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IanT

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    Storms and Convective discussion- 7th August onwards

    in Storms & Severe Weather

    Posted

    9 minutes ago, Paul Sherman said:

    In a Nutshell as follows

    Storms will initiate over Inland parts of Sussex and Surrey and move through the M4 Corridoor and as the Convergence zone sags south more will form this evening over South Hants, IOW, West Sussex before dying off in the early hours.

    So its game over for the COS & TOD but the SOR may well fall today

    I recognise that these are parochial terms, but could someone please offer a rough reminder / definition of each for those of us who have missed a few posts along the way...?

    • Thanks 1

    Storms and Convective discussion- 7th August onwards

    in Storms & Severe Weather

    Posted

    12 minutes ago, SqueakheartLW said:

    No wonder it's so severe in Wales and the SW Midlands. This is a snapshot of surface winds over the UK

    Untitled.thumb.jpg.99796cae7276bf3d71abb6ecbaf05397.jpg

    Look at all those various wind directions. Northerlies over Wales, Southerlies over SW England and easterlies over the rest of England. Also there appears to be an independent rotational feature showing up too, maybe due to supercell action.

    Good spot / interesting post. Thank you...

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    Cold hunt - models and chat

    in Spring Weather Discussion

    Posted

    38 minutes ago, SLEETY said:

     

     

    So close you can't rule it out with the back tracking going on this morning. Terrible performance by all the models whatever the final outcome. 

    Terrible performance? 

    What is going to happen next week may well have changed. To suggest that a forecast updated by 48h more data is “terrible” is a viewpoint underpinned by an implicit assumption that the weather is a deterministic system in which no randomness is involved in the development of future states. A deterministic system being one that will always produce the same outcome from a given starting condition or initial state.

    But I think that we all understand innately that weather is far from deterministic. There are chaotic elements - the proverbial light left on in Fairbanks etc. - which influence outcomes. As forecast horizons extend the scope for these to have an impact increases. Given a specific set of starting conditions, multiple outcomes are possible. The weather that we get is one realisation, but there may have been other equally plausible outcomes at the point the forecast was made.

    Criticism of NWP outputs on this board (and others!) often follows a pattern. Posters will say that the models are "struggling to tie down" a feature, or that the models are "always poor in this type of setup". With the benefit of 20/20 hindsight, posters will say the model was "wrong because it failed to pick up ... [height rises over the Azores?]".  Any verification gap is thus attributed to modelling weakness/failure, overlooking the possibility that something changed between the point at which the forecast was made and its realisation.

    What is the point of all of this? I guess it's a plea for acceptance that the models aren't "searching for a solution". We shouldn't criticise them when they "flip-flop" from one outcome to another, especially where longer time periods are involved. There simply isn't one path that the models have to "lock-on" to. Weather has chaotic elements. If the models this afternoon don't show the extended cold spell they were showing yesterday it might not be because they are inaccurate. Something might well have changed!

    NWP models are wonderful tools for forecasters. I'm sure that they have massively improved forecasting accuracy in recent years, and that these improvements will continue. But we should always remember that weather is not a deterministic system, and modulate our expectations and responses accordingly.

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    Model output discussion - 7th January onwards

    in Forecast Model Discussion

    Posted

    2 minutes ago, bluearmy said:

    rather than looking for reasons that the pattern will establish, we should be looking for reasons that it wont …...the modelling is rock solid now on this last third of jan becoming v cold and wintry ….. what can scupper that ?  we are usually searching for bits of evidence that it might become cold at some point going forward ……. i suggest we are now entering a spell where the opposite is true. as such, posters shouldn't become exercised by posts which illustrate how this cold period might not verify …….. 

    “...rock solid...”?

    Whilst I understand and support the sentiment behind your post, I think “...rock solid...” is WAY too strong a phrase for UK weather that is still 12 days away...!

    • Like 18
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