Archives For stingjet

Whilst Storm Katie was not a record-breaker by any means she did provide some evidence of stingjet winds in the wrap-around feature that showed up especially on IR satellite photos and rainfall radar in the later stages of her track across the SE and UK.

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Here is some analysis from local Reigate Surrey records of these winds to explore this feature more. Stingjets can be the most damaging winds, in this case they were not especially strong.  Professionals, like Matt Hugo (NorthWstWx services) and Simon Lee (MMetReading), saw hints of stingjets during the passage of the storm but what evidence of stingjet winds is there “on the ground” and how can amateur observers watch out for these potent weather features in future?

Katie’s rapid cyclogenesis prior to landfall over the UK was a precursor to the formation of stingjet winds.

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Stingjets are associated with vigorous bomb-depressions developed in a process called rapid cyclogenesis: in RaCy depressions central pressure falls very rapidly and slows the horizontal surface speed of the cyclone.  A stingjet is a narrow band of gusty winds that descend from high altitude in the latter stages of RaCy depressions.  They arrive from some 3-4km above the ground and are associated with descending stratospheric air into the low core.  Ironically stingjets are associated with weakening fronts and aging cyclones.  This descent of upper air pushes the jetstream lower and this can deliver extremely gusty conditions to the surface in a narrow band that is shown by a cloud hook and matching rainfall pattern.

Watervapour satellite loop from 00hrs to 8am showing dry descending stratospheric air forming a dark slot – dry intrusion – that wraps into the developing low core: a sure signal of rapid intensification.

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The hooked rainfall signature below matched a marked increase in the strength of wind and the modest strongest gusts of the whole event of 52mph in Reigate. In exposed places like Redhill aerodrome this exceeded 60mph and over the North Downs at Kenley 68mph was recorded with 70mph in places. 2016-03-28_07-32-34 Arriving from high altitude, stingjet winds are unsurprisingly cold with low dew points. So you’d expect a dip in temperature at the surface.  Parts of Wales and Shropshire had snow in this airstream wrapping round the back of the low as it moved into the North Sea. Stingjets match the mature stage of the Shapiro-Keyser cyclogenesis model.  In all respects, they are aptly named as the “sting in the tale”.

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metoffice stingjet info

Storm Katie had some generally strong winds (for the SE of the UK!) associated with the southern edge of this cyclone where the tightest pressure gradients developed as pressure fell across the SE: in the narrow warm sector especially.  The lowest central pressure was 971mb.  Our Wight-Wash Oscillation reached 16 or 17mb at one stage: the difference between the pressure over the Isle of Wight and the Wash.

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Most significant storms to impact SE England have similar NE tracks through the Bristol Channel and exiting through the Wash. This was certainly the case with October 1987, St Jude 2013 and Storm Katie March 2016.

A complex warm sector and bent-back wrapped occlusion make it tricky to identify the normal Norwegian model of warm-cold frontal passage.

It is more likely that Katie, like other RaCY depressions, developed according to the Shapiro-Keyser model of rapid cyclogenesis which involves a break away cold front fracturing away from the depression core: T-bone.

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The wrapped rainfall radar and cloud hook above both suggest a stingjet feature associated with this kind of development.

Now, onto evidence stingjets… the passage of the strongest gusts shown below do not match the passage of a “normal” cold front but suggest some other process was at work to deliver the strongest gusts.  The strongest winds occurred 3 hours after the passage of the front that started to deliver the expected colder polar air behind the wrapped occlusion. This can be seen from the chart of Reigate wind speeds and temperature below. 2016-03-28_22-25-38 The chart shows the persistent warm sector gales proceeding from midnight on 28 March through to about 5am.  These gales do not exceed 80kmh but they modestly peak just before the passage of the front, an expected pattern.  Fronts then pass through Reigate at about 5am and temperatures fall as expected, as do gusts.  However, from 7am gust strength sharply increase, this matches the timing of the sting jet cloud feature on satellite photos.  This increase in wind speed had no front associated with it and therefore suggests evidence of a stingjet process: arriving out of the blue!

Note the temporary drop of temperature to a minimum and recovery after the departure of strongest winds.  This again suggests these winds are not frontal in origin but are part of the stingjet process.  Are they associated with a sting jet of descending high altitude air originating some 3-4km in the troposphere?  I’d like to think this is a sting jet signature but will need confirmation from official sources to pin this down. 2016-03-29_07-29-56 For comparison shown above is a “normal” cold front passage from the previous weekend when a cold front squall line of some note passed through.  Note there is no dip to minimum temperature associated with the maximum gusts and temperatures remain cold after the passage of the front because of the insurgence of cold polar air.   This is more typical behaviour when it comes to frontal passage.

 

 

Other charts and references support this idea, but some not unfortunately not quite with same timings.

http://www.metoffice.gov.uk/media/pdf/2/p/Sting_Jet_Flyer.PDF

The popularly named St Jude storm (officially named Christian) of 27-28 October 2013 was the most severe to hit southern Britain for over a decade. Whilst it was less powerful than the 1987 Autumn storm, St Jude lived up to its forecasted strength and caused an estimated £1 billion worth of damage and losses across the southern half of the UK.  Here is a round up of  causes and some impacts of this severe mid-latitude storm.  Locally, @RGSweather covered the storm continuously overnight, providing updates and advisories on twitter as things developed minute by minute.  This is a summary of causes, what happened and what we have learned from this storm…

Causes

The “ingredients” for the birth of St Jude include…

1. A big warm soup:

A warm Atlantic Ocean, some 3-5c warmer than 30-year average, acted as a perfect birthing pool and nursery for StJude. The warm sea surface temperatures provided plenty of extra water-vapour, heat energy and lift ready for stirring up a potentially big storm.

Warmer than usual Atlantic

Warmer than usual Atlantic

2. Add some extreme pressure!

The North Atlantic Oscillation is a measure of the difference in pressure between Iceland and the Azores and it was in increasingly positive phase during the storm build-up.  Whilst this is more of a measure than a cause, a positive phase nevertheless indicates LOWER than average pressure over Iceland and higher pressure over the Azores, which usually indicates a strong zonal west to east flowing jetstream across the Atlantic and fast moving weather with the potential for plenty of low pressure systems from the west.  This rapid west-east flowing weather was a necessary ingredient in the set-up for St Jude.

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bubble bubble toil and trouble

bubble bubble toil and trouble

3. Throw in Mr Muscle

A very strong jetstream: blowing at 240mph across the Atlantic towards the UK acted as the main ingredient in the birth of storm St Jude. The jetstream directs weather on the ground.  The jet over the Atlantic in the days leading up to St Jude was extremely strong and blowing directly across UK latitudes. The jetstream is a product of the temperature and pressure contrast between cool Polar air to the north and warm Tropical air to the south.  The temperature difference between polar air and tropical air is particularly marked at this time of year: with the tropics still very warm, while the Polar ice sheets seeing a marked fall-off in temperatures with their attendant air masses.  This builds steep pressure gradients and a strong jet. The jet is also a key factor in creating and guiding LOW and HIGH pressure systems on the surface.  Like a dog on a lead, St Jude was dragged across the Atlantic by it’s angry owner, the jetstream. At times during the passage of the storm wind speeds above the Channel exceeded 180mph.

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4. Whisk up some bad parents!

So all the ingredients are ready, but no storm yet!? The mother of St Jude was a deep robust LOW south of Greenland.  This formidable storm produced hurricane force polar winds directed from the NW in the days before St Jude was even a twinkle in her eye.  The father was a weakening and slow moving tropical storm called Lorenzo.  He had spent the week meandering slowly in the Mid-Atlantic but Lorenzo, despite his old age. still arrived with plenty of hot air from the Tropics.  Their respective air masses collided in the mid-Atlantic some 1000 miles off the SW coast of the UK and, encouraged upwards by the jetstream, they produced their only child, St Jude!

baby bomb is born

baby bomb is born

5. Give it a stir!

Cyclogensis is the process of rapid growth of a baby storm in the mid-latitudes: due to converging warm and tropical air and, encouraged by the jetstream, air rapidly lifted off the surface and Jude’s central pressure, as predicted by the UKMO fell steeply.  This so-called meteorological BOMB exploded (or perhaps imploded, as air was dragged into the low pressure causing all that wind) formed a deep wave depression LOW that charged across S Wales and England in less than 12 hours.  It made a rapid exit from the UK via the Wash and then continued to deepen across the North Sea before smashing into Scandinavia. Arguably the storm did not deepen over the UK quite as spectacularly as some models forecast, but nevertheless, the track and winds were much as predicted and the storm went on to cause significant damage. The chart below shows the pressure falling at a NOAA weather buoy in the development zone of St Jude some 300 miles SW of Cornwall. Note the INCREASE in pressure before the sudden drop-off.  This is entirely in line with cyclogenesis: pressure builds ahead of rapidly developing warm fronts as isobars are buckled up ahead of the storm.  This is popularly known as the “calm before the storm” where winds die down before the maelstrom hits.  This was marked across the country on Sunday evening. At that stage people wondered “what storm?”.

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calm before storm

The satellite picture below of St Jude still in development phase shows the characteristic wave form kink of a rapidly developing storm.

6. Watch out for that sting in the tale!

More immediate “causes” of storm damage from StJude, making it extra-powerful, include the relatively newly discovered weather phenomenon called a stingjet wind.  These are isolated fierce gusts of wind experienced behind a departing deep area of low pressure, often behind a cold front. Oddly, they tend to occur as conditions more widely are improving. In very tight depressions descending air from the upper troposphere pushes gusts to the surface and, like a giant invisible hand, these can, in a careless whim, push down whole swathes of mature forest, take rooves off houses, rip down scaffolding, push over cranes, roll over double decker buses and blow trampolines clear out of your garden!  The sting in the tale is an appropriate analogy, as the curl of winds descending round from the NW of the departing LOW are frequently the last hurrah for these storms. **NOTE: Stingjet NOW confirmed!**

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The storm was perfectly forecast by the UKMO up to a week before the event.  It was always going to be hit-and-miss up to the last minute, not least with inevitable media-hype; but the consequences of playing this down would have been potentially disastrous.  Overall, it was well predicted and people were warned effectively days beforehand.  Whether they prepared effectively or took warnings seriously is another matter.

Below is a slide-show of synoptic charts showing the progress of the storm.  Note that the central region of LOW pressure experienced light winds like a hurricane “eye” (but not as extreme in contrast!). Below this is a brief list of impacts.

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Impacts

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The highest winds were largely restricted to places south of the M4, excluding Wales which had 80mph+ winds in the run-up to the storm on Sunday but calmed down for a time overnight Monday as the centre of the LOW passed over the Severn Estuary and S Wales. Across S and SE England wind speeds were widely 40-60mph and 70-80mph+ on the south coast.  Remember that average wind speed across an area can seem surprisingly low during a storm.  The average wind speed across Reigate from 5-8am during the height of the storm was only 16mph! It is, of course, the random gusts that cause the most damage. The highest gust in Reigate was 48mph at 6:20am on Monday morning.  The highest official max gust was 99mph on the Needles, Isle of Wight, other notable wind speeds were Heathrow 70mph and 62mph at Redhill aerodrome.  Reigate, as predicted by @RGSweather, was spared the worst as our max wind gust was 48mph. Our location in Surrey is away from the coast and locally the town is low down in a vale with low wooded hills to the south, Priory Park, that shelters the town from S or SW gales such as the ones StJude produced during the worst of the storm.  More exposed parts of our local area certainly experienced higher wind speeds.  Rainfall was intense for a period of time and caused localised flooding. In Reigate 25mm of rain fell overnight, which is more than for the whole month of July or, August, in less than 10 hours!  St Jude crossed at night and only keen meteorologists were awake to see it go through.  If this had been a daytime storm, impacts listed below are likely to have been worse with more people getting out and about, or attempting to.

Here are some of the impacts from St Jude in the UK:

  • 3 people were killed by falling trees, 1 boy was very sadly swept out in rough seas on s coast in the lead-up to the storm
  • 147 flood alerts, 17 flood warnings issued by Environment Agency, including our own River Mole
  • In the English Channel and approaches there were 20-30 foot waves and storm force winds.
  • power cuts in SE across 270,000 homes, some for 2 days
  • 5 train companies cancelled all their trains in SE
  • 130 flights from Heathrow cancelled, delays at Gatwick
  • Port of Dover closed, horrifying stories of Channel ferry crossings
  • crane collapsed onto Cabinet office
  • Major bridges were shut in high winds including Severn Bridge and QE2.
  • Dungeness B nuclear powerstation had a power cut in 90mph winds and had to shut down both reactors
  • In Suffolk a double decker bus was rolled over by a gust of wind
  • Clacton pier helter skelter was blown down
  • 1000’s of trees blocked roads and caused travel delays and closures
  • other impacts across the SE here http://www.bbc.co.uk/news/uk-24699748
  • here http://www.bbc.co.uk/news/uk-24700611
  • and costs here (sorry, only one I could find 🙂 http://www.mirror.co.uk/news/uk-news/uk-weather-storm-st-jude-2651458

Emergency service response was predictably very effective in dealing with thousands of calls.  Public were advised not to call 999 for tree falls, and only call in real emergencies.

The storm went on to cause significant damage and some 15 people in total died across the UK, N France, Netherlands, Belgium, Germany and Scandinavia.  It deepened across the North Sea and became more intense with stronger winds, with 120mph reputedly being recorded in Denmark (http://en.wikipedia.org/wiki/St_Jude_storm)

So… hundreds of trees down, some scaffolding torn apart, helter-skelters blown away and some very unfortunate people killed out in the storm… plus £1billion lost through days off and travel chaos.  Inevitably, forecasters are stuck between over-blowing storms and under-playing them so as not to cause panic.  Personally, I think they got this spot on from the start, so congratulations UKMO! The fine balancing act between under-playing and exaggerating potentially serious events is not an enviable task for forecasters.  Despite being very powerful, computer forecast models were still flip-flopping 24 hours ahead with the exact track and severity of this storm.  It was an on-then-off affair right down to the line!  For the future it is worth raising awareness in the public that, despite computers producing forecasts (and who trusts them!?), predicting the weather is still based on the judgement of experts at the UKMO and elsewhere. (photo of clouds over Channel below left was taken by an airbus pilot on his way over Channel during storm). other resources for this storm:

http://www.theguardian.com/teacher-network/teacher-blog/2013/nov/03/st-jude-storm-extreme-weather-teaching-resources?CMP=twt_fd

http://www.independent.co.uk/news/uk/home-news/the-30m-supercomputer-that-helped-the-met-office-predict-st-judes-storm-8911510.html

Which was bigger, 87 or 2013? http://www.bbc.co.uk/news/uk-24708614

Finally, the old chestnut “was it a hurricane?”… Despite getting winds exceeding hurricane force ((74mph+) UK storms cannot be classified as hurricanes. Hurricanes are tropical weather phenomenon and do not form in the Mid-Atlantic at our latitude, neither do they ever get to the UK.  We may experience hurricane force winds in extreme low pressure systems which are confusingly also called cyclones, although they are NOT tropical cyclones!  At our location, on this side of the Atlantic and this far north, we have never experienced a true-hurricane.  Even the ’87 storm was not technically a hurricane despite having even stronger winds. We sometimes get “old” hurricanes impacting the UK but this is not the same, and neither St Jude nor 1987 were one of these characters. Handy pic below illustrates this nicely. Below this are a selection of photos posted on twitter mainly from our local area in E Surrey.

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Lead-up to storm in Reigate on Sunday pm

WEATHER WARNING (but with a note that some models are downplaying this in latest runs 🙂

The explosive LOW arrives Sunday night on the back of the parent LOW moving across today.  There is a risk of significant disruption, albeit there is also a risk of NO disruption as the LOW trails further north or south and some model runs are downgrading the storm inland to 60mph max which is blowy but not as severe… in any case check forecasts regularly for updates. It’s a very tight little low so changes in track will make an enormous difference to your experience on the ground from “oh, is that all?” to something more akin to “OMG!”.  In any case, the advice is to avoid unnecessary travel if at all possible and expect disruption to all forms of transport due to high winds in the SE and heavy rain further north. The strongest winds are located in the southern part of England and Wales.

Here is an outline of what we can expect in the SE and Reigate in particular.

The relative overnight calm will continue through the small hours until after breakfast when winds will slowly pick up, with gusts over 30mph.  Bands of squally, sometimes heavy showers will pass over during the morning. More rain and gusty winds will continue through Sunday pm making it a miserable afternoon with temps barely exceeding 14c and feeling more like 9c.

The highest winds exceeding gusts of 50-60mph arrive in the SE and Reigate around midnight on Monday and the winds quickly increase thereafter.

High winds with gusts exceeding 50mph will be a feature of Monday morning from 0300hrs through to around lunchtime when they will die down quickly.

Heavy rain will arrive with the highest winds overnight but become lighter through Monday daylight hours.

A NASTY STING IN THE TAIL? RED LINE ON MAP SHOWS POTENTIAL FOR 90-100MPH SUDDEN GUSTS IN AREA AT RISK FROM STINGJET: UPDATE: the 99mph winds at Needles were an example of stingjet winds

THERE IS A RISK OF VERY STRONG DAMAGING GUSTS IN THE SE FROM 09:00hrs – 12:00hrs Monday, ESPECIALLY NEAR THE COAST AND IN THE FURTHEST SE CORNER i.e. Sussex and Kent; but everywhere below the red line on the map is especially at risk from this atmospheric phenomenon discovered after the 1987 storm and only observed and studied retrospectively a few times. A stingjet is a newly discovered feature which can develop in rapidly forming storms such as this.  It is when cool dry air sinks from high in the troposphere and can accelerate winds wrapping around the back of an intense LOW, in this case to potentially over 90mph inland and even 100mph+ gusts are a slight possibility over the coast.  There is a risk of such winds occuring anywhere along the coast of Southern England and inland towards the RED line drawn on the map above.  These gusts are like a giant fist from the sky and can flatten whole forests and take off rooves as they did in 1987.  if one occurs it will be one of the rare occasions when it has been modelled.  The characteristic tell-tale of a stingjet is a loop or hook in the wind field or water vapour satellite photo.  Weathermen will be keenly looking for real-time evidence of this throughout the storm.  A hook-like feature on the high resolution model from NMM seems to suggest a stingjet-like formation.  You can see the hook travelling along the zone of highest winds on the animation below (courtesy of NMM netweather). Read below to see what sensible precautions you should make tomorrow.  OK, so this is not a hurricane and is not anywhere near the force of Sandy or Katrina but hurricane force winds MIGHT be embedded within the maelstrom, so we ought to make sensible preparations to avoid unnecessary damage.

PREPARATIONS:

  • secure anything that can blow around e.g. bird feeders, bins, toys
  • cut back any loose branches and trees: store these in a garage
  • check where your car is parked … avoid parking under suspect trees
  • check roof tiles and chimney pots
  • make sure your pets are safe overnight
  • clear loose and clogged drains and gutters
  • ensure all outside doors and windows are shut and secure
  • locate torches and check they WORK in case of power cuts
  • charge your phone in case of power cuts