: Convective Outlook Mon 22nd July 2013 -

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Convective Outlook Mon 22nd July 2013

#1 User is offline   Tony Gilbert 

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Posted 21 July 2013 - 14:31

Posted 3pm Sunday

AM Period; Slight Risk of Elevated Thundery Showers 00Z-12Z Dorset northward through to NW Wales, and SE UK (also Channel Islands).

PM Period; Slight Risk of Thundery Showers UK & Ireland. Moderate Risk of SB Tstorms NW Ireland, Wales/ NW Midlands through to SW Scotland 12Z-00Z

A rather mixed bag of potential events hence divided into am and pm periods. Realistically there is a general risk of thunder activity any where across the UK and Ireland Mon. Prime risk for severe weather pm period. High pressure moves slowly eastward allowing for marginal though still significant influence of the upper trough to the west. A surface low pressure regime develops slowly through the afternoon across UK with elevated convection turning surface based. My main concern for this outlook is available surface moisture. There would now appear to be less available than earlier predictions. Though it would appear still enough for isolated Tstorms. Upper shear rather marginal but still influential; Whilst only partial separation of updraft to downdraft will likely occur the fact that the storms look to move in a steady transit north suggest that favourable warm inflow can be sustained from the NE sector of any building cell. So in short whilst storms might look rather messy they are likely to adopt a cyclic life span. Cyclic storms can develop tornadoes if the low level shear is in place but this does not look to be the case here ATM. Whilst a small number of funnel clouds are likely along any sea breeze convergence zones I do not see the potential for fully fledge tornado development Prime risk looks to be cg's on a local scale and quite prevalent nearer the stronger CAPE's. In fact lightning can on occasions be more deadly than others. This could well be the case here!
The strongest storm activity looks to occur around Herefs, Shrop and Worc counties

Overall confidence is high for potential results but low for locating regions to be affected!

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This post has been edited by Tony Gilbert: 21 July 2013 - 14:56

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#2 User is offline   Uskys 

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Posted 21 July 2013 - 17:45

A few GFS 12z charts for 15z tomorrow.

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  • Attached Image: precipital water 22nd.png
  • Attached Image: cloud top temp 22nd.png

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#3 User is offline   PJB 

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Posted 21 July 2013 - 18:12

PJB's Convective Outlook - Monday 22nd July (also includes overnight into Tuesday)


Models - HIRLAM, GFS, NAE, UKMO GM and ECMWF


Main Risks -

1. Scattered Thunderstorms and Heavy Showers 00Z to 09Z - SW England and CS England (Elevated Destablisation)
2. Home GRown Storms, Monday 14Z to 23Z, Midlands, SE England (North of London). Cambridge, Midlands, E England & NE England
3. Destabilisation of the 850mb Elevated Plume by differential Thermal Advection 18Z Mon to 06Z Tues - CS England, SE England, Midlands, E England, E Anglia

A complicated forecast largely because of the various different elements which have to come into being to make this forecast a success. However good agreement from the Hirlam, NAE and 00Z ECMWF for some confidence.

1. ) Thunderstorms over France have largely fired due to insolation and will not migate over the English Channel. However Hirlam and NAE indicate some instability assoc with a weak shortwave now approaching the Isles of Scilly (See WV Imgery) this shortwave moves NE and may provide enough cooling aloft and combined with continued 850mb heating to produce some release of mid level instability. Any storms firing here will not be routed in the B/Layer and will instead be elevated. Main Risk across E Devon, Dorset. Somerset and some other parts of CS England by 06Z. These fading away as the morning continues

2.) Surface based storms on Monday. High levels of heat on Monday - Expected to exceeed 30C Widely over the SE tomorrow and 32-35C over London and the SE and imported higher DPs making for some very juicy air tomorrow. Mositure is not overly extensive, but with some forced ascent from Sea Breezes we can expect some home grown convection to be sparked inland over the SE and Central England and then moving North. Very heavy rain possible in any localised storms. More people staying dry than seeing them but where they do occur some locally very heavy rain falling. Main Risks. M3/M4 northeast around and to the West of London and then NE through E Midlands, E England and into the NE England by evening . Lightning and Hail possible.

3.) Destabilisation of the high Theta -W Plume tomorrow night. Greater than 18C WBPT moves Northeast tonight and tomorrow and it will be within this zone which destabilises completely tomorrow night as the sharp upper trough moves NE tomorrow night. Storms will develop across E Devon, English Channel, Channel Islands and move NE becoming more widespread across CS England, Central England, London and the SE. Storms have the ability to produce lightning, and whilst initially will be elevated, have the ability to re-root to the b'l as further destabilisation occurs. NAE and HIrlam hint at a MCS or Mesoscale Convective System moving NE across CS England and into London and the SE overnight with potentially for locally very heavy rain (>25mm in a few hours possible) Even the Crude NAE (by Mesoscale standards has 15-25mm across CS England) UKV would suggest even higher amounts locally. Hail. Gusts, Flooding and widespread lightning possible as the system develops and moves NE. Most likely from Somerset and Devon to the Midlands and Lincs SE wards. (Within the 18/19C WBPT)

By Tuesday morning storms still rumbling across N and E England with further storms developing across E and N England during the day.

Issued PJB

19.12

21.7.2013
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#4 User is offline   Sam Jowett 

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Posted 21 July 2013 - 20:55

Interesting to note the STP (significant tornado parameter) and SCP (supercell composite parameter) values being forecast on Lightning Wizard for tomorrow's surface based activity. Important not to take too much stock in indices like these, but I'm curious why the SCP is so high in particular, given the lack of decent wind shear. I'd have expected updraught and downdraught separation to be quite limited, although the greater altitude EL (equilibrium level) creates more scope than we often have. The 0-1km wind veer for this neck of the woods looks to be around 90º, with around 120º of 0-3km wind veer, so perhaps this is creating the scope for up/downdraught separation and the potential for storms to go severe? Surface convergence looks to set up in a line from London to Liverpool later tomorrow... if this convergence is able to coincide with the easterly sea breeze front anywhere, I'd expect strongest conditions for a storm to go severe... perhaps E Leics, S Lincs, Cambs? 36mm of precipitable water available too... so expect flash flooding!
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#5 User is offline   Tony Gilbert 

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Posted 21 July 2013 - 22:29

As expected, the models are still shifting around somewhat. The lightning wizard tornado potential is based on a formula. This formula is a good combination of indices but nevertheless should not be relied upon to make a risk assessment. There are just too many other influential factors at play. We might estimate the tornado risk for Monday as slight based on the high level of CAPE and convergence. So in short looks like any tornado risk based on the now forecasting would be a brief event along the convergence boundary and probably nowhere else. Updrafts might be strong but are unlikely to be sustained for any length of time hence deep layer vorticity or mid level mesocyclones are unlikely to develop.Dangerous lightning looks to be a high risk issue.For directional shear at low levels to create some displacement of updraft to downdraft we would probably need to see the greater veer between 800mb and 700mb.

This post has been edited by Tony Gilbert: 21 July 2013 - 22:38

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#6 User is offline   John Mason 

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Posted 22 July 2013 - 05:33

Just looking around the soundings - some juicy air about for sure - for example, PWAT at Camborne for the 00z ascent was 27, Nottingham and Herstmonceaux both report 30, while down in the deep west at Valencia it is a very respectable 37. The latter would translate into a veritable deluge if a storm developed within the airmass!
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#7 User is offline   Tony Gilbert 

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Posted 22 July 2013 - 07:25

UPDATE 8.24am Mon

As per earlier forecast with further risk of elevated storms moving north from the continent effect all eastern regions over night.
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#8 User is offline   smartie 

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Posted 22 July 2013 - 08:05

Experimental 10km WRF Ensemble Init GFS 20130721 18Z run. 24 hr accppn to 00Z 23 July.
Awaiiting deterministic 1.5km run.
D
Attached Image: 20130722_ukens10_accppn.png
Attached Image: 20130722_ensmaxmn_accppn.png
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#9 User is offline   PJB 

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Posted 22 July 2013 - 08:34

Just to add from my side. I have nothing to update this morning, my forecast remains as above. Overnight guidance from Hirlam, NAE and ECMWF largely agrees and is followed with some confidence. Expect large scale destabilisation of the 850mb plume overnight.
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#10 User is offline   Matt D 

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Posted 22 July 2013 - 09:29

View Postsmartie, on 22 July 2013 - 08:05, said:

Experimental 10km WRF Ensemble Init GFS 20130721 18Z run. 24 hr accppn to 00Z 23 July.
Awaiiting deterministic 1.5km run.
D
Attachment 20130722_ukens10_accppn.png
Attachment 20130722_ensmaxmn_accppn.png


Hi David - thanks for posting these up. Just wondered what your opinion is on WRF's capability to simulate thunderstorm development from elevated plume situations such as tonight? I have seen WRF output from a night earlier this year and it failed to produce CAPE and ppn over the English Channel and Isle of Wight, when indeed there were widespread thunderstorms breaking out from the 900mb level with frequent lightning and heavy rain.

On the other hand, it often seems to excel at marginal surface based situations (e.g. from what I saw, it did very well and simulating the isolated TS that formed to the N and W of London last Wed).
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#11 User is offline   smartie 

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Posted 22 July 2013 - 09:44

View PostMatt D, on 22 July 2013 - 09:29, said:

Hi David - thanks for posting these up. Just wondered what your opinion is on WRF's capability to simulate thunderstorm development from elevated plume situations such as tonight? I have seen WRF output from a night earlier this year and it failed to produce CAPE and ppn over the English Channel and Isle of Wight, when indeed there were widespread thunderstorms breaking out from the 900mb level with frequent lightning and heavy rain.

On the other hand, it often seems to excel at marginal surface based situations (e.g. from what I saw, it did very well and simulating the isolated TS that formed to the N and W of London last Wed).

Matt - cross-posted from the chat thread.
The posted ensemble output posted only goes to 00Z Tuesday so is limited.
The ensemble is rather simple ATM - future possible enhancements possible include adding a stochastic element to some of the runs. The most recent version of WRF includes stochastic back scatter (SKEB) for vorticity which can aid in accounting for MCS generated PV anomalies- a known source of model error in these situations.

WRF (in common with many models) may not handle the elevated well, or at all. If you compare model vertical profiles with RAOBs you''ll see the model lacks many details of the EMLs and real vertical structure- these may be crucial to convection aloft (and within the PBL). This is partly due to lack of vertical resolution, but also lack of detail in the initial conditions - the GFS driving model has 62 levels upto 1 hPa. The former is easily remedied (with more computing power) but the latter is more difficult- you can increase the levels in your driving model, assimilate more vertical level obs- profilers, RAOBS etc. But this is difficult on a small-scale and in real time.

I'll run a grid with enhanced vertical levels for tonight and tomorrow later today.
D
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#12 User is offline   smartie 

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Posted 22 July 2013 - 10:24

As an example here I compare this morning's Camborne ascent with a model sounding from the ensemble control run. The model only has 35 levels, this has been shown to produce reasonable results for ensembles in the US, the overall vertical structure is depicted quite well , but stratification apparent in the ascent is not reproduced eg pronounced dry layers at 800 and 500 hPa.
D
Attached Image: 201307220600_skewt_camb03808.pngAttached Image: 2013072200.03808.skewt.gif
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#13 User is offline   Matt D 

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Posted 22 July 2013 - 10:41

Hi David, thanks for that, very interesting. As you say the subtleties in the structure of the Elevated Mixed Layer that is not always resolved well in model output can sometimes make the difference (especially with regards to dry mid-level air).
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#14 User is offline   smartie 

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Posted 22 July 2013 - 10:56

Accumulated ppn at 18Z from a 1.5km/45 level run init with the 18Z GFS 21 July.
Must say I'm not too convinced by the small totals here in central England , as the model does develop showers of some sort- perhaps it's a bit late.
No graupel at surface.
D
Attached Image: 20130722uk1.5d021800d02_accppn.png
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#15 User is offline   smartie 

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Posted 22 July 2013 - 12:09

View Postsmartie, on 22 July 2013 - 10:56, said:

Accumulated ppn at 18Z from a 1.5km/45 level run init with the 18Z GFS 21 July.
Must say I'm not too convinced by the small totals here in central England , as the model does develop showers of some sort- perhaps it's a bit late.
No graupel at surface.
D
Attachment 20130722uk1.5d021800d02_accppn.png

I see that the UKMO have now (12Z) issued a yellow warning for Cent England/East Midlands. The model output there doesn't develop much convection prior to 18h and then it is rather isolated. It could be both runs are developing convection rather late. they agree on ppn over N Wales.
D
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#16 User is offline   PJB 

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Posted 22 July 2013 - 12:32

The 00Z ECMWF developed a lot of precip tomorrow and esp tomorrow evening across the Whole of E England with some Torrential rates in parts of SE Scotland.
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#17 User is offline   vince 

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Posted 22 July 2013 - 12:53

View PostPJB, on 22 July 2013 - 12:32, said:

The 00Z ECMWF developed a lot of precip tomorrow and esp tomorrow evening across the Whole of E England with some Torrential rates in parts of SE Scotland.



and there was me hoping that was meant to read SE England :P . fat chance as usual
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#18 User is offline   PJB 

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Posted 22 July 2013 - 17:17

Storms are now firing in the 19C WBPT plume from Dorset and Wilts and into HAnts and will move NE wards this evening. Further destabilisation is indicated over the English Channel overnight as the Shortwave to the SW of the UK moves NE. Heavy Rain and Thunderstorms develop around Midnight on the Hirlam over the Central Channel and move NE into CS England and then across London and the SE. GFS far less keen. Hirlam very keen with some torrential totals over CS and Midlands on its hourly frames.
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#19 User is offline   Tony Gilbert 

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Posted 22 July 2013 - 17:48

Worth noting that the 12Z cambs skew-t showed a solid cap possible inversion at 850mb. Steering winds look to suggest these conditions could be representative of the lack of 'Surface Based' storms so far over Wales and west Midlands.

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#20 User is offline   John Mason 

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Posted 22 July 2013 - 18:30

Tony - that fits in with obs here. Cu congestus formed but failed to really get going, with a few light showers originating from the field ca. 1630 onwards. Cloud-tops lacked the hard, 'Cauliflower' appearance indicative of vigorous convection but instead had that smoothed appearance that towers get when they cannot bust the cap.

Multicell line heading towards the Midlands has some fairly impressive lightning in association, I note.
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