Convective Outlook - Sat/Sun Jan 17/18th

[martinastro]

Some early discussion on the weekend's convective potential from various sources on the UKweatherworld...

http://www.ukweatherworld.co.uk/forum/forums/thread-view.asp?tid=28061&posts=10&mid=409731#M409731

UKMO are consistant with the development of a deep depression west of Ireland. Predictions are going for eventual drop to 442 mb! Could be the deepest we've seen in a year or two (If you know different then please specify!)

Orientation of streamlines suggest a classic polar maritime flow with a very moist unstable fetch from mid Atlantic. SST anomalies present 10 deg C west of UK & Ireland. These temps have not yet troughed fully this year and will add impetus to any broad scale weather system. Too early to be specific and UKMO synopsis continue to give two scenarios. Though GFS currently give exceptional divergent mid level jet with the main cold front.

Confluent upper troughs are more likely to bring the risk of line convection/ana cold front potential - 23rd Feb 1995, 22 Sept 2003 and 24th Sept 2007 cold fronts with severe line convection and tornado reports were great examples of this. As far as I remember though and looking back at a few basic charts from the date, the 3rd December 2006 line squall occurred under diffluent conditions (or perhaps a negatively tilted diffluent trough that was gradually tending to neutral). Guess there is a greater risk of the cold front splitting when the flow is diffluent, it just then depends whether the potential instability close to the surface front can be released.

Diffluent troughs seem to be more reliant on there being some inherent instability in the pre-frontal airmass, as they are somewhat less likely to produce a strongly forced line from the cold-air 'scoop effect'. Certainly diff. troughs are what you'd look for in the Great Plains, but of course, we're not there! Even so, some squally winds seem very likely, and if convection can develop, tornadoes are possible.

More information closer to the time.

[martinastro]

From Tony Gilbert (TORRO)

Whilst it is too early to make any accurate predictions. It is still worth noting that January and sometimes early February have periodically delivered some violent tornado events to the UK & Ireland. These are logged within the long term data base within TORRO. ATM we can atleast say that the models are in fact building a picture which could well be conducive to such conditions. The winter years will not always only produce the vortices from 'Ana' or squall type cold fronts. The air mass ahead and behind the cold front ATM looks to be fairly unstable. Though what is most significant is the level of vertical speed shear which could arrive. A mention earlier of the slightly above average SST SW of UK & Ireland. This fact will have overriding influence on the level of severity for building convection, ahead of the cold front, on the cold front and within the post frontal air mass.

Certainly one to watch for and to follow carefully. So lets not put the book down quite yet and keep the discussion rolling

[brianb]

It is still worth noting that January and sometimes early February have periodically delivered some violent tornado events to the UK & Ireland.

Including the one that smashed up Patrick Moore's observatory IIRC. Though the damaging winds that can occur over large areas are probably more of a risk .... force 11 usually causes some structural damage, apart from keeping the TV ariel repairers in business! The chances of actually getting hit by a tornado are small, and the chance of seeing one are not great either seeing as there is a 70% chance that it will be dark at this time of year even if one does pass within "visual range".

[martinastro]

Very true indeed. The hours of darkness will certainly impair any visual detection at some distance but it could be well worth a look anyway. From my own location it's amazing how well you can see even at a generous distance due to the amount of lights reflecting on the base of clouds.

The chances of getting hit by one in the UK are low but not that though...there are already two members on these boards who encountered tornadoes at close range from Ireland. I came close to seeing one myself during 2007....it didn't make ground contact but it was close. I hope to some day get an image of a tornado touch down in this country. That's one of my goals  ...what a challenge!!!

The main threat as you said will be powerful wind gusts. The post frontal air mass will be worth watching too due the unstable air. Some nice cumulonimbus cells could be on the cards. There's plenty of wind shear around for one of those to spwan a funnel or tornado to.

Latest info places the low closer to Scotland now, a little early yet for details. At least there's some interesting weather on the cards. ..depending on your view of course.

[martinastro]

UPDATE 11pm Thurs

Current forecast synopsis drops low much further south than earlier predictions. Net effect; surface vector derived from warmer seas ultimately increasing instability ahead of the cold front. Though this is yet to be reflected within the models. If the models are in fact playing catch up then we should see the now almost zero CAPE ahead of the cold front, increase somewhat. Upper divergent jet is dipped further south placing the best forced lift further south. Concerns regarding potential instability (dry air intrusion) remain and is currently limiting my expectations regarding the cold front specifically.

The overall risk pattern, though transitional should last till late Tuesday. It will therefore be necessary to break the 3 day risk period down into separate outlooks closer to the time. The alternative will see utter confusion regarding which cold front or trough may be associated with any severe weather...Tony Gilbert