Archives For March 2016

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.

2016-03-29_08-26-04

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.

This slideshow requires JavaScript.

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.

This slideshow requires JavaScript.

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”.

2016-03-29_07-45-03

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.

This slideshow requires JavaScript.

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.

2016-03-28_07-18-49

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

2016-03-27_08-43-08

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”.

2016-03-25_21-01-45

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.

2016-03-27_10-44-49

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

2016-03-26_13-56-12

Baroclinic leaf in the Atlantic, jetsream cirrus to the UK

 

2016-03-27_07-35-43

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.

2016-03-27_08-36-49

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.

2016-03-27_10-23-46

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.

2016-03-27_10-25-29

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.

2016-03-27_11-45-58

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).

2016-03-27_10-41-42

Update: Amber warning issued for Storm Katie.

2016-03-27_18-37-03

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

2016-03-27_08-28-44

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

2016-03-27_19-14-09

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

2016-03-28_10-51-34

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.

2016-03-28_07-38-22

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..

2016-03-28_12-13-54

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

2016-03-21_19-02-57

The Atlantic is showing off some classic visible cloud features of cyclone birth and decay today.  Systems labelled 1-4 on the satellite photo above show different features including stages of cyclone / mid-latitude depression formation and decaying high pressure ridge all on one satellite picture.  The chart below shows the same view with fronts.

2016-03-21_19-01-49

Starting with LOW number #2 (why not?!): the spectacular classic cloud spiral of LOW #2 indicates a mature low occluding and filling.

2016-03-21_19-15-13

This maturing occluding LOW has a couple of interesting extra vortices near the low core.

2016-03-21_19-05-09

Despite their angry look, classic cloud spirals like this on satellite photos are actually decaying and filling lows, losing their strength as pressure rises in the low core.  This particular LOW has a spectacular cold front of over 1500 miles stretching from 60N to the sub-tropics. The red colours on the RGB false colour eumetsat image below shows the cold continental polar air surging in behind the cold front.

Low #1 is a rapidly intensifying LOW off the coast of Labrador.  It looks harmless as a smudge of cloud but this shape … a so called “baroclinic leaf” indicates the birth of an angry developing storm: rapid cyclogenesis.

2016-03-21_20-33-50

This will deepen and pressure will fall rapidly in the next 24 hours as frigid continental air collides with humid maritime air under the influence of an active 200mph jetstream.

2016-03-21_19-28-26

LOW #1 is expected to form a big storm in the Labrador Sea by Wednesday. The fronts on this storm are then forecast to stretch clean across the Atlantic and bring the SE our first frontal rain for over a week by later Thursday.

This slideshow requires JavaScript.

System #3  on the top satellite photo shows the HIGH pressure lingering over the South of the UK but regressing into the Atlantic.  This ridge has dominated mid to late March weather in the UK but brought a lot of anticyclonic gloom to the SE.  The deflating ridge will allow a more unsettled Atlantic westerly regime to dominate late March and early April weather.

2016-03-21_18-52-38

Low pressure #4 is an interesting developing depression in the Mediterranean, courtesy of a southward limb of the jetstream. Currently a disturbance dumping snow over the N Atlas in Morocco, this LOW is set to deepen across the Mediterranean through the week.  It will track directly ENE through the Med and bring snow to the Atlas mountains, rain to N Algeria and foul wet, windy and wintry conditions to Italy and then more snow and wintry weather to the Balkans.

Finally, for the UK our weak ridge is deflating to the SW and this will open the door to zonal westerlies and frontal systems bringing rain and wind from later Thursday and into the weekend.  Ensembles below show the dry spell ending this week and some notable rainfall spikes in the days to come, especially over the weekend.

2016-03-21_18-39-02

 

2016-03-20_09-26-16

High pressure has dominated the last week of our weather but it has turned out disappointing here due to cloud cover lingering under a persistent temperature inversion, not unusual for this time of year.  Lingering decaying fronts have caused drab stratocumulus cloud to spread out beneath an anomalously warm upper air mass causing anticyclonic gloom for much of the SE.

2016-03-20_09-22-51

Any convection has been limited to the lowest 1km and been unable to break the inversion, so cloud, unable to rise into cumuliform tufts associated with the stronger sunshine in April, simply spreads out into a boring grey blanket, especially when the flow arrives from the North Sea bringing additional moisture in the lower layers. An inversion is when temperatures increase with height through a part of the atmosphere, usually referring to a lower layer.

So, whilst upper air temperatures have been anomalously warm, the surface temps have been kept disappointingly low.  Somewhat “upside-down” weather.

2016-03-20_09-22-20

This is because the Spring sunshine has been unable to break through the cloud and warm the surface.  The exception has been the north and west of the country, especially the hilly parts of Wales and NW England, which have enjoyed more sunny days.

2016-03-18_21-29-22

anticyclonic gloom

Today it was the turn of the E/NE coast to get the sunshine as weakening fronts shifted south around the high.

2016-03-20_14-38-23

The coming week sees the HIGH slowly deflating, like a sad party balloon, into the sub-tropical Atlantic.  A couple of powerful late winter storms emerging out of the NE US and Newfoundland start the onslaught to break a westerly unsettled flow back across the Atlantic by Easter.

For us in the SE this change to unsettled conditions happens slowly but ensemble runs are showing around 20mm of rain is possible before the end of March. So we might expect some wet and windy days before the end of March, including a risk of a wet bank holiday period.

2016-03-20_10-06-59

A return to more mobile frontal conditions is not all bad news, especially in the SE.  A westerly flow with weakening Atlantic frontal systems will break the gloomy cloud cover and bring sunny intervals and showery episodes to clear the pollution phase we have experienced lately.  The risk for us is any fronts stalling over the SE in front of a European HIGH – this situation can dump fairly large amounts of rain. Chart below shows wind speed and an active cold front for Sunday, too far off to be reliable but worth watching.

 

The multi-model charts below show this change from HIGH to LOW during the next week.

2016-03-20_09-16-50

Note the model agreement below by Saturday for SW winds, bringing temperatures up possibly into the mid-teens in the SE.

2016-03-20_09-16-30

In short, we can expect the weather to turn the “right way up” again and we should enjoy more mobile, fresher, brighter conditions, albeit with breezy episodes of potentially heavy rain at times. The charts below show temperatures rising in the SE but rain returning by Friday.

2016-03-20_08-48-19

2016-03-20_08-48-40

Further into April there are hints that pressure rises across the Pole which will bring the potential for further unsettled cooler conditions during the school holiday period. RGSweather is off to Iceland (East fjords) again so this could mean some nice cold conditions for our trip there as the AO is expected to turn negative and the flow northerly, at least for the N Atlantic.  The UK appears to get stuck in an unsettled trough for early-mid April. Worth watching as JMA and CFSv2 both agree on this blocked pattern.

2016-03-20_14-14-26

2016-03-12_13-32-30

February 2016 weather statistics for Reigate, Surrey, England.

  • Tmax 13C
  • Tmin -3C
  • T Average 5.3C
  • Rainfall total (CoCoRaHs) 48.2mm
  • Sunshine 99.6hours
  • Average MSLP 1010mb
  • rain days 18

Feb CET 4.9C (anomaly +1.1C)

Mean UK temp 3.9C

Storm Henry opened February weather with inclement wet windy conditions for the far NW but no significant impact here in the SE.

Storm Imogen on the 8 Feb impacted Wales and the SE more with the highest gusts of the month exceeding 41mph here in Reigate, with a max gust of 81mph on the Needles, Isle of Wight.

2016-03-12_13-39-57

February was another mild winter month, though not on the scale of record breaking December 2015.  February UK average was 3.9C, the Central England temperature (CET) for Feb was 4.9C, an anomaly +1.1C above the long term average. Locally our February average was 5.3C showing that the SE was warmer than the rest of the UK.

February was the sunniest winter month with nearly 100 hours of sunshine, compared to 94 hours in January and a dull 80 hours in December.

2016-03-12_13-32-51

The mean 500mb heights for the month (above) shows a trough placed over the UK for much of February and an overall Atlantic flow bringing milder than average temperatures, mainly to the south and SE. The continent was even warmer with some anomalous temperatures exceeding 10C in E Europe and Russia.

Rainfall for February was about average for the month at 48.2mm, similar to the MetOffice official SE England precipitation figure of 49mm.

2016-03-12_17-33-40

Global mean temperatures turned out warm again for February, in fact 0.47C warmer than the previous record.

2016-02-01_11-10-14

stratospheric warming February 2016

A notable atmospheric event that went mostly unnoticed at the surface was a significant stratospheric warming in February (and March).  The sudden warming of temperatures high in the stratosphere, much anticipated by the weather community and long range forecasters as a key tool to forecast winter conditions in the Northern Hemisphere, came too late to cause much cold winter weather in Europe.  El Nino winters are often accompanied by Sudden Stratospheric Warmings (SSW) events in late winter.  Such events can cause cold weather in mid latitudes as pressure eventually rises in the troposphere some 2 weeks after a SSW.  The warming during an SSW is truly remarkable, with temperature rises exceeding 80C in a matter of days at 10hPa (30km), from -80C to +10C in some SSW events.

An SSW is like inflating a balloon in the high atmosphere, pressure is expected to propagate down and eventually rise in the lower atmosphere some weeks after an SSW. Importantly, the pressure rise often disrupts the zonal westerly flow of the jetstream.  This appears as negative anomalies on the zonal wind chart below (blue colours). Unfortunately for this winter, the SSW came too late for much significant winter cold to penetrate into Europe.  You can spot that the anomalous easterly winds only got going in March… too late for deep cold to push into Europe.

2016-03-12_14-03-38

The JMA snow charts below also show how February came out as an anomalously un-snowy period for much of Europe.

2016-03-12_14-07-49

http://www.metoffice.gov.uk/climate/uk/summaries/2016/february

 

2016-03-07_19-50-17

After a pleasant dry and sunny day in the SE, the satellite photo from this evening spells trouble ahead for mid-week with a deep depression over Iceland and an increasingly active Atlantic with a long frontal boundary trailing across the ocean into thick bands of cloud.

This slideshow requires JavaScript.

Reigate and the south of England are set for a potentially very wet day on Wednesday as shown on the Euro4 chart below showing just 12 hours of rainfall during the morning.

Unusually high rainfall totals could mount up, possibly to around 30mm for the day on some models. However, models usually exaggerate rainfall totals but it is likely to be soggy!

A strengthening Atlantic jetstream is causing the convergence of moist sub-tropical and polar air at the Polar Front over the Atlantic Ocean.  The winds can be seen converging on the Atlantic chart below.

2016-03-07_19-35-27

The convergence of Polar and Tropical airmasses can also be seen on this chart showing the trajectory of winds arriving in the UK on Wednesday.  Note the surface tropical airmass circulates round the Azores high and meets the incoming Polar air from Canada.  It’s the less dense moisture laden maritime tropical air which is lifted over the cold, enhancing rain on frontal boundaries.

The 850hPa (15oom) temperature chart below shows the steep temperature gradient between contrasting airmasses across the Atlantic.  The water vapour Meteosat satellite picture shows a broad sweep of moisture laden air crossing the Atlantic from the Sargasso Sea.

2016-03-07_21-23-45

The boundary of the contrasting moist Polar and humid Tropical airmasses causes lift and this is set to rapidly form a depression over the UK later on Tuesday into Wednesday courtesy of a jetstreak to the west of the UK.

Large amounts of Atlantic moisture are set to converge in this low pressure as airmasses meet at frontal boundaries.

2016-03-07_18-53-29

The result over SE England is an unsually steep rise in dew point to 6-7C, indicative of increasingly moist air.

The atmospheric column looks to become exceptionally moist on Wednesday and saturated through to a height of 25,000 feet.  Cloud depth will make it a very dull day.

2016-03-07_19-46-39

Rainfall charts look impressive and, at the moment, show the rain arriving on Wednesday morning. Here is a medley of rainfall charts from 3 different models showing the potential for a deluge, though do note that models tend to exaggerate these totals this far out.

The 6 hourly total chart from GFS shows an extraordinary 26mm over parts of SE on Wednesday morning.  This would lead to local surface flooding on roads.

2016-03-07_18-49-31

Note this is not a convective event so no thunderstorms are likely, which makes such high rainfall even more unusual. It could be the biggest daily rainfall total for quite a while, over 36.6mm of rain in a day was recorded on 24 August 2015. Keep posted on twitter and check MetOffice forecasts for updates if travelling.  Some disruption could occur if this comes off as models suggest.  Milder and settled conditions are expected into the weekend after our mini-monsoon!

2016-03-07_18-51-40