Tornado Risk - N. Ireland - Sat April 11th

[martinastro]

From Tony Gilbert of the UKWW



Slight Risk of Isolated Sharp Convective Showers Across Ireland 12Z-18Z Sat 11th April 09

Further Risk of Small Hail and a Brief Weak Isolated Tornado Report for Northern Eire and N.Ireland same period.

UKMO Forecast currently applies a rather slack regime across Ireland through Saturday. Progged surface trough expected to develop showers from the west by morning some of which could become quite sharp in nature.

Rather weakly sheared environment limits severity of any building cell. Though, it is noted that late in the day we might expect some surface convergence to develop central northern Eire which could dramatically increase directional shear within the lowest levels hense the suggestion of some tornado potential. Whilst 500 mb shows -27 degC, this is advected into the region within very dry air enhancing potential instability to the maximum. GFS go for a very moist surface profile with mild temps bolstering the current predicted level of CAPE.

The overall opinion at this point suggests that based on the current model output, Ireland probably has the first real chance at some true Spring convective showers!   with the bonus of a weak tornado or funnel cloud report, maybe!

[martinastro]

Not so good update from Tony...

As per earlier forecast with outlook remaining a rather low key Showery regime typical for April across much of Eire. Both NMM and GFS continue to develop the best CAPE along a SW to NE line of moisture convergence. NMM projects a vale of cloud cover and NVA for much of southern and SE Eire suppressing most convection for this region. This restriction shows up rather clearly on the projected skew-t soundings with signs of an elevated inversion. Updated models are now much less consistant with the progress east regarding the mid level dry air. Overall opinion still gives for an interesting build up of isolated showers early afternoon to the west of Eire and N. Ireland by afternoon as the best solar heating gets going. Though, soon after surface temps look to modify quite rapidly under cold outflow as showers get going pretty much destroying any earlier instability. ATM given the level of weak convergence I'm not so sure about the risk of tornadoes today.

[brianb]

Overall opinion still gives for an interesting build up of isolated showers early afternoon to the west of Eire and N. Ireland by afternoon as the best solar heating gets going.

Not much doing up here, Martin - essentially overcast for the last couple of hours which has killed the convection which was starting to develop. Might not even get showered on ... and if it dissipates later in the afternoon, perhaps another clear night

[martinastro]

Same here Brian, overcast and extremely cool with no heat at all to fuel things. Having said that, the big showers have not formed yet, there are some forming in western areas but the surface trough as yet to cross the country, late afternoon or evening may be the time to see something. Don't have high hopes at the min but there's plenty of cu updraughts around which could produce a funnel anywhere. I hope it does clear tonight, last night it was good so I spent half an hour looking for Yi-SWAN but since it was in close proximity to a street light my attempt was unsuccessful. Maybe get round two tonight. Not long to the Lyrids now..which reminds me...must start a thread 

[martinastro]

The SW to NE convergence line over Ireland is beginning to form now as seen on the radar images. A line of showers stretching across the country have now formed with light to moderate rainfall, don't know if they are convective though but they should be arriving near my area soon.

http://www.metoffice.gov.uk/weather/uk/radar/

[martinastro]

Big convection to my W now. Sun is breaking out...showers falling from a kind of gust front. Time to go out!

[martinastro]

Impressive skies here during the late afternoon. Seen big showers organizing on the radar and creeping closer to my area so I headed out to good spot to watch. It was well and truly worth it. A huge black shelf cloud moved slowly in from the SW and covered the entire skyline with nasty precip curtain behind. Intense rain and hail dropped from it for a long time. A great looking structure. I was amazed there where no c-gs from it because it looked exactly like a big thunderstorm, only without the sparks. Large area of rotation observed under the shelf where it joins the precip core. This was followed by very strong convective updraughts over the Sperrin Mountains. A very interesting day indeeed!. Lots of images and video clips taken of the shelf.

[brianb]

Hmmm ... about 10 mins ago just checking out the back for signs of clear sky, did a quick double take & grabbed for the camera:

OK, I can see now it's not a funnel, but what exactly is going on?

The orange colouration is obviously low sun (the image is facing WSW). BTW there was a moderate shower a few minutes later.

[martinastro]

I can answer that no problem. Recall I mentioned the massive shelf cloud passing over here earlier. Well it was moving from SW to NE and by the looks of things it has arrived at your location later in the day at a gorgeous low Sun angle!. On the image that's a portion of the gust front/shelf caused by the outflow/downdraught pushing cool moist air down and forward from the storm (minus sparks).  Tornadoes can form on the leading edge of these on occasions, including the rear of the system, within, or behind the rain curtain. Worth checking that area in daylight for any disturbance.

When it arrived over the Sperrins it was black-grey and extended across the entire N to S sky and was menacing looking. Maghera was covered in hail stones.

Thanks very much for sharing that stunning image Brian, it looks very menacing and photogenic with those colours!. That shelf was the biggest and most organized convective structure which occured in the UK today. Quite a beast. I checked the radar and there where intense red echoes from within it.  Tony's forecast turned out good afterall....due to a period of brief heating and a convergence zone.

Did anyone else observe or image this shelf cloud?

[martinastro]

http://en.wikipedia.org/wiki/Gust_front

Here's a link with some info Brian. That feature is also known as an outflow boundary, as well as gust front (or guster) or shelf cloud.

[brianb]

That feature is also known as an outflow boundary, as well as gust front (or guster) or shelf cloud.

Thanks ... no gusts though, at any rate not above force 4. Totally calm overnight, with the late evening shower making the ground wet there was intermittent fog & very heavy dewing ....

[martinastro]

Last night was very clear but I seen stunning fog creeping through the country fields and roads lit intense white with the Moon complete with fog crep rays when the Moon viewed through trees. I observed a complete lunar fog bow which was amazing with fog below and stars above around 02.30 UT. Image didn't turn out so well though. Great meteor activity around also. I tried for Yi-SWAN and Cardinal last night in the 8.5" as the Moon was rising during the evening so the sky was fairly dark, but I could not see either due to these horrific street lights which happen to be placed beside both comets. I can see stars in the fov but no fuzzies...very frustrating. Round 3 sometime next week, doesn't look good for Sunday night.

No gusts of wind came from the shelf here either but the intense rain and hail made up for that. 

[Roman White]

I tried for Yi-SWAN and Cardinal last night in the 8.5" (...) I can see stars in the fov but no fuzzies...very frustrating.

Just curious - what was the stellar magnitude limit, Martin?

[martinastro]

Hi Roman, I didn't take note of it but the sky was in very good shape. All other DSOs where visible except the comets. The sky was quite dark too with good clarity. The proximity of the street lights would be just enough to render a comet invisible though. I will keep trying. The N is a bad area for me from here. As the comet moves towards the double cluster it will be even more difficult for me.