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Stratosphere watch 2020-21

  • 17-11-2020 3:19pm
    #1
    Registered Users Posts: 12,554 ✭✭✭✭ sryanbruen


    Relatively late with starting this new thread. Before we look at the foreseeable future, let's have a look at the previous season although I don't think many want to :P

    The season was dominated by a much stronger than normal stratospheric polar vortex (SPV) which meant the Northern Hemisphere had a very mild winter, in fact the mildest on record for many. There was very little blocking on offer in the high latitudes and there was very little warming to disturb this strong SPV from developing. There were 2 minor warming events through the season, first one in early November and the other in early February but the latter did not do much at all.

    The final warming took place on 29 April which was 14 days later than average. This was unusual as winters with a major SSW event tend to have very late dynamical final warmings (such as 2019 did) but 2019-20 had no major SSW.

    It certainly was no varied season like the previous two, 2017-18 and 2018-19, were but it was yet another notable season for the stratosphere.

    gaknl1f.jpg

    You can look at other years back to 1979 here for yourself if you so wish to.

    For those that are new, the earth's atmosphere is divided into different layers. The troposphere is where weather occurs and the stratosphere is just above the troposphere. Much like the weather around the earth, the stratospheric temperature varies by season with it reaching its lowest point in December on average. Each autumn, a phenomena known as the stratospheric polar vortex forms within the Arctic Circle. This is an upper-level area of low pressure formed by the temperature difference between the equator and the pole. The vortex weakens and strengthens from year to year via dynamic events like stratospheric warming or the annual summer hibernation where the SPV "goes to sleep" due to a lower temperature gradient between the equator and the pole. The SPV is well defined when it's strong with a single vortex lying within the Arctic Circle.

    Normally, the troposphere and stratosphere are in sync with each other which results in downwelling. This is when planetary waves are propagated to either level of the atmosphere via the weather patterns that occur. For example, an Aleutian Low (low pressure over or around Alaska) / Scandinavian High (high pressure over Scandinavia) combination can be a sign of disrupting the Polar Vortex and a precursor to a SSW event as a result. Another example is that with a cold stratosphere and healthy SPV, the North Atlantic jet stream is powered up bringing mild and moist conditions to western Europe with low pressure centred over Iceland. Tropospheric patterns that can have an impact on the SPV are defined by zonal wavenumbers which I would like to direct you to GL's excellent post from a 2018-19 thread here as I couldn't have explained it better myself.

    There are different types of warming that occur and can have different degrees of impacts on the weather around the Northern Hemisphere if a tropospheric response occurs.

    There is a minor stratospheric warming which involves the stratospheric temperature rising but less dramatically compared to a major warming and the zonal mean zonal winds do not reverse.

    A major sudden stratospheric warming involves the zonal mean zonal winds at 60N 10hPa in the stratosphere to reverse from westerly to easterly. The SPV is completely disrupted and it will either be split into two or more vortices OR displaced from its normal location over the North Pole. Major SSW events are forced by tropospheric patterns disturbing the SPV but sometimes, these events can propagate back down into the troposphere and result in anomalously blocked patterns which gives a higher chance of colder weather for Europe. However, even if a SSW downwells successfully into the troposphere, the positioning of the blocking is a factor to consider also.

    Cohen and Jones (2011) did a good paper on tropospheric precursors and categorising past SSW events into splits and displacements here, it's free access to everybody.

    Met Éireann highlighted previous Irish cold spells and if a SSW could have caused them in their Storm Emma paper here.

    Other warming events that occur include final warming events which indicate the transition into stratospheric summer hibernation mode and Canadian warming events which occur earlier in the winter.

    The foreseeable future shows the stratospheric polar vortex continuing to strengthen at a greater amplitude than normal. No signs of weakening any time soon.

    Ia2u2g9.png

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«1345

Comments

  • Registered Users Posts: 12,554 ✭✭✭✭ sryanbruen


    GFS continues to forecast a stronger than normal stratospheric polar vortex (SPV) and set to develop further in the foreseeable future. As a result, the zonal mean zonal winds will stay firmly westerly in the stratosphere.

    Ptfymrl.png

    The FI period of the past few GFS runs have started to show a minor stratospheric warming occurring over Siberia displacing the strong SPV from its normal position somewhat and maybe some slight disruption if the GFS were to go on a bit longer.

    However, we see minor SSW events (where the zonal mean zonal winds at 60N 10hPa do not reverse from west to east) a few times per winter on average. Yes, even last winter had 2 such events as illustrated by the first graph shown in the initial post. So nothing unusual going on here and it is ultimate FI stuff in the GFS.

    s7BXuPV.png

    ECM's extended ensembles do show a weakening SPV that is presumably caused by the same minor warming which the GFS shows above.

    yEOHhfZ.png

    The strong SPV is indicated by the positive Arctic Oscillation (or as it's called in this case, the Northern Annular Mode) in the stratosphere with the orange colours in the below chart. The trop and the strat are disconnected at the moment with the trop not really feeling the impacts of the strong polar vortex within the stratosphere. This is set to continue according to the latest GFS with the exception of the first few days of December when positive AO conditions are forecast at 1000hPa.

    VCy5sVj.png

    All in all, not much different from the previous post. The SPV remains healthy for now.

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  • Registered Users Posts: 87 ✭✭ witzky


    I wish I knew what any of that meant 😕


  • Registered Users Posts: 12,554 ✭✭✭✭ sryanbruen


    witzky wrote: »
    I wish I knew what any of that meant ��

    I tried simplifying the best I can in the original post under the first graph.

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  • Registered Users Posts: 1,858 ✭✭✭ Artane2002


    Ventrice is beginning to mention the possible warming being indicated by the GFS. I'm no expert but surely we wouldn't want a warming over Siberia? Wouldn't it force the PV back to Canada and then make it difficult for cold weather to occur?

    https://twitter.com/MJVentrice/status/1331963606943084551


  • Registered Users Posts: 8,195 ✭✭✭ Gaoth Laidir


    witzky wrote: »
    I wish I knew what any of that meant ��

    Basically, the chart below shows the value of an index called the Northern Annular Mode, which is an indicator of how strong the polar vortex is. A positive value means the vortex is strong (with a strong westerly jetstream) and therefore the cold air is "trapped" up over the Pole. A negative value means that the vortex is weaker, with some slowing down and north-south meandering of the jetstream. This can theoretically allow cold air to flow further south and bring cold weather to lower latitudes.

    Moving from left to right shows how it's been gradually shifting towards a more positive value, especially in the stratosphere (above ~200 hPa), which is a natural shift during winter as the stratosphere cools and the vortex spins up. We'd expect these strong westerly stratospheric winds to continue throughout the winter and up until the spring, when the rising sun warms up the polar stratosphere again, causing the vortex to break down and the stratospheric winds to shift to easterly for the summer.

    It's not that simple, of course. During the winter, warming sometimes occurs in the stratosphere, temporarily breaking down the vortex and flipping the NAM to negative. If conditions are right, cold outbreaks can occur. The chart below does show a slight weakening of the index in the 2nd week of December. Exactly what that will mean for our weather, if anything, remains to be seen.

    VCy5sVj.png


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  • Registered Users Posts: 12,554 ✭✭✭✭ sryanbruen


    Can't show charts right now but the signals continue for that minor warming and getting closer to the more reliable timeframe and as a consequence, the SPV will get a disturbance and be displaced from the pole as well as weaken somewhat to less or greater extent.

    No changes from the last update for the more immediate future but GFS has began to back off a bit from what was looking like an agreement with the ECM on quite a prolonged weak vortex. Saying that, the GFS has had some wild swings in FI as of late, think the focus should be on a much sooner timeframe than that for now.

    Strat and trop continue to look disconnected too again to less or greater extents on different models/timeframes.

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  • Registered Users Posts: 14,307 ✭✭✭✭ nacho libre


    So hopefully any return to a positive NAO state by mid month is short lived. How are the zonal winds looking for later in the month?


  • Registered Users Posts: 12,554 ✭✭✭✭ sryanbruen


    10hPa is starting to warm up. Now just slightly under average

    iFAEPae.gif

    Looking at the GFS for 1300 today, this should be no surprise as there has been a minor warming towards northeastern Asia which has been enough to displace the stratospheric polar vortex (SPV) from the pole. This is why there has been a slight spike and a little more warming is likely in the coming week before flatlining and drop somewhat next week again.

    The GFS has been playing around in FI with another minor warming that would disturb the SPV further. Like previously mentioned, the runs show disturbances to varying degrees but the trend is definitely for a weaker vortex due to these minor warming events.

    HtWMhqh.png

    qJQkFDp.gif

    The GFS zonal mean zonal wind forecast at 60N 10hPa confirms what I was saying about the weakening of the vortex but as it's quite minor, not a reversal.

    Ob8byh0.png

    No sign of a major SSW but a weakening of the SPV is on the way.

    The disconnection between the strat and trop goes on regardless with the tropospheric polar vortex very messy. Some recent models have been playing around with the possibility of a split here which would be somewhat unusual to see as it would likely be all tropospheric driven.

    aSd21w0.png

    EC 46 day forecast has yet to update.

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  • Registered Users Posts: 14,307 ✭✭✭✭ nacho libre


    You would have to think events in the troposphere such as the major Russian high is having some influence on the stratospheric vortex longer term. Some strat' experts are of the view that tropospheric forcing can eventually lead to significant warming in the stratosphere.
    Whether it leads to a split longer term remains to be seen, but some are of the view we may get a split sometime in January


  • Registered Users Posts: 12,554 ✭✭✭✭ sryanbruen


    You would have to think events in the troposphere such as the major Russian high is having some influence on the stratospheric vortex longer term. Some strat' experts are of the view that tropospheric forcing can eventually lead to significant warming in the stratosphere.
    Whether it leads to a split longer term to be seen, but some are of the view we may get a split sometime in January

    Yes, the Russian high is the key player here to propagating atmospheric waves upward and disturb the SPV.

    I would argue that the very end of the GFS run is the kind of situation you would want going into a SPV split. It looks prone to doing so but a matter then would be of getting it into the reliable timeframe. Fortunately, the stratosphere is easier to forecast than our actual surface conditions so more reliability but caution always required regardless.

    I remember the February 2018 major SSW event and how initially models picked up on a weakening after an active January and that it was relatively minor. The split and extreme warming that occurred was suddenly picked upon only in early February if I recall correctly. Remember teasing in a post saying there would be a risk of the Beast being unleashed if this split were to occur then and of course that indeed happened. February 2018 was an extreme example however but shows what can happen and why I mention it.

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  • Registered Users Posts: 12,554 ✭✭✭✭ sryanbruen


    sryanbruen wrote: »
    Yes, the Russian high is the key player here to propagating atmospheric waves upward and disturb the SPV.

    I would argue that the very end of the GFS run is the kind of situation you would want going into a SPV split. It looks prone to doing so but a matter then would be of getting it into the reliable timeframe.

    Just for fun for now... the latest CFSv2 members show this series of wave-1 displacement minor warming events becoming a major SSW event in early January with the possibility of a wave-2 split.

    I see this as a realistic possibility however given the current model trends between the GFS and ECM so certainly wouldn't rule it out. Apparently UKMO Glosea5 (which we do not have access to) is singing from the same hymn sheet too... take from that what you will.

    I might have let the cat out with the pigeons but here here, let's remain calm until we get to then shall we :P Then it's a matter of seeing if we can get downward propagation after.

    WJKThL9.png

    Despite the above, I haven't felt this excited since 2017-18...

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  • Registered Users Posts: 1,085 ✭✭✭ Billcarson


    If an ssw were to occur in early jan I presume that would mean the possibility of a cold spell in the second half of Jan. We haven't had a second half of Jan cold spell in yrs . It would be nice to get a cold spell then right in the heart of winter.


  • Registered Users Posts: 12,554 ✭✭✭✭ sryanbruen


    Billcarson wrote: »
    If an ssw were to occur in early jan I presume that would mean the possibility of a cold spell in the second half of Jan.

    If we get a downward propagation and it's a relatively quick tropospheric response which is usually the case with splits, the straightforward answer is yes.

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  • Registered Users Posts: 14,307 ✭✭✭✭ nacho libre


    sryanbruen wrote: »
    I might have let the cat out with the pigeons but here here, let's remain calm until we get to then shall we :P Then it's a matter of seeing if we can get downward propagation after.

    5425869312

    A good sign if Glosea5 thinks there will be a SSW.
    If there is one, let's hope for a quick response like February 2018, a beast from the east in the heart of winter would be the ideal scenario


  • Moderators, Science, Health & Environment Moderators Posts: 15,175 Mod ✭✭✭✭ Gonzo


    GFS parallel going for a more significant warming compared to the operational run, almost verging on SSW levels and the PV getting stretched and displaced towards Europe, looking good.

    gfsnh-10-336.png?0

    gfsnh-10-384.png?0


  • Registered Users Posts: 17,796 ✭✭✭✭ hatrickpatrick


    sryanbruen wrote: »
    If we get a downward propagation and it's a relatively quick tropospheric response which is usually the case with splits, the straightforward answer is yes.

    I think one of the reasons people are more cautious about this recently is because people remember the last time we had a major SSW resulting in a split vortex during December, only for it to downwell so ridiculously slowly that it was already Spring before any blocking occurred. This may have been early 2019?

    Do we as yet know why that particular split took so long to make its way down to the troposphere, and whether we can predict that in advance based on teleconnections?


  • Registered Users Posts: 12,554 ✭✭✭✭ sryanbruen


    You didn't get that information and charts from GavsWeatherVids by any chance did ya Gonzo ;)

    But for real though, the developments right now in the works are very interesting from a stratospheric standpoint.

    The prospects are for a series of minor wave-1 displacement events to occur through mid into late December before possibilities increase of a wave-2 split occurring in the New Year. I've seen some comparison between evolution of this and what happened in 2012-13 which was quite a blocked December in the higher latitudes and this eventually resulted in a major wave-1 displacement of the stratospheric polar vortex during the first few days of January 2013 before it was ripped apart by a wave-2 split through the second week of January.

    Every event is unique however gives us an idea of what can happen and how events unfold.

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  • Registered Users Posts: 12,554 ✭✭✭✭ sryanbruen


    I think one of the reasons people are more cautious about this recently is because people remember the last time we had a major SSW resulting in a split vortex during December, only for it to downwell so ridiculously slowly that it was already Spring before any blocking occurred. This may have been early 2019?

    Do we as yet know why that particular split took so long to make its way down to the troposphere, and whether we can predict that in advance based on teleconnections?

    That wasn't anything to do with the January 2019 major SSW event, that was from the annual final warming which somewhat unusually had the dynamic wave forcing of a mid-winter major SSW event and has been discussed by Lee and Butler in the link below. I quote:
    Final stratospheric warmings are radiatively driven as the sun returns to the Arctic pole, but can also be driven by dynamic wave forcing akin to a major SSW. The FSW in April 2019 had a substantial dynamic component, with high wave activity preceding the event. This developed an unusually intense Aleutian high which displaced the weakening SPV and produced date-record strong easterly U1060 in early May (a minimum of -20.4 m s-1 was reached on 4 May). Although the envelope of variability becomes smaller into the summer, U1060 remained close to date-record minima through June.

    http://centaur.reading.ac.uk/87110/1/lee_butler_weather_accepted.pdf

    Unlike February 2018, the January 2019 major SSW event did not couple with tropospheric weather patterns to produce a negative NAO whilst the strong vortex event that occurred in February/March 2019 did produce a strongly positive NAO.

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  • Registered Users Posts: 14,307 ✭✭✭✭ nacho libre


    Speaking of Amy Butler, she seems confident of an SSW event sometimes in January. So along with glosea5 things do look very promising.
    We just have to hope that if the Russian High is still on the scene, it does not turn out to be a spoiler- sometimes major warmings can actually flip a promising troposhere pattern, whereby we end up on the mild side of arctic air plunging south.


  • Registered Users Posts: 1,085 ✭✭✭ Billcarson


    [QUOTE=nacho libre;115552981
    We just have to hope that if the Russian High is still on the scene, it does not turn out to be a spoiler- sometimes major warmings can actually flip a promising troposhere pattern, whereby we end up on the mild side of arctic air plunging south.[/QUOTE]

    I think that's what happened in feb 09. A decent cold pattern during the first half of that month but a mild second half which I think was caused by a ssw which ruined the pattern.


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  • Registered Users Posts: 12,554 ✭✭✭✭ sryanbruen


    Can't show charts at the moment but looks like models now showing a more significant re-intensification of the stratospheric polar vortex in the short term following the very minor displacement.

    However, despite the above, the trop and strat remain disconnected with one another.

    As for longer term with prospects on the chances of a polar vortex split or a more significant warming, the CFSv2 has backed off that significantly. Looks to me that the ECM and the UKMO Glosea5 have fared the same with any ideas of such being pushed back to at least mid-January. This is a very long time away and when I mentioned it, it was a wild card.

    To end on some good news - well if you're a stratosphere observer like me, seasonal stratospheric zonal wind forecasts have been added to the Copernicus site with the UKMO Glosea5 included.

    https://twitter.com/SimonLeeWx/status/1338174948519649281?s=20

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  • Registered Users Posts: 14,307 ✭✭✭✭ nacho libre


    Not good news at all. I did note a recent UK Met Office up date hinting at something happening in Mid January. We just have to hope if there is a ssw in mid january that it downwells quickly. It would be just our luck to get a cold spell in Mid to late February.


  • Registered Users Posts: 14,307 ✭✭✭✭ nacho libre


    image.png.e24517070985ebb2212091a3a41f7476.png

    The extent of the blocking is impressive. Of course it probably won't happen like this, but if it did the vortex would be under pressure big time


  • Registered Users Posts: 12,554 ✭✭✭✭ sryanbruen


    Not good news at all. I did note a recent UK Met Office up date hinting at something happening in Mid January. We just have to hope if there is a ssw in mid january that it downwells quickly. It would be just our luck to get a cold spell in Mid to late February.

    Been holding off on posting about the wave-1 warming that the GFS has been showing for a little bit for the last week of the year. This morning it has gone to the brim with a situation where a split would be inevitable in my opinion. However, that's only an interpretation from the 0z run, will we get to that extreme?

    A fairly significant stratospheric warming is supported by the ensembles nevertheless. Remember that this is only an average of all perturbations, there would be ones that show a weaker warming but also ones that show a more intense warming.

    WNrrTLB.png

    Still no signal for a reversal just yet which is no surprise really with the hemispheric profile.

    6ITXSIQ.png

    Reversal in the very upper strat shown on that run.

    https://twitter.com/SimonLeeWx/status/1339874890544013315?s=20

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  • Registered Users Posts: 14,307 ✭✭✭✭ nacho libre


    I see some strat' experts are indicating a SSW may occur just after the first week of January. I would love to know what the glosea5 model take on it is.


  • Registered Users Posts: 475 ✭✭ bazlers


    sryanbruen wrote: »
    Been holding off on posting about the wave-1 warming that the GFS has been showing for a little bit for the last week of the year. This morning it has gone to the brim with a situation where a split would be inevitable in my opinion. However, that's only an interpretation from the 0z run, will we get to that extreme?

    A fairly significant stratospheric warming is supported by the ensembles nevertheless. Remember that this is only an average of all perturbations, there would be ones that show a weaker warming but also ones that show a more intense warming.

    WNrrTLB.png

    Still no signal for a reversal just yet which is no surprise really with the hemispheric profile.

    6ITXSIQ.png

    Reversal in the very upper strat shown on that run.

    https://twitter.com/SimonLeeWx/status/1339874890544013315?s=20

    Sryanbruen, you fill us with so hope : )

    If you look back to 2018 can you see any correlation between now and then, as in was it showing something similar a month or so pre split? Or is each lead time to an event different?


  • Registered Users Posts: 1,858 ✭✭✭ Artane2002


    The GFS 12z is showing the strat temperatures reaching 4c on the 30th, think I saw a similar chart either on the 6z or the 0z today. I'm not sure if we want an SSW though if the trop pattern the GFS has been showing on the last 3 runs happens.


  • Moderators, Science, Health & Environment Moderators Posts: 15,175 Mod ✭✭✭✭ Gonzo


    just about getting to a SSW on the latest GFS run:

    gfsnh-10-288.png?12

    By January 3rd the PV very much under strain and almost split.

    gfsnh-10-378.png?12


  • Registered Users Posts: 1,085 ✭✭✭ Billcarson


    It would be ironic and just our luck if we got into a decent cold pattern which is possible from what the models are showing...........

    .... and then for an ssw to change the pattern to something less cold. I think I'd cry......


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  • Registered Users Posts: 12,554 ✭✭✭✭ sryanbruen


    bazlers wrote: »
    Sryanbruen, you fill us with so hope : )

    If you look back to 2018 can you see any correlation between now and then, as in was it showing something similar a month or so pre split? Or is each lead time to an event different?

    2018 was an absolute perfect evolution for a major SSW event in my eyes and getting high latitude blocking on our side of the globe with a severe cold easterly airflow. Only thing was the timing of it being in early February so the severity of the succeeding cold spell was limited somewhat by the time of year but still a truly exceptional period we had. The pattern being so extreme and blocked kind of played against us in a way as the high retrogressed over to North America and the negative NAO became western based. This allowed Storm Emma to successfully undercut the block and turn us relatively milder. So although eastern parts of the US was very mild when we were in the freezer, March and April would become extremely cold and snowy in the US. It's difficult to compare to that.

    In late January 2018, we were seeing signs of a SSW event in deep Fantasy Island of the GFS operational run. At first closer to mid-January, this was looking like a minor wave-1 warming with the PV displaced from the pole which was a start as January had been an active month for the stratospheric polar vortex (SPV). I had fairly good confidence that something more notable would come from this closer to the timeframe as MJO phasing was looking good, heat flux was good etc. I made quite a bold statement on 25 January 2018 below:
    sryanbruen wrote: »
    The question on our minds at the moment is that will the beast be unleashed in February 2018? When it comes to the bigger picture - i.e. my methodology I use to make my forecasts - it does look pretty compelling.

    The significant stratosphere warming (again not a SSW though) is still up for grabs in early February on the GFS runs, see the latest 12z for example below. Even the ECM is showing some warming going on in the stratosphere (though only at 10hPa and not at 30hPa at all).

    I will not put any bets (you should never do gambling on weather anyway) on the Beast From the East coming but what I can say is that something is certainly up in the air here.

    I also remember closer to February, some GFS runs would show the SSW event much weaker. This was back when I looked at every single run regularly which I don't do as much nowadays. I had headaches from these flips and flops despite the overall trend being evident and forcing myself to have a break just to come back immediately because the GFS would show a "massive upgrade" on the warming and a big wave-2 split to occur around 12 February followed by an even bigger split around 17 February.

    That bold statement verifying was all luck to be honest. The trop and strat could easily have not linked with one another or as in the case of Jan 2019, not project onto the NAO.

    Split events tend to have quicker tropospheric responses than displacement events.

    And has been posted above kindly by Gonzo, the latest GFS is indeed on the cusp of a split towards the end of its run. It does not quite have a reversal at 60N 10hPa (perhaps down to around 8hPa in the upper strat) but is on the verge of one. That is a fairly significant sudden stratospheric warming. It follows on from the 0z and to some extent the 06z too that looked prone to a SPV split. The 12z confirms my thoughts. If we were to get a major SSW event (zonal mean zonal winds at 60N 10hPa reversed to easterly), could be a big spanner for any seasonal forecasts which have already been poor for December.

    As for Glosea5 nacho libre, it does not show a major SSW event but is in line with the ECM of showing a weaker than average vortex.

    We shall see how it evolves but as I said in a prior post, before the longer range models backdated on the possibility of a major SSW event, this is the most excited I have been since 2017-18.

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