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thestixx

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    Model Output Discussions 12z 03/05/2016

    in Forecast Model Discussion

    Posted

    2 hours ago, cheeky_monkey said:

    Don't forget the models are not modelling the weather just for the UK but the whole globe..we are just a very small area so even if the models are 1% inaccurate anywhere this has a huge implication on the weather predicted and the actual weather observed

    It never dawned on me, I just presumed there'd be a programme for the North Atlantic/Euro region. 

    Model Output Discussions 12z 03/05/2016

    in Forecast Model Discussion

    Posted

    29 minutes ago, Vorticity0123 said:

    June has been an interesting month for many parts fo Western Europe, featuring excessive rainfall and floods for several parts of e.g. Germany, Switzerland and the Netherlands. This was caused by relative weak pressure differences over mainland Europe, on average rather low pressure and relatively moist air. However, July has shown a turnaround in the pattern. A steady westerly flow has brought a series of troughs and low pressure areas towards Europe, resulting in mostly changeable weather. Will this westerly flow persist, or will we see a significant change in the weather patterns to come?

    Westerly flow - but ridge to come

    The westerly flow has also been dominating today with low pressure to the north of the UK and high pressure to the south. However, change is on the way if we look upstream - just south of Greenland.

    GFScurrent_EDIT.jpg

    GFS analysis of 500 hPa heights (colours) and surface pressure (white contours) for Sunday 12Z.

    To the south of Greenland, a weak ridge can be identified mainly in the upper levels (orange colours pointing northward). Often such high pressure areas (or ridges) tend to stabilize the weather for some time.

    - But for how long?

    The ridge will pass by the UK around this Wednesday, but it does not seem to be a rather transient feature. This fits very well in the pattern which we have observed over the past week or so - with ridges and troughs quickly alternating in a meandering westerly flow.

    GFSwed12_EDIT.jpg

    GFS forecast of 500 hPa heights (colours) and surface pressure (white contours) for Wednesday 12Z.

    From the west a new low pressure area (green colours) is again approaching the UK. But will it also reach the UK? The models agree that this will somewhere around the weekend. However, one can also see notable differences developing between these models on the details of the low pressure area.

    (Un)certainty and phase differences

    The further outlook is what one could call certain as well as uncertain - depending on the point of view. Although there is a lot that can be said about the general pattern - it is about nigh impossible to have a certain local forecast in 6 days out or more! And we can caputure this (un)certainty in just one plot.

    GFSspaghetti.JPG

    GFS ensemble spaghetti forecast of 500 hPa height for Monday (8 days out) 12Z.

    The image above shows an ensemble of GFS forecasts for 8 days out. Here one should focus on the bottom lines, indicating the height of the so-called 5760 dam (5.76 km height) surface. Each line represents one model run.

    First of all, it can be seen that all lines are generally very close together for the 576 dam line, suggesting high certainty in the overall forecast. This forecast suggests low pressure to the north and high pressure to the south, with a potent westerly flow in between. One can also see this from all lines running west-east. See also the forecast of the CPC (Climate prediction center) for day 6-10.

    However, there is one huge caveat if one wants to apply this to a local weather forecast. If you examine all the lines individually, you can see that there are slight meanders in them. The position of these weak meanders (which can be interpreted as ridges if they point northward and troughs if they point southward) varies from run to run. However, the exact position of such a small scale feature is very important for the weather on that day. For example, a ridge for a given day (a northward pointing line) would argue for settled conditions, and a downward-pointing line would suggest a trough and thereby unsettled conditions. This uncertainty is what one could call phase differences, as depicted in the figure below.

    Phasediff.png

    Illustration of phase differences between various model runs. Note how the location of the ridges (H) and troughs (L) differ strongly, an indication of a phase difference.

    Summary

    It appears that the westerly flow which has been present over the past few days will continue to persist for at least the next week (and probably thereafter). This will create changeable weather (with a possible drier interlude on tuesday-friday). Details at long range are hard to gauge due to the phase differences discussed above, but it is pretty certain that the main theme will be changeable weather.

     

     

    Thankyou for taking the time to out together such an informative post. 

     

     

    • Like 1

    Model Output Discussions 12z 03/05/2016

    in Forecast Model Discussion

    Posted

    Hi everyone, it's been a long time since I last posted on here, but as a long term lurker I'm seeing some pretty good signals for our summer.... High pressure is in an abundance which is always good news for heat lovers like me. I might be off kilter here but are there any similarities with 2003 with regards to the scale of HP ridging in the from the Atlantic/Azores?

    • Like 2

    Model Output Discussion

    in Forecast Model Discussion

    Posted · Edited by thestixx

    I view this forum daily and have done for many years, gaining lots of insight along the way, but I'm getting really wound up by posts of FI charts from 1 run from 1 model, It's a waste of everyones time (in my opinion). RANT OVER

    The fact we've got High pressure sniffing around the British Isles again after such a miserable June/Early July (here in Leeds) puts a huge smile on my face. I'm looking forward to the next few relatively settled days too, The sunshine felt very pleasant today. I've got a hunch that High pressure will be around for a while too....

    Model Output Discussion

    in Weather reports

    Posted · Edited by thestixx

    No sign of heat but worryingly the trend is for little or no rain going out into the mid timeframe. If high pressure continues to assert its dominance, this year surely has a chance at being the driest on record.

    Talk about putting the commentators curse on matters, Talk like that can surely only bring rain and lots of it in August

    Glastonbury Forecast 2010

    in Spring Weather Discussion

    Posted

    I'm just hoping that with this high pressure being slowly pushed back from fully covering the UK, by the time it finally does it's influence will still be felt come Gladstock. This years is my 11th and I've experienced all aspects of weather, Altho I'd love sunsheeiinne throughout, as long as it stays dry I'll be a happy chappy!! Fingers crossed. I'm going down Tuesday night in readiness for the England game on weds too so not long for me to wait to see the fest weather in more reliable time frame.

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