Tornadic Ana Cold Front Potential - Tues/Wed Nov 24/25th

[martinastro]

This is interesting, from the UKWW...

Deep low expected to sweep cold front eastwards across UK later Tuesday and during Wednesday. Driven by confluent trough, front is expected to be of the rearward sloping 'Ana' variety with the risk of waves running along it. This is very conducive to LEWP elements developing along the frontal boundary with the risk of squall and/or tornado development. There is a risk that this particular cold front may spin up a considerable number of tornadic vortices during its passage across the UK.

All parts of England, Wales and Ireland are at risk.

[martinastro]

This LP is fast becoming another Atlantic storm with NW areas getting strong winds, perhaps much stronger than the weekend.

[martinastro]

Just checked out the convective maps for 36 hours into the future. Very strong vorticity, tornado, and supercell parameters over N. Ireland and Britain. Could be interesting/nasty over the next few days!

[martinastro]

Ireland is out of the picture again, Britain is getting all the action - darn!. Even the TV weather forecast today mentioned the risk of tornadoes tonight...that doesn't happen very often. Latest from TORRO...

13:45GMT - Tuesday 24th November 2009 - TORRO CONVECTIVE OUTLOOK

A strong cold front will move south-eastwards across the British Isles this afternoon, evening, and tonight. Across England and Wales, this front is expected to be marked by a line of intense rainfall, along with very squally winds and possibly tornadoes. Wind gusts could reach 60mph, perhaps 70mph in places.

Furthermore, a trough may well follow the front during tomorrow morning and early afternoon, mainly across southern parts of Wales and England. Should this occur, it also brings the risk of severe winds and tornadoes.

The situation will be monitored through this afternoon, and a discussion or watch is likely to be issued later.

Forecaster: RPK

[martinastro]

Here's the tornado mention...

[martinastro]

TORRO TORNADO WATCH 2009/013

A TORNADO WATCH has been issued at 17:15GMT on Tuesday 24th November 2009

Valid from/until: 18:00 - 12:00GMT on Tuesday 24th/Wednesday 25th November 2009, for the following regions of the United Kingdom & Eire:

Parts of (see map)

All of England and Wales

Channel Islands

THREATS

Tornadoes; wind gusts to 70mph; hail to 15mm diameter; CG lightning (hail/lightning post-cold front)

SYNOPSIS

Strong cold front will move east/south-east across the area tonight. Whilst deep convection is not expected along the front, strongly-forced shallow convection is simulated, with a good chance of LEWP/bowing segments, and misocyclone development. Such development brings the risk of wind gusts of 60-70mph, and short-lived tornadoes. There is a chance of a number of tornadoes overnight.

Tomorrow morning, one or more post-frontal short-wave troughs is expected to race eastwards, especially (but not exclusively) across southern parts. If convection fires on these, hail/thunder/strong winds and (perhaps) tornadoes will be possible - indeed, a stronger tornado (T4) is possible should this convection develop.

The major driving trough is expected to move across late morning into the afternoon. The watch may need to be extended/re-issued at 1200GMT tomorrow.

Forecaster: RPK

[paulster78]

Saw that forecast this afternoon as well, nearly fell off my chair when I heard them mention the possibility of tornadoes- very rare indeed!

[martinastro]

ESTOFEX forecast for Wed/Thurs UK and Ireland....



Forecast Update
Valid: Tue 24 Nov 2009 19:00 to Wed 25 Nov 2009 06:00 UTC
Issued: Tue 24 Nov 2009 19:58
Forecaster: VAN DER VELDE
A level 1 was issued for the UK for a chance of tornadoes and severe gusts. Excessive non-convective rain amounts are possible.

SYNOPSIS

A cold front passes slowly over the British Isles, and strong moisture transport causes jet another rainy episode for northern UK and Scotland. The front shows a double structure in surface convergence fields, with the jet right between them. The unstable maritime cold airmass will touch upon Ireland and western UK in the early Wednesday morning hours. Forced convection may occur along the eastern convergence band.
At 00Z the right entrance to the >70 m/s 300 hPa jet maximum is over the central UK.

DISCUSSION

12Z GFS model has now tipped over to producing convective precipitation and colder EL heights along the cold front during over UK during the late evening and night. The very strong flow creates potential for wind gusts up to about 35 m/s, but the orientation of winds and shear vectors along rather than perpendicular to the front decreases the potential for a convective squall line. A forced line may nevertheless exhibit wave patterns with small bow echoes oriented more favorably. 0-1 km shear over 20 m/s, 0-6 km shear over 35 m/s and SREH3 over 300 m²/s² in interaction with a (forced) updraft can yield mesocyclones which can produce tornadoes.
The period during which GFS suggests some CAPE, as well as positive QG forcing, is limited to around 20-00Z for the western UK. Around 00Z negative QG forcing (descent) is predicted over the region. Near-neutral lapse rates will remain and continuous strong forcing may at times result in slight realease of instability, in the form of convective cells or narrow lines.

Towards the end of the night, western UK and Ireland receive the cold airmass cells, under a very steep pressure gradient creating a severe large scale wind field. This convection should not be linearly organized and provides little enhancement other than turbulence. Deep and low level shear over W UK will be very large, still, but also unidirectional (lack of veering/SREH), but a tornado cannot be ruled out.

[markt]

The line of the front is just about to pass through here, and it has become very gusty to say the least!  It's (obviouslly!) dark for me now, but i'll still keep an ear out for any tornadic activity...

[martinastro]

Good man Mark, keep us informed of any developments. The bowing segments sure look good on radar, great text book example of a LEWP, haven't heard any severe weather reports yet but time will tell. 

[martinastro]

That cold front looks excellent on radar, wouldn't be surprised if a tornado came from that, it's huge too!

T-storms in NW N. Ireland coast areas.

[martinastro]

Check out the radar of that cold front over Britain

http://www.raintoday.co.uk/

LEWP with classic broken S sections.

[paulster78]

Looks very interesting, there has to be some damaging squalls or tornadoes from that!

[martinastro]

ESTOFEX updated forecast for Wed daytime. We get a mention in this one...



Storm Forecast
Valid: Wed 25 Nov 2009 06:00 to Thu 26 Nov 2009 06:00 UTC
Issued: Tue 24 Nov 2009 23:00
Forecaster: PUCIK
A level 1 was issued for England, Northwestern France, BENELUX, N Germany, Denmark, SW Sweden mainly for tornadoes and severe wind gusts.

A level 1 was issued for Ireland mainly for tornadoes and to the lesser extent, marginally severe wind gusts.

SYNOPSIS

The major feature on the weather maps is a large cyclone over the Eastern Atlantic, which is expected to reside over the area also during Wednesday. On the forward flank of the cyclone, a southerly strong mid and upper level jet is observed with windspeeds over 50 m/s from 500 to 300 hPa layers. Jet will gradually weaken during the day as the cyclone slowly spreads eastwards. Ahead of the cyclone, a ridge will retreat to the east with its axis stretching from the Mediterranean to Central Europe and Scandinavia around 12 UTC. At the surface, center of the cyclone will cross Northern Scotland with pressure under 960 hPa and then turn northeastwards. At 06 UTC, surface cold front will be passing the British Isles and during the day we expect that it will have passed BENELUX and much of France and Germany.

DISCUSSION

...England, Northwestern France, BENELUX, N Germany, Denmark, SW Sweden...

Strong cold front will make its way across the region. Ahead of the front, very strong flow will be present, veering from southerly to south-southwesterly direction with height. At 925 hPa windspeeds above 20 m/s are anticipated and at 500 hPa, values will vary between 40-50 m/s. Models are predicting a belt of enhanced SREH values both in 0-1 and 0-3 km layer. Due to the above mentioned facts, favorable kinematic setup is in store with wind shear values above 40 m/s in 0-6 km layer and from 10-15 m/s in 0-1 km layer.

Strong mesoscale ascent is anticipated in the frontal region and high PV values are advected at upper levels behind the front. One of the detrimental factors to the situation will be a lack of well-developed latent instability. Both GFS and ECMWF predict very marginal values of CAPE, around 200 J/kg in the vicinity of the front. But frontal ascent can at least a bit compensate for the instability. There is a slight chance that with strongly forced convection, meso or misocyclonic circulations manage to develop with attendant threats of severe wind gusts and tornadoes. Due to these threats, Level 1 is issued.

... Ireland, Northern Ireland, Wales, Western England...

In the post-frontal environment, cold mid-levels combined with relatively warm SSTs lead to the marginal destabilisation of the maritime polar airmass. CAPE values around 500 J/kg are expected, mostly over the sea, but models simulate certain values of CAPE also inland. Due to the strong flow at lower levels of troposphere, strong windshear at 0-1 km is forecast, with values up to 15 m/s. Enhanced SREH is also simulated, due to the ageostrophic flow component due to the friction. Slight chance of tornadoes will exist as well as marginally severe wind gusts with stronger convection.

[Padraig OBrien]

Some quite havey showers here today very stormy this morning heard in the news Roof completley blown off and apartment block in Dublin's Southern Suburbs'.