean and Spain has brought temps up to 19.4c in Reigate this weekend and 20c in London.  A warm sunny Saturday was especially pleasant.  The breezy S/SE wind is bringing Saharan dust falling over the UK, watch for this in any showers that might come our way on Monday.  Check your car for any dust.

Warm plumes from Spain can also introduce unstable moist air and these produce thunderstorms and showers when moist warm air converges or fronts  undercut the plume with Atlantic air creating lift. There is good potential for heavy convective downpours of rain this week due to these scenarios.  Whilst not necessarily a classic Spanish Plume the synoptic situation is very similar to May 1998 when large super-cell thunderstorms drifted to the North and caused torrential rain and flooding in parts of the North.

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Various indicators are used to establish the potential for heavy convectional rain and thunderstorms.  At times the charts show several of these indicators at unsually high levels for end of March / early April this week.  CAPE (convective available potential energy), LI (lifted index) and ThetaE (potential equivalent temperature) … these are all technical charts that are commonly used to assess how likely thunderstorms will be.  ThetaE rarely goes above 19c in the UK so temps of +12c in April are unusual so early. Unfortunately, the development of convective rainfall and thunderstorms is difficult for forecasters to predict for any one location for a particular time.  Thunderstorms and showers are, by nature, hit and miss affairs: one place might get large hail and a deluge while, a mile down the road might remain sunny and dry.  Nevertheless, watch out for some potentially heavy rain, especially if travelling this week.  Watch out for interesting cumulonimbus clouds too!

Total rainfall remains highest in the west and away from the SE where pressure remains higher and frontal action is more limited.  So Reigate may escape the worst of all this convective rain action but still worth keeping an eye out for rogue storms that may well come our way drifting most likely from storms in the Channel around mid-week.

Finally, models are hinting at high pressure building back in for the school holidays after this wet unsettled week. Whilst not yet represented on charts very convincingly, the models suggest a possibility of some reasonable holiday weather in the UK with pressure rising and temps above the 30 year average on the GFS ensemble mean.

 

 

 

Soil moisture: possibly the most under-rated meteorological measurement!  Rarely do weathermen get animated about the extent of wet sod across the country.  Nevertheless, soil moisture, usually measured in centimetres of water in the top two metres of soil or as % saturation (see maps below), has been found to control continental scale weather patterns, summer maximum temperatures and even heat waves and the extent of droughts.  So we ignore soil moisture at our peril, especially as soil moisture also controls vegetation growth and death and the ability of farmers to grow food.

How wet the soil obviously relates to how much it has rained recently.  During winter, in mid-latitudes, soils usually become increasingly saturated with a surplus of water building up as inputs of precipitation exceed evaporation which is reduced in the cooler months and shorter days.  During the summer, soils tend to become increasingly depleted of their moisture content as evaporation (output) exceeds precipitation (input).  This input and output of moisture forms an annual balance known as a soil moisture budget and is shown in the graph below.

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High rainfall during the winter builds up a lot of water in the soil.  In the spring time a high soil water content “uses up” more energy from the sun in the process of evaporation.  The more energy “used up” in evaporation, the more energy is lost from the system to produce sensible warming at the surface. Hidden energy, or latent heat, is required to change liquid water into water vapour.  So “latent cooling” reduces the amount of energy available to warm the atmosphere as long wave radiation.  So local temperatures can be depressed over areas of wet soil especially during a spring when wetter-than-usual soils might take a long time to dry out.  It might also be expected that, after a wet winter, there could be a cooler period until such time that the soil dries out locally and more energy becomes available to produce a sensible heat flux at the surface.

In fact, the effects of soil moisture go far beyond these micro-climatic changes and can have impacts that are continental in scale.  In a 2007 study (see below) it was found that 25% extra soil moisture could reduce continental Europe-wide temperatures by up to 2c from average summer maximums. Likewise, a 25% reduction in soil moisture could raise temperatures across continental Europe by 2c in the study period.  It was also discovered that higher winter and spring soil moisture could raise summer precipitation levels and change continent-wide pressure patterns.

The study from 2007 used reanalysis of computer weather models to investigate the impact of soil moisture on the European 2003 heat wave, the warmest for 500 years.  This heat wave killed over 20,000 people and caused crop damage.  The study found that by re-running computer models just with different soil moisture values, the maximum temperatures and heat wave intensity varied greatly.  Dry soils during the spring increased summer heat wave intensities while wet soils reduced the maximum temperatures.  The difference was significant, and in some localized regions the intensity of heat anomalies varied by 40% simply due to different soil moisture content at the outset of the model runs.  The largest differences were mainly located over central Europe.  It seems that differences in soil moisture have most impact across central continental Europe and progressively less impact on summer temps with increased distance further north.

Not only did soil moisture control temperatures, it also had a control over continental pressure patterns.  Dry soils built pressure through the middle troposphere, while wet soils could lower pressure.  This has numerous positive feedbacks: wetter soils reduce pressure which increases cloud formation and summer rainfall that enhance the wet soils.  Dry soils build pressure, reducing cloud formation, reducing summer rainfall, further drying out soils.

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So, in summary:

  • If the soil-moisture deficit is high, the dry soils raise the sensible heat flux, producing a deeper, warmer, drier low-level atmosphere: raising temperatures and enhancing surface heating and drying.  Increased drought risk.
  • If soil moisture is high, the latent heat flux by evaporation and transpiration dominates, enhancing cloud formation and a tendency for cooling.  lower temperatures and enhancing rainfall and further wetting of the soil.  Increased wet summer.

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The full article is here: Fischer_heat_waves_2007 (1)

A ridge of HIGH pressure brought excellent fair weather cumulus cloud formation over the SE today.  The sat pic shows cloud streets of cumulus over the SE where local convection, capped by an inversion at 5000 feet created beautiful fluffy bubbly cumulus for much of the day. This came after the “coldest” night of the year at -1.2c in Reigate and -6c in the far north of the UK under clear skies and polar air.

Another point to note was the opposite wind directions at different altitudes today: surface SE flows giving way to NW flows above 7000 feet.  The skew-t chart below illustrates these features of a super-lovely day here.  The fairweather Cu convection was surpressed later in the day as an advancing occluded front spread high level cirrus and cirrostratus across the sky thereby creating an enormous regional inversion of warmer upper air that effectively stopped further convection and the cumulus gradually faded with incoming cirrostratus and altostratus later in the day.

 

skew-t chart fair weather CU formation

skew-t chart fair weather CU formation

 

The rest of the week for Reigate deteriorates and becomes cooler and damper and more gloomy.  A large cut-off low is set to form over the continent, while a HIGH over Scandinavia will combine with this COOL-POOL to drag NE and easterly winds over the UK.

Usually such winds bring cool, damp and showery weather across the East and SE, including Reigate, with moisture picked up from the N Sea and temps from a cool early spring-time continent often creating layers of cloudy gloomy stratocumulus widely.  Thurs/Fri may see brighter spells with showers.

A LOW from Denmark mid-week is modelled to drift across the N Sea and this is likely to exacerbate the cold by bringing an extra-cool shot of cold continental air (-6 at 850hPa) over the UK mid-week: snow is possible over high ground of the Pennines and NE at this time.

Later in the week and at the weekend a subtle shift in the location of the Scandi HIGH and Euro-low could allow warmer SE winds sourced from the Mediterranean to filter into SE UK edging temps to the upper teens once again.  This is a fair way off still but will make the weekend a more pleasant prospect if this comes off.

Model predictions for the start of April look uncertain still with ifs-and-buts over how settled it will be.  Several sugges high pressure building at times early on but other models put a LOW over the UK for this period… so charts definitely worth watching for Easter school hols.

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The weather has been mercifully quiet, with mostly warm and sunny conditions for Reigate over the last 2 weeks. This is gradually about to change but no particular drama is afoot other than some perky and windy frontal rain arriving in Reigate on Thurs night /Fri am, followed by showers and cooler breezier weather with hopefully some interesting clouds over the weekend!

Our two week dry spell (well, 13 days) ended briefly today with a light shower.  As the blocking high sinks away to the S, a cooler NW flow will swing by over the weekend ushered in by an active cold front on Thurs/Fri which will mark the arrival of some cooler days and frosty nights through to early next week.

The charts show a typical early spring flip-flop in temperatures as warm air from the SW on the back edge of HIGHs is replaced by cool NW and even N winds as LOWS sweep across the North of the UK.  The back-edge of the LOW moving over Scotland in the next few days will drag down chilly Polar air especially on Saturday and Sunday.

Daytime temps will struggle to double figures which, when last weekend hit 20c might feel a bit of a shock!  This cool airtstream, fresh from Polar regions, will also be warming through from below and therefore get rather unstable: this means thermals will have a tendency to rise, and keep rising through the chilly upper air, creating towering cumulus clouds and showers.  The chart below shows a typically unstable temperature height diagram with some ingredients for a showery day capable of building some nice cumulus clouds: steep lapse rate, saturated airmass through a large column of the atmosphere, some wind sheer (change of wind speed or direction with height).

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By the afternoon especially on Saturday cumulus congestus clouds might even be tall enough for some sharp showers, possible hail in the cold upper air and the odd clap of thunder is not impossible.  Nevertheless, total rain predicted is low and there will be sunny intervals. April showers, but a month early!  Next week sees a continuation of flip-flop spring weather.  April has the possibility of some high pressure returning with sunny and dry weather but the exact timing of this remains uncertain.  There is also some indication of unsettled pattern in April too, so take your pick!

LOW over Scandinavia

LOW over Scandinavia

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.

Quick stats summary for February and a medley of photos below…

Tmax 13.3c

Tmin -0.2c

Average temp 9.8c (5.9c 2013)

sunshine hours 101.7hours

Total rainfall  139.1mm (42mm 2013)

Strongest wind gust 43 mph

The sun rose for the first time over Svalbard, Arctic on Feb 15 after months of darkness.  Whilst the news from Reigate was not quite as dramatic, the weather in Surrey continued wet and stormy for much of the time.. check the sat pics below for the amazing run of spectacular low pressures that swept across the UK.

On the ground in Reigate this meant higher than average rainfall of nearly 140mm for the month, compared with 42mm last year.  February rainfall for the whole of SE England was 273% above average, so the Reigate total was not too far off that figure and highlights our generally sheltered location relative to other places in the region (i.e. the coast). Whilst not quite as bad as the Dec/Jan storms for Reigate, February wind gusts locally in the town peaked at over 40mph mid-month, and were considerably higher over the Downs (50mph+).

A notable feature of the month were big showers and occasional claps of thunder in cumulonimbus clouds (Cb) produced in unstable streams of polar maritime air often sweeping in behind occluded fronts.  These Cb shower clouds sometimes produced splendid rainbows and clearing skies at night, ideal for star gazing.  An unusual aurora (from solar coronal outburst) late in the month caused spectacular displays of the northern lights to be seen all over the UK.  In Reigate the aurora could be seen as a faint red glow away from the lights of London.  Although plainly less spectacular than the amazing aurora displays further north, this was nevertheless a special sight considering the rarity of such events.

http://www.surreymirror.co.uk/Excitement-Northern-Lights-seen-Surrey/story-20727615-detail/story.html