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General Category => Weather & Atmospherics => Topic started by: martinastro on August 18, 2009, 07:12:59 pm

Title: Convective Potential Ireland & UK - Wed/Thurs Aug 19/20th
Post by: martinastro on August 18, 2009, 07:12:59 pm
There may be a possiblity of some active weather over parts of the UK, Ireland, and N. Ireland over the next few days. On Wed a cold front sweeps in from the Atlantic with heavy rain and gusty winds with some weak instability. There are strong supercell and tornado parameters from this so there's a chance of isolated strong gusts of wind, and perhaps even the odd rumble of thunder from embedded convection. There might be a chance of tornadoes however other factors are needed.

Thurs will have an unstable post frontal air mass over Ireland/N. Ireland with good CAPE and LI. Again there is some wind shear with the supercell and tornado parameters so isolated heavy showers and thunderstorms could develop if the cap is not too strong provided we get some solar heating. There may also be a risk of funnels and weak tornadoes according to the lightning wizard convective maps.  This is subject to change though.

Warm air advects over the S of the UK from France, there's much uncertainty but there is a chance of t-storms over S/SE Britain. There could be many restrictions with the set up so waiting to see what the experts think. Certainly it's the first potential for convective weather since July.

Title: Re: Convective Potential Ireland & UK - Wed/Thurs Aug 19/20th
Post by: martinastro on August 18, 2009, 10:09:29 pm

Storm Forecast Issued: 2009-08-18 21:28:00
Valid: 2009-08-19 00:00:00 - 2009-08-19 23:59:00

Regions Affected
extreme Southeast England ( all of the UK, excluding Outer Hebrides and SW England are including in the WATCH )

A complex area of LOW pressure between Scotland and Iceland, and a large area of HIGH pressure over the continent will dominate the weather across the United Kingdom on Wednesday. Ahead of a cold front, aligned through central Scotland and the Irish Sea at midday Wednesday, a very warm and humid airmass is advected northwards from France. A strong cap appears to be present over France for much of the daylight hours, preventing convection from occurring - however, during the evening hours the pressure gradient over northeast France is expected to slacken, creating a surface LOW to develop. This may cause initially scattered or isolated thunderstorms to develop. There is a risk of one or two of these clipping the extreme southeast corner during the mid-late evening hours - this is uncertain due to disagreement between the models. Later in the night the storms are expected to expand and merge into an MCS-type feature across Benelux and the southern North Sea. These storms pose a risk of supercellular characteristics. Elsewhere, although some heavy and locally torrential pulses of rain are expected to move northeastwards along the cold front, the risk of embedded thunderstorms seems unlikely given rather limited cloud height.

Title: Re: Convective Potential Ireland & UK - Wed/Thurs Aug 19/20th
Post by: markt on August 19, 2009, 10:25:48 am
Well today, for me, is the first day for ages where the WHOLE of the sky is blue!  8)

I'm going to make the most of it and get some solar observation done today, though I suspectel sun will be decidely blank.  Ho hum, best get the rays when I can as the break down is on its way!  ::)

Title: Re: Convective Potential Ireland & UK - Wed/Thurs Aug 19/20th
Post by: martinastro on August 19, 2009, 12:17:35 pm
Great to hear it Mark - make the most of it and enjoy the good weather when it's there.  :)

Title: Re: Convective Potential Ireland & UK - Wed/Thurs Aug 19/20th
Post by: martinastro on August 19, 2009, 07:02:58 pm
For Thurs


Extended Forecast
Valid: Thu 20 Aug 2009 06:00 to Fri 21 Aug 2009 06:00 UTC
Issued: Tue 18 Aug 2009 21:51
Forecaster: TUSCHY
A level 1 was issued for Belgium, the Netherlands, Luxembourg, NW and central Germany, Denmark and parts of Norway and Sweden mainly for large hail and severe wind gusts and to a lesser extent for tornadoes.


A 985 hPa vortex east of Iceland is the steering mechanism for most parts of the European weather. Intense WAA downstream causes hot conditions over central Europe with scattered thunderstorms along the frontal zone. Cold and stable conditions preclude thunderstorms over eastern Europe.


... Benelux, W/NW-Germany, Denmark, parts of Norway and Sweden ...

An unseasonably strong trough regarding its expansion and intensity is forecast with model guidance being in line, showing a sub-990 hPa between Iceland and Scotland. Impressive WAA is forecast downstream, which overspreads an area all the way up to north Sweden and Norway. The upper trough continues to move eastwards on a slow rate and acquires a slight negative tilt. Along its eastern fringe, an active frontal zone pushes eastwards in the form of a cold front, which runs from the Azores all the way to the west coast of Norway. This front approaches Benelux during the evening hours and W-Germany during the morning hours (21st August). However, models agree in the development of a sharp prefrontal convergence zone ahead of the cold front, which will be the focus for the most widespread thunderstorm initiation.

This airmass modification is accompanied by steep mid-level lapse rates and a seasonably moist airmass at lower-levels. GFS again is the outlier with dewpoints in excess of 20C over Benelux and NW-Germany, but we stick with this model for now due to the prolonged time for the return flow with mid-/upper tens already in place over France (evening hours of the 18th August). Beside the advective component, strong nature of the forecast convergence zone indicates good moisture pooling along this zone, probably maximized over Benelux and NW-Germany. Dewpoints decrease somewhat to the north and south of those areas. MLCAPE will be maximized over Benelux ( probably greater 1kJ/kg) with lower values to the north and south.

The 0-6km bulk shear vector is parallel to the frontal zone over Benelux and north-northeastwards and supportive for rapid thunderstorm organisation. Latest indications point to an enhanced supercell risk over Benelux, extreme NW-Germany and Denmark but later model outputs will be evaluated before issuing higher probabilities. Large hail, severe wind gusts are the primary risk, the latter one could become more serious, if 20m/s 0-3km shear indeed verifies. The tornado risk still depends on the strength of a weak surface trough, which is not present in all models. Instability north of Denmark decreases, although enough elevated instability remains for a large hail risk all the way to central Norway. A flash flood risk arises beneath nort-northeastward moving storm clusters.

Initiation is quite complex, given numerous, strong short waves, which cross the frontal zone and the intensifying prefrontal convergence zone over the southern part of the level 1 and the elevated nature over the northern parts. Numerous rounds of thunderstorms are probably the most likely scenario, with the most active one occuring during the afternoon and evening hours.

An uncertainty could be the near front-parallel storm motion, which could cause rapid storm clustering. However, even with this scenario in mind, the severe risk still exists during the initiation phase, where thunderstorms are still more discrete.

... Eastern France, Switzerland, parts of the Alps and rest of Germany ...

During the day, daytime driven thunderstorms occur along the southern Alps and parts of Switzerland. Shear is too weak for anything organized, but a level may be issued due to slow storm motion and therefore an augmented flash flood risk. For now, the risk looks too marginal to go with a level 1 in an extended outlook.

During the evening and night hours, thunderstorms also evolve over E-France, SW/S and central Germany, along the eastward preceding convergence zone and cold front. Instability is on the decrease but the severe risk increases to the north, where shear is the strongest and also over SW-Germany along the cold front during the morning hours. Isolated large hail /severe wind gusts are possible but only central Germany was included into a level 1, where coverage of wind/hail reports could be more widespread.

... UK, Ireland and Scotland ...

Beneath cold mid-levels, daytime driven thunderstorms are forecast. Shear is too marginal for anything severe, but the potential for augmented LL CAPE release has to be monitored, which dictates the tornado / funnel risk.

Title: Re: Convective Potential Ireland & UK - Wed/Thurs Aug 19/20th
Post by: martinastro on August 20, 2009, 05:00:05 pm
Quite a wild day here with nasty weather which felt more like Autumn than Summer. Although I did not witness any lightning I did get on some cool stuff. Encountered an impressive gust front over Toome which looked very nasty and dark with intense curtains of rain and hail with dynamic curling motions evident on the outflow/shelf interface. This was red and white on radar. Got ahead of it and later got in its way over the NW shore of Lough Neagh at Ballyronan Marina.  It hit with brutal intensity with torrential rain and intense squalls of wind which sent bow waves rippling across the Lough - very impressive stuff. The temp drop was phenomenal in the icy outflow which felt like I was doing photography in the snow. The same guster produced lightning on the E side of the Lough. Lots of mammatus around after this. Lively stuff indeed! Also lots of flooding today in the fields with an impressive height to the River Ban after last night's nasty rainfall

Title: Re: Convective Potential Ireland & UK - Wed/Thurs Aug 19/20th
Post by: martinastro on August 20, 2009, 05:01:06 pm
Surprised there are no other reports so far, there's plenty of lightning in E NI and into the Republic including parts of Britain.

Title: Re: Convective Potential Ireland & UK - Wed/Thurs Aug 19/20th
Post by: martinastro on August 20, 2009, 05:07:36 pm
Today's lightning so far from ATD sferics. The lightning on the E side of Lough Neagh came from that gust front I  reported.


Title: Re: Convective Potential Ireland & UK - Wed/Thurs Aug 19/20th
Post by: brianb on August 20, 2009, 07:56:55 pm
Surprised there are no other reports so far
The weather this week has been so awful it's hardly worth commenting on....

Title: Re: Convective Potential Ireland & UK - Wed/Thurs Aug 19/20th
Post by: martinastro on August 20, 2009, 08:43:01 pm
For Astronomy...then no, it hasn't been good - that aurora activity last night would have been visible from here I reckon if we had clear skies.

If you are into storms, or weather photography though, today has been a nice treat. I quite enjoyed that cold front last night, it's good to see serious rain again which means business rather than that patchy drizzle nonsense we have been betting for so long. At least things are more lively now. It's not over yet though - ex-Hurrciane Bill may affect us is the near future, however lots of uncertainty at this time.

Title: Re: Convective Potential Ireland & UK - Wed/Thurs Aug 19/20th
Post by: martinastro on August 20, 2009, 09:55:20 pm

Storm Forecast
Valid: Fri 21 Aug 2009 06:00 to Sat 22 Aug 2009 06:00 UTC
Issued: Thu 20 Aug 2009 20:28
Forecaster: TUSCHY
A level 2 was issued for parts of Germany and extreme western Czech Republic mainly for large hail and severe wind gusts.

A level 1 was issued for S-Sweden mainly for large hail.

A level 1 was issued for Switzerland, E-France, S-Germany and parts of Austria mainly for large hail, severe wind gusts and excessive rain.


A huge upper trough over NW-Europe continues its slow eastward motion and will be placed over the North Sea during the end of the forecast. Impressive WAA tongue further downstream still covers parts of central and northern Europe. The Mediterranean remains hot and stable with chilly conditions north of the Black Sea.

A north-south elongated cold front is moving gradually to the east, but decelerates during the forecast. Abundant precipitation is forecast along this front and a prefrontal convergence zone.


... Most parts of Germany ...

Not much change in the forecast philosophy. A cold front advances eastwards, faster over NE and E-Germany with the trailing part over southern Germany. Next to this front, a prefrontal convergence zone / surface low pressure channel is situated already over the far eastern regions of the level 1 area, still dependent on the magnitude and extend of the overnight activity. Upper trough axis draws near during the forecast from France and Benelux and enhances mid-level forcing, but this may only play a timing issue in respect of initiation.

Marked off by the convergence zone and the cold front, a warm and moist sector covers the level 1 area with surface dewpoints in the upper tens and temperatures between 25C and 30C. Strongest moisture advection and highest averaged mixing ratios are likely along the more active part of the front, over NE and E-Germany. Despite a weakening EML due to active convection the night before and diminishing WAA, a prefrontal tongue of 500 - 1000 J/kg MLCAPE looks still reasonable, probably too narrow to be captured by the sparse sounding mesh. Values, exceeding 1000 J/kg may evolve over SE-Germany, extreme SW-Czech Republic and extreme N-Austria, where mid-level lapse rates remain a tad steeper.

Obviously, shear is the best along the cold front and most parts of the aforementioned sector, between the front and the convergence zone. 15m/s and 20m/s shear in the lowest 3km and 6km respectively cause rapid thunderstorm organisation with large hail and severe wind gusts. Convergence zone further east is a bit displaced from the best wind field and forcing, but still adequate for an augmented risk.

Thunderstorms, probably disorganized and non-severe, continue during the early morning hours along the cold front, but undergow a diurnal minimum in organization and activity. Already during the late morning hours / noon, thunderstorms over NE-Germany re-intensify with isochronic initiation further to the south but also along the convergence zone over far east Germany, west Poland and the Czech Republic. Initiation over SE-Germany holds up until the afternoon and probably even until the evening hours. Models hold firm on the development of two thunderstorm clusters. The first one out of multicells and supercells over E/NE-Germany (afternoon and evening hours) and a second one over S-Germany/W-Austria during the night hours, probably a quite disorganized cluster of storms with a rapidly developing more stratiform rain shield. In-between (e.g. central Germany), neither forcing nor instability impress us, but shear remains strong, so more isolated multicells and supercells are still forecast during the afternoon hours.

The level 2 was adjusted to those regions, where storm mode and environment looks promising for a more widespread large hail and severe wind gust risk. The level 1 was issued for various reasons, which can be seen in the header.

The level 1 was expanded well to the north over S-Sweden to reflect an isolated hail risk with elevated thunderstorms. Favorable overlap of MUCAPE/shear diminishes betimes and so does the hail risk.

Excessive rain is likely with those storm clusters over the western Alps and NE-Germany /NW-Poland, but also over E/SE-Sweden.

... UK and Ireland ...

An active, diurnal driven thunderstorm day is in store, but there are no hints of an augmented funnel/tornado risk. Isolated events can't be ruled out, given some low-end LLCAPE and low LCLs, but not enough for a level 1. Thunderstorms decay after sunset.

... Belgium, Netherlands and the SE-North Sea , 18-06 UTC ...

Beneath the base of the upper trough, conditions improve for a few evening storms over Belgium and the Netherlands with an isolated hail risk. However, the thunderstorm activity shifts offshore and conditions for a few waterspout events improve. Expected risk is not yet enough for a level 1.