Detailed convective forecasting discussion

[Nick F]

Chance of heavy downpours with thunder and lightning today over central-S/SE England and East Anglia today. 00z Cambourne ascent suggested that the airmass over Sern England is already unstable - and will become increasingly so with insolation, WV imagery shows dry air intrusion overspreading Sern areas - so we are seeing showers already over Sern counties.

MLCAPE 18z:

With breaks in the cloud South and SE of the Midlands, surface heating will develop further heavy downpours - dry air intrusion present moving slowly East will allow thunderstorms with hail to develop, convection likely to be helped/organised by a strong upper trough shown on 300mb charts and also surface wind convergence:

Wind convergence will possibly allow enough vorticity to stretch the stronger updrafts of convective cells into short-lived funnels, otherwise Low-level and deep layer wind shear looks too limited today for tornadoes.

[stormchaser1]

Morning everyone

Just a quick question [if someone can answer this one i will be much appreciated}

Question is ,

Why is`nt the Cin and K index hardly mentioned ??

ive been reading about Cin and the K index and have found that these readings are near enough the same as CAPE,

From what i have read the K index was composed for forecasting air mass thunderstorms or thunderstorms with no Dynamic triggering mechanism.

hERE ARE A FEW INDICE WHICH IVE FOUND

0 TO 15= 20% PROBABILITY

21 TO 25= 20% TO 40% PROBABILITY

The higher the K index the best chance for a storm 40 = 100% probability

Skew charts this morning are showing for my area 23 which there is a 20 to 40% probability

Now the CAPE is only showing 53 which to my Knowledge is very light rain

CIN = Convection Inhibition

This can be measured as how unlikely thunderstorm development is , or the energy needed for thunderstorms to develop . also the Cin can be signified as the "CAP" if the cap is weak which this morning Skew charts are suggesting it is , its showing 0 for this area it may be easily eroded early in the morning by heating and the energy expanded to form a Cumulus field. If the CAP is too strong ie 200J /kg the energy might not be overcome and no development may occur. CIN between 50 and 150 j/kg is favourable

Can someone help try and understand the readings for Cin and K index please

Nigel

[Nick F]

Yes, the formula used for CIN is very similar to the CAPE calculation. The difference is where to find CIN and CAPE on a sounding, CIN is the 'negative' area or stable layer of air on a sounding between the surface and LFC (Level of free convection) and CIN calculates the amount of work required to lift a parcel through a layer of air that is warmer than the parcel itself, this calculation is measured (like CAPE) in j/kg. The higher the CIN value, the more stable the the layer of air is between the surface and LFC. For a parcel of air to break the negatively buoyant or stable layer of air - a forcing mechanism is usually required. Cape on the other hand measures the cumulative buoyant energy (positive area on sounding) between the LFC and the EL (Equilibrium level) towards the top of the troposphere, above the EL a parcel will become negatively bouyant and decelerate in upward motion.

The K index is a measure of thunderstorm potential based on vertical temperature lapse rate, moisture content of the the lower atmosphere, and the vertical extent of the moist layer. Unlike CAPE though the K index is arithmetically derived and therefore doesn't require to be calculated from a plotted sounding. As the K index works on the 850mb dew point to provide information on the moisture in the lower atmosphere and uses the 700mb temperature dew point depression to verify the vertical extent of the moist layer, it tends to work better for forecasting non-severe pulse type convection/tunderstorms and also forecasting heavy rains, so it has its limitations compared to CAPE with regards to convection measured at different levels i.e. SBCAPE and MLCAPE.

[stormchaser1]

Morning Nick

Thankyou for that , much appreciated

Nigel

[Nick F]

Chance of heavy showers with hail and thunder developing over Scotland perhaps the far North of England this afternoon. Upper low/cold pool over Ern Scotland will steepen lapse rates over these areas and allow general ascent, strong upper trough rotating around upper low will create lift along with occlusion.

Forecast skew-ts over Scotland suggest several 100 j/kg of CAPE by this evening, cloud tops towards 300mb suggest thunderstorms are likely. Winds through the troposphere generally light with little/no change in direction, so weak low-level and deep layer shear suggest non-severe pulse-type storms will develop from sfc heating.

Wind charts suggest some low-level convergence possible over Ern Scotland this afternoon/evening ... so low-level vorticity may allow for funnel development of any strong updrafts.

[BrickFielder]

Some interesting parameters showing up for an upper trough following the rain band on thursday. Some moderate wind shear and helicity icnreases the risk of some moderately severe storms developing.

More storms tomorrow for Northern Ireland and Scotland.

[Supacell]

Some interesting parameters showing up for an upper trough following the rain band on thursday. Some moderate wind shear and helicity icnreases the risk of some moderately severe storms developing.

Hi, does this forecast for tomorrow still stand?

[johnholmes]

what seems likely to happen is that, put simply, a 'wave' development, to the south of the main low centre, will run quite quickly ne and Occlude as it does so, Its this which is likely to give the increased rainfall.

hope that helps

[BrickFielder]

As john mentions there is an occluding front which will cross the UK giving some moderate rainfall amounts. Behind this may well be an upper to mid level trough.

Certainly we have some steepening mid and upper level lapse rates giving a fairly unstable environment.

Cloud tops should be cold and cloud bases fairly low.

Forecast SkewT's show quite a lot of instability (in UK terms) with drier air coming in at mid levels. This should act to enhance any updraft regions of storms. What is also noticeable is a lot of wind speed shear which will help to seperate the updraft and downdraft meaning any storms should be longer lived than a normal pulse cell. The environment is not quite right for supercells and tornadoes although they can not be ruled out if bowing multicell segments develop.

The main risk is likely to be from localized flooding and strongly gusting winds.

Convective precipitatin charts do not show much in the way of rain but this is likely to be concentrated into small areas.

As ever with storms we will not really get a clear picture untill tomorrow morning. All too often some small parameter can change and the likihood of storms diminishes. I am a little confused about the modelling of the trough and the triggering of any storms so I expect the forecast to change somewhat as we get closer to tomorrow.

Edited July 25, 2007 by BrickFielder

[johnholmes]

to add to Bricks' post and my earlier one.

This is the sat picc for this morning

notice the area of cloud out in the Atlantic. This is the area which will 'spawn' if that is the right description, both the occluding system and the trough referred to by Brick

below is th 06z Met O chart showing various frontal systems strung out along the Atlantic beneath the main surface low.

and below is the Met O version of how this will develop by 06z tomorrow/Thursday.

It seems as good an idea of what is likely to happen as any other.

So there is the occluding frontal system and the trough behind it which together are the reason we issued an Alert a couple of hours ago.

As often is the case with systems approaching from the south west, albeit from around 47-50N in this instance, the fine detail is probably not going to be certain until its almost happened. A case of us, Met O even more so, and of course the various emergency control centres, radar watching in the early hours and through tomorrow.

Edited July 25, 2007 by johnholmes

[VillagePlank]

Morning everyone

Just a quick question [if someone can answer this one i will be much appreciated}

Question is ,

Why is`nt the Cin and K index hardly mentioned ??

ive been reading about Cin and the K index and have found that these readings are near enough the same as CAPE,

From what i have read the K index was composed for forecasting air mass thunderstorms or thunderstorms with no Dynamic triggering mechanism.

hERE ARE A FEW INDICE WHICH IVE FOUND

0 TO 15= 20% PROBABILITY

21 TO 25= 20% TO 40% PROBABILITY

The higher the K index the best chance for a storm 40 = 100% probability

Skew charts this morning are showing for my area 23 which there is a 20 to 40% probability

Now the CAPE is only showing 53 which to my Knowledge is very light rain

CIN = Convection Inhibition

This can be measured as how unlikely thunderstorm development is , or the energy needed for thunderstorms to develop . also the Cin can be signified as the "CAP" if the cap is weak which this morning Skew charts are suggesting it is , its showing 0 for this area it may be easily eroded early in the morning by heating and the energy expanded to form a Cumulus field. If the CAP is too strong ie 200J /kg the energy might not be overcome and no development may occur. CIN between 50 and 150 j/kg is favourable

Can someone help try and understand the readings for Cin and K index please

Nigel

Hi Nigel,

K Index

0-15 No thunderstorms

18-19 Thunderstorms unlikely

20-25 Isolated thunderstorms

26-30 Widely scattered thunderstorms

30-35 Numerous thunderstorms

36-39 Thunderstorms very likely

40+ 100% chance of thunderstorms

Total Totals (TT)

43 Thunderstorms unlikely

43-44 Isolated thunderstorms

45-46 Scattered thunderstorms

47-48 Scattered thunderstorms/ isolated severe

49-50 Scattered t-storms/few severe/isolated tornadoes

51-52 Scattered-numerous t-storms/few-scattered severe/isolated tornadoes

53-55 Numerous thunderstorms/ scattered tornadoes

56+ You don't want to know

I reckon, though, the best way of forecasting thunderstorms is to (i) Assume that there will be one, (ii) Look for reasons why there won't be one.

If you can come up with one reason then the thunderstorm is unlikely.

[johnholmes]

I'm old fashioned I still prefer the t-phi or skew-t, after all its that basic sounding from which all these terms come from!

for instance

CIN CAP=inversion to oldies like me

so all you need to do is find the temperature to break that.

[VillagePlank]

What would be nice is to have a single chart which shows the ratio between wind at different heights (lower/upper) For instance in Rochester tomorrow the ratio is something like this:

00z 0.34

03z 0.34

06z 0.38

09z 0.39

12z 0.39

15z 0.40

18z 0.38

21z 0.34

It'd be a nice objective shear scale!

[johnholmes]

nice of you to volunteer to make one to give to Paul to use in Extra!

[Nick F]

Heavy showers with hail and thunder are likely to develop over SW and central S England, S Wales during the morning/afternoon - these perhaps propagating across the SE this evening before slowly dying out late evening/overnight. Upper long-wave trough is disrupting across the UK with a cut-off upper low and cold pool breaking South to be over the SW and NW France this afternoon, this upper cold pool will steepen lapse rates sufficiently for convection to initiate with help of surface heating.

Already there are showers approaching Cornwall and Devon, the 00z Camborne ascent suggested that the airmass over the SW and central S England will become readily unstable to sfc heating this morning:

GFS suggests a sea breeze front developing along the South coast this afternoon against the general light NW flow, a convergence line is likely to set up somewhere along Sern counties and could be a focus for heavy shower and storm development, large upper trough aloft is likely to further help lift and associated dry air intrusion will increase chance of hail and strong wind gusts in any cells:

90-100 kt jet winds aloft towards the SE is likely to bring strong speed shear towards the SE and may organise any convection into multicell clusters later this afternoon/evening, otherwise light upper winds further West suggest pulse type showers/storms for the SW/Central S England. The presence of the wind convergence zone suggests that low-level vorticity may be enough to stretch strong updrafts into funnels or weak spouts over Sern counties this afternoon.

[Nick F]

Convective/Severe weather potential Wednesday 15th August

SW England, S Wales, Central Southern England, Midlands, Lincolnshire, East Anglia 12z-18z

Upper trough currently over Nern England and collocated occlusion with cold front characteristics will slowly move SE during the day, ahead of trough occlusion over much of Central, SW, S and E England is rather warm and moist Tm air with sfc dew points of 15-16C currently over Sern England.

A lobe of strong vorticity advection aloft along the forward side of the upper trough is likely to induce strong synoptic lift over the areas mentioned above, steep lapse rates along and ahead of the trough over the warm moist air are likely to yield 200-350 j/kg of CAPE, cloud tops should be sufficiently high for thunderstorms to develop along the occlusion.

Dry air intrusion aloft of the area likely to see t-storms develop is likely to bring the threat of hail and strong wind gusts in any storms:

A powerful jet streak moving NE across Sern England will bring strong vertical shear, 0-6km deep layer shear of up to 70 knts, suggest multicells will develop with perhaps bowing characteristics:

The combination of a backed surface flow ahead of the trough and a wind convergence line suggested look to produce strong low-level shear and vorticity enough to warrant the risk of a tornado or two,in the areas mentioned, more particularly on the Sern side of the area likely to see t-storms:

The far SE/London may stay dry this afternoon, the line of showers and storms moving erratically SE may weaken by the time they reach the far SE this evening, though chance that the cells may stay active.