Archives For CAPE

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.

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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.

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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/

May 19 thundery showers was not a “severe” weather day but 10mm of rain from 7 showers in 7 hours, several with some moderately intense rainfall rate (30-40mm/hr), small hail and episodes of thunder, was of interest locally and deserves a review of some charts leading up to the event.

Often our area of the SE seems to miss convective action but this time showers perked up as they converged across East Surrey and some developed into reasonably impressive cumulonimbus clouds (cb).  As a bonus mammatus clouds were spotted at the end of the day (see photo above). Below is a summary of some key ingredients for the day.

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Synoptic chart thundery 19 May 2015

Low pressure: This synoptic set-up saw an unusually cold plunge of upper air flood across the country during May18-19. The synoptic chart above shows the invasion of cold polar maritime air behind a cold front, itself not especially dynamic across the SE. The LOW over the North Sea dragged this unseasonably cool air from the NW: the basic ingredient for instability on this occasion.

meandering jetstream

meandering jetstream

Note the u-bend wiggle in the jetstream around the LOW pressure in the chart above. The northward limb of the jet on the inside of the U-bend can become the location for instability, though this was not a fast jetstreak occurrence.  The base of the trough also crossed the SE of the UK during the day encouraging lift.  Showers were forecast on the charts days before.

Steep lapse rates: Very cold upper air temperatures fell to below -30C at 500hPa (5500m) across the UK.  In Spring a cold airmass like this can become unstable over land especially if the sun warms the surface to create thermals of warm air able to lift through the cold environmental air. The 850hPa temperatures show comparatively warm air at 1500m which increases the lapse rate.  The morning of May 19 was sunny and the surface heated with 5 hours of May sunshine to a modest 14C, just enough to release thermals and rising parcels of air.  Convection requires heat as a key ingredient to steepen lapse rates and create thermals, rising parcels of air.

Low lifted index: LI forecast for Heathrow (below) showed a LI of -3.  The lifted index is a measure of instability in the atmosphere and shows how readily bubbles of warmed air will rise from the surface to a great height (convection).  It is the difference in temperature between the environmental air at 500mb level (around 5500m) and the temperature of the theoretically lifted bubble of air (parcel).

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lifted index (UK rarely exceeds -5)

Negative LI numbers are good for thunderstorms because they show that the parcel is significantly warmer than the air around it at altitude, which means that at 5km the air is still bouyant. http://www.weatheronline.co.uk/reports/wxfacts/Thunderstorm-Probability.htm

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Moderate CAPE: convective available potential energy is the energy available to push air vertically up and create those surging bubbles of cumulonimbus clouds.  The higher the CAPE the greater uplift potential (above).  Note it is potential energy and a high CAPE often confounds convective activity if other ingredients are lacking, such as heat or moisture.  19 May had marginally unstable values for the UK of 800j/kg.

In the USA Tornado Alley forecast CAPE values exceed 2,000 j/kg in super cell outbreaks.  CAPE on a skew-t chart is the area between the parcel trajectory and the environment temperature.  In the SE of the UK we often just get “skinny cape”, where rising parcels are only just warmer than the surrounding environmental air, whilst in Tornado Alley “fat cape” is frequent which indicate parcels much warmer than the surrounding air, increasing bouyancy.  http://www.weatheronline.co.uk/reports/wxfacts/CAPE—Convective-Available-Potential-Energy.htm

The colourful charts above from lightningwizard.com show streamlines of wind and air mass equivalent potential temperature.  Streamlines show where the airmass is coming from and, if they converge at the surface, can indicate potential for lift.  Theta e charts show moisture and temperature characteristics of airmasses.  Where there is a contrast of colours between high theta e and convergence at the surface and lower values and divergence of air at altitude this can bring on convective weather. The moisture chart shows that the airmass supplied sufficient moisture to the UK to provide for cloud formation and release latent heat.  Dew points stayed relatively low and this showed moisture (and heat) locally was lacking for bigger storms.  Compare these forecast charts with the actual occurrence of lightning on the day.

curved forecast shows showers

curved forecast shows showers

Forecasts for the day (Euro4) showed typical lines of rainfall accumulation which indicate showers.  The actual radar showed “popcorn convection” (@metmanjames) over a wide area drifting east.

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Recommend following @convectivewx for UK convective forecasts.  Their forecast from 3 days ahead was spot on.

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analysis chart shows HIGH edging out with moist Atlantic winds ready to pounce

The analysis chart above shows a weakening ridge of HIGH pressure over the UK being edged out north by a slow moving Atlantic LOW to the W/SW.  Reigate is still currently (Saturday am) in cool dull easterly winds generated by the HIGH but a significant switch in wind direction will take place over the next 12 hours into the bank holiday period as a wholly different mild and humid Sub-Tropical Atlantic air mass, with a source region round the Azores, takes hold from the SW.

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weather cross-section

A mild moist S/SW wind drives in from the Atlantic as the LOW edges north east tonight. The first mass of rain is edging onto radar from the SW and is expected to arrive in Reigate by around mid-late pm today.  Most rain is likely for places further north and west but the SE is still likely to pick up plenty of wet weather overnight with low cloud and rain into Sunday morning when it could turn heavy and showery for a while in the early morning as the trough passes directly overhead and pressure continues to fall. Things are expected to clear to brighter conditions later in the afternoon as pressure rises and winds turn more westerly. Cloud cover will hopefully break and cloud height will lift during the afternoon becoming more cumuliform.

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trough and fronts migrate north, showers follow

If the sun comes out then there could be a low risk of an odd heavy shower Sunday afternoon, possibly thundery, but these are more likely further north of our area where more unstable air makes progress across the Midlands and East Anglia.

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During Sunday winds will be occasionally blustery with moderate convective gusts possible, especially on hills and nearer the coast, and make the mild temperatures Tmax 16C feel considerably cooler. Temperatures overnight could hold up to a balmy 12-13C.

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Overnight Saturday-Sunday rain could linger as showers through the morning

Winds turn from SW to more southerly through Monday and pressure should up-tick slightly giving a mostly dry and warm day and less windy as things stand currently.  Troughs could progress east during Monday and build cloud and produce some showers.  More importantly there is a looming threat of something special for later Monday-Tuesday night.

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As the northern block (high over Greenland) holds on, the Atlantic LOW just west of Ireland will usher in a mild and moist S/SW flow of air from the continent.  An unstable LOW brewing in the topical Atlantic today (Saturday) is forecast to sweep up and intensify from Biscay later Monday and into Tuesday and this might bring heavy rain and winds to the south and SE and a possible thundery episode later Monday but more likely overnight into early Tuesday for SE.  The jetstream is dipping well south and is forecast to perk up and approach the UK from an unusually southerly direction by Tuesday.  If this happens the jetstream could deepen this low considerably, as modelled by some charts (latest UKMet shows 980mb).

Depending on the evolution we could find ourselves in the unstable left exit region of a jet where divergence aloft enhances convective action and creates heavy rain.  Warm air from the south will also contain more moisture.  A dry slot at mid-levels might also enhance instability (rising dry air cools more quickly increasing lapse rates and CAPE, enhancing lift).  High dew points near the surface temperature also encourage condensation and indicate extremely moist warm surface flows.

So all these ingredients stirred up could be a good recipe for some briefly moderate-severe weather in our region especially some briefly torrential rain, though totals are unlikely to amount to more than 10mm.  Gusty winds and gales near coasts could also accompany this system.  Latest metoffice chart shows pressure dipping to 980mb in the North Sea which is significantly LOW pressure for the time of year.

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coastal gales and convective gusts inland

However these episodes have a habit of tracking across Holland and merely clip Kent with thundery showers and miss us entirely.  Models also generally exaggerate these early on and then things flatten out nearer the time considerably.  Nevertheless, it is worth watching this develop as our first potential “warm plume” of the year.  If we take a direct hit the SE could have some heavy rain.

The GEFS summary below clearly shows the two main rainfall spikes tonight and Monday night.

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GEFS 850hPa temperatures and rainfall London

Later mid-week the LOW is expected to drift east across the UK bringing in a more westerly pattern so unsettled showery weather is likely for a while. Thereafter, a rise in pressure from a developing Euro high pressure may then take place from the south and settle things down for us in the SE, though this might only make faltering progress.

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faltering pressure rise later next week