Archives For low

A system stirred up by a low pressure tracking out of Mexico into the Gulf of Mexico this week is on track to bring unsettled conditions for part of the weekend to the UK.  It’s nothing too severe for most but is an interesting feature that will bring some wind and rain everywhere.  Below is a satpic showing the development of this system as it interacted with a lively jetstreak on Saturday 28/02/2015.

development of LOW on jetstreak

development of LOW on jetstreak

 

This LOW illustrates nicely how extra-tropical systems can rattle clean across the Atlantic in a few days if they are picked up by an active jetstream.   This one does precisely that.  Spot the system leaving Florida on the chart below for today and the sat pic.  This system started as a low pressure crossing from Mexico into the Gulf of Mexico mid-week, so for weather systems it will be an aged fellow on arrival here in the UK.  Its’ longevity is partly due to the exceptional COLD over NE US which interacts with the warm tropical air and causes further deepening.

The Gulf low pressure is tracking quickly NE skirting the US east coast before being picked up and deepened further by an active jetstream.  The jetstream itself is particularly powerful at the moment due to intense cold spilling out of an exceptionally wintry NE USA meeting warm tropical air issuing from a strong subtropical Azores HIGH pressure converging with the moist Gulf airmass.  A result of the powerful jetstream is a positive North Atlantic Oscillation: the NAO is a measure that indicates the difference in pressure between Iceland and the Azores.  In positive NAO conditions the jetstream is often active, producing a strong westerly zonal flow keeping Europe mild and unsettled especially in winter, or early spring!

Our Gulf LOW is due to pass over Reigate fairly rapidly through Saturday pm and overnight into Sunday am and bring some moderately wet and windy weather, likely to go unnoticed because of the nocturnal transit.  Winds gusting in excess of 40mph are possible for Reigate into Sunday am in exposed places.  Notably, due to the TROPICAL origins of this airmass the temperature overnight Sat-Sun could climb to double figures in Reigate.  Tropical air crossing the Atlantic also picks up a tremendous amount of moisture so attendant fronts are likely to bring a lot of rain too, possibly exceeding 10mm overnight, which is a moderately wet night.

Here are the synoptic charts showing the Gulf low progress across the Atlantic, deepening and occluding into the North Sea.  Note the secondary wave which could bring additional rain later on Sunday to the South.

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The additional rain later Sunday afternoon / evening looks potentially heavy for the South and SE.  It’s a rapidly developing wave feature that needs attention as, on the northern edge, it looks to raise the possibility of snow across the Midlands.  Heavy rain is possible for the SE and #Reigate with a period of gales on the south coast.

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The outlook for next week is for the Azores HIGH pressure to extend a ridge to the north and cause a NW then northerly airflow for the UK.  This will bring cooler temperatures to the UK.  Whilst it is likely to be mostly dry for Reigate and the SE with pressure rising, wintry showers especially on east facing coasts of the North Sea could be possible depending on how the HIGH develops.  Frost is likely with temperatures dipping below freezing at night from mid-week.

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Atlantic ridge builds to the west and brings in northerlies

How long this Atlantic block persists, and the cooler weather, is uncertain.  The coldest scenario would depend on the HIGH moving north and east and building over Scandinavia to pull in easterlies from a cold continent.  This scenario is preferred by the ECM by later next week whilst the GEFS topples the high to the SE and brings back a zonal mild westerly flow from the persistent Icelandic LOW pressure that erodes the edges of the HIGH from the NW.  The charts below show the uncertainty as a wide spread of possible pressure and temperature towards the end of the first week in March.

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Gulf low for the weekend, then pressure builds

The cool start to March is shown below.  The overall outlook is for a persistent positive NAO and Arctic Oscillation to persist and this would suggest a brief cool epsiode without the formation of a persistent Scandinavian High.  Models have flirted with possible easterly winds by the end of next week but the outlook is for the positive NAO to persist and this rather suggests a quick return to milder zonal westerlies.  As the high builds in early in the week various troughs and fronts could even push some wintry precipitation as far as the SE on Tuesday (spot the pink on the rainfall chart below for Tues)

First week of March starts cool: MetOffice synoptic chart for mid-week shows Azores high briefly ridging north to block mild zonal westerlies and usher in a cold polar airmass, albeit briefly as this ridge looks to topple SE and by next weekend we could be in a pretty mild SW flow hitting mid-teens possibly.  So a cool, mostly dry middle part of the week for Reigate and much of S England but precipitation, some even wintry, pulling in on NW / N winds is not ruled out with a North Sea low possible.  As the Atlantic is likely to push westerlies back in later in the week we can expect more purposeful frontal rain pushing east across the whole country.

2015-02-28_09-19-25

 

link to accuweather take on this system

http://www.accuweather.com/en/weather-news/winds-to-whip-uk-north-sea-coa/43063930

 

IMPORTANT UPDATE 11AM Fri: UKMET extended AMBER warning to include all of SE inland including Reigate, Surrey, London: gusts of 60mph possible overnight. 

Latest update: (pm Thurs): one HiRes model just HALVED rainfall totals for storm tomorrow: if others follow that’ll be good news for flooding.  This storm is not resolved perfectly yet, some models flip and flop before the event.  Nevertheless, one to watch for signs that, perhaps, the rain is being over-done. In fact, rainfall totals have rarely matched model runs in recent events… fast moving fronts have simply zipped by so fast that rain totals have been rather low.  example: 8.8mm on 12 Feb front yesterday.

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Now you see it…?! Rain totals just halved on latest HiRes model run (left). GOOD NEWS for floods!

Update for Reigate and SE England on UKstorm Fri 14 – Sat 15 Feb:  the storm tomorrow is yet another serious weather event, this time focusing most intensely on SW, Southern coasts and SE England and Wales overnight Fri-Sat.  Below are some satellite pictures from today showing the rapidly intensifying storm in the Atlantic to the SW of the UK.

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The wind and rain forecast on NWP models for inland SE areas are severe: often the forecast winds and rain do not transpire, coming in at a good deal less than the modeled projections especially for sheltered towns where most people live.  However, the magnitude of figures consistently being predicted for gusts and total rain associated with this storm in our area are consistent enough to be of some concern, especially for places exposed to southerly winds (on hills like North Downs, for example) and at risk of flooding, either surface water flooding or in the flood plain for the River Mole or tributaries.

Rainfall will be heavy throughout Friday and could total over 20-30mm in 24 hours in places.  If this exceptional rainfall does occur then extensive local flooding is likely to result rapidly, especially along the River Mole catchment which has a flashy response to rain within 10hours.  Winds are due to possibly exceed those experienced this winter so far, forecast to peak at over 60mph gusts inland early Sat am.  Heavy rain is also likely to add to the problems of flooding.

UKMO warnings stand at Yellow alerts for Reigate and Surrey from Fri through to Sat 15 Feb and AMBER alerts for all places nearer the coast.  The storm is rapidly intensifying and will bring heavy rain all day to Reigate and Surrey and the SE as a whole through Friday, totals could be over 30mm, which will cause the River Mole to flood in places and plenty of surface water on the roads. Strong winds build gradually through the day and will peak overnight in the small hours of Saturday morning.  The winds, even inland, could even gust at 60-70mph and for southern coasts 75-80mph.

These are exceptionally strong gales for inland and, if these NWP modeled magnitudes transpire, it would be exceptionally bad news for the region already suffering from flooding. Remember our strongest gust for YEARS in Reigate was 52mph in January.  So any wind speeds over 60 mph will be exceptional. The UKMO also forecast wind gusts over 60mph around midnight.  ESTOFEX have issued a Level 1 tornado warning and for isolated gusts from afternoon onwards due to warm air advection and presence of sub-tropical air wrapped in the system.  This will add lift and energy to frontal rain bands which might cause thunderstorm activity, the odd clap of thunder embedded in fronts might be a sign of tornadic potential activity so keep an eye our for funnel clouds or drops in cloud base or tornados.  If you see any of this please tweet or email a picture!

The saving grace is that it passes over relatively quickly and winds will abate on Saturday leaving a bright possibly showery and blustery day. The remainder of half term week looks less dramatic and with a ridge of high pressure flirting with the South it should be more settled, albeit with another LOW early in the week, but this is much less intense.

RGS weather is on holiday for the remaining week, inspecting the other side of the jetstream in the Atlas Mountains of Morocco. Intending to fly out from LGW at 4am on Sat morning! Close up and personal with this storm but sadly no tweet updates are likely.

By the way, press details can be hopelessly inaccurate, especially certain papers!  The pic below is wrong: indicating several storms that were not a special problem for the UK… handle with care!

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

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

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

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

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

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

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

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

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

PREPARATIONS:

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

A stubbornly slow moving “dishwasher” LOW spinning up west of Spain and just NE of the Azores has been abandoned by the jetstream. This LOW has sat there all week in a cool pool near the Azores. The tropical Azores have seen Tmax temps only a tad higher than Reigate this week as a cool polar pool gradually fills in that region. It is the position of the Atlantic dishwasher that controls our weekend weather.
Most models agree it will edge closer but this is a slow stuttering move without purpose or direction. Infact, it’s likely that this LOW will never move across the UK because of the stubborn ridge of HIGH pressure to the NE, causing the LOW to remain stuck out west sending occasional fronts and heavy showers our way as the pressure falls. The UK will remain in a relatively warm upper southerly flow with Tmax staying into the 20s through the weekend and possibly into next week, though things are uncertain at that range at the moment.
Expect windier conditions through the weekend, especially to the south coast.
Weekend weather could possibly see thundery showers edging over Reigate sometime Saturday through to Sunday but this is dependent on the location of the LOW. Current model runs suggest the heaviest rain will stay south in the Channel but check back as updates occur.
Between now and then… sluggish, overcast high pressure days with anticyclonic gloom for much of the week until the wind picks up.
Reigate was, again, the warmest place in the UK today… at 23.7C matching a rounded up figure for Charlwood, Gatwick at 24c.

This weekend a big LOOP in the jetstream is set to rip up the summer heat still lingering over the southern part of the UK. In a dramatic weather battle in the upper atmosphere above our heads the southerly blowing limb of the jetstream will bring POLAR air to west of the UK to fight it out with the continental tropical heat lingering over the south and east. Heavy rain and strong winds will be the result of this battle but the exact location of the heaviest downpours is tricky to be precise about. Broadly speaking, the heaviest rain will start in the South of the UK on Friday and move North, rotating over to the NW and falling heaviest over the northern hills as the LOW pressure drifts NW during Saturday and Sunday. The South could have comparatively drier days after any heavy rain on Friday, but stay tuned for details on that.  Friday rainfall for Reigate could exceed 10mm starting in the afternoon – so possibly very heavy rain for a few hours Friday pm, with lighter rain through Saturday and possibly none at all on Sunday as the LOW moves away to the NW.

Very warm air drifting up from the continent over England by a northward blowing jetstream will be forced to rise over the markedly colder polar air invading from the NW through Friday and Saturday. The difference between these air masses is very marked: the warm 28ºC surface air currently over Reigate equates to -12ºC at 5000m while the invading cold Polar air mass is -28ºC at 5000m, which will push down surface temperatures on Friday to a Tmax of only 16ºC! It is this contrast between the air masses which is a hallmark of autumnal weather and the key to creating lots of rain: polar air meets tropical air, forcing it skyward, forming rain with gusty winds spiraling round LOW pressure: typical autumnal scenario (except this weekend’s scenario is actually quite unusual: called a trough disruption with the surface low drifting off in an unusual direction: from SE to NW, unlike usual LOWS which track west to east across the UK along a zonal jetstream).  Wind speeds for Reigate this weekend could gust at 25mph at times on Saturday. No thunder is now forecast this weekend as the polar air is relatively stable, being on a return leg to the Poles.  

Thereafter, as you can see from the ensemble below, next week looks to remain cool and unsettled but with some improvement in the south possible later in the week as pressure could rise by next weekend, but no return to hot conditions is expected.

ensemble sept 4

The “something and nothing” weather in the South East of the past week and the uncertainties in the forecast are set to continue for a while. Some met-people call these conditions “unforecast-able”. Models seem to be unreliable beyond a few days and even hours. Rainfall has been especially hit and miss to forecast in the SE: predictions have been varying wildly for specific days between torrential, heavy, some and then no rain arrives at all! The reason is possibly the lack of the usual “zonal flow” in the jet stream: i.e. west to east flowing jet.  The jetstream is meandering north-south and weather systems are more or less STATIC: the UK has been stuck in a low pressure trough for over a week.  The normal procession of low pressure systems (depressions) and brief sunny HIGH pressure ridges seems a distant memory: it simply hasn’t been a feature of our weather for ages. Forecast models seem to struggle with this.

meridional flow jetstreamThe overall synoptic weather situation remains the same. That is: a big blocking HIGH over the Atlantic and very weak westerlies with the jetstream in a North-South pattern (meridional) bringing down cool northerly winds direct from the Arctic which “pool-up” across Northern France and Southern UK creating a LOW pressure trough.  LOW pressure in Spring with a stronger sun can mean pleasant warm sunny spells but showers: it is the showers which, fortunately, have barely troubled Reigate.  However, (and this does look more certain!), a significant little LOW is set to spiral down the N Sea Thursday – Friday, deepen along the way and strengthen Northerly winds and bring rain, especially to the SE: some frontal rain and showers are predicted to accompany this LOW but again – it could be rather hit and miss depending on how close the LOW gets to the SE England and the strength of accompanying fronts.

storm risk thursThere is a 30% chance of thunderstorms over Reigate area through Thurs and Friday afternoons (as the sun heats the surface which creates bubbles of warm air through the day which convect upwards through the cool Polar airmass creating tall cumulonimbus clouds).  With upper air temperatures at 5000 feet as low as -12ºC later this week any vigorous showers may fall as hail.  Frontal rain attached to the LOW will certainly feel chilly in the wind. Night time temperatures could fall as low as 4ºC and any wind will make it feel distinctly cool.  A weak ridge over Sat and Sun may bring pleasantly warmer and drier weather but some models are showing a return to LOW pressure in a storm arriving from the NW on Monday bank 

Meanwhile, sincere sympathy and thoughts to those caught by the terrible EF5 tornado which caused such terrible damage in Moore, Oklahoma yesterday.  Following events on Twitter and news reports on destroyed schools was very sad and upsetting indeed.  The tension between following exciting weather and the potential for witnessing terrible disasters unfold in front of their eyes was palpable for the storm chasers and met-enthusiasts involved.  Unfortunately, the weather in MidWest continues to threaten areas with tornadic conditions: lately in New York state too.  Take care out there.  Our UK weather is usually mercifully benign in comparison.  

A text book depression / low pressure system approaching the UK from the west might signal the beginning of the end for this long run of bad weather. It’s a big low pressure system tracking NW rather slowly and will take all weekend to clear off but most of the rain will fall in the west and not much will reach Reigate. The warm front is due to pass over Reigate around midday on Friday bringing some brisk SW winds and cloud and some light rain. The “warm sector” following the warm front comprises an air mass called “Tropical Maritime”, bringing balmy +10°C temperatures to Reigate over night Friday through to Saturday morning. The cold front is due to pass over Reigate after lunch on Saturday when temperatures will drop by several degrees to 3°C overnight . Despite the slow movement of the depression as a whole, winds will be pretty strong especially as the cold front passes. The cold front will bring gusty winds, possibly up to 50mph on Saturday morning, and heavier showers on a cooler NW wind. The air mass following a cold front is called “polar martime” and will feel significantly cooler in the wind, even in sheltered Reigate.
There are signs that a HIGH pressure will build up over the south of the UK by the new year week bringing drier weather, at last! The ensemble forecast below shows a drier spell to start off 2013. An ensemble forecast is one which combines several computer weather forecasting model “runs” and sees how well they match.  The closer the match, the more confident the forecast and more probability that it will be correct.  Each “model run” is “perturbed” which means tiny differences in starting data are used to simulate the error and vagaries of real chaotic weather systems.

27-12-2012 22-58-26 dry spell in january

Winds in the North Sea exceeded 60 mph today, creating waves over 30 feet high. On the Beaufort Scale winds topped out at violent storm force 11. The strongest winds preceeded a frontal system attached to the same LOW pressure which brought the heavy rain to the UK yesterday.  Every region of the country has flood alerts and almost every sea area surrounding the UK has been issued with gale warnings. 

With serious flooding elsewhere in the UK it seems that Reigate and the South East fortunately escaped any serious problems from this storm. The River Mole has burst its banks but this seems rather trivial in comparison! The low pressure and unsettled conditions are set to continue.

dec 23 storm force gales in north sea

Most agree that this cold snap-ette will be beaten back by a return to a more “normal” run of LOW pressure systems from the Atlantic with attendant milder, windier and wetter conditions from the end of the week. Some models show low pressure systems dominating UK weather for weeks to come and maybe all the way up to Christmas. Low pressure usually means mild, wet and windy. The temperature chart (called an 850hPa ensemble showing atmospheric temperatures at 1500m) shows a collection of temperature predictions from the Global Forecast System which spread out towards Christmas showing increasing uncertainty but overall warmer than right now.