MattTarrant

NHC (Advisory 29) Key points: . Increase in vertical shear (25 - 30kt estimates) has caused degradation of Florence's structure (Intensity set at 90kt/ Category 2) . Deep convection no longer symmetric (low level circulation visible) . Nuances of environmental shear forecasting causing complications in intensity predictions . Continued weakening over the next 24hrs likely as strong vertical shear continues (Category 2 > Category 1) . Movement has slowed (9kt), with a NW movement . Development of mid - level ridge will steer Florence (Post 24 - 36hrs) increasingly westwards . Florence expected to regain major hurricane status by day 4/5 (Re - Strengthening post 36hrs) . Movement post day 4 uncertain with major model suites diverging on ridge strength (ECMWF & UKMET vs GFS & HWRF)

Indeed, 3 rumbles of thunder and a serious amount of rain, this is more rain than we have had in at least 2 months.

Just for reference, raw operational data showcases temperature peaking at 45C! Impressive (understatement) given tendency to under play temperatures by 1 - 3C, temperature record of 47.3 would surely be under notable threat if those synoptics were actually transferred to 0h.

One issue today has seemingly been the continuation/minor development of weak elevated convection from this morning. It has seemingly co existed alongside surface convection, hence why the further west broader areas of rain have been present extending cloud cover (cloud top heights were better further east)! I mentioned the possibility of a messy transition from elevated to surface convection yesterday in my forecast. The more intense cells have located in regions that exited the elevated convection in the morning!

*Mid - Day Up Date* (Self Forecast) *Early elevated convection was only weakly electrified, degrading far quicker that initially forecasted. The remaining cloud cover, in line with model trends has lead to the shift of the MDT region, covering more of Wales and removing parts of the South West. The SLGT region has been extended to cover North Wales, while being removed from the far SE. This doesn't relate to the severity of the storms, isolated storms still may produce frequent lightning, large hail and excessive rainfall within the SLGT region!*

I'm not referring to saturation of soils, simply their mindset is a little more cautious after several episodes of flash flooding with only yellow warnings.

Though I believe the Met Office are urging on the side of caution with recent flash flooding, an amber for thunderstorms isn't that common!

Thunder Fog... Has a ring to it

@TJS1998Tom It is difficult to pin point the exact likelihood of Lincoln receiving a thunderstorm, though I would argue under current model guidance that an elevated batch of storms may progress far enough north to produce some sporadic lightning (Early Afternoon). It is important though to stress that a weakening trend will exist! A very isolated surface based storm could develop (Late Afternoon) also, though this considered a low to very low threat . Please use this only as a guide!

Day 2 Convective Outlook VALID 06:00 UTC Thu 31 May 2018 - 05:59 UTC Fri 01 Jun 2018 ISSUED 21:17 UTC Wed 30 May 2018 ISSUED BY: Chris A widespread thunderstorm outbreak is expected across southern Britain on Thursday. Mid-level instability will spread into southern England during the morning sparking the potential for thunderstorms by mid-morning across parts of Kent, Sussex and areas south of London. As the day progresses thunderstorms will develop more widely across southern England and eventually into Wales, the Midlands and parts of East Anglia. These storms will experience several different forcing factors. The aforementioned mid-level instability will spread further northwards into the Midlands, but should mix with surface based instability into the afternoon in response to daytime heating. CAPE values >1000 J/kg are forecast quite widely through the MDT and much of the SLGT area with PWATs in the low 30s mm. Both low-level and deep-layer shear will be weak, meaning storm modes will be messy, and largely pulse variety. Development of thunderstorms will likely occur in response to weak surface convergence, that is noted in the surface wind fields in the model, particularly from London across Bucks, Oxfordshire and Glocs. Weak steering flow will mean that thunderstorms likely sit over the same locations for an extended time, leading to the risk localised flooding. Some areas could receive in excess of 50mm of rain in less than 1 hour. The larger thunderstorm updrafts may also be capable of supporting hail to 2.5cm. Some uncertainty over MDT risk area as extensive low-level cloud during the morning could limit temperatures into the afternoon, and lead to lower CAPE values than the models are suggesting. A few isolated thunderstorms are possible in northwestern Scotland as well, with CAPE values over 700 J/kg forecast, however dry-midlevel conditions and capping at around 750mb will limit potential there. *Similar in nature to my forecast

Thanks...

*This is an update in response to the 12z runs (My Own Forecast) VALID 6:00 UTC Thurs 31 May 2018 - 11:59 UTC Thurs 31 May 2018 The coverage of elevated thunderstorms in the beginning of the forecast period looks increasingly uncertain, with some models reducing the extent (Euro4 most pronounced in coverage). Increasing confidence though in the extent & location of surface based storms within the 12z model suite (MDT Region On Map). This is, however, a dynamic setup, radar & satellite will be useful tools in this forecast period. Criteria Key: LOW (Grey) = 10 - 20% of lightning within 25 miles SLGT (Green) = 20 - 45% of lightning within 25 miles MDT (Yellow) = 45 - 60% of lightning within 25 miles High (Red) = >60% of lightning within 25 miles Severe (Dotted) = Notable threat to life and/or buildings

What makes you believe that?

The Somerset, Dorset & Wiltshire region look favorable, though given the time range there is sufficient wiggle room to migrate the main risk areas west or east.

*Self Forecast (Produced By Myself) Day 2 Convective Outlook VALID 6:00 UTC Thurs 31 May 2018 - 11:59 UTC Thurs 31 May 2018 There is reasonable signs (model consensus) for an area of elevated convection to be drifting across S/SE at the beginning of the forecast period, continuing to migrate NW - wards through the day. Questionable as to the extent of organisation, with forecast profiles presenting limited wind shear across the risk zone. Subtle weakening is likely as the convection transfers NW - wards, due to weakening mid level instability. Despite minimal wind shear, storms will have frequent lightning and *occasional hail*(Largely SE). Storms have the potential to produce local disruption with flash flooding, with precipitable water values >30 mm. Elevated convection could transfer into surface based (root in boundary level) as the trough extends NW - wards, though this could be a messy transition and is currently regarded to be unlikely due to excessive cloud cover (hence subtle weakening on NW - wards trajectory favored). Behind the trough, insolation will yield around 1000 J/kg of CAPE (1,500 J/kg GFS). This will potentially lead to the development of isolated thunderstorms, again producing; frequent lightning, occasional hail and substantial local rainfall. Any one region could see a storm, though development is dependent on the progress of the earlier convective activity.

The recent period of convective activity has seen numerous episodes of elevated convection, providing frequent lightning and substantial local rainfall amounts. However, my level of understanding about convective dynamics is very basic, especially with regards to elevated convection. My struggles often evolve understanding forecasted 'Soundings' (GFS), and using this as a predictive tool. This thread is intended to provide an area where those whom understand the dynamics of elevated convection can help guide enthusiastic learners on this forum, by sharing explanations and literature on the topic. I recall 'Skew T's' has been discussed on a broad scale in previous threads, though a more detailed breakdown in there use in forecasting elevated outbreaks would also be very helpful! Any responses would be greatly appreciated by me, and I suspect many others on this great forum. *Please can this thread stay formal in tone, it is intended as an area for learning (not chit chatter about storm prospects)!

Yes that is another area of concern, these issues could be discussed within a learning section of the forum. 'Basic model interpretation techniques & automated/text forecast use'...has a ring to it . Anyway, enough conversation around this, do not want to clutter this thread.

The comments made ('The PIT') highlight the difficult position the Met Office hold in the UK. Comments made about contradiction also bemuses me, the Met Office is not an organisation compiled to support the enthusiasm of forum users! In essence there job is forecasting in relation to public safety, hence the often broad nature of warnings and occasional deviation away from other organisational forecasts. It often infuriates me when people compare warnings and automated forecasts in the same sentence, I feel there is a real need for a learning area on this forum, one that teaches the basic inferences that can be made from automated forecasts in conjunction with warnings or associated storm forecasts!

Indeed, I find it also weird with the potential for thunderstorm's not heralding a breakdown. Very much breaks with tradition here in the UK.

That is not really a logical argument unless this idea was placed to all the models currently being analysed. Why exactly would the GFS in particular being out of kilter? I'm not having a go on a personal level, but we need more science to back these claims up!

HOTTEST EARLY MAY BANK HOLIDAY WEEKEND EVER NOW 28.7c REACHED AT NORTHOLT

Gosport, Hampshire has reached 24.7c (BBC Weather)

Yes...Yes that is the case . In all seriousness, 23.6c seems rather low even with the common dull weather that accompanies bank - holiday weather!

Indeed, such a conclusion follows the dialogue of 6 -30 day Met Office forecast . The biggest question (within the 10 day range) is the local of the focus of increasing heights, with the; ECM, GFS, UKMO & GEM all showing variations on a theme. The ECM on a synoptic level is ideal for sustaining the anticyclonic spell while maintaining above average(perhaps notable) temperatures.

22 max in London, you guys really need to gain some experience .