Detailed convective forecasting discussion

[Bobby]

A convergence zone down the west of the country is focal point for thunderstorm development today. A fair bit of energy around in that area too.

Here is the risk map for today:

[Nick F]

Chance of some heavy showers with thunder across SE England and East Anglia today before the cloud and rain to the NW over Wales and N Midlands moves Eastwards. Cold pool aloft over SE shown by 500mb charts is steepening lapse rates:

Already some heavy showers across Sussex, more are likely to move NE from the Channel and further ones developing inland where insolation occurs. WV imagery currently shows a dry air intrusion across the SE and East Channel which will enhance convection through evaporative cooling and also increase the chance of hail and also thunder. Strong vorticity from the nearby jet over Nern France is likely to create lift combined with general rising motions due to the deepening low currently over South central England expected over East Anglia this afternoon.

0600 WV imagery shows dry air intrusion across SE:

London fcst skew-t shows 300-400 j/kg of CAPE around noon, cloud tops limited perhaps to 350mb, boderline for thunderstorms:

By the afternoon, CAPE is suggested to be limited towards East Anglia, maybe due to increse of cloud and rain curling East from the West, so heavy showers and thunderstorms becoming confined here by the eveming:

[BrickFielder]

Possibility of some storms tomorrow as a mid level trough swings south across the UK.

Winds patterns show some upper air diffluence over the area due to being under the left exit of a small jet streak this should provide some extra lift. Over in the north east current forecasts show some moderate veering of upper winds perhaps enough for mesocyclone formation. 850hpa winds are around 25mph while low level winds are pretty calm. It is marginal but there may just be enough low level speed shear to spawn an isolated tornado.

SkewT's show limited instability limited along with surface temperatures by low cloud.

I am not sure about this and other models suggest temperatures may be higher with more broken cloud. Limiting factor seems to be cloud tops with just heavy showers predicted rather than thunderstorms even with the underestimation of surface temperatures.

Main risk is for some localized flooding again, the finer details will become clearer tomorrow.

[Nick F]

Still the chance of some heavy showers developing over Eastern areas today, more general showery rain for Wales, Southern and central areas of England as shallow low with warm sector trundles SE across Sern UK. Behind this low a cold pool/upper low aloft over Scotland moves South with associated deep upper troughing which will both steepen lapse rates across NE and Eern UK and also create lift:

CAPE looks to be on the meagre side this afternoon down the Eastern side - fcst skew-ts suggest cloud tops look rather limited too, with heavy showers rather than thunderstorms more likely.

However, dry air intrusion is expected to overspread Northern and NErn areas this afternoon looking at WV imagery, so this should enhance convection and increase the threat of hail in any showers here:

Threat of heavy showers may extend to East Anglia and SE England later this evening - as the low retreats Eastwards towards the Low countries, and lapse rates increase with upper cold pooling spreading South.

[BrickFielder]

Complicated pattern today with a strong jetstreak crossing central Ireland during the day and placing England and Wales under the left exit region of the jet streak. This will create some lift and an area of cloud and light rain is expected to form under this diffluence of upper air. I expect this to move quite fast south east ,quicker than the 00Z models suggest. Behind are some troughs which should sweep south east across the country.

These troughs could be a focus for convective developments today.

Forecast SkewT's suggest dewpoints around 9 or 10C which seems consistent with current readings although temperatures of around 13C look a little mean probably due to cloud expected to linger around.

Cloud tops look limited although this is based on forecast upper temperatures which have not been sampled by sounding so we will not really know until the midday soundings come in how accurate it is.

At the moment things look limited to heavy showers mainly in the north east. Some low level convergence is expected

and veering upper level winds mean an isolated tornado or funnel can not be ruled out if instability materialises and cloud top can reach a little higher than forecast.

Lightning Wizard Charts

Main risk will be some isolated heavy showers perhaps causing some localized flooding. It is a fairly dynamic situation so things can change during the day.

[BrickFielder]

Chance of a heavy shower mid morning through to early afternoon tomorrow across England as an occluded front moves through. More heavy showers for Scotland and Northern Ireland as well.

Interesting wind field showing up on the forecasts for northern midlands at the moment but it is still too early to say much more.

[Nick F]

As BF mentoined above, chance for some heavy showers developing late morning as the rain clears, more particularly SE England and East Anglia. Occlusion is expected to move ESE across SE England this afternoon which will enhance shower activity with some insolation as skies clear, also dry air intrusion/dry slot is likely to continue to stream aloft over Sern England, this will steepen lapse rates over a moist boundary layer and create an area of strong vorticity and upper lift.

Cloud tops look rather limited on forecast skew-ts and CAPE fairly meagre, so any thunderstorms are likely to be isolated, GFS suggests some LL shear and 0-3km storm-relative helicity over Sern England this afternoon - so there is a small risk of short-lived funnels/tornadoes with any strong updrafts.

[BrickFielder]

GFS is forecasting an area of vorticity to approach from the south west at mid and upper levels late afternoon.

By nature of positive vorticity advection areas just ahead of this vorticity could be a focus for thunderstorm developments.

Away from the south west a band of heavy showers is currently moving west to east across the country but lack of insolation will tend to limit the severity of any storms devloping here. The main risk is likely to be heavy slow moving rain.

Vertical velocity charts show some mid to upper level lifting should occur.

But there also looks to be some moisture convergence at that level which could make things rather messy and mid level lapse rates drop off rapidly.

Winds look unidirectional and quite light all the way up so no indication of super cell or tornado development although winds may be light enough for some none mesocyclonic weak funnels to form.

My best guess would be that a few storms will develop over higher ground in the south west and southern wales mid afternoon as well as those showers towards the north east picking up as temperatures rise.

[Nick F]

00z Camborne and Herstmonceux ascents suggest a very moist profile upto 600-700mb, this airmass is likely to become readily unstable to surface heating this morning and afternoon.

00zCamborne ascent:

... additional forcing of this moist air is likely from strong vorticity generated by a strong 150-160kt jet over the Channel/N France and also a short-wave trough rotating around the upper low over Nern UK, dry air intrusion across Southern counties associated with the jet stream may additionally enhance convection, bring strong upper winds towards the surface as wind gusts and increase hail threat:

Several 100j/kg of CAPE are likely across Sern and Ern areas of the UK this afternoon to tap into, even if GFS are overestimating CAPE values:

Strong speed shear across Sern counties may augment cells into bowing multicells, otherwise poorly organised cells elsewhere, but with very moist profiles, heavy downpours with high totals in a short space of time may cause localised flooding. Lightning wizard suggests little in the way of LL shear and SR Helicity over England and Wales, so the threat of tornadoes looks low, though strong updrafts are likely to increase the threat of funnels where vorticity exists.

Widespread heavy showers are likely over much of the UK, thunderstorms most likely across central, southern and eastern England this afternoon and evening IMO.

Estofex have a level-1 threat for Sern UK - but their risk of flash flooding rather than funnel/tornado risk is more likely to be realised IMO.

[BrickFielder]

Satellite pictures show a broad band of descending stratospheric air north of the jet stream. This will cover much of the southern half of England and more than the forecasts suggest. Descending air at higher levels is not a particularly good environment for air to rise up to form thunderstorms which is why storms in the south are likely to be more isolated.

06Z Larkhill ascent show dry cold air at upper levels which makes things pretty unstable for temperatures around 17 -18C. I also not that there is some directional shear in the lower levels and perhaps a touch more speed shear at upper levels. Not an envronment condusive to the formation of super cells and associated tornadoes.

As Nick suggest bowing multicells seem a possibility across southern areas and these should move at a fair pace, but only if shear picks up a little. The northern and southern most cells of which may be moderately severe giving a risk of strongly gusting winds and hail with perhaps a very low risk of even a tornado.

Further north upper air will be moister and slightly warmer but not descending. Here we should expect some high precipitation slow moving storms. A upper trough moving slowly west to east is giving some heavy rain across parts of the north midlands and southern northern areas. We should expect further storms after this with things being rather messy and congested. There may be a chance of some non mesocyclonic weak funnels along east coasts if some sea breezes can pick up ,particularly late afternoon as cloud bases come a little lower.

Towards north east scotland late afternoon a low level convergence zone develops and this could be a focus for some more weak funnel development although it is not till late on that cloud bases start to descend below 500 meters.

Main risk is likely to be localized flooding across parts of the midlands and northern England. Slight risk of gusting winds and hail across southern areas and the most electrified storms will probably be in the south but these are likely to be more isolated. Slight risk of Weak funnels/spouts (probably not touching down) for east anglia, lincolnshire and north east scotland late afternoon.

[BrickFielder]

Surface low just to the north of Northern Ireland looks set to cross the north of Uk and fade. Swinging round with it will be an occluded front forecast to cross the midlands and northern England. Satellite pictures suggest that the low is a little further north than expected and has more structure still than expected.

Winds are expected to pick up quite strongly in many areas with the possible exception of an area near the borders of scotland where the surface low crosses. This means that non mesocyclonic funnels are unlikely away from the borders and northern England. Mid level shear is poor as well so super cells are unlikely which means tornadoes are very unlikely.

Rather messy and overcast conditions look probable across the Midlands and Wales and these are likely to keep temperatures and instability down a little here. Some heavier showers are expected across this region with a risk of localized flooding.

Further south skies should be a little clearer giving more isolated and stronger storms especially with the increased shear although from midday onwards negative vorticity advection should begin to kill activity off.

Another round of storms for many , some heavy downpours but nothing to suggest particularly severe storms.

[Nick F]

Upper cold pool (seen on 500mb chart below) shifts towards SE UK today, this will tend to bring the steepest lapse rates towards the Eastern/South-Eastern side of England today, this is where highest MLCAPE is indicated this afternoon:

00z Camborne and Herstmonceux ascents were unstable and suggest airmass over Sern England will be readily unstable to surface heating today, some heavy showers with thunder have affected the SW and Channel coasts already this morning:

UKMO shows an upper trough moving SE across Midlands and Eastern England this afternoon, GFS also has a slightly differently alligned sharpening upper trough moving East - this upper trough will likely enhance convection together with insolation, temps reaching 20-21C in the SE today. Dry air entrainment within the trough will perhaps increase the threat of hail too.

CAPE charts and Forecast skew-ts suggest the Midlands/Central-S England eastwards most likely to see cloud tops high enough for thunderstorms. Heavy showers and thunderstorms look to be slow moving, so some high rainfall totals in a short space of time may lead to localised flooding. A lack of deep layer and low-level wind shear over Eastern England suggests storms will unlikely have tornadoes, though local wind convergence from outflow boundaries in the light winds may induce a funnel in the strongest updrafts.

More general area of heavy rain arriving across SW England, Wales and the West this evening as a surface low with wrap around occlusion crosses SE from Ireland.

[BrickFielder]

Complicated pattern today with a number of interesting features. Upper levels show an area or vorticity moving west to east with it currently placed over the irish sea. Satellite pictures suggest this is moving a little faster eastwards than forecast.This I think means NVA is likely to limit cloud tops increasingly from the west during the day.

At mid levels a marked trough currently forecast over north wales and cumbria will weaken and disappear. This will initially give some extra impetus to developing storms but its effects will weaken.

Precipitatable water forecasts show a moist tongue slowly moving west to east and I think this is likely to be the focus for any developing storms.

One thing that does interest me is an area of wind divergence from 850hPa up to 500hPa which develops over northern England. Although winds are quite light this will give extra lift in these areas and these are perhaps areas with an increased risk of hail.

Cloudbases are forecast to be quite low but maybe a little high for non mesocyclonic funnels.

Forecast SkewT's show some moderate directional shear which when combined with high cape values could give an isolated tornado with areas north east of London perhaps favoured. None mesocyclonic tornadoes are more likely with low level convergence likely across east anglia mid afternoon.

Steepest 850-500hPa lapse rates will be concentrated in the east and this is where storms are most likely.

[BrickFielder]

Overnight soundings from France suggest that 23C required for surface convection. This rather limits any thunderstorms to the warm sector with the focus being northern France. If I had to pick a place for a thunderstorm then it would probably be western Northern Ireland late afternoon where there is a slight risk of non mesocyclonic tornadoes developing. Second choice would be east anglia late afternoon where there is a very small risk of mesocyclonic tornadoes.

Models have done pretty well with the broadscale events today but have made a bit of a mess of the small scale details. I am interested in a surface convergence of winds which will move north across the UK. Ahead of this feature winds will be from the east which will be in direct contrast to winds from the south at upper levels. Positioning of this feature and any clearance of the skies may give a slight tornado risk. I am not sure the models have got these details right and indicators suggest that there will be only a short period where convection could be surface based.

Across the south east there is much more Cape available although there is no real mechanism to trigger activity. Although this does not seem to be a problem currently around the isle of wight which I am guessing is triggered by an area of 850hPa vorticity movinhg north east.

There is a suggestion that a triple point will cross this area but as yet it is not clear where this will form.

There is quite a high risk that a super cell will form late on over north western France which is likely to travel north west across the south eastern corner of the UK during the night. Models do not agree well on where it will be and it may be slightly elevated when it reaches the UK. Some very strong upper winds could make things interesting.

Watch the radar and satellite pictures for a better picture.

[BrickFielder]

An area of mid level vorticity moves up from the south west ahead of which will be sparked some storms.

Forecast SkewTs show a pretty unstable atmosphere without much inhibition. Iguess things could be messy convection wise ahead of the trough making storms a little weaker across central areas before picking up again late on in more northern areas. I have noticed that a small 850hPa jetstreak across the south east has been consistently forecast which provides adequate low level speed shear for tornados to form. Directional shear and helicity looks pretty poor though so we may have to look to storm splitting for mesocyclone formation.

Lightning wizard charts are sort of picking up on this.

I am not convinced and much will depend on the actual location of the 850hpa jetstreak. Too far south and low level winds are too high ,too far north and mid and upper level winds are too light. Much more realistic is the threat of non mesocyclonic tornadoes across Northern Ireland and parts of Scotland.

Again today I feel that the models are always a little behind the action and small changes will make all the difference so watch the radar and satellite for up to date news.

Steerage winds seem quite strong so storms should have a fair pace reducing the risk of localized flooding today. However cloud tops should be pretty high increasing the risk of lightning and hail. Strong gusting winds could also be a trheat especially in more souther counties.

[BrickFielder]

Risk of storms again today but more concentrated in the north and east. I think GFS's suggestion that all the moistire has moved north and east is probably not correct looking at current dew point readings.

However these areas are likely to be a focus of storms due to low level convergence.

If there is a surprise package there it is likely to be across the north midlands late afternoon.

Winds through the atmosphere do tend to turn and there is a touch of an 850hPa jet confirmed from the Larkhill 06Z ascent. Light winds at 500hPa tend to suggest super cells are unlikely although can not be ruled out. Updraft and downdraft seperation looks largely unlikely.

It was interesting that Estofex picked up on rather weak low-level instability as this showed up quite well on the cells I saw yesterday. Mammatus drooping from mid and upper levels , good strusture , but rather weak low level inflow tails. Warmer temperatures today may combat this a bit.

Non mesocyclonic tornadoes (weak not reaching the surface) are possible again especially where convergence zones occur.

further storm risks tomorrow but probably in more western areas where lapse rates are steepest. Again a risk on friday with the risk of an import early morning in the south east and perhaps some fireworks as as rain band moves north.

[SMU]

Good Morning All-

I hope the mods dont mind but ive opened a thread to discuss the damage that could be caused by the Shallow Low pressure system fast approaching from the SE-

From the radar returns at 100 & 130 am the echoes are beginning to develop into some distinct deep well organised circulations & I would expect that thunder & lightning could become widely reported in the early hours-

The first crosses beginning to appear in the SE-

http://meteocentre.com/lightning/map_sfuk....e=0〈=en

Combined with the thunder & lightning is the threat of the heaviest rain of the Summer yet- Where as the MSC just fell apart last week this event will hold together a lot better apon landfall & VERY heavy rain is likely-

24 Hour estimates from 00z is around the 60/70mms in some areas- thats up towards 3 inches--

http://www.wetterzentrale.de/pics/Rmgfs24sum.gif

With already sodden ground in some areas the areas prone to flooding could be in for a worrying 24 hours-

Discuss here:

Edited July 20, 2007 by Steve Murr

[SMU]

130 radar update

slight jog east of the main Precip-

London could be in the direct firing line.

S

[londonsnow]

130 radar update

slight jog east of the main Precip-

London could be in the direct firing line.

S

intersting for us steve, but potentially quite a serious rain event in store

[pottyprof]

Steve. We already have a thread in the storms area but if this was to discuss the technical aspects only, then I'm sure this would be great. :o

Going to move it into the storms area though.

[tinybill]

morning all still dry here not for long radar its geting itself together plus there a huge thunder storm,in the north of france not sure if that got our name on it yet, will be back this afternoon hopefully before the proper heavy rain kicks in here in the east

Edited July 20, 2007 by tinybill

[johnholmes]

see my ideas for Friday blog, updated just now.

[BrickFielder]

Aside from the heavy rain there is likely to be some storms brewing late afternoon across central southern Uk today.

Satelite pictures show a complicated pattern of short wave troughs crossing the area and this is likely to be the trigger for storm development.

Winds tend to be light going up but in places swing wildly in direction.

Signs of a surface low and convergence later as well although not across the region of best instability (Cape).

Check out the Estofex forecast for further details and keep an eye on the radar and satellite pictures.

Estofex Forecast

[Bobby]

There is a convergence zone developing across much of southern Britain this afternoon, it may be triggering some of the storms across the south east at the moment. Could be some very intense slow moving downpours from it, meaning local fooding is possible.

[BLAST FROM THE PAST]

130 radar update

slight jog east of the main Precip-

London could be in the direct firing line.

S

Hi Steve

We were!!!!

Fred