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What’s a Polar Low?

January 29, 2015 — 1 Comment

2015-01-29_18-19-06

Update: this feature is under discussion on met-forums as to whether it was technically a polar low or not!  Some features (e.g. satpic) suggest it was but other features, such as upper temperatures, suggest it may have been a small scale feature with some similar characteristics (referred to below). References below contain more details. This post still valid as it describes an interesting weather feature with several polar low characteristics, albeit jury is out on final definition here!

Polar lows are small but intense maritime meso-scale cyclones, a few hundred km across, that form quickly in cold polar or arctic air advected (moved) over relatively warmer water.  They are much smaller than our usual mid-latitude frontal depressions.  They usually occur in winter in Arctic or Polar airmasses streaming Equatorwards and they form characteristically beautiful swirls of cumulus cloud and a comma cloud formation visible in satellite pictures (see above VIIRS sat pic 29/01/2015).  Sometimes these swirls form an eye reminiscent of tropical cyclones, to which they have surprising things in common.  Heaviest snow / rain occurs near the “eye”.  This similarity is why they are sometimes called “Arctic Hurricanes”, though they do not always produce hurricane force winds.  Despite their scary name they are relatively common over ice-free mid-latitude waters, they can produce rain or snow (due to “warm core” see below) and one was recorded moving down the North Sea in Dec 1995, so they are uncommon but not unheard of around the UK.

early evolution spotted on MetOffice fax

early evolution spotted on MetOffice fax

Polar lows tend to go through a rapid life cycle of a day or two which previously caught Arctic explorers unawares with hurricane force winds blowing up from nowhere and creating high seas in hostile Arctic waters.  Gale force winds wrap tightly into these features.  Polar Lows were invisible to meteorologists previous to satellite pictures and only with very recent upgrades in Numerical Weather Prediction models have Polar Lows been “visible” at all on charts.  They are still hard to predict and models sometimes struggle to track their intensity and path.

Polar Low circulations do not last long and, like their tropical cyclone cousin, they tend to decay rapidly once they move from warm sea over cooler land, because the convective energy and steep lapse rates driving the system are lost.  Cold upper air temperatures and a warm sea assist steep lapse rates that can cause thunderstorms with active convection.   Lightning strikes were recorded in this 29/01/2015 low as it came ashore over Northern Ireland, typical of polar lows.

Charts modelling the evolution of this low, albeit rapidly in the last few hours, have predicted tracks moving the system SE across the UK overnight.  Areas especially at risk from snow include Northern Ireland, N Wales and Midlands and possibly parts of the Southern England into early Friday.  However, the system is likely to fill rapidly overland as the sensible heat flux available for convection is lost over the colder land.  Additionally, the land is rougher than the ocean and this increased roughness increases surface convergence (air arriving faster than it lifts) and this causes the central pressure to rise and the system to decay.

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decaying polar low

Like hurricanes, Polar Lows form over oceans and gain much of their energy from them.  Polar Lows usually form in places where there is a rapid change in temperature and/or pressure horizontally, this is known as a baroclinic zone.  Edges of ice sheets or simply where warm and cold air meet are prime locations.

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baroclinic zone west of Scotland: breeding ground for this polar low

Warmer ocean water is another essential trigger for polar lows.  The warm surface water provides essential mositure and lift that creates convection, condensation in frigid air and the release of latent heat that develops cumulus and cumulonimbus clouds. The clouds then wrap into a tight circulation around a rapidly deepening low core not dissimilar to a hurricane.  Polar lows are much smaller and more transient than a regular mid-latitude depression.  Polar lows tend to form on the eastern side of a high pressure ridge and to the east of a decaying occluding mother low.  Both these features can be seen on charts.

An important feature of a polar low is the formation a warm core.  Charts below show this as milder surface temps at 950hPa and the theta e chart which appears to show warm air too.  Internal evolution of this system seem to suggest it has a warm core perhaps comprising Arctic flow chasing down a long-track of milder airmass originally sequestered by the extremely active cold front that swept through this location yesterday secluding a warm pool in the core of the mother low sat over Scandinavia.  This warm pool appears to have advected east to meet incoming Arctic air.  An area of positive vorticity (spin) contributed to the evolution by adding spin to the air that caused the low to form.

The polar low that developed today NW of Scotland has many of the hallmark characteristics of a Polar Low and seems to be generally accepted as such mainly because of the defining characteristic swirl of cumulus clouds round the low pressure core.

Note:  met community not all agreed whether this was a Polar Low or not : has many characteristics but some consider it too warm with rain in some areas rendering it a meso-scale slide low or similar. 

some links on Polar Lows

http://www.weatheronline.co.uk/reports/wxfacts/The-Polar-low—the-arctic-hurricane.htm

http://rammb.cira.colostate.edu/wmovl/VRL/Tutorials/SatManu-eumetsat/SatManu/CMs/PL/backgr.htm

https://polarlows.wordpress.com/tag/climate-change/

http://www.keesfloor.nl/wolken/sat/polarlow.htm

So far December weather for Reigate and across much of S England has been much the same for a while: breezy westerly winds and exceptionally mild on occasions with December temperatures breaking records in Reigate: the highest December temperature of 12.7C was recorded on Dec 18, the highest average and highest minimum were also records broken this month in the town. The December CFS chart for Bournemouth temps show how much time they have been above average this month so far.  Whilst it has been mostly relatively mild this month so far in the UK, large parts of the rest of the N Hemisphere has been even warmer with much of N America and parts of Russia being anomalously warm too.  On the other hand E Siberia has been very cold with Verkhoyansk falling to -50C at times.

Whilst a colder spell is expected to arrive soon after Christmas this will not be sufficiently extreme or long enough to move 2014 away from a record breaking warm year.  The Central England Temperature records go back before 1780 and 2014 looks almost certain to be the warmest ever recorded CET with an anomaly of +1.49C above the long term average.  This matches 2014 as a warm year globally, if not quite the very warmest.

So, apart from the rather underwhelming “bomb” depression earlier in the month, why has the weather been so mild, uneventful and unchanging for much of December, so far?  An immediate cause of our quieter weather this December is the jetstream being less active than last year.  On the charts below spot how much more active the 2013 jet was at the same time last year across the Atlantic.  A succession of Atlantic storms were driven headlong at the UK by a powerful jetstream which sat further south.  This year the jetstream has mostly been further north than the south of the UK and has been more meandering (meridional) and less powerful: hence fewer and less powerful storms for us in the south, most of the storms have tracked to the north of Scotland which has had an “ordinary” December of high winds and heavy rain!

2014-12-22_21-01-41

The reason for our milder than usual December is that much of the time our winds have arrived from a warm SW direction round a persistent HIGH over Europe.  We have had few incursions of Arctic northerlies, as yet.  Nevertheless, for parts of the month especially for the NW of the UK, there has been a sustained NW wind associated with high pressure over the Mid -Atlantic and LOW pressure over Iceland and this has pegged down our overall temperature anomaly to 1C above long term average.  This pattern of broadly westerly winds is called zonal and is indicated by a positive North Atlantic Oscillation (pressure low over Iceland and high over the Azores).

While the jetstream controls our synoptic scale pressure patterns, a more remote driver of our overall weather in the Atlantic mid-latitudes is the rather more geographically far-flung “Madden-Julian Oscillation”.  The MJO is a cycle or wave of weather disturbances (clusters of convective storms) that start in the Indian Ocean and move east through the tropical oceans into the Pacific and beyond.  It usually peters out across the Atlantic but can be observed to continue as an unbroken oscillation into the Indian Ocean where it starts another Phase 1 again. The MJO connects up with observed known weather patterns in the Atlantic and has therefore got an impact on UK weather, especially apparently, in years of weak or neutral ENSO (el-nino southern oscillation).  This year is currently a weak El Nino.  With the MJO in Phase 3-4 then a positive NAO is usually favoured but is not fixed of course, yet more drivers are likely to influence proceedings in the new year, not least “sudden stratospheric warming”.
MJO Phase 3-4 = +NAO

MJO Phase 3-4 = +NAO

DIY forecast for the possible storm 27-28 December

When upper winds (including the jetstream at 300-250hPa) match the direction of those at the surface, the weather tends not to change very much over quite long periods, like recently.

winds the same at height: no change

winds the same at height: no change

Wind direction and air masses can tell us a lot about how the weather will change and sometimes this is helpful when forecast changes are uncertain or simply fail to arrive when expected.  To an observer on the ground clouds at different heights can sometimes appear to be moving in different directions.  If on such occasions, the observer sees the clouds moving perpendicular to each other then a rapid change in weather is likely to be on the way, either good or bad!  The so-called “crossed-winds rule” is handy if you want to know how quickly expected weather events will arrive, especially if you are out and about on a day when expected changes are afoot but the timing is uncertain from forecasts.  This could prove handy later this week when a large storm might upset our run of benign conditions but the exact timing of fronts and events is uncertain, especially at this range!

2014-12-22_21-38-47

The storm, expected to intensify rapidly in the Mid-Atlantic during Boxing Day (yes, a possible rapid cyclogenesis bomb depression) could take a track modelled by the GFS that runs from the N of Scotland down through the N Sea. This SE track could pull in strong Arctic winds and, combined with high spring tides and high waves, this could cause a storm surge event for E coasts of the UK.  Note this is too far off to be certain at this stage.

Other models, such as the ECM and UKMO have different tracks that take the LOW on a less potentially damaging course.  Nevertheless, the models are worth watching because on some tracks the synoptic situation is similar to the 1953 storm which caused a lot of coastal flooding.  Of course, sea defences are much improved since 1953 so, even if the track was similar, it would be unlikely to cause anything like the same impacts.

compare 1953 storm surge with models 2014

compare 1953 storm surge with models 2014

It is also noticeable how similar the zonal pattern preceding the 1953 storm is to the current synoptic pattern: a strong positive NAO was present with both (see below).

2014-12-22_16-48-17

Back to the present… a weak cold front moving south during Christmas Eve will introduce a cool N/NW polar air flow across the country by Christmas Day. No snow is forecast for the SE with this cold front and it will be noticeably colder but brighter on Christmas Day.  A high pressure ridge on Boxing Day could start the day bright and frosty but, depending on the final track of the RaCy depression, things could go downhill rather quickly thereafter.  If you are out for a walk on Boxing Day then this is where the Crossed-Winds rule will come in handy to do some of your own forecasting!  Here’s how:

Bozing Day crossed-winds rule

Bozing Day crossed-winds rule

It will be necessary to see clouds at different heights – especially those wispy cirrus ones to check high altitude jetstream winds.  If it is completely overcast then forget this unless you happen to see breaks in the low cloud.

Find the surface wind direction: look at lower cumulus clouds or just local wind vanes or weather stations. It’s important to get the correct surface wind direction though so don’t rush this bit.

Stand with your back to the surface wind : the simple rule means that any low pressure will be on your left.  You can also tell how quickly weather will change for you locality thus…

Watch the direction of any cirrus clouds: check this with an aerial or static object.  If they can be seen to move at all then they will be shifting fast, because cirrus can be 10km up! The best cirrus for this is hooked mares tales or jetstream cirrus which appears in long impressive streaks along the core of the jetstream or similar upper winds. The more fuzzy edge is on the tropical warm side and the firm sharp edge to cirrus, if this can be spotted, is on the polar air side.

 

Now, the rule: stand with your back to the surface wind and if the cirrus (upper winds) come from the left then the weather will get worse.  The more perpendicular the winds (right angles) the more rapidly the weather will deteriorate.

If the surface and upper winds are going the same way, then there will be be no immediate change, or just very slow change.

If the upper winds come from the right then the weather will start to improve.

On the back edge of the storm, chilly Arctic air could bring down an active cold front with snow showers on the back edge into the SE, but more likely these will be restricted to NE/East coast.  It’s ages away so no details are possible but this storm looks one to watch.

Finally, the longer range outlook is rather uncertain!  Signals such as stratospheric warming, forecast to take place into early January, favour a cool January with the break-down of westerly flows into occasional cold Northerlies / easterlies.  In contrast, the MJO seems to favour more of the same unsettled westerlies for a while at least.

Quite possibly it will probably be a mix of the two!  This is suggested by lastest output by the CFS showing some zonal westerly weeks and some potential cold northerlies with the subtle position of HIGH and LOW pressure being critical to the outcome.  Lots of interest over the Christmas period and beyond, keep watching the weather!

 

Monday 25 Aug

Bank Holiday heavy rainfall wash out!

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Nasty LOW

Like much of the rest of England and Wales, Reigate and Surrey and the SE will see an August Bank Holiday wash-out tomorrow with lots of rain through the day, likely to be heaviest in the afternoon.  This has the potential to be a moderately severe wet weather event with 48 hour rainfall accumulation possibly topping 20mm in our region with some models taking this even higher to 30-40mm in places but this is probably over-egging things.  The WRF and NMM models push totals to 40-50mm in places while Hirlam and Euro4 (UKMET) keep a lid on rain at 20-30mm.  Anything approaching 40mm in 48 hours would be highly unusual for us in SE in Atlantic frontal depressions but it’s by no means impossible.  Remember that a “wet day” for us in the SE usually adds up to only around 10mm, so any totals over 20mm tomorrow will seem a significant deluge. Check the rogues gallery of models below for the worst rainfall offenders.  By the way, always Check the UKMET forecast for a professional look at the situation, this site is for a local perspective on weather and to generate enthusiasm and interest and share some more understanding of the air around us, especially in these times of such uncertainty regarding weather and climate.  As we approach another autumn and winter please share any of your weather stories and photos here or follow us @RGSweather on twitter.  Twitter is an amazing portal for sharing, learning and discussing the amazing fast moving world of weather… join it if you can.

Either way, surface water local flooding is likely to be a feature on roads tomorrow and the UKMET has issued a yellow warning for heavy rain.  You can expect a cloudy, dull, blustery, cool and wet day: so maybe snuggle up with the TV guide and check out some bank holiday films 🙂  and watch @RGSweather on twitter for exciting weather updates of course!

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The bank holiday soaking is courtesy of a low pressure in the Atlantic set to deepen to 981mb overnight as an unseasonably fast jetstream blasting through the Channel draws very moist air from the surface and lowers the surface pressure throwing all types of fronts our way and tightening isobars cross the SE during the course of the day. 

Today, the system lurked in the Atlantic as a significant band of cloud, as shown here by the VIIRS satellite at 13:00hrs.

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The system will bring rain first thing tomorrow to Reigate.  Initially rain looks to remain fairly light but will become progressively heavier through the day with even a low risk of heavy thundery showers as the cold front passes later pm and into Tuesday night (updated) before things gradually calm down Tues am. Whilst breezy through the day, max wind gusts are likely to arrive later in the afternoon but only 20mph.  This is certainly more a wet event than a wind event!

Into Monday morning: update:

Satellite pics show a rapidly developing occluded low pressure with tightly wrapped fronts.  The dark are on the sat pics represents high altitude air that is subsiding (sinking) into the depression.  This dry intrusion can cause increased instability around the cold front where it is forced to rise once more nearer the surface, increasing cloud formation.  Sometimes is can cause thundery doinwpours and some of the models are picking up on this by increasing rainfall totals even further for the SE as the frontal system passes over.  Interestingly this does not count as a “bomb” cyclogenesis which requires a 24mb fall in pressure over 24 hours (1mb per hour).  This has managed about half that pressure fall so far. 

Quick update here focusing on SE especially: please note this applies mainly to Reigate in Surrey and is an amateur analysis for educational purposes.  For updates through the storm please see @RGSweather on twitter for the Bertha story as it unfolds for us in Reigate and SE.  This is called NOWCASTING (as opposed to “forecasting”).

Ex-Bertha is turning out to be rather interesting meteorologically!  A convective potential has emerged today, which means there is more possibility of thunderstorms of some significance as the LOW passes across the UK, especially to the south of the system.  Convective gusts of 50-60mph could be possible and the odd tornado cannot be ruled out, though no need to panic because these are quite common and not usually powerful or damaging in the UK.  So it is still the case that the overall impact of this storm is still not likely to be extra-ordinary or wreak widespread havoc Daily Express style.  It is more likely to be underwhelming for most.  Nevertheless, rainfall totals in a short space of time for some places might be high and there could be some interesting weather phenomena associated with active fronts.

UPDATE Sunday 7:30am

Estofex and TORRO have issued severe convective weather warnings for the S UK. Estofex Level 2 storm warning is most unusual for the UK and TORRO do not issue tornado watches lightly.

(back to yesterdays update:) The UKMO fax chart below for Sunday midday shows a “triple point” of three fronts meeting near the SE (warm front, cold front and occlusion) Between the warm front and cold front the warmest humid air is wrapping into the centre of the LOW in the warm sector: this contains much of the moisture to fuel the storm as condensation releases latent heat driving up parcels of air.  On top of this a conveyor of cooler drier Polar air that flows over the cold front and warm sector and this increases lapse rates further encouraging lift throughout the system. The warm air eventually flows to the core of the storm as it occludes.

2014-08-09_20-43-56

 

All the time the jetstream to the south is lifting air off the ground (by a process called divergence in the upper atmosphere) and lowering the central pressure causing air to converge into the centre of the LOW… this results in the surface wind rushing into the centre.   Converging air at the surface has nowhere to go except up.  Rising air, especially where tropical air meets polar air at the fronts, creates condensation, thick cloud and potentially plenty of rain.  The potential water available in this storm is large.  In addition, cloud top temps, with the influx of cold air aloft, are likely to be as low as -50C causing ice to form in turbulent air that can create charge up thunderstorms.  Such storms are only a risk and may not happen at all.2014-08-09_20-59-21

For Reigate and the SE it seems we can expect more rain during the morning than was previously the case in earlier models and forecasts.  Latest models suggest widepsread rain in the SE of up to 20mm and discrete patches of high totals possibly exceeding 50mm in the SE.  This is about a month of rain in one day, so local flooding could be a problem.

Rain will arrive tonight, after midnight, and persist throughout the morning.  Wind speeds, probably 30-40mph max gusts inland, possibly more gusty in any thunderstorms, will increase towards the middle of the day and potentially be highest as the cold front moves away which, on current models looks like early afternoon.  Strongest gusts will be associated with any thunderstorms.  The good news is that by pm the cloud should break rather rapidly, however, scattered showers could follow in the brisk westerly. This regime will continue for much of the early part of the week.

Needless to say, apart from the rain potential, Reigate is less at risk from tstorms than further N during this episode (Reigate storm shield!)

Even now much still remains uncertain about this storm and it is causing lots of interest and headaches for both professional and amateur meteorologists.  The nature of the fronts may produce some organised squall like features and some organised thunderstorms for places but predicting these is extremely difficult.  Any such storms can have the potential to deposit a lot of rain in a short space of time.

Late this afternoon Saturday Bertha split in two: one rain system moving north, the other pushing more ENE.  This was unexpected.  Currently the rain moving north across Ireland is the more significant but things can change.

 

Let’s finish with the UKMO forecast for Reigate.  It shows lots of rain, potential for thunderstorms and some unsually strong winds for the time of year.  As leaves remain on trees this might cause loose branches to fall and peak rainfall totals, if met, could cause some local flooding.  Certainly nothing to panic about but do look out for any interesting weather features and send them in to @RGSweather!  Sadly for @ridelondon the prospects are not terriibly nice in the morning.

2014-08-09_21-06-51

 

Photo mosaic of the squall line that passed over Reigate on 10 Aug afternoon: quite a feature!

and tornado reports of damage from various locations including Hull, plus other news here:

http://metofficenews.wordpress.com/2014/08/10/sundays-rain-and-wind-data/

http://www.bbc.co.uk/news/world-europe-jersey-28746723

 

 

Even in her hey-day Hurricane Bertha was not a powerful or well organised storm and she disappeared as a discrete tropical storm a few days ago. What is left crossing the Atlantic is a significant “blob” of heat and moisture that she dragged into the dog-days of the mid-latitudes.  Unfortunately for meteorologists the energy injected “intravenously” into the mid-latitudes upsets super-computer weather model forecasts. Models cannot handle the excitement!  The result is that various weather forecasts have struggled to agree and have been producing significantly differing outcomes as to the strength of Bertha and to when, where and IF she “makes landfall” in the UK.  Usually, the differences in model forecasts gradually reduce and forecasts increasingly agree, but in volatile atmospheric conditions it often goes down to the wire, like on this occasion!  This is why meteorologists have been unable to confidently pin down the exact track and strength and impact of Bertha on the UK.

Today there remains uncertainty, despite being only 48 hours out from her arrival.  The main cause of uncertainty is how she eventually interacts with an unseasonably strong and southerly jetstream.  Where a low pressure like Bertha arrives and moves under a jetstream makes a big difference to where she goes and whether she strengthens or weakens. The so-called “left-exit” region of a jet exerts most influence on deepening low pressures into significant storms.  Where the jet is slowing down or speeding up or curving can create extra lift, dragging air off the surface causing pressure to drop.

Here are three of the possible forecast outcomes from the GFS (including WRF/NMM which are similar), UKMET and ECMWF models. (Thanks to @BigJoeBastardi for the ECM model output from @WBAnalytics)

#1 Bertha could travel more or less down the eye through the English Channel from the SW.  This would take most of her highest winds and gales through France but would clip the south and SE of England with some heavy rain as the low crosses directly over this area.  The highest winds in these tight LOW pressure systems usually occur to the south of the LOW centre, associated with the warm and cold fronts.  The heaviest rainfall is modelled to fall to the north of the low associated with the occluded fronts and low pressure centre, hence the heavy rain clipping South and SE England. This outcome is currently favoured by the BBC and UKMET office models.

#2 Various other models, the GFS and WRF-NMM amongst them, favour a more northerly route and take the LOW from the SW approaches, through St George’s Channel, across Wales and into the heart of England before exiting into the North Sea.  Broadly speaking, this would be a worst-case scenario because the UK would take the brunt of both heaviest rain (to the north) and the highest winds (to the south).  Nevertheless, for @ridelondon and #Reigate and the SE this might be the preferred route because any fronts and poor weather would pass over our area relatively quickly on Sunday, the worst of it probably from around 9am through to 2pm.  It would also leave only trailing fronts depositing up to 10mm or so of rain across our region and some gusty conditions, wet for a while but nothing too drastic to speak of really.

#3 A third model, the ECMWF has, until recent runs been quite the outlier amongst all competing tracks, sending Bertha further south through France in earlier runs.  More recent ECM runs have played catch-up with GFS and now sends the LOW NE through the UK.  Significantly, ECM drops pressure to 987mb in the Irish Sea.

The outcome could, of course, be somewhere in between these two or something totally unexpected!  It is worth also pointing out that extra-tropical storms arriving in the UK are not uncommon and this one is likely to underwhelm in many places inland where gusts are unlikely to exceed 40mph and, in the SE maybe 30mph gusts will be the widespread maximum.  Coasts, hills and exposed areas are likely to see the worst of it with gusts up to 60mph or more.  Even here it will not be anything like as potent as our storms last winter where winds exceeded 90mph on occasions.  Nevertheless, with leaves on the trees there may be some blow-downs and loose branches and rain in some places might disrupt travel but it is doubtful that wide-spread chaos will ensue as compared with the morning after St Jude last October which was a lot more potent. See below.

 

The best advice is to watch weather warnings and updates carefully on Sunday for any changes.  RGSweather will be posting NOWCAST updates on twitter for Reigate and environs.  Here is the latest from NWS/NCEP showing storm force winds in the sea around the UK, note the unseasonably LOW central pressure on 985mb. In comparison, St Jude storm had a lowest overland central pressure of 976mb back in October 2013.

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Interestingly, the longer term weather impact of Bertha into next week is probably better modeled than her immediate track on Sunday.  She is forecast to move slowly up the North Sea and merge with and deepen her “parent” LOW to the N of Scotland.  This will introduce cooler unstable showery NW winds to the UK for the early part of next week. Showers and some more organised bands of rain are likely to be frequent visitors, especially to the NW and west coast.  It will also feel cooler and more breezy for the whole country.  So, put the balmy warm days of summer on hold next week.

http://www.bbc.co.uk/weather/features/28707422

http://www.bbc.co.uk/weather/features/28707422

 http://metofficenews.wordpress.com/2014/08/06/ex-bertha-more-likely-to-miss-uk/

Update: Fri pm

More model agreement now on a northerly track.  Potential for UK worst-case scenario. Wind and rain crossing the country.  Inland 30-40mph, coastal and hills poss 50-60mph.

2014-08-08_20-37-10

There is some interesting weather potential this week, but indications that the SE will see least of any drama which will be mostly further west. The well-established warm plume of S/SE wind from Africa/ Mediterranean and Spain has brought temps up to 19.4c in Reigate this weekend and 20c in London.  A warm sunny Saturday was especially pleasant.  The breezy S/SE wind is bringing Saharan dust falling over the UK, watch for this in any showers that might come our way on Monday.  Check your car for any dust.

Warm plumes from Spain can also introduce unstable moist air and these produce thunderstorms and showers when moist warm air converges or fronts  undercut the plume with Atlantic air creating lift. There is good potential for heavy convective downpours of rain this week due to these scenarios.  Whilst not necessarily a classic Spanish Plume the synoptic situation is very similar to May 1998 when large super-cell thunderstorms drifted to the North and caused torrential rain and flooding in parts of the North.

2014-03-30_18-37-25

Various indicators are used to establish the potential for heavy convectional rain and thunderstorms.  At times the charts show several of these indicators at unsually high levels for end of March / early April this week.  CAPE (convective available potential energy), LI (lifted index) and ThetaE (potential equivalent temperature) … these are all technical charts that are commonly used to assess how likely thunderstorms will be.  ThetaE rarely goes above 19c in the UK so temps of +12c in April are unusual so early. Unfortunately, the development of convective rainfall and thunderstorms is difficult for forecasters to predict for any one location for a particular time.  Thunderstorms and showers are, by nature, hit and miss affairs: one place might get large hail and a deluge while, a mile down the road might remain sunny and dry.  Nevertheless, watch out for some potentially heavy rain, especially if travelling this week.  Watch out for interesting cumulonimbus clouds too!

Total rainfall remains highest in the west and away from the SE where pressure remains higher and frontal action is more limited.  So Reigate may escape the worst of all this convective rain action but still worth keeping an eye out for rogue storms that may well come our way drifting most likely from storms in the Channel around mid-week.

Finally, models are hinting at high pressure building back in for the school holidays after this wet unsettled week. Whilst not yet represented on charts very convincingly, the models suggest a possibility of some reasonable holiday weather in the UK with pressure rising and temps above the 30 year average on the GFS ensemble mean.

 

 

 

No, not a record low pressure this one but the stormy weather continues for the UK with another deep area of low pressure sweeping up past the NW of Scotland tomorrow, Friday.  This one, called Anne, has violent storm force westerly winds out in the Atlantic building high waves and matching high tides, causing unusual storm surge conditions for the west coast. It is the orientation and track of this storm that appears to be causing most trouble: spot those isobars directed straight at the UK and building high seas with those high tides.  The distance which wind travels over the sea is called FETCH and the longer the fetch the greater the possible wave height.  Note also the waves and wind that build in the Channel.

There are currently 17 severe flood warnings from the Environment Agency for the west coast of Wales and SW England.  The River Severn estuary is also at risk as as it faces the SW winds, funnels tides and has high river flows all to contend with.

Envt Agency are even warning people to evacuate if possible from vulnerable locations on West coast.  N Ireland also adopting this too.

http://www.bbc.co.uk/news/uk-northern-ireland-25580692#TWEET1001676

http://www.bbc.co.uk/news/uk-25572390

For Reigate and the SE (inland) heavy overnight rain will pause for a while in the morning only to resume as winds increase to gusts possibly 30-40mph, gusting higher on hills, and with possible heavy showery rain.  Rain totals for Fri-Sat 48 hours could amount to 30-40mm, but more likely 10-20mm for Reigate and Mole Valley.

02-01-2014 22-10-19

rain totals fri – sat

Is there any sign of improvement? Met experts watch the high atmosphere for some long range forecasting. In particular, the stratospheric Polar Vortex is a a possible indicator of how the tropospheric jetstream might be acting in weeks to come.  The stormy weather we have been experiencing has been “caused” by an extremely powerful polar vortex: a great contrast in temperature difference between high and low latitudes sets up high winds in the stratosphere which act like a belt to hold in cold air to the Pole.  The North Pole stratosphere is extremely cold this winter and this has encouraged a powerful polar vortex.

02-01-2014 23-32-17

 

You can see from the chart below that the vortex is forecast to change shape: split or squeeze, meteorologists are watching for this to tell them if the jetstream will weaken, and there are signs that it will in mid-Jan.  This may also have the effect of allowing polar leaks of cool air to reach the UK: so maybe less stormy but a tad cooler. Check these guys for further details: http://www.weatherweb.net/wxwebchartsecmopmsl.php

01-01-2014 13-59-58

Finally, the Express is unusually conservative with this article: the wind speeds are too low!

02-01-2014 21-38-31

 

Should the UK start naming storms?

The number, frequency and rapid movement of recent UK storms has meant that, for many people and the media, one anonymous unnamed storm has merged into another. Identifying impacts and explaining the specific nature and different characteristics of each storm is often confusing.  The current procedure of UK storm nomenclature is by date, such as “the 2013 Dec23-24 storm” or, for more infamous specific high impact events, randomly adopted names are used, such as StJude.  Storm identification in the UK seems, therefore, to be rather an ad hoc, random, informal and possibly even disorganised process.  For a country that experiences a lot of storms and, seemingly, an increasing number of intense meteorological events, it would seem like a timely idea to START NAMING UK STORMS!

Naming storms that impact Germany and Central Europe has been a tradition at the Institute for Meteorology of the Free University (FU) Berlin since 1954. They run an interesting adopt-a-vortex scheme that allows users to (pay?) to adopt a LOW. more here…  http://www.met.fu-berlin.de/adopt-a-vortex/ Some forecasters in the UK use the Berlin vortex Euro-names already, especially on twitter.  Other professional forecasters fear that using such storm names will not help public understanding of meteorological events (not least because some of the worst UK storms are wave depressions that are spawned by rapid cyclogenesis from more persistent parent lows; this is more complex than a hurricane which tends to stay as one system).

Better known are the names given to hurricanes and typhoons in the Atlantic and Pacific since 1945.  Names like Hurricane Katrina, Superstorm Sandy, and Typhoon Haiyan are iconic in the public imagination and formally arranged by the World Meteorological Organisation using a formal alphabetical list.  http://www.wmo.int/pages/prog/www/tcp/Storm-naming.html. Whilst UK storms are not usually on the same scale as tropical hurricanes, they nervetheless have the potential for significant economic, environmental and social impacts.

http://en.wikipedia.org/wiki/Tropical_cyclone_naming

There are several benefits of naming storms, not least a heightened public awareness if storms have unique names.  With storm damage and intensity set to increase for the UK and considering Atlantic storms arrive here first in Europe, it seems like a good idea to start our own system of storm naming. The UKMO or http://www.rmets.org/ could run such a scheme and funds raised could be charitable or go towards funding the scheme.

A scheme similar to that of Adopt-A-Vortex where the public can pay to name a storm would help fund it.

Have your say below and, as a community we might be able to ask the UKMO to consider this as a proposal, you never know!

Quick interim storm summary graph with data from our Reigate weather station. Analysis and links to follow.  Below is a medley of images… analysis to be continued after festive season!  Meanwhile… hope you survived unscathed and Happy Christmas!

70mm total rainfall (manual rain gauge reading) now confirmed for Reigate during the storm 24hrs.

which was bigger, StJude or Caspar? (our name, not Dirk!) Caspar of course! Caspar goes on to Scotland today… 24 Dec

24-12-2013 05-07-51

24-12-2013 14-36-26

Dramatic VIIRS sat pic from 24 Dec midday: note boiling clouds along Atlantic front

http://www.getsurrey.co.uk/news/local-news/fallen-trees-floods-power-cuts-6444107

http://www.environment-agency.gov.uk/homeandleisure/floods/riverlevels/136484.aspx?stationId=7262

River Mole 3.45m above normal level: severe EA flood alert issued: the only one in the UK at the time.

Flanchford Bridge wall collapse

Severe weather warnings out: Gales and heavy rain across the South…

UKMO warnings page: http://www.metoffice.gov.uk/public/weather/warnings/#?tab=map&map=Warnings&zoom=5&lon=-3.50&lat=55.50&fcTime=1387756800

Flooding map: http://www.environment-agency.gov.uk/homeandleisure/floods/142151.aspx

Brief update on storm “Caspar” (our own name, middle one of Three Kings ): possibly worst in 130 years for UK (BBC source).

WIND: Reigate can expect possible gusts of 50-60mph, though the town itself is sheltered from southerly and SW winds by Priory Park hill and woodland so that we can shave off 20% from max wind gusts, usually.  The max quoted gust from UKMO is 66mph in small hours of Tues am as cold front goes through. StJude, the last big storm in October, had a max gust in town of 48mph during high winds that lasted only a few hours.  Caspar is set to be a longer duration, across a wider part of the country and is a bigger beast, the storm being centred off the NW of Scotland, rather than running through the Midlands like StJude.

Strongest ever jetstream? Chris Fawkes BBC weather

23-12-2013 05-20-02

Winds will spring up early tomorrow and by 11am could be gusting at 50mph.  They remain strong for the next 12 hours, peaking as the cold front arrives overnight/early morning Tuesday.

The emerging storm is now visible in the Atlantic as a familiar hook shape develops with cold and warm fronts.  The storm is set to explode over the next 24 hours in a process called cyclogenesis, caused by a very powerful 270mph jetstream “sucking” air off the ground as air diverges aloft.  This lowering of pressure by some 50mb in a few hours will cause air to “rush” into the “void” (converge) and this, in a simple way, is the cause of the maelstrom.  The spin of the Earth causes the winds to rotate into the centre of the Low pressure in a process called coriolis effect.

928mb

928mb

The surface pressure is expected to fall to somewhere around 926-928mb, one of the lowest pressures recorded near mainland UK (lowest land surface pressure 925.6mb, 1884, Perthshire). The BBC explains the development here:

http://www.bbc.co.uk/weather/features/25485532

RAIN: Reigate can expect almost continuous rain for 12 hours, starting early Monday morning, around 6am, through to around the same time on Tuesday. Heavy rain is caused as air rises, cools and condenses especially where warm air is forced to rise over cooler polar air.  This will happen especially at two moments as the storm passes…  first the warm front and then a cold front (overnight, probably Tuesday early am).  They have different characteristics.  Of most concern to us and the Environment Agency tomorrow, is that this storm will be causing more or less non-stop rain from 9am Monday through to about the same time Tuesday.  Rainfall totals 20-30mm can be expected during the course of the storm and, on saturated ground, this will cause flooding and plenty of surface water on roads.

22-12-2013 20-57-13

The “good thing” is that cold fronts pass pretty quickly and moderated wind conditions arrive soon thereafter, this means Tuesday is likely to see brightening ameliorating conditions arrive during the morning for any clear up. Flooding will be a problem by then and it’s likely that our River Mole will flood key back-routes and surface water on more major roads could be a problem.  Winds of more than 50mph can also causes trees to be knocked down and other damage is possible.

http://www.bbc.co.uk/news/uk-25484998

Finally, waves in the Atlantic are expected to reach near “phenomenal” heights with significant wave heights of 13m by Christmas Eve (which means waves double this height could occur once an hour).

After a quieter rest over Christmas things sadly don’t get a lot better… another storm is heading our way for Friday and the weekend.

23-12-2013 00-11-59