Archives For heat wave

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/

After a cool unsettled end to May with a strong zonal jetstream, early June weather prospects are getting interesting for Reigate as models build a tantalizing “heat wave” with potential for warm plumes and thunderstorms, quite a contrast to the start of the week!  The warming is just beginning to cook up on the mean temperature anomaly chart and recent GFS model runs shown below with Tmax temperatures in the high 20’s and some approaching 30C by next weekend/ Saturday especially. Here’s a review, rather than a forecast, of some of the synoptic features unfolding early June weather.  The scenario can change a lot by next weekend of course, so stay tuned, especially to twitter and the fantastic UK weather community (both amateur and professional) for updates.

In the short term, the end of May and start of June will continue to be dominated by LOW pressure to the north sweeping active fronts across the UK with attendant rain and wind, especially Sunday and more on Tuesday, though as usual Reigate and SE will be sheltered from the worst of this which will impact the NW mainly.  Tuesday’s Atlantic depression has an unseasonably low forecast central pressure of 972mb (UKMET) and 968mb (GFS) due to a strong jetstream across Scotland of 160mph.  Expect windy, gales in west and coasts, showery and unseasonably cool weather everywhere but especially in the NW during these episodes. Reigate on Tuesday could have gusts exceeding 40mph.  Update Sunday: MetOffice have issued a weather warning for gales countrywide for this event.

968mb would be close to the lowest June central pressure to impact the UK since 2000**, though the MetOffice chart below showing 972mb is probably nearer what will happen in reality.  Highest winds (40mph inland, 50mph coastal) in Reigate and SE are expected Tuesday am as a daughter frontal wave-low whizzes across the Midlands trailing her bigger parent.

**

 

By the end of this week models build a meridional (wiggly) flow in the jetstream as the North Atlantic Oscillation (NAO) turns negative which means pressure over Iceland rises relative to that over the Azores which falls below average. This weakens the pressure gradient and reduces jetstream strength.  The NAO is not a driver of weather but is an indicator of Atlantic patterns that controls incoming weather for the UK and Europe.  A negative NAO often means fewer Atlantic based LOWS with a less aggressive and frequently more meandering polar front jetstream, with more chance of continental weather impacting the UK as pressure builds to the north (in winter this can bring cold weather from the continent).

A negative North Atlantic Oscillation usually indicates a weaker jetstream and one that meanders with high amplitude waves across latitudes, a so called meridional pattern (rather than zonal which blows purposefully west to east across Atlantic along strong pressure gradient between Azores and Iceland: dragging in frequent LOWS).  A meridional jetstream can slow-down and fix weather patterns into place, especially if a HIGH builds to the north as a so called blocking pattern.  Such a pattern looks possible with current model runs, though with steep temperature gradients building over a heating up USA/Canada, a return to an active unsettled Atlantic pattern seems possible later into June which could breakdown any blocking pattern and finish off our balmy continental flow, but that is way off so remains to be seen!

The meandering jet developing from mid week will encourage a warm / hot continental easterly / SE flow for the UK as pressure builds initially to the north east and pressure lowers to the south from Wednesday.  A cut-off LOW to the west of Iberia sandwiched between the Azores HIGH and the HIGH further NE is also a prime ingredient to waft warm unstable Spanish plumes our way as the HIGH pressure drifts east over Scandinavia (see above charts).

2015-05-30_20-18-19

Spanish Plume June 2015

The threat of heat and thunderstorms peaks next weekend, notably on Saturday, with temperatures peaking as high as 30C. The 850hPa chart above shows the warm plume arriving from Spain.  Upper air temperatures exceeding 15C would yield hot daytime temperatures approaching 30C in sunny conditions.  Such warm plumes of continental air, meeting Atlantic air injected from the cool HIGH offshore, could lead to unstable thundery episodes (more on Spanish Plumes here) On the other hand the pressure is quite high in the East so this could suppress convective action here, the detail will be critical.  On the skew-t chart below spot the steep lapse rates, negative lifted index and high CAPE, high dew points (moist air) and precipitable water content below, all lively indicators of an unstable atmosphere.  It’s a long way off though so things can change a lot and frequently do!

2015-05-30_09-33-32

This set-up is the source of excitement over “heat” by the end of next week: a warm continental plume.  Pressure in this scenario would be highest in the north of the UK so the SE could see more unsettled conditions.  Interestingly, the current Madden-Julian Oscillation Phase 1 and 2 (a tropical disturbance pattern used to forecast patterns in medium range) correlates with this emerging pattern, with P1 and P2 often linked to HIGH pressure to north, LOW over Europe and an unsettled S/SE UK.

Despite this, a prolonged heat wave does not seem to be a strong possibility.  Technically a UK “heat wave” is when daily maximum temperatures for more than five consecutive days exceed the average maximum temperature by 5 °C.  For the SE this usually means exceeding 30C daytime Tmax and 16C night time Tmin, whilst forecasts for the end of next week are warm, sustained heat of this nature does not seem likely.  Pressure and 850hPa temperatures rise this week with some models in the high 20’sC Tmax. Both medium and longer range models suggest either a flat-line or fall in both as June progresses.

Models also suggest a weakening of pressure and some play with a thundery breakdown bringing in wildly high CAPE values (convective potential) into June. This suggests a breakdown as pressure falls. Some CAPE values forecast are extreme for the UK and would not be out of place in a Mid-West tornadic supercell! However, often these scenarios fail to materialise as forecast and often the instability and thunderstorms simply brush past the SW of the UK or stay in France, perhaps clipping Kent alone.

Models in the second week of June seem to suggest the HIGH builds out to the NW, regressing from the location over Scandinavia.  This would put the UK on the cooler side of the HIGH with a northerly flow, thus ending any heat spike. This is just one GFS run and at the unreliable end of the model but a possible solution.

2015-05-30_12-11-00

Finally, over the long term average June is rarely “hot” for a prolonged period and this brief hot spell declining into “warm” would seem to match the emerging pattern.  On average, the hottest day of the year falls in June only 25% of the time and June has shown climate trends of becoming rather duller and wetter bucking the trend of overall warming for other months due to climate change.  The frequency of Atlantic westerlies, on average, also picks up in June making sustained “heat” a rarity.  Nevertheless, a brief warm spell is likely as shown by the end of this week as shown by increasingly convincing model trends. Thereafter, the location of the HIGH and jetstream activity will probably control unfolding June weather events.

Update from @wansteadweather supports idea that June may not live up to any hot start, HIGH dominates but the position may feed average to cool flow from N.  Will be interesting to see how this pans out!

http://www.metoffice.gov.uk/learning/learn-about-the-weather/weather-phenomena/heatwave

http://www.metoffice.gov.uk/public/weather/forecast/gcpg7rs0t#?tab=fiveDay&fcTime=1432944000

Reigate should see 30ºC Tmax for the first day of August and high 20’s on Friday. The cause is a warm tropical plume of air flooding over the south of the UK from Iberia (Spain) and Africa brought in as the jetstream moves north and a LOW to our west and HIGH to the east builds a brisk Southerly airstream. Thursday will be dry and bright and the night will feel very muggy with high dew points and lows of over 20ºC at midnight: warm and damp, the air will feel close! Friday will be warm too but slightly less so for Reigate, dawning bright but with cloud spilling in associated with a weakening cold front pushing east across the country. Little frontal rain is expected but the cooler upper temps could release the huge heat energy built up in the lower atmosphere and kick off heavy / thundery showers for a while, especially Friday morning. Confidence is low currently but this might occur, the potential is there with all that heat.  

Every thunderstorm indicator is giving green lights to thunderstorms in the SE sometime in the next few days BUT the airstream is capped with warm uppers: this appears to be the only factor stopping convective action. This means heat building in the lower atmosphere will stay put until forced to rise into the unstable layers above (see sounding graph above). The approaching cold front might be sufficient to produce something of convective interest on Friday, morning especially… before the warm Euro air retreats back to east. So far Reigate has seen little of the impressive thundery activity elsewhere in the UK this summer. You might be forgiven for thinking it must be our turn soon! It’s unlikely to be in the next few days as the risk is small for any major thundery outbreak.
The synoptic situation remains similar over the next few days with HIGH pressure over the continent and LOWS edging close to the NW of the UK bringing rain and wind to the NW. The SE should escape with any trailing fronts weakening and Reigate should remain largely dry but for any threat of isolated thunderstorms in building heat.

The HIGH pressure now building over the UK looks like persisting for at least a week and possibly for longer with a breakdown only hinted at from 15 July or later on current model runs. High pressure brings warm, dry weather with mostly clear skies as air sinks from aloft and warms and dries out as it does so. High pressure has pushed the jetstream (which guides LOW pressure storms at the surface) well north towards Iceland. So GREAT summer weather for Reigate and the UK over the next week and beyond!

Temperatures will mostly be in the high 20’s and could climb to 30ºC locally. Night time temperatures are also going to remain well in double figures for most of this period.
One potential spoiler for east and SE England could be the build up of brisk NE and E winds swinging round the edge of the HIGH as it continues to build in pressure during next week and slips north. The forecast Northerly / NE winds running south down the North Sea and swinging east into the Channel could reach 20mph and inland could be 10-15mph (supergeostrophic = surprisingly breezy in high pressure as winds swing out round the pressure gradient). The North Sea is still only around 13C sea surface temp and this will have a cooling effect on the breeze and east coast areas especially.  This wind might even pick up some low cloud and mist for the east coast. How far inland this effect reaches remains uncertain but most likely the cooling will be restricted to the immediate coast. So, warm everywhere but west is certainly best!

SE heat 7 JulyUpdate: scorchio for Sunday but thereafter in SE and Reigate it will be pleasant and “normally” summery and warm.  The NE breeze will take edge of scorching temps in SE and especially near coasts.  The NE may also bring cloud which will further peg back temps to a more normal feel at mid-20’s rather than high 20’s. 

monday 8 july

The predicted heat wave (or should that be “warm wave”?!) is showing up nicely on latest runs with 30C quite possible in some parts of the South. The lack of rainfall over the next 10 days is also significant. High pressure stays in charge for at least a week with the jetstream being shifted north to Iceland where it belongs in summer!  

pressure builds july

Pressure will build right over the UK from the weekend bringing warm, sunny and dry conditions possibly for another 10 days!

july high blocksAn anticyclone building over the UK will dominate our weather for at least next week and possibly beyond.  Upper air flow over the weekend will direct warm tropical air from the south and sw over the UK. Thereafter upper air flow will shift to a more northerly flow, slightly cooler.  So hottest temps are likely this weekend.  Surface winds will be more NE and easterly as the HIGH slips to the North. Whilst winds inland will be light, E and NE winds in the Channel could build through to Monday as easterly winds drain away from the HIGH located over the North Sea. 

Sea surface temperatures are reaching the low teens around the UK.

Very strong sunshine and high temperatures: enjoy and don’t forget to slip, slap and slop! (slip on a Tshirt, slap on a hat and slop on the sun cream!).

To become a “Heat wave” the weather must reach a technical threshold and, whilst Sunday will be v warm for the UK, the rest of the week will be just a bit above average: a heat wave is where daily max temperatures for more than 5 consecutive days exceed the average max temperature by 5°C.

For the SE England Max temps must exceed 31°C and min temps exceed 16ºC at night.  So technically this is unlikely to be labelled a heat wave, more like a warm wave 🙂 http://www.metoffice.gov.uk/weather/uk/heathealth/

Confidence now high for the HEAT WAVE starting weekend then continuing thru early next week and possibly beyond.  Updates later.

Early days still, but there is some indication of July warming up considerably from next weekend after a breezy mid-week. Models are coming together showing the Azores High building right across the UK from next weekend (7 July) bringing anticyclonic sunny and warm weather and a warm upper air flow. Some charts show temperatures in the south east exceeding 25°C and some even show 30°C! Whilst the UK is set to be much warmer than normal (positive anomaly), much of Europe is cooler than usual, still stuck in the trough of cool air.

warm UK 7 July

 It is worth remembering that the sun is very strong this time of year and, although a weather advisory seems tedious, it is worth slapping on the sun cream and adding a hat: slip,slap, slop!  Of course, our own heat wave will be nothing compared to the temperatures the SW USA is experiencing right now.  Record breaking heat is pushing temps up to the hottest ever recorded 54ºC.  That beats previously discredited records in Libya. ttp://www.nypost.com/p/news/national/western_issues_vegas_could_treated_Z7IWo9K7JwgXdDcqNOJ7jO

UV levels