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minus86BriJ

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    Model output discussion 10/02/21

    in Forecast Model Discussion

    Posted

    It is only the 14th of Feb and with semi reliable FI, from an overall trend perspective, only going out to about 10 days anything could pop up come the 24th.  I seem to remember a recent, not so successful from a cold return POV, SSW that dragged on for months bringing below average temperatures all the way right through May.  I suspect more cold is to come.

    • Like 1

    Model output discussion - The run up to Christmas

    in Forecast Model Discussion

    Posted

    2 hours ago, snowking said:

    There's been a lot of focus on 850mb temperatures over the last few days and will they/won't they support snowfall, so just wanted to create a post for any of the less experienced members who are probably left scratching their heads about the conflicting views amongst different members and want to know what to believe.

    Let's start with why everyone looks for the -6c 850mb (or hPa) line and it's a bit of a history lesson I'm afraid. I believe in terms of these (and other) forums, the magical figure of -6c was really coined as the "snow line" back in the bad old days of the late 90's and early 00's over on the old BBC Snowwatch forums, when things were so desperate that we were crying out for any sort of 2 day northerly toppler just to look forward to (which is why I do find it amusing that some members can't see the potential in the upcoming period, I think unfortunately December 2010 distorted the expectations of some). Anyway, because back in those days we were primarily looking for Northerly topplers for any sort of cold, given an almost complete lack of Easterlies between 1996-2005, the 850mb temperature needed to be significantly below 0c for the airmass to be conducive for snow falling (this is a bit of an oversimplification, but let's keep it light!). Why? Well in the lower part of the troposphere we generally expect that as we travel from the clouds to the surface of the earth, the temperature increases. However we also have to remember that cold air falls, and so what we expect is the colder air from the 850mb level to fall down closer to the surface of the earth over time. The rate of this temperature increase (or "thermal gradient") varies hugely due to an overwhelming number of factors, however if we consider this in a simplified form again, the answer lies, for us at least, in the oceans.

    Because from a North or North-Westerly airflow the air is travelling a long distance over a generally warm Atlantic ocean, there is more likelihood of warmer pockets of air close to the surface of any airflow from the N/NW cancelling out some of the colder air dropping from aloft than there would be from say the East, where the air is travelling a far shorter distance over the North Sea and so is less likely to include these warmer pockets of air. These warmer pockets of air can affect many of the parameters that we won't go into in this post, but the two that are worth calling out as they are mentioned a lot are the 2m temperature and the 2m dew point. 

    So if we try and summarise the above two paragraphs, in a N/NWly airflow the air is travelling over warmer oceans and so warmer pockets of air can be expected to be found closer to the surface. To override these warmer pockets of air we need the cold air aloft (at the 850mb level) which falls down towards the surface of the earth to be cold enough to override these warmer pockets of air and make the air from cloud to surface cold enough throughout to support all of our lovely snowflakes falling from the clouds remaining as snowflakes rather than falling through a warmer pocket (or layer) of air and melting into horrible rain, which from experience is much more difficult to make snowballs from.

    I think the primary reason we chose the -6c line was that back in those bad old days where Wetterzentrale was the choice of most members to consume their daily dose of GFS from, the dashed isotherm line which indicated the 850mb temperatures were spaced out at approximately every 5c (although for some reason I seem to remember Wettzentrale often showing -6c instead of -5c as one of the dashed isotherm lines, correct me if I'm wrong if anyone can remember), and so we would often look for the -5/-6c isotherm on the 850mb temperature charts as our guaranteed snow line, as it allowed for enough headroom for some warmer pockets of air at the surface to be overridden by the falling colder air from above. At this time I think it's fair to say that none of our collective weather knowledge was anything like it was today, and so looking for something simple like a single "snow line" helped us in our search for snow.

     

    So, on to the next part, what exactly is the 850mb (or hPa - they both essentially mean the same thing) temperature chart. Again I will keep this as simple as I can, so apologies to anyone offended by the oversimplified statement I may be about to make. Well to answer the first part, it's exactly what it says - it is the temperature of the air at the point in the atmosphere where the air pressure is equal to 850 millibars (mb) or hectopascals (hPa). But the exact height above the surface of the earth of where pressure is equal to 850mb can change, and that is what all of the pretty colours on the "height" charts we see are. So we've been talking about the 850mb level of the atmosphere, let's take a jump to the other common charts we see - the 500mb charts, such as the one below:

    image.thumb.png.03cf9b7c39f819a2dadfc6c37e7a5ffb.png

    The colours here represent how high above the earth the 500mb pressure level is. The more towards blue/purple the colour is, the lower the height (or closer to the surface of the earth) the 500mb pressure level is, and vice versa for the greens/yellows/oranges. This also has a knock on effect on how high/low the 850mb pressure level below it is.

    So what we are saying in the context of our upcoming cold spell is that under that huge low pressure system, the height above the surface of the earth of both the 500mb and the 850mb pressure level is lower. What this means is that the colder air aloft, at the 850mb level, does not have as far to fall down towards the surface, and therefore more colder air is able to come down and help override any warmer pockets/layers of air towards the surface. This means that we don't necessarily need the 850mb temperature to be below -6c to support snow falling to the surface. We also then need to bear in mind that as that low pressure falls further South, and we change our feed of air from being from the North/North-West across the Atlantic ocean to instead coming from the East from the near continent, fewer of these warmer pockets of air are likely to exist.

    Just to give you an example of this from the most recent 6z run, here are two 850mb temperature charts, the first one at +138 hours, and the other at +168 hours:

    image.thumb.png.008364409c3017ef0a765f9396ad0fe8.pngimage.thumb.png.e859f8ea6bdd946d9b05e14427dbf953.png

    It would stand to reason that if it was as simple as colder 850mb temperature = colder surface temperature, then we would expect to see a lower 2m dew point temperature in the corresponding +168 chart right? Well see for yourself:

    image.thumb.png.56e7fd2837bf969b98a4f38023ffdf48.pngimage.thumb.png.36cbd243605930e16c3bc602cf24548b.png

    You can clearly see that despite the 850mb temperature being 1-2c warmer at +138 than at +168, the dew point temperature is 1-2c lower at +138c. 

    You can again see the reason for this by looking at the 500mb height charts:

    image.thumb.png.b9e0f99d4b5c2dfdc4e51a7cef589341.pngimage.thumb.png.235a86caf979ba2e3f63c7e83d64f996.png

    We can see here that the lower heights at +138 aid the cooler 2m dew point temperatures.

    So in summary for what is a very long post, you do not require -6c or colder 850mb temperatures, even in a North-Westerly airflow sometimes, in order to guarantee all of the relevant parameters for snow being the right side of marginal, it is significantly more complex than that, and that is why the upcoming period could well deliver a lot of surprises - the word could being crucial in there!

    I’ve reported this post for being way too excellent, as a lurking layman these kinds of posts are pure gold ??

    • Like 5

    Cold hunt - models and chat

    in Spring Weather Discussion

    Posted

    12 minutes ago, Djdazzle said:

    Who knows what will happen in Feb? It does seem that there is little in the models to suggest anything substantially cold. We were told to be patient, it will come etc. Are those people so confident now?

    As much as I want cold and snow, part of me also would like a mild Feb just to serve as a concrete example as to how unreliable long term preeictions can be, regardless of background signals and the EC46 etc.

    A lot of egg will be scraped off faces!

     

    Look, everyone knows getting a decent cold snowy spell in these parts is nigh on impossible in any given winter.  Hope casting to me isn’t even a derogatory term, it describes those members who are genuinely searching with hopeful optimism for the elusive white gold.  The naysayers that prod and jibe at others in here might be right 80% of the time past day ten but even in failure it’s 100% more fun and rewarding being a Jedi.

    • Like 2

    Ireland Regional Weather Discussion 13/12/2018 Onwards

    in Regional

    Posted

    Met Eireann 11:04 Update:

    "Thursday remains very uncertain but current indications suggest it will start cloudy in most areas with a band of rain spreading from the southwest. The rain possibly turning to snow as it pushes northeastwards but there is a lot of uncertainty in relation to this feature at the moment. It will be very cold with temperatures of just 3 to 6 degrees. Frost and ice will develop on Thursday night along with further wintry showers."

    Not a wholly inspiring update but you never know.  Anyone on higher ground in the heavy precip zone are looking good for a day off work on Friday, the rest of us will have to wait and see.

    Cold hunt - models and chat

    in Spring Weather Discussion

    Posted · Edited by minus86BriJ

    The infinitely complex interaction of macro drivers will always keep FI at 4-5 days followed by flip flopping ensemble chaos from run to run, especially if your focus is on the pimple that is the UK and Ireland from a global perspective.  My point is winter is NOT over just yet, yes we need a huge slice of luck in the next couple of weeks but no one can deny that there’s always a chance.  Feb91, March18 did actually happen.

    • Like 1

    Cold hunt - models and chat

    in Spring Weather Discussion

    Posted

    Is the downwelling data entered into the model algorithms manually or how does that work?  I don't know what percentage the models rely on historically analogous set ups/data sets to produce their runs (massive overly simplified I know!) but being as rare an event as it is I would have thought a trop downwelling would cause absolute model chaos!  Models flipping within 24 hours where over-stating even the smallest variable would effect T-12 never mind T-144 and beyond.  Was the failed beast last week a victim?  Are we all victims?  Why do I care???  I'm now lying down...

    Cold hunt - models and chat

    in Spring Weather Discussion

    Posted

    12 minutes ago, danm said:

    Correspondent on the BBC from Brussels, snowing heavily there. That's from the front that swept through the UK last night, rain mostly here, as soon as it hits the contintent it turns to snow. If only we had a more continental climate! 

    Yeh snowing away nicely in Amsterdam as well.

    https://balticlivecam.com/cameras/netherlands/amsterdam/dam-square/

    I lived there for 2 years recently without a flake of snow.. send me to Russia in a box and you'll all be guaranteed 2 foot plus ?️

    • Like 1

    Model output discussion - cold proper on the way?

    in Forecast Model Discussion

    Posted · Edited by Polar Maritime
    To move discussion on.

    Schadenfreuders dream here this morning, you gotta love em though

    The influence of the beast from the west coming out of states may be overstated by the ECM after the initial data is fed in, it seems hard to believe one system can disrupt two large HP cells hell bent on a love affair so drastically.  I’m gonna check back it in at 18:24, I don’t look at the models anymore, it’s much more fun just catching up in here ?

    Model output discussion - cold proper on the way?

    in Forecast Model Discussion

    Posted

    Is it fair to say that the 12z ECM might have misconstrued or over/underinflated one or even a percentage of one of the macro variables to a point where any resultant output is scuppered from the outset? Would this error (for want of a better word) not have the same effect on the entire ensemble suite to boot?  Like many others I’ve been following this forum and the models for donkeys years and without question the only constant is inconsistency.  Entire ensemble suites can and have turned from one extreme to another before you can finish your cornflakes.  The ECM threw out a “downgrade” or two already this week, personally I think the overseers are just trying to mess with feb1991s head (it’s working (only poking F91))

    Roll on amarach

     

    • Like 1

    Model output discussion - 7th January onwards

    in Forecast Model Discussion

    Posted

    6 minutes ago, shaky said:

    Dont know what to make of the ukmo!!dunno if anyone gets any snow out of that slider at all!!and gfs is still stubborn!!all the yummy charts back into fi again and everything flat!!give us a break!!

    UKMO looks like cold rain for my part of the planet as progged by the Irish met this morning from Thursday through the weekend. I’m still confident of favorable HLB setting up down the line. Is it behind door number 1, perhaps it’s door number 3?!.....find out after the break

    Model output discussion - 7th January onwards

    in Forecast Model Discussion

    Posted

    8 minutes ago, Jonathan Rhodri Roberts said:

    Personally, I actually think the GFS 0z is very good for January with frequent high pressure bringing plenty of dry weather, certainly much better than the past several weeks when it was low after low bringing gales and floods here in South Wales ??

    ? Did you accidently repost this, surely this was posted before santy came?

    • Like 6

    The Hunt for Cold continues 02/01/19

    in Spring Weather Discussion

    Posted · Edited by minus86BriJ

    1 hour ago, carinthian said:

    Yes shaky, out in Vancouver they still waiting for their first taste of snow. Think parts of Canada having milder temps than normal so far.

    C

     

    I get the impression over the last week that your a bit disappointed with missing out on the serious/dangerous snow totals back in your adopted home.  What's it at now, 5, 6 feet?!! 

    I'm not going to say I have a 'feeling' that some juicy charts are about to emerge from the long grass over the next few days but....actually scrap that, I can feel it in my waters!

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