Archives For stormkatie

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

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Before we get to Katie, it’s worth mentioning recent and current weather for Easter Sunday: the first convective day of 2016.  A cold pool of polar upper air followed the impressive cold front yesterday which developed a very long squall line.

Thundery conditions followed the cold front in an unstable Polar airmass with notable CAPE (convective available energy) and negative 2 Lifted Index for the time of year – both indicators of potential thundery heavy shower activity.  The limiting factor on multicell thunderstorms locally was lack of wind shear and an anticyclonic jetstream, both subduing anything truly impressive or sustained. Nevertheless, some hail and thunderstorm activity is rife across the UK today in this unstable polar maritime airmass.

Storm Katie is riding a powerful jetstream and deepening rapidly over the Atlantic through today.  She is some 600miles away to the SW but approaching and deepening rapidly as she undergoes rapid cyclogenesis (RaCy) falling from 999mb to 975mb:  24mb in 24 hours, qualifying as a “bomb depression”.

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Katie emerged out of Canada earlier this week as cold continental air met humid sub-tropical maritime air.  She engaged with an active jetstream to be launched across the Atlantic.  Storms tend not to move fast and deepen rapidly at the same time, so her rapid progress across the Atlantic will now slow as she now expends more energy lowering pressure on her approach to the UK.

The evolution and track of this storm is similar to St Jude October 2013, but Katie is forecast to be less powerful, due in part to the slacker pressure rise on her departure into the North Sea.** (update: in the event Katie pipped StJude, see below!)

Katie also has some similarities to the October 1987 but again is significantly less powerful than that rare 200 year event.

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The water vapour sat pics below show the evolution of Katie and cirrus cloud associated with the jetstream.

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Baroclinic leaf in the Atlantic, jetsream cirrus to the UK

 

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dark slot shows rapid intensification stage as strat air descends into LOW, showers over UK

Storm Katie is forecast to deepen through the SW approaches to below 980mb on an expected track overnight Sunday-Monday from Biscay into the Bristol Channel through to exit around the Wash.

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The pressure gradient between the Isle of Wight and the Wash could be as much as 12mb… the highest of our Wight-Wash Oscillations recorded, and greater than the 10mb WWO recorded in StJude, if this comes off.

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However, the pressure rise behind Katie is expected to be less rapid and sustained than St Jude, so this critically reduces the potential for highest gusts, sting jets aside.

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The highest wind speeds for Storm Katie will be south of the low core in the occluding warm sector in the small hours of Monday morning.

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Max gusts of over 80mph are possible in the Channel, 70mph along Channel coasts and 60mph further inland.  For Reigate this means a brief episode of gusts possibly exceeding 50mph and more over exposed hills.  This could wake people up momentarily as gusts roar through trees and round houses.

Rainfall will be significant too and some hi res models put down >40mm over parts of the North Downs, showing the significance of small hills in generating orographic rainfall.

Rainfall more widely is more likely to be around 20-30mm, enough for local flooding. The MetOffice have issued a yellow warning for Storm Katie.  Check the weather impact matrix showing moderate likelihood of moderate impact (low yellow).

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Update: Amber warning issued for Storm Katie.

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The shipping forecast is less equivocal and shows storm warnings right around the UK associated with Katie and other low pressure systems.  Storm Force 10 is possible in the Channel for a while. http://bbc.in/1RtwvyS

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Furthermore, evolution so far matching StJude and the Wight-Wash pressure gradient forecast seems to be matching or beating St Jude.

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Below is a Meteosat picture (courtesy Dundee sat receiving station) showing Katie moving into the North Sea, deepening again slightly to 972mb,

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Meteosat visible photo of Storm Katie moving into North Sea on Monday 28 March

Post-Katie we enter a broadly zonal westerly flow but turning cooler as slack winds swing northerly through the week as LOW pressure moves into North Sea/Scandinavia. Unsettled and showery activity continues through the week but with pressure slowly rising from the west by Thursday. No further significant storms are expected for the time being. Into the start of April, pressure over the Atlantic is expected to rise and progress over the south of the UK into Europe to give a settled period for the start of April.

RGSweather is off to East Iceland, so expect occasional tweets and updates from cooler climes.

Storm Katie: In the Event:

various meteorological impacts show Katie exceed StJude in power but both fall well short of October 1987 Great Storm.

  • Max wind gusts Needles 105mph (St Jude 99mph)
  • Reigate max wind gust 51mph, widely 50-60mph and 70mph on North Downs (Kenley) Gatwick 59mph
  • Redhill aerodrome recorded 70mph gust at 04:50
  • Lowest pressure: 971mb (St Jude 2013 976mb; Oct’87 958mb)
  • Reigate lowest pressure 976mb (St Jude 980mb; Oct ’87 est
  • Reigate rainfall 20mm
  • Wight-Wash Oscillation 14mb (St Jude 12mb, Oct ’87 storm 20mb!)
  • Rapid rise in River Mole catchment – no significant flooding but overtopped bankfull stage.

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Note, compared with the Oct ’87 storm Katie and St Jude are found wanting.  The Wight-Wash Oscillation for the 200 year storm of October 1987 was a whopping 20mb! See the chart below for the exit of the Oct 87 storm through the Wash on 16 October 1987.

Finally, which of these big SE storms are most powerful? Play SE storms TRUMPS to find out..

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http://www.bbc.co.uk/news/in-pictures-35910151

http://www.bbc.co.uk/news/uk-24708614

http://www.chichester.co.uk/news/update-more-than-600-incident-in-sussex-as-storm-katie-batters-the-south-1-7299324?utm_medium=twitter&utm_source=dlvr.it

http://www.telegraph.co.uk/news/weather/12205613/Storm-Katie.html?frame=3601967

http://www.bbc.co.uk/news/uk-england-35909651

http://www.dailymail.co.uk/news/article-3512119/Storm-Katie-batters-Britain-Gales-100mph-dozens-flights-cancelled-UK-wakes-Bank-Holiday-washout.html

http://www.theguardian.com/uk-news/2016/mar/28/storm-katie-flight-diversions-wind-rain-batter-southern-england-and-wales

http://www.mirror.co.uk/news/uk-news/see-full-carnage-storm-katie-7644830

local coverage

http://www.dorkingandleatherheadadvertiser.co.uk/Storm-Katie-Trees-power-Surrey-storm-hits-PHOTOS/story-29006658-detail/story.html