Archives For hazards


The ingredients for our Monday storm are stirring in the Atlantic, as shown by the sat pic from today.
The Mother of LOW pressures south of Greenland is whipping up hurricane force winds and 50 foot waves in the Atlantic.
A warm plume of tropical air is being brought up by a weakening ex-tropical depression Lorenzo.
They meet at the polar front where an extremely powerful 240mph jetstream is due to lower surface pressure rapidly forming the storm which will cross UK on Monday.  The birth of our storm is called cyclogenesis: it is rapid and will cross the UK rapidly overnight Sunday to Monday.  It will continue to fall in pressure and intensify as it does so.  Conditions will be worst along the south coast and SE where wind gusts could reach 80mph, inland gusts of 60mph will decrease north of the M25. The beaufort scale below shows what these winds mean to you and your house. Sustained winds around Reigate will probably hover around 20-30mph, which are certainly not damaging. Gusts are a different matter and these could lift tiles, bring down branches or even weak trees ready to fall.  Loose fences and garden furniture not tied down may also blow about in gusts and cause additional damage.  Umbrellas will certainly be “difficult to use”, (I love this description!) which is, of course, a critical meteorological threshold occurring when winds exceed Force 6, 25mph. 

Heavy rain alert and UKMO weather warning issued for Reigate and SE Thursday 3 Oct from afternoon and overnight through to small hours Friday.

An unstable plume of warm moist air will edge across the SE during Thursday afternoon and bring the potential for heavy rain as fronts provide lift to the air to produce possible thunderstorms through Thursday pm and evening.  Rainfall totals from showery outbursts will vary a lot locally, probably most further towards the south coast and further east, but could be as high as 20mm over the 24 hour period almost anywhere in S and SE corner during this time; most places will see a lot less, 6-10mm being widespread.  Winds will be 15mph initially from the SE but in heavy showers gusts could reach 25mph. The wind will veer to the S overnight. It will be increasingly humid during the day with Tmax of up to 20c, staying rather warm overnight but cooler and much brighter on Friday at 17c as cooler polar air follows the cold fronts with a few showers lingering as fronts move away leaving a brighter showery day.

Many ingredients for producing decent thundery activity are met during this period: including…

  • high dewpoints rising to 17c in the warm sector and as the cold fronts approach on Thursday evening: high dewpoints indicate a saturated atmosphere with plenty of moisture available for cloud and rain formation
  • warm air advection: which means a warm plume of air will be provided for several hours during Thursday pm but will be replaced by cooler air on Fri am.
  • wind shear: winds at the surface will intially be SE but at height will be a brisk warm southerly.  This change in direction and speed with height storms will not “stagnate” over ground they have just cooled with a soaking, so that thermal activity will be maintained with further warm air available to lift and grow bigger cumulonimbus clouds which roll through rapidly.  On the other hand, high rainfall totals will be subdued as showers move over relatively quickly.
  • Jetstream overhead (arriving later on Thursday evening) providing strong upper air divergence: this means air will be dragged off the surface as air rushes UPWARDS to fill the gap left by the rapidly disappearing air aloft.  This convergence on the surface means rapid uplift… perfect for building thunderstorms!
  • Some ingredients are missing or rather lackluster.  The most obvious one is a lack of heating during the day.  Thursday will be mostly cloudy and, being October, the heat from the sun will not provide as much buoyancy as in mid-summer. This is a critical ingredient for big thunderstorms.  This episode is also short duration and a cold front will rapidly replace the warm humid plume with cooler polar air, increasingly less showery, during Friday.
some ingredients are good to go

some ingredients are good to go

jetstream 3 oct

Super Typhoon Usagi is blasting through the Luzon Straight and heading for Hong Kong. The Joint Typhoon Warning Center (JTWC) in Pearl Harbor expects it to make landfall in 24 hours, probably early Monday in HK. Wind speeds are over 100 knots and max wave height is 43 feet. Hong Kong sea port has been shut down and over 300 flights have been cancelled. All flights are expected to be cancelled from this evening (Sunday). Track Usagi with links below.

Updated location :usagi landfall

Typhoons in Pacific Sept 2013

Typhoons in Pacific Sept 2013

NOAA Usagi data

NOAA Usagi data

Warning Red Alert USAGI

Warning Red Alert USAGI

Hong Kong Observatory issued No8 storm warning signal (on a scale 0-10)  with the following alert as landfall occured 100km N of HK:

Hong Kong No8 signal

Meanwhile, in total contrast, the UK can expect the benign and balmy warm air feed from the SW to continue as a trough in the Atlantic deepens and brings warm air from Iberia. If the anticyclonic gloom shifts then temperatures will soon lift into the low 20s and maybe 23c is possible.  Models after about wednesday remain uncertain.  A general breakdown of the current HIGH over the UK with a return to zonal conditions with a strenghtening jetstream by next weekend is likely.  Some models bring on quite siginifcantly unsettled Autumnal weather next weekend, with heavy rain and gales, but this is too far off and models remain too wobbly with extra-tropical uncertainty still lingering in the N Atlantic.  The Atlantic hurricane season remains very quiet and no further incursions by ET storms are expected this week so Humberto maybe the first and last ETS affecting the UK this year … but the hurricane season has a while to go yet. ETS=Extra-Tropical Storm.

Blocking HIGH for the UK… opposite of Typhoon!

block high sept 22 2013

Some useful links on Usagi…

Devastating tornadoes hit Oklahoma suburbs schools and towns today.

News at

supercell thunderstorm

Info @RGsweather


Saturday am: heavy snow falling across our region: check twitter for updates.  A major wintry weather event is unfolding tomorrow and over the weekend across the UK.  This could bring potentially hazardous weather Friday through Sunday almost anywhere in the UK so if you are travelling please take care and check weather warnings for your destination. Reigate remains comparatively sheltered throughout but even here we have the threat of seriously miserable and cold weather with an evil mix of heavy rain, cold winds and sleet turning to snow over this weekend.  Snow amounts and depths for us still uncertain though.

Cold Polar Easterly winds will increasingly undercut and push back the advancing warm air brought in by a vigorous depression out west in the Atlantic. Where these air masses meet is essentially where the heaviest rain and snow will fall: the fronts move north on Friday and then move south on Saturday as the cold air wins out over the warm: it is the move south on Saturday which could bring some SNOW to Reigate. The SW is due to get torrential rain, whilst anywhere north of the M4, especially on high ground, could see heavy snow and drifting in high winds. Reigate remains comparatively sheltered throughout this episode BUT if you are travelling anywhere else do heed weather warnings.
Whilst Reigate will escape the worst of all this, we are due to have a pretty cold, windy, very wet and sleety and at times snowy weekend weather wise!
Friday sees fronts crossing the country and bringing increasing rain to Reigate during the day. Overnight into Saturday might see this rain turning increasingly icy and sleety. Wind chill 0°C to -4°C.
Saturday will be a truly horrible day with temperatures falling throughout. Cold winds (feeling -5°C) and heavy rain will turn increasingly sleety and turn to snow anytime but with greatest risk in the afternoon.  By the afternoon, however, the heaviest precipitation should be dying out over Reigate so any snowfall should be light by that stage.  Saturday could see totals over 20mm of precipitation (mostly in the morning as rain); snow lying could amount from 0cm to a few cm especially on the Downs by evening. Any snow conditions will be worse to the north of the region and over high ground.
Sunday sees even colder weather and a threat of further snowfall from nearby fronts to the south.  Still uncertainty around this so keep watching forecasts. Wind chill -8°C so feeling very cold indeed.  Drier later though.
The rest of the week looks cold, frosty and dry with gradual recovery of temperatures and the hint of more spring-like temperatures next weekend!
Take care this weekend if you are travelling anywhere in the UK: floods in the SW, blizzards and snow up North. If you don’t need to travel, you should probably wait until Sunday or Monday when things have calmed down.

What a difference a year makes: check out the daffodil pictures: taken one year apart!

update Friday pm: much as stated above with main snow zone staying north of London for longer on some models: filthy horrible day mostly sleet icy rain Sat; poss light snow later pm as cold air arrives; cold wind; colder on sunday and into next week. Cold and frosty next week. #notspring 

Here is a summary of the causes of the March 11 2013 “Channel Blizzard” which brought extra-ordinary “Spring” weather to SE England and the Channel Islands and N France. At RGS we had record low wind chill temperatures of -10°C at 9.30am, 36mph gusts and sub-zero temperatures all day.  Follow the numbers on the map to get a quick view of why it all happened!

1. Cold source region for Polar Air: the NE winds originated from the Polar regions with temperatures below -15°C and traveled across an extremely cold continent to reach the UK.

2. On their journey, the North Easterlies warmed a little over the North Sea (by now +4-5°C) which caused showers to form in unstable air (warming causes air to rise, clouds form and it snows). These showers formed lines called “snow streamers” which fed snow to the SE for most of the day and into Tuesday morning.

3. Much warmer SW winds at +10°C met the frigid cold Polar air mass somewhere over the Bay of Biscay but they didn’t mix well and they certainly didn’t get on!  In fact, the cold, dense polar air pushed the lighter, less dense tropical air right up off the ground, to over 4000m, where all the moisture condensed, formed cloud and snowed!  The high winds experienced across SE England were “squeezed” like toothpaste between the HIGH over Iceland the LOW over France (a high “pressure gradient”: look how close together the isobars are over SE England and the Channel!).

4. Next? Things will stay cold during mid-week as the UK remains firmly in Polar air and enjoys some dry weather courtesy of a HIGH over Iceland (unusual).  Clearer skies and frosts.  However, a Polar Low forming SE of Greenland at the moment is due to break through the Icelandic HIGH later in the week and bring unsettled conditions to the end of the week / weekend: as Polar air is still involved this may bring further snow and sleet.

Greenwich Lightship is a UK Met Office weather station in the middle of the English Channel. 3 metre waves, 50mph winds, sow and 10 foot waves were recorded.  Unbelievably horrendous conditions which the Channel Islands experienced as a blizzard.

Watch the satellite animation below and spot the storm winding up in the Bay of Biscay before it hits the Channel and says “Hello” to France and the UK!

A severe blizzard in the English Channel will brush perilously close to Reigate today and overnight. Sub-zero temperatures for the next 48 hours and strong gusty easterly winds of >20mph will make it feel like -10ºC and potentially much lower (see wind chill charts posted earlier).  Whilst the UKMet-Office model refuses to bring any significant snow to Reigate other weather models certainly do!  In fact, some forecast as much as 10cm of snow to our region falling later Monday and overnight especially. With cold temperatures and strong winds this snow will blow around and be very unpleasant indeed, and could amount to 10cm, especially south and east, less west and north.  Updates @RGSweather on twitter.

Sunday final analysis update: crossing the Channel Monday?…be warned of v cold blizzard conditions!  LOW centre set to track thru N France most likely bringing only light snow to Reigate Monday morning but increasing risk of significant accumulation later in the day and overnight into Tuesday.  Any shift further north in the fronts could bring heavy snow to S counties pm.  Latest hi-resolution NMM model (below) shows light snow showers all day but risk increasing to heavy snow pm extending into southern counties of England, reaching Reigate late pm or evening….; UKMet Office predicts the least snow of all the models.V V tricky forecast.

So: Reigate forecast for Monday-Tuesday: emphasis on bitterly COLD, snowy day with extreme wind chill for (any) time of year with a low risk of heavier snow later and overnight to Tuesday, especially for places further South. so…accumulations 0 – 10cm by Tuesday am.

sunday 06Z NMM model

Cold weather alert! Reigate should prepare for an impressive weather change this weekend into next week.  The temperature will drop like a stone over Sunday night: shown by these Reigate maximum temperatures at midday on Saturday max +11ºC, Sunday max +5ºC, Monday max -1ºC with wind chill even as low as -18ºC (yes) possible in forecast 40 mph gusts in exposed places.  Winds will average 20 mph but even this will cause wind chill of -8ºC.  It will feel cold on Monday!  Light snowfall could start Sunday pm and continue overnight through to Monday. It may seem odd, but this snow will be caused by warm air from the South over-riding the frigid COLD Polar air blasting in from the NE: meeting at a front.  Check the temperature height diagrams to see how this works: warm, moist air circulating around the LOW in the Channel will be undercut by the dense freezing cold polar air from NE. The warm air forced to rise causes cooling, condensation, cloud formation and, as upper air temperatures are so cold… SNOW is likely to fall. Possible accumulations Sunday – Monday around Reigate could be 0-3cm (higher figures on hills) and more possible to the south of the region where precipitation could be more intense nearer the fronts.  So…positioning of the LOW is critical.  The snow maps give an idea of potential snow areas in the south on Monday.  There could be light snow showers on and off in Reigate and another moderate fall is possible Monday pm as the LOW tracks nearer to the south east of our region.  It will feel cold in strong gusts from the East: wind chills down to an outrageous -18ºC are theoretically possible if you are caught in strong easterly gusts with air temps below 0ºC all day. Temperatures will actually fall through the day on Monday as the Polar air takes a grip.  Lowest temperatures all winter are possible overnight Monday-Tuesday. Hopefully, this weather change will not take anyone by surprise: it has been on the cards for over a week and flagged up here on 2 March. The rest of the week will remain cold, but feel less cold in light winds, and drier with pressure building from the north, but more snow might precede rain later in the week as things warm a little next weekend.  More updates later as details emerge for Monday which is still v changeable!

Reports of unusual “thunder snow” coming from NW England tonight with the passage of bands of heavy wintry showers moving south associated with a front. They appear to be dying out as they move South but might just still produce a heavy shower of snow or most probably sleet (as temps still 5°C and falling) over the Downs in the small hours. Unlikely to add up to much though and any showers should largely clear off by the morning rush, but take care of ice if driving later tonight. Surrey gritters are out.