Archives For convection

05 june plume fax

Analysis chart on the day: tstorms arrived early Fri SE and grew across East Anglia

A brief, rather marginal, teasing Spanish Plume is developing in the next 48 hours and might cause the first significant thundery activity of the year in the SE Thursday into Friday this week.  A Spanish Plume (written up here previously from 2014) is a plume of warm air that rides north, out of Iberia, through France and over the UK.  It is often accompanied by a preceding south easterly from a warm or hot continent.  This mix typically causes humid weather and thunderstorms, but plumes vary in quality!

This particular episode is marginal for the UK, and taunting forecasters, because the heating episode is very brief and most of any heat and resulting instability is forecast to remain over the continent impacting N France and Benelux with decent thunderstorms before they migrate into the North Sea. The UK might see little of this in comparison.

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plume edges into UK

Whilst it is not certain that thunderstorms will develop over the UK, some action is likely in the S/SE and East Anglia during Friday. A MetOffice warning was issued earlier today but note the very low likelihood of moderate impact.

It’s worth recording here because it is a similar synoptic set up to the first plume event of June 7 2014 which created relatively perky thunderstorms to the SW and elevated rumbles and thundery showers over SE.  The charts below show streamlines at different levels associated with the plume on Friday.

The plume on Friday is a heavily cut-down version of the “heat wave” that the charts flirted with last week.  The heat wave is not going to happen but a day of warm / hot conditions will brush fleetingly past the SE.  Upper air over 15C is set to push in from the continent through Thursday night into Friday morning and will quickly waft over the S/SE and raise temperatures to Tmax 27C before quickly being pushed east by cooler Atlantic air arriving later Friday.

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Brief heat

This warm “heat” spike for the SE is caused by HIGH pressure (currently over the UK) slipping NE towards Scandinavia, followed closely by an Atlantic LOW edging in from the west.  The result is a humid surface S/SE flow from the continent with an upper airflow directed from Iberia meeting cool polar maritime from the Atlantic.

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The convergence of these airmasses can cause steep lapse rates (rapid drop in temperature with height) and instability, in which saturated air rises freely to a great height creating tall thunder clouds.  The charts below show some of the parameters involved: note the relatively high (but brief) CAPE values and PWAT (water content).  The limiting factor might be low SST temperatures over the Channel which could subdue any imported storms from France.

Instability can be caused locally if the sun comes out and heats the surface sufficiently to erode the “cap” and release parcels of warm air that rise, condense and form cumulonimbus clouds. Upper air is cold enough to cause hail as updrafts keep driving precipitation aloft into freezing cloud top temperatures below -30C.  If it stays cloudy we can still get elevated thunderstorms imported in unstable upper air from the continent.  Skew-T diagrams show the relatively unstable situation forecast for Friday.  A dry slot / hydrolapse (where dew point departs markedly from air temperature in a mid layer) can increase potential instability because evaporative cooling in any dry mid-layers can increase instability by increasing the difference in temperature between rising thermals and environmental air (though 700hPa dry layers I think are more significant than the lower layer shown below).

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skew t plume Friday 5 June 2015

It is most likely that the plume will be cut off rapidly before heat can establish and cause whopping storms.  Nevertheless, heavy thundery showers are likely here and there.  The latest GFS charts from lightning wizard suggest a low risk of meso-scale convective systems earlier on Friday being imported from France into the south coast in the LOW pressure moving North from Spain.

Later in the day any action is likely to migrate further east into the SE / East Anglia.  Most action will be on the continent where some significant thunderstorms are likely. The charts below show how rapidly the cold air makes progress east.  Once this arrives the plume is cut off and convective activity associated with it ends.

Looking ahead, pressure is expected to rise into the weekend in the south so it will be dry and pleasant, but not hot. Into next week things look similarly benign with high pressure mostly in charge in the Atlantic feeding a flow of cooler air keeping us a tad below average in terms of temperature. Into mid-June things become more unsettled.

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

May 19 thundery showers was not a “severe” weather day but 10mm of rain from 7 showers in 7 hours, several with some moderately intense rainfall rate (30-40mm/hr), small hail and episodes of thunder, was of interest locally and deserves a review of some charts leading up to the event.

Often our area of the SE seems to miss convective action but this time showers perked up as they converged across East Surrey and some developed into reasonably impressive cumulonimbus clouds (cb).  As a bonus mammatus clouds were spotted at the end of the day (see photo above). Below is a summary of some key ingredients for the day.

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Synoptic chart thundery 19 May 2015

Low pressure: This synoptic set-up saw an unusually cold plunge of upper air flood across the country during May18-19. The synoptic chart above shows the invasion of cold polar maritime air behind a cold front, itself not especially dynamic across the SE. The LOW over the North Sea dragged this unseasonably cool air from the NW: the basic ingredient for instability on this occasion.

meandering jetstream

meandering jetstream

Note the u-bend wiggle in the jetstream around the LOW pressure in the chart above. The northward limb of the jet on the inside of the U-bend can become the location for instability, though this was not a fast jetstreak occurrence.  The base of the trough also crossed the SE of the UK during the day encouraging lift.  Showers were forecast on the charts days before.

Steep lapse rates: Very cold upper air temperatures fell to below -30C at 500hPa (5500m) across the UK.  In Spring a cold airmass like this can become unstable over land especially if the sun warms the surface to create thermals of warm air able to lift through the cold environmental air. The 850hPa temperatures show comparatively warm air at 1500m which increases the lapse rate.  The morning of May 19 was sunny and the surface heated with 5 hours of May sunshine to a modest 14C, just enough to release thermals and rising parcels of air.  Convection requires heat as a key ingredient to steepen lapse rates and create thermals, rising parcels of air.

Low lifted index: LI forecast for Heathrow (below) showed a LI of -3.  The lifted index is a measure of instability in the atmosphere and shows how readily bubbles of warmed air will rise from the surface to a great height (convection).  It is the difference in temperature between the environmental air at 500mb level (around 5500m) and the temperature of the theoretically lifted bubble of air (parcel).

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lifted index (UK rarely exceeds -5)

Negative LI numbers are good for thunderstorms because they show that the parcel is significantly warmer than the air around it at altitude, which means that at 5km the air is still bouyant. http://www.weatheronline.co.uk/reports/wxfacts/Thunderstorm-Probability.htm

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Moderate CAPE: convective available potential energy is the energy available to push air vertically up and create those surging bubbles of cumulonimbus clouds.  The higher the CAPE the greater uplift potential (above).  Note it is potential energy and a high CAPE often confounds convective activity if other ingredients are lacking, such as heat or moisture.  19 May had marginally unstable values for the UK of 800j/kg.

In the USA Tornado Alley forecast CAPE values exceed 2,000 j/kg in super cell outbreaks.  CAPE on a skew-t chart is the area between the parcel trajectory and the environment temperature.  In the SE of the UK we often just get “skinny cape”, where rising parcels are only just warmer than the surrounding environmental air, whilst in Tornado Alley “fat cape” is frequent which indicate parcels much warmer than the surrounding air, increasing bouyancy.  http://www.weatheronline.co.uk/reports/wxfacts/CAPE—Convective-Available-Potential-Energy.htm

The colourful charts above from lightningwizard.com show streamlines of wind and air mass equivalent potential temperature.  Streamlines show where the airmass is coming from and, if they converge at the surface, can indicate potential for lift.  Theta e charts show moisture and temperature characteristics of airmasses.  Where there is a contrast of colours between high theta e and convergence at the surface and lower values and divergence of air at altitude this can bring on convective weather. The moisture chart shows that the airmass supplied sufficient moisture to the UK to provide for cloud formation and release latent heat.  Dew points stayed relatively low and this showed moisture (and heat) locally was lacking for bigger storms.  Compare these forecast charts with the actual occurrence of lightning on the day.

curved forecast shows showers

curved forecast shows showers

Forecasts for the day (Euro4) showed typical lines of rainfall accumulation which indicate showers.  The actual radar showed “popcorn convection” (@metmanjames) over a wide area drifting east.

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Recommend following @convectivewx for UK convective forecasts.  Their forecast from 3 days ahead was spot on.

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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.  

The jetstream meanders like a river of air in the high atmosphere. Much of the time the jetstream blasts pretty straight west to east, at other times it loops wildly north to south. This week the jetstream is due to loop wildly, like a meandering river and threatens to form a special weather feature called a “cut-off low”.

How do cut-off lows form and why are they special?

1# A loop of the jetstream descending over the UK is forecast to form a TROUGH of low pressure over Europe this week. The southward limb of the jet directs cool polar air towards Europe and LOW pressure.

2# The loop becomes so sinuous (bendy) that, like a meandering river, the neck is cut-off as the jetstream re-forms to the north.  The LOW to the south becomes “cut-off” as HIGH pressure builds to the north.

3# The cut-off LOW over Europe is left as an “ox-bow” of cool unsettled weather, especially near the centre of the low pressure.

4# The surface warms and CONVECTION occurs through the cool air: air rises forming big convective storms.  These are forecast to migrate from the continent and effect especially the South and SE of England.  Higher pressure should keep the north more settled. 

5# Cut-offs fill gradually as warm air convects aloft, reducing instability.

6# The rest of May looks to have more rain and cool unsettled conditions as LOW pressure dominates.

(pics courtesy of netweather)

A cool LOW pressure tracking across from Greenland and Iceland will move over the UK on Friday and sit over Reigate for the weekend and into much of next week. Atlantic depressions usually have some warm tropical air circulating with cold polar air but not this one!  With high pressure to the west any warm SW air flow is being blocked out, leaving Reigate on a “cold-washing-cycle” with mostly swirling cold polar air circulating around the low. Cool air coupled with low pressure even in this cool Spring weather causes unstable airmasses.  Instability means that big showers can develop as thermals rise from warming surfaces even in the weak Spring sunshine we are experiencing.  Convective thunderstorms are a remote but interesting potential risk for Reigate over the weekend.
Various fronts and troughs will circulate around the LOW pressure which is blocked by a HIGH over Scandinavia and E.Europe and won’t move much. The LOW will bring showery rain, some of it heavy with the possibility of hail and even thunderstorms, especially over the weekend (50% chance of convective thunderstorms on Sunday over Reigate).  Temperatures will never climb much above 7°C and, with gusty winds of over 30mph, it will feel cool, but nothing like what we had earlier this week. Some models predict a return of cold Easterly winds as the LOW slips south over the UK and drags in cold continental air again. There is still uncertainty about this but it will certainly be an unsettled week. Other models see glimmers of spring after around the 23 March as winds turn more SW and a ridge builds over the country bringing drier conditions.  More on this later, keep your fingers crossed!