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marksiwnc

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    Wales/Cymru Regional Weather Discussion

    in Regional

    Posted

    4 minutes ago, Ian Price said:

    that is strange isn't it!!

    However I tend to ignore those computerised "table type" forecasts and look at the synoptic charts instead such as this GFS one :)

     

    GFSOPUK06_6_9.png

    It was snowing in penygroes this morning deffo.  Although Llantrisant is quite far from here lol

    Model Output Discussion 29th December - Into mid-Winter.

    in Forecast Model Discussion

    Posted · Edited by marksiwnc

    The Arctic Oscillation (AO) is currently positive and is predicted to first trend positive week one and then trend negative back to neutral week two.

    The weakly positive AO is reflective of mostly negative pressure/geopotential height anomalies in the central Arctic and mixed pressure/geopotential height anomalies in the mid-latitudes. With mixed heights over Greenland and Iceland but positive heights across the mid-latitudes of the North Atlantic, the North Atlantic Oscillation (NAO) is currently positive.  With heights predicted to rise slowly near Greenland the next two weeks, the NAO is also predicted to slowly trend negative. 

    With strong oceanic ridging/blocking cold temperatures have become widespread across the Northern Hemisphere (NH).

    In general, Eurasian temperatures are predicted to follow the AO trends first turning milder and then colder again.

    Much of North America is in a clear trend to milder weather as a strong polar low is predicted over Alaska that will result in a mild, southwesterly flow of air downstream over much of North America.  However the mild trend will be briefly interrupted by an Arctic outbreak at the end of this week.

    With near normal poleward heat transport, the stratospheric polar vortex (PV) has remained relatively strong.  However the weather models are predicting the PV to become more disturbed over the coming weeks.  A more highly perturbed PV could have an important influence on surface weather but for now that remains highly uncertain.

    I have been anticipating a stratospheric PV disruption beginning in the latter half of January that will have an impact on the weather; however, as far as I can tell the stratosphere and troposphere remain mostly uncoupled.  Therefore weather patterns are more transitory and that will likely continue unless there is a significant disruption to the stratospheric PV.

    Impacts

    It is my opinion that the stratospheric and tropospheric PVs were coupled in the fall and through much of December.  In December they were coupled in such a way that resulted in cold temperatures in western North America and western Asia.  Later in December they were coupled that the PV was strong in both the stratosphere and troposphere that resulted in an overall milder pattern across the NH.  But since the end of December the stratosphere and troposphere seemed to have gone their separate ways.  The polar cap geopotential height anomalies (PCHs) warmed in the troposphere characterized by blocking in the northern latitudes of the two ocean basins that resulted in colder temperatures for both continents.  In contrast the PCHs have stayed cold in the stratosphere and for now the Global Forecast System model (GFS) is predicting that the two remain mostly uncoupled.

    In the absence of coupling between the stratosphere and the troposphere the weather is likely to remain transitory.  North American weather is sensitive to variability in the atmospheric circulation in the North Pacific.  So far in January there has been strong blocking/positive geopotential height anomalies near Alaska resulting in cold temperatures downstream in Canada and the United States (US).  However the pattern is predicted to reverse with a deep polar low centered over Alaska that will result in a southwesterly flow of mild, maritime air from the North Pacific across much of Canada and the US.  My expectations are that this pattern will be transitory as well with a cooling trend towards the end of January.

    Across the larger Eurasian continent, the weather has been more stable with cold temperatures across Siberia and warmer temperatures across South Asia.  The cold air from Siberia has been transported to the southwest into southwest Asia and Southeastern Europe by blocking in the Barents-Kara seas.  Closer to the blocking/positive geopotential height anomalies across Northern Europe including Scandinavia and the United Kingdom (UK), temperatures have been milder.  Variability in the pattern is expected but the overall pattern looks to persist in my opinion.

    My expectations have been for the stratospheric PV to become more highly perturbed the second half of January.  There are signs that the stratospheric PV will become more perturbed in the coming weeks but I feel that it remains highly uncertain how much and what will be the impact on the weather.  I don’t see an ideal pattern to excite strong upward Wave Activity Flux (WAFz)/poleward heat transport in the next two weeks.  Until the stratosphere and troposphere become coupled, in either direction with strong or weakly coupled PVs, my ability to anticipate multi-week changes in the weather is limited.

     

    Brief update by judah 

    Model Output Discussion 29th December - Into mid-Winter.

    in Forecast Model Discussion

    Posted

    1 minute ago, nick sussex said:

    NCEP just updated. Good news.

    SO OVERALL...THE 12Z AND 00Z ECMWF APPEAR TO BE THE BEST FIT
    OVERALL TO THE PREFERENCE AT ALL FORECAST HOURS...BUT THE 12Z
    UKMET/CMC LOOK REASONABLY SIMILAR TO THE ECMWF. CONFIDENCE IS ONLY
    AVERAGE GIVEN MODEST CHANGES FROM RUN TO RUN AND MODERATE ENSEMBLE
    SPREAD.
     

    Thankfully no room for the GFS! Remember this is just the short term but crucially this is where the differences start with the GFS.

    Let's just hope the ECM is correct and that high pressure will continue to rise in the Atlantic so everyone can have snow.  I'm not feeling confident on snow in my area Thursday/Friday even is there's weather warnings 

    Model output discussion - 5th December 2016 - Into Winter we go

    in Forecast Model Discussion

    Posted

    Just a quick question.  With waters quite warm around the UK and the burst of frigid cold coming down would this not promote some polar lows?  If polar lows do develop then surely the outlook could be anything.  I remember a low dropping from Iceland was a possibility yesterday could this low be a polar low development and the models are not able to pick it up until last few hours??? 

    • Like 3

    Model output discussion - 5th December 2016 - Into Winter we go

    in Forecast Model Discussion

    Posted

    Ian f did mention that they (met)  thought that a zonal period was probable between the high switching from our East to our North East I believe.  I think this might be the way this pans out. Hopefully it would be able to dig a bit further north west..  hope gp puts a post the next few hours to see where he thinks we could be heading.  give us (sacra members) some good news boys 

    • Like 3

    Model output discussion - 5th December 2016 - Into Winter we go

    in Forecast Model Discussion

    Posted

    56 minutes ago, Glacier Point said:

    12z data showing a shift in Equatorial low level westerly winds and convective anomalies much further east compared to 00z and 06z data. Be interesting to see what if any difference this makes to tonight's models.

    Upper level winds still more easterly dominated but two out of three tropical forcing facets demonstrating an eastward trend [and therefore less likely to be destructively interfering on the downstream pattern].

    What impacts could this cause for us in the UK?  what's your take on things regarding stratospheric warming? 

    • Like 2

    Model Output Discussions 12z 30/11/16

    in Forecast Model Discussion

    Posted

    2 minutes ago, nick sussex said:

    I think if we had the normal PV stuck over Greenland and jet on steroids then it would be less frustrating because in that case you know the NH pattern is really against cold. However we seem to be in this situation where the outputs should be delivering much better synoptics and we're getting zip. The chart you posted highlights this well, somehow with the main PV in Siberia, a negative AO the other chunks of the PV well to the nw you'd expect the bit in the middle to not be so underwhelming.

    Still about 10 days away I think so a lot can change.  Remember what's been said today.  it may change round again soon as models struggle with quite different scenarios to what we are used too.. 

    Model Output Discussions 12z 30/11/16

    in Forecast Model Discussion

    Posted

    7 minutes ago, Glacier Point said:

    Theoretically, EC46 dayer should show continuity with previous, possibly shifting back a little on time scales if the MJO component is being modelled correctly.

    EMON_phase_51m_full (2).jpg

    It is showing some dawdling of the convective signal in the Indian Ocean and then eastward propagation. 

    Thanks to you and tamara for your amazing input.  I did say in the last page that I believe the weekend of Xmas is when we could see the transition.  but it's just a guess at the moment.  what do you think gp?  it seems that all changes in weather happen around Xmas every time lol 

    • Like 3

    Model Output Discussions 12z 30/11/16

    in Forecast Model Discussion

    Posted

    15 minutes ago, Singularity said:

    Concurs with GP's take on things and, quite surely, that of GloSea5.

    Also suggests that even the Met Office can't resolve the tropical situation just yet. Standing by my suspicion (well, hope!) that better output should be with us by early next week.

    This was his reply to me this morning. I'm thinking the transition to mobility and back to blocked weather is causing mayhem.  personally I think cold will come but I'm thinking around the weekend of Xmas rather than mid month.  we shall see 

    • Like 1

    Model Output Discussions 06z 04/11/16

    in Forecast Model Discussion

    Posted

    Models are settling down after a hard time finding out where the high would settle.  I think it may travel a bit more ne during nxt week dragging colder air down to us.  Hopefully there will be some snow chances from the middle of nxt week onwards.  probably only grazing the SW and south maybe.  what do you think gp,fergieweather??! 

    • Like 2
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