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Storms and Convective Discussion - 20th April 2019 onwards

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1 Year ago today!

myPAJZM.thumb.jpg.734c96c8bd5f8dc1a0cad4bca53c54c3.jpg

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Very good to see the storm season beginning. 

As supacell has mentioned above, some decent chances of afternoon storms for Tuesday. Watch out for the shortwave into Wednesday aswell, for more east/southeastern parts, before Thursday looks like a very typical late April day with sunshine and showers, but Tuesday and Wednesday could well see “weak” Spanish plumey type storms occur. 

Not a bad start! 

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2 hours ago, TomW said:

1 Year ago today!

myPAJZM.thumb.jpg.734c96c8bd5f8dc1a0cad4bca53c54c3.jpg

Oh I remember that, sat on the coast watching them come in with my sister.  Hoping for more of the same this year!

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Was the last storm of note here, very sparse electrical activity in the past year.

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Day 3 Convective Outlook

VALID 06:00 UTC Wed 24 Apr 2019 - 05:59 UTC Thu 25 Apr 2019

ISSUED 08:42 UTC Mon 22 Apr 2019

ISSUED BY: Dan

Negatively-tilted upper trough will span from the Atlantic to Biscay on Wednesday, while pivoting gradually northwards. As the forward side of this trough moves steadily northwards across the British Isles, increased forcing / cooling aloft combined with surface heating will create an unstable environment, with 500-800 J/kg CAPE ahead (to the north) of the surface cold front. A few scattered thunderstorms may develop over Wales / Midlands / East Anglia, these drifting northwards into northern England during Wednesday afternoon, weakening during the evening as instability wanes. These may initially be elevated, but provided there is sufficient surface heating then there is potential for these to become rooted within the boundary layer. 

However, the speed of the cold front, and cloud amounts thrown ahead of the front, will both determine the areas at risk of thunderstorms - both of which could inhibit deep convection. A SLGT may be introduced nearer the event if confidence improves.

 

The post-frontal environment will become increasingly unstable as the next shortwave, and associated cooling aloft, arrives from the south. As such, showers may affect the English Channel and into southern Britain during Wednesday evening and night, though the extent of any lightning activity is questionable given marginal instability.

 

http://www.convectiveweather.co.uk/forecast.php?date=2019-04-24&fbclid=IwAR2piVGU0r3DLIoqrkSca9SEY9h3j2pBlx-jX6JeF9Wzh0NsuTuBh3d1jS0

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Day 1 Convective Outlook

VALID 06:00 UTC Mon 22 Apr 2019 - 05:59 UTC Tue 23 Apr 2019

ISSUED 21:08 UTC Sun 21 Apr 2019

ISSUED BY: Dan

Atlantic longwave trough will continue to dig southwards during Monday towards western Iberia. As a result, southerly flow aloft across the British Isles will gradually back southeasterly, advecting relatively high ThetaW airmass from France into southern and central Britain. Areas of medium/high level cloud will occasionally drift northward courtesy of weak mid-level instability and increased moisture aloft.

Deep mixing will likely occur through the day, resulting in steep low-level lapse rates. However, this will also likely mix drier air down to the surface, resulting in low dewpoints and large dewpoint depressions. Subtle forcing aloft will drift over S / SW Britain on Monday afternoon, coinciding with peak surface heating. This combined with low-level wind convergence and orographic forcing may allow a few isolated heavy showers / thunderstorms to develop late afternoon or evening, primarily Somerset/Glocs and into Wales. There could also be one or two over Ireland.

However, the vast majority of NWP guidance remains sufficiently capped to prevent any thunderstorm development. GFS and derivatives (WRF, NMM) often produce profiles that are excessively moist, and hence suggest much higher surface dewpoints than most other models for Monday afternoon. Consequently they often favour thunderstorm development - especially over mountainous areas - when in similar events in the past such storms have failed to materialise. UKV and AROME have also been known to be too keen for thunderstorm development in fairly capped environments. 

As such, it is plausible that no thunderstorms may develop at all. That said, a broad LOW threat level has been introduced to cover the risk - at this stage it is difficult to be more precise where an isolated shower/storm may develop, though S + W Wales is most favoured. If confidence does improve a small SLGT may be introduced. Should a storm develop, given forecasting soundings and the potential magnitude of CAPE, there could be quite a bit of lightning locally. Any activity would then drift NW-wards towards eastern Ireland during the evening hours.

On Monday night, pulses of showery rain will move into S / SW England containing the remnants of deep convection originating from northern France. Weak instability suggests any lightning activity with this should be fairly isolated.

http://www.convectiveweather.co.uk/forecast.php?date=2019-04-22

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Convective initiation in a couple of places around Central and Southern Wales by the looks of satellite imagery.

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storm over lundy island!

Capture.thumb.PNG.b27fd5ee005092241b6ec3c0ce6f981b.PNG

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Only showers are over the southern Ireland and northern France at the moment. However we've certainly got the high level cloud here making the sun hazy.

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Lightning strikes over Lundy Island.

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Greedy people anyways no doubt the storm shield will power up over Sheffield during Tuesday evening and Wednesday.

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Lots of new TCu cloud showing over West Midlands and through Wales now. I'd imagine something is going to give shortly.

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Yes, looking quite convective here in Walsall, some dark bases and some pretty large Cumulus about.

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Plenty of mid-level convection to the SW here with some good wave clouds forming, models suggesting that if the cap was to break today it would be doing so right about this time.

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Early cloud clearing here and Radar showing what showers there were and the rogue storm are dying away.

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Radar showing instability but without daytime heating it's not anymore more than light showers cropping up here and there, at least there's the Saharan Dust to look forward to tomorrow.

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Day 1 Convective Outlook

VALID 06:00 UTC Tue 23 Apr 2019 - 05:59 UTC Wed 24 Apr 2019

ISSUED 06:01 UTC Tue 23 Apr 2019

ISSUED BY: Dan

... TUESDAY AFTERNOON / EVENING ...

Negatively-tilted longwave trough will continue to be located to the southwest of the British Isles on Tuesday, creating a southeasterly flow aloft with subtle disturbances running northwestwards along the forward side of this trough. As a result the overall pattern is very similar to Monday, with pulses of medium/high-level cloud associated with weak elevated instability, but a generally capped environment to surface-based convection.

However, provided sufficient moisture pooling can occur along a zone of low-level convergence from Glocs/NE Somerset northwestwards across S + W Wales, then a few isolated heavy showers / thunderstorms will be possible during the late afternoon and evening hours. This is rather dependent on surface dewpoints reaching 12-15C, and it must be noted that on Monday afternoon dewpoints generally remained below 10C when GFS simulated 12-14C, hence proving the point that GFS (and associated WRF/NMM) produce excessive moisture and are therefore too "trigger-happy" with developing deep convection.

Nonetheless, deep convection does seem more likely than was the case on Monday - and a low-end SLGT has been introduced to better highlight the area of interest, though it is certainly possible no thunderstorms may occur. There is better model agreement on the potential for a few isolated showers / thunderstorms during the late afternoon / evening across NW Ireland, more especially as they move offshore, hence the inclusion of a low-end SLGT here - this dependent somewhat on sufficient cloud clearance during the afternoon hours.

... TUESDAY EVENING / NIGHT ...

During Tuesday evening / night, increased forcing aloft as the upper trough approaches from Biscay will encourage elevated convection to develop over NW France / English Channel and drift towards SW England. However, this will tend to turn increasingly dynamic with time as instability weakens. Nonetheless, pulses of showery rain will push into southern Britain and southern Ireland during the overnight hours, capable of producing a few lightning strikes from elevated instability. However, this is unlikely to be active or widespread enough to warrant a SLGT by the time such convection reaches land.

http://www.convectiveweather.co.uk/forecast.php?date=2019-04-23

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Waiting for tomorrow - there’s too much of a SW bias for any major excitement today.

Good luck to anyone near a SLGT tho, perhaps good results from this plumette, let’s wait and see 🙂

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I'm feeling slightly different areas today - leaning more towards the East. Chance of something stronger around the Manchester area maybe. All in all, pretty low risk.

23.04.2019.png

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31 minutes ago, Azazel said:

I'm feeling slightly different areas today - leaning more towards the East. Chance of something stronger around the Manchester area maybe. All in all, pretty low risk.

23.04.2019.png

Interesting thoughts. However, just a quick scan of the models shows that the areas you have highlighted are well outside of any type of risk.

TT index ties in with convectiveweather's forecast.

viewimage.thumb.png.7a454a583dba8bc2e8e7e049f2a35079.png

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Let the forecast battle commence... ;)

Personally I'm feeling if anything happens in will be well west.  I've nothing technical to base it on other than that the sky in London right now looks stable enough to transport 1000 eggs

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