Archives For air masses

synoptic chart for heat spike

synoptic chart for heat spike

In the event: July 1st 2015 Heathrow Airport recorded highest ever July Tmax at 36.7C. More here

http://blog.metoffice.gov.uk/2015/07/07/on-the-record-observing-a-heatwave/

Reigate and the southern half of Britain could see some very high temperatures from mid-week next week.  Technically this will probably struggle to become a “heatwave” because it looks like too brief a hot spell, ending by or through the weekend, to exceed the 5 day threshold for an official designation (see below). Nevertheless, a significant HEAT SPIKE is certainly on the cards.

Some very high, possibly record breaking upper air temperatures are due to arrive aloft (forecast 23C 850hPa temperatures for South are quite unheard of in recent years) and, if it’s not too cloudy, 2m surface temperatures could soar to over 30C and possibly even nudge up to 35C.

This is not a forecast, and models will ebb and flow with the event intensity up to the wire, but it’s a review of some factors that will play a part in this heat spike episode.  The synoptic set-up, on charts below and top, shows a blocking HIGH developing over Scandinavia (an omega block) and a trough in the Atlantic with a heat LOW over Iberia. This set-up brings the well known “Spanish Plume”.  Typically this involves a warm, dry upper air flow from the South, drawn up by a perky northward limb of the jetstream and accompanied by an easterly or SE surface flow.  The combination is associated with heat and thunderstorms, though not necessarily extremes of either.

Whilst the Scandinavian Omega Block persists, the easterly continental wind and drier conditions should prevail over the UK but we are close to the edge of the anticyclone and in the line of fire from Atlantic fronts nibbling at the edges and thermal LOWS from Spain running north under the jetstream. The interaction of the advancing cool Atlantic air with the warm upper flow and the increasing surface heat can crank up convective instability as the plume migrates north across France to the UK.  The CAPE and skew-t charts below show the possibility of (elevated) thunderstorms by mid week.  Interestingly at times there is a strong cap near the surface, but great instability aloft, so storms likely to be elevated AcCast (altocumulus castellanus) with potentially strong lightning shows but maybe little rain getting to the surface, at least at first / Wednesday. CAPE is through the roof on some runs but too dry at lower levels and too strongly capped to yield widespread storms as shown below on the skew-t chart.

2015-06-27_08-35-10

upper level instability for potential elevated thunderstorms

Later in the week cool Atlantic air from the west is likely to interact with the plume, descending behind cold fronts and this process can cause CAPE values to increase bringing the chance of more organised thunderstorms that usually herald an imminent invasion of a cooler westerly regime.  Cool tropical maritime air behind fronts typically descends and causes increased lift as it runs into the unstable plume. Recent runs show the GFS wants to hang on to the heat longer while the ECMWF brings back westerlies more promptly by the weekend.  (update: now reversed!) This is not, therefore, likely to be a completely dry hot episode, because thunderstorms threaten especially after any really hot days.

The cross section below shows the flow of upper air clearly swinging round from a southerly direction.  Note the surface flow from the SE.  This combination, brief though it is, raises the risk of unstable conditions and thunderstorms indicated by the raised lifted index (LI) and Total Totals at the foot of the chart.  The average weekly 2m temperature anomaly charts below show how brief the heat might be… the second 5 day average returns back to normal.

Despite the likely short duration, it could be a notable period due to other factors playing a part.  Important ingredients that can be thrown into the mix of heat and thunderstorm potential are sea surface temperatures and soil moisture content. Despite a cool Atlantic, the seas immediately surrounding southern Britain are currently anomalously warm and ripe for transport of heat and thunderstorms across the Channel.

Similarly, dry soil enhances potential heat build up by reducing evaporative cooling and possibly kicking up temperatures higher as less heat is “used up” evaporating soil moisture.  Also, dry soil can enhance instability due to the rapid surface heat build increasing lapse rates.  I’m not quite sure how models handle these factors when producing their 2m temperature forecasts.

Locally, it’s worth noting the frequency of summer days exceeding 30C in Reigate, Surrey. Since 2009 only 29 days have reached or exceeded 30C and only 2 of these have been in June, most in July.  Also note the increase in hots days recently, though of course this is not a big enough sample to be significant.

2015-06-25_20-44-03

It’s still too early to be precise about when and how hot and how thundery it might get next week but the ingredients are at least in place for some very interesting weather!.  Further ahead this looks like a brief heat spike as westerlies resume promptly.  However, the overall pattern seems to favour a blocking HIGH over Europe which could continue to feed the UK with warm Southerly or at least SSW winds for a while.  A cool Atlantic also favours higher pressure so this ought to reduce the chances of very wet conditions, especially here in the South East. Phase 6 of the MJO is also correlated with blocking HIGH patterns over Europe.  So July ought to continue warm, dry and occasionally sultry but with possible thundery episodes.

Media references:

http://www.metoffice.gov.uk/news/releases/archive/2015/hot-july?utm_source=twitterfeed&utm_medium=twitter

http://www.theguardian.com/uk-news/2015/jun/25/mini-heatwave-forecast-for-uk-next-week-temperature-30c

http://www.dailymail.co.uk/news/article-3138816/Glastonbury-Wimbledon-UK-weather-Britain-set-hottest-temperatures-year-week.html

http://blog.metoffice.gov.uk/2015/06/25/hotter-weather-for-the-start-of-july/

http://www.theguardian.com/uk-news/shortcuts/2014/sep/17/continental-blow-torch-warm-weather-britain

http://www.telegraph.co.uk/comment/11733731/Met-Office-caught-out-over-its-hottest-July-day-ever-claim.html (twaddle)

http://www.express.co.uk/news/uk/587218/Heatwave-UK-weather-forecast-summer

https://notalotofpeopleknowthat.wordpress.com/2015/07/05/new-data-from-ruislip-casts-more-doubt-on-heathrow-record-temperature-claims/

http://www.carbonbrief.org/blog/2015/07/met-office-wind-data-dispels-doubt-about-cause-of-heathrow-high-temperatures/

2015-06-11_18-35-59

synoptic chart spanish plume event on 12/06/2015

Review: In the event: this was largely a fizzle! Few thunderstorms emerged until later and the SW missed out completely, which was the favoured area for action by some forecasts. One or two heavy tstorms impacted Sussex / Kent and East Anglia later in the day and into evening.  Surrey largely missed out, probably due to too much cloud cover, which left insufficient surface based heating to trigger home grown storms.  Also, a lack of soil moisture is significant too and humidity was left crashing by the afternoon.  Tmax Reigate barely scraped 25C here and, at 18C, dew points were on the low side all day, with RH similarly unimpressive hovering as low as 67% for a good while.

2015-06-12_18-20-06

complete lack of humidity in SE by evening

Imported storms attempting to cross the Channel generally couldn’t make it and, those that did, clipped Kent and Sussex late in the evening.  An interesting local convergence line in the evening popped up convection late on but this was mostly not thundery and missed Reigate anyway.  This was always forecast as an isolated tstorm event and heavy showers did occur but nothing of note over Reigate.  Many forecasts were found wanting and many convective storm specialists and enthusiasts will probably be smarting over this partial non-event.  Possibly the biggest “miss” in forecasting for a number of years.  In reviewing the charts below it’s worth noting that the individual ingredients were modelled but did not come together on this occasion and others were not solid: GFS CAPE and dew points significantly lowered in runs prior to the event.  There was little wind shear to drive storms along in a very slack flow. It wasn’t a classic Spanish flow from the SSW – drier easterlies dominated. Finally, the initial flurry of Channel storms left a good deal of cloud clag across the region and did not permit surface heating.

original post:

Southern England, including Reigate, is likely to have a first proper convective event of the year tomorrow including moderate to possibly even severe isolated storms with some heavy rain in a short space of time and a chance of hail, thunder and lightning.   Not everywhere will get a storm but, if you do, it could be a pretty big one.  For us in Reigate thunderstorms are most likely towards the afternoon and into Friday evening, though they could “pop” almost anytime during the day, especially if it is sunny and heat builds early.  If it stays cloudy through the morning the set-up could fizzle out spectacularly!  Expert convective forecasters say that this is a highly complex situation and forecasts, even at this late stage, are prone to inaccuracy when pinning down potential storms like these.  This post is not a forecast but outlines some key ingredients for thunderstorms and sees how tomorrow is set up to deliver the goods, at least some of them.

A MetOffice weather warning has been issued because the rain or hail could be heavy in a short space of time and cause local flooding and some disruption. The worst conditions are not expected here but further to the SW and S Wales. Severe thunderstorms have been occurring in France and Spain all day and, indeed across S Europe for much of the week, ours will be pretty moderate by comparison.

Thunderstorms need 3 things to get them going: moisture, heat and lift.  Here’s a quick review of some of these ingredients thrown into the Reigate and Surrey weather mix tomorrow with some charts to illustrate.

thunderstorm characteristics

thunderstorm characteristics

1. Heat

You may have noticed that today warmed up considerably reaching nearly 25C in Reigate . Tomorrow will be warmer still. This is due to a warm “plume” of air arriving from the continent, from as far away as S France, Spain and the Mediterranean.

This imported heat alone will raise the “airmass” temperature to over 15C at 850hPa (1500m).  Any sunshine, of course, will further heat the surface and this could raise temperatures on the ground to over 25C.  This is a critical ingredient for thunderstorms: air needs to be warmed so that it will rise into the atmosphere.

In conditions of potential severe weather it is useful to have sunhsine to heat the surface.  The cloud cover tomorrow looks broken and, if it remains like that through the morning, this will build bigger afternoon storms.

Whilst heat at the surface is a good thing to create warm rising bubbles of air, a comparatively cooler atmosphere through which the warm parcels of air can rise, is also a useful ingredient.  The air high up is unusually cool at the moment and this will create steep lapse rates… a rapid reduction of temperature with height.   Lapse rates can be shown on charts like below and on skew-t diagrams: these look tricky but show a cross-section through the atmosphere.

Steep lapse rates encourage warm parcels to stay warmer than the surrounding air.. so they will keep rising creating tall clouds such as cumulonimbus. It’s also useful to have droplets freeze at the top of clouds: it’s these ice particles that bounce around through the cloud, rising and falling to build a charge that causes lightning.

thundery skew t

thundery skew t

The red line is the “environment” air temperature: imagine taking the temperature of the air at regular heights as you rise up on a balloon flight: you’d expect the temperature to go down… usually by about 0.6C per 100m.  Now consider how a warm rising air parcel will rise, expand (as less pressure), cool and condense at a different rate: this is the dashed line which shows how rising parcels sometimes stay warmer (tomorrow) than the environmental air right the way to the top of the chart at some 30,000 feet.  So long as the rising parcel stays warmer relative to its surrounding then it will rise! The ultimate height of some well developed cumulonimbus clouds exceed 10km.  Freely rising bubbles of warm air (thermals) is known as an “unstable atmosphere”: like heating soup on the hob.. bubbles rise through it.

2015-06-11_22-41-54

Channel sea surface temperatures

The Channel is only 12-13C sea surface temperature at the moment and this can subdue thunderstorms attempting to cross from France. Nevertheless, storms currently approaching the south coast are pretty active still but are not expected to reach far inland to the SE as pressure is still comparatively high.

2. Moisture

Without moisture there will be no clouds and certainly no thunder.  The humidity and high dew points on the charts above shows how tomorrow there is plenty of moisture being advected into the country on the humid plume after the warm front passes north.

2015-06-11_20-56-23

theta e charts show airmasses well; spot the plume!

The synoptic chart shows the warm moist wedge that is due to pass over the UK tomorrow; it has large quantities of precipitable water (over 30mm) which could fall all at once in the right conditions.  Water vapour is a key ingredient: as water vapour condenses it releases latent heat which can drive upward lift in thunderclouds yet further (saturated air cools less slowly, so increases instability).

3. Lift

Lift can be any forcing mechanism that encourages air upward.  Surface based heating (diurnal heating) is important tomorrow, but so are fronts.  An advancing cold front sometimes cools and dries out the upper atmosphere and this can increase lapse rates dramatically and encourage further lift.  Charts showing CAPE (convective available potential energy) and lifted index can both show the tendency for air to lift.  Higher CAPE numbers are good, negative Lifted Index numbers are good for storms too. LOW pressure is also important to encourage the mass ascent of air.  An upper ridge early tomorrow could inhibit thunderstorm development in the SE until the trough arrives later.

2015-06-11_18-24-22

CAPE and lifted index

A further source of lift is surface convergence of airflow.  Convergence is where air arrives in a location quicker than it leaves.  When surface winds converge air “piles up” at a location and has nowhere to go except UP.  Surface convergence, with divergence aloft, is a good set up for lifting.  Sometimes hills or coastal breezes can cause convergence and enhance lift too.

What’s missing tomorrow?

For the very biggest storms more of all the above is good.  Moderate storms and isolated severe storms might arise but there is a lack of wind shear to organise storms into supercells.  Wind shear is increasing wind speed or change in direction with height.  Tomorrow is a slack flow until a moderate jetstream appears later in the evening.

lacks shear strong upper flow

lacks shear strong upper flow

Wind shear has the effect of hoovering air up from the surface and separating the warm storm inflow from the cold outflow. An organised storm will thus keep hoovering up warm air which feeds further development into a severe storm or supercell (which starts rotating and is a precursor of tornadoes).

2015-06-11_22-38-36

organised thunderstorm formation

With little wind shear the inflow feeding the storm can be disrupted and stopped by cold air descending from the tops of thunderclouds and cutting off their heat supply.  Such storms die naturally after a few hours and are known as single cell storms.  Multicell, supercell or meso-scale convective systems (MCS) require some shear to keep them strong and well fed. The lack of isobars on the synoptic surface pressure chart below indicatea the slack flow.  This can cause high rainfall totals because  storms sit and soak the same place rather than move on.

slack plume

slack plume

The overall synoptic development of this plume shown below is good-to-go for storm action tomorrow for some places in the south and SE including Reigate. A separate article discusses Spanish Plume development here.

So, some factors favouring storm formation tomorrow include:

  • Heat: strong advection of very warm humid air across southern England.
  • Low pressure: allows air to rise on mass.
  • Winds from the SE / ESE: can import storms from France in a NW direction. (they often miss us and go off to Kent otherwise)
  • Moisture: humid air across the south throughout the day.
  • Lift: sunshine will hopefully lift temperatures and permit thermals to rise to develop home-grown storms later in the day.

Do watch out for the King of Clouds… cumulonimbus or any of the development clouds like altostratus castellanus, and share pics and stories of any storms that come your way. Meanwhile,check professional forecasts for updates of course if you are making decisions.

Note: this is not an expert convection article, but an educational outline of storm formation. Comments always welcome.

Try Lightning Camera app https://play.google.com/store/apps/details?id=com.pluto.aftershot&hl=en

2015-04-27_20-21-59

cold plunge of polar air to end April

The newspapers have this colder than average week billed as a “polar plume”.  Cold air cannot really be said to “plume” like warm air (spanish plume).  Nevertheless, it is certainly cold up North with settling snow over the hills and wintry precipitation elsewhere too.

The GFS ensemble (several model runs combined at once) chart below shows that the colder than average (upper air) temperatures will last until the weekend at least.  Thereafter temperatures rise but note the rainfall spikes indicating unsettled conditions.

2015-04-27_22-22-51

upper air temperatures

For us here in the sheltered SE it is just cooler than average with patchy overnight frost and bright days with light showers Tuesday pm. A blustery cold front on Wednesday is likely to bring more purposeful rain for a while, as could further active fronts on Thursday when the trough axis moves through our region accompanied by the jetstream nearby to the south.  Friday is likely to see a transient high pressure ridge bringing settled weather and then a change in wind direction by the weekend.

The overall cool set-up is due to “northern blocking” which is when pressure builds over Greenland and the Pole and the, previously strong, Azores HIGH slackens off and nudges south. At the same time the usual Icelandic low pressure weakens or is dominated entirely by a HIGH pressure (see chart below).

2015-04-27_19-24-24

In this situation the jetstream works its way south of the UK.  The result is that cold polar air is able to leak south out of the polar regions and into the mid latitudes.

2015-04-27_19-28-07

The index shown above summarizes the overall pressure pattern in the Atlantic. It is called the North Atlantic Oscillation. The NAO measures the pressure difference between Iceland and the Azores.  It has been mostly positive this winter: meaning that pressure is LOW over Iceland and HIGH over the Azores.  Typically a positive NAO indicates a strong zonal jetstream and mild often wet conditions for Europe with relatively fast moving LOWS passing through.  The chart below shows how the NAO has gone negative recently and this indicates that pressure has built over the Pole, creating a blocking situation.

The charts above show theta-e temperature which shows cool airmasses over the UK clearly.  These are 850hPa temperature charts which represent temperatures at 1500m (1.5km) above the “boundary layer”.  This height is used to avoid disruptive temperature changes which occur nearer the surface caused by day and night, mountains and water bodies etc that upset the overall temperature pattern for analysis.  The situation shown below by this weekend is quite different, though still unsettled.  Note the warmer flow from the SW.

After the transient ridge on Friday it looks like pressure will fall into the bank holiday weekend as a low pressure nudges in from the Atlantic.

2015-04-27_22-38-16

pressure falls into weekend

The milder humid air brought from the SW by the Atlantic low could potentially cause some significant rain at times around the weekend and into early next week as this meets cold air over the country.  Despite the milder upper air arriving from the SW, it is unlikely the “milder” temperatures will be noticeable in such wetter and windier conditions. In winter this set-up could have brought big snow events but in early May it will simply bring rain. For the detail on timing and amount of weekend rainfall we will have to wait and see, but it certainly looks potentially quite wet, though models suggest pressure building briefly thereafter.

2015-04-27_20-10-05

The weekend sees an atmospheric battle taking place overhead between tropical and polar air masses. The video below shows the stage being set for this battle: watch the westerly winds pushing against the cold polar North-Easterlies sweeping down from Scandinavia and meeting over… well, Reigate!! Anyhow, this weekend temperatures will fall throughout Saturday from 3°C to freezing by late afternoon. Rain could turn sleety from mid-afternoon onwards as the coldest air pushes in and light snow is possible, dying out early Sunday morning. Sunday looks dry and cold, feeling like -7°C in a chilly northerly wind. Monday is set to be even colder, staying near freezing all day and feeling like -8°C in the wind, but dry (though latest is for snowy front moving south according to UKMO). The rest of the week is unclear but likely to stay cold until at least Wednesday with more significant snow likely Tuesday. Longer term predictions suggest January will stay cool, with a few milder interludes.

It’s all set for a battle right over the SE between tropical and polar air over the weekend. Models are struggling to tell which will win – some say any cold snap will be brief and milder tropical air will return next week, others predict polar air dominating for a while with the jetstream shifted south of the UK. Who knows?
Meanwhile, as we wait to find out, enjoy the contrail photos from 8am this morning over Reigate! What are contrails and why do they thicken up sometimes and disappear at others? Posts on clouds and cloud types coming soon!

Air masses arriving over the UK at this time of year are influenced by the sea temperature they travel over. Dry, very cold easterly winds from a frigid continent arrive at the UK by travelling over a warm-ish North Sea (currently 9C). These winds are warmed from below and can pick up enough moisture to deposit snow over eastern counties.
Note Atlantic sea surface temperatures are still over 10C even well into December, hence any winds from this direction are likely to be relatively warm and wet but not snowy.