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Youcan'tbecirrus

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Posts posted by Youcan'tbecirrus

    Model Output Discussion - Into the new year

    in Forecast Model Discussion

    Posted

    7 minutes ago, Tim Bland said:

    Stunning! Glad it dropped the naff output from earlier! 

    C61B46F4-634E-4691-8555-C93E07E8C812.png

    To my very untrained eye that looks a stunning chart. A couple of days ago, and with a SSW apparently in the wings, I think members would have been salivating over such a chart but it is interesting to see how there seems to be a lot less positivity in here this morning.     

    • Like 3

    Model Output Discussion - Into the new year

    in Forecast Model Discussion

    Posted

    14 minutes ago, Man With Beard said:

    I know we're being asked not to raise expectations, but the honest truth is, we are in the game for a memorable winter period in the weeks ahead. Sometimes the potential reaches fruition, sometimes not, and usually we don't find out until very close to T0. But the potential is there for a big freeze, undeniable.

    It's a good point. Based on absolutely nothing more than years of failed potential in here, I will reserve any excitement until T72. If the models are still showing these synoptics at that stage then I'll be more on board. Upcoming Countryfile should be worth a look.  The '87 event began on 12th January.

    Model Output Discussion - Christmas and New Year

    in Forecast Model Discussion

    Posted

    54 minutes ago, General Cluster said:

    One thing I did notice about the 0Z, was that it 'tries' to get -30C Uppers into Siberia... Just where we want them be, during runup to an SSW?

    Now that would get me interested. I don't know if the '87 event (akin to lake effect snow ?) came from such a set up and whether it was the result of an SSW but there has been nothing like it since in my area (21" snow). 91 or 92 was good and I was too young to remember 62/63 but I know that was off the scale. When the models are showing something like -20 in Moscow -12 Berlin and -6 in Amsterdam that's when I will get really interested but this upcoming spell does look pleasingly cold with enough precipitation around to satisfy many at some point.  

    • Like 2

    Model output discussion - Closing in on Christmas

    in Forecast Model Discussion

    Posted

    29 minutes ago, bluearmy said:

    It’s no longer part of the pv but it contains some air that once was ........  have had this discussion before on here - some think it still is part of the pv but I think description of it as such misleads people as to what conditions really would be in winter actually under part of the vortex ....

    Out of interest how often has that set up happened before. Not suggesting for one moment it would be the same this time but would the 47/63 winters have come from such a set up ? Was 2010 similar ? From what I recall the 87 event came from the East so I assume that was a totally different set up.  

    Model output discussion - the beast arrives

    in Forecast Model Discussion

    Posted · Edited by Youcan'tbecirrus

    5 minutes ago, tight isobar said:

    Gfs 18z says no to the rash-mild inflow.

    And lets quickly remind ourselves how well this suite (18z) has been modeling of late.

    The spell has just gone up another notch!!!

    To the knowledgeable ones amongst you, could the less cold air associated with the low be affected by the surface cold where snow already lies ?    

    • Like 3

    Model output discussion - here comes the beast!

    in Forecast Model Discussion

    Posted

    56 minutes ago, nick sussex said:

    It may or may not turn milder by next weekend, before that there is some quite exceptional cold shown for this late in the winter, and plenty of snow chances.

    Theres also the possibility of a big snow event as the low to the sw moves ne, the margin for error even if every model agrees at day 5 is enough to markedly change the conditions on the ground for parts of the UK.

    So my advice just wait and see what the trend is over the next few runs. 

    With regards possible temps at the end of the week Nick, what are your thoughts about temps where there is lying snow ? Are those numbers factored into the models so could milder temps be suppressed at the surface ?   

    • Like 1

    Model output discussion - here comes the beast!

    in Forecast Model Discussion

    Posted

    1 hour ago, nick sussex said:

    Let’s see where we are by tonight.

    The actual block is expected to head further west but you only need a small correction in where the low goes to bring quite different conditions.

    There’s been some corrections this morning from the outputs which have improved slightly from last night in terms of snow.

    As long as you maintain that surface flow from the east even if the 850s become less cold it will still be cold at the surface .

    Wouldn't the surface cold be enhanced by lying snow ? I am not sure how much the models factor this in. 

    • Like 1

    Model output discussion - here comes the beast!

    in Forecast Model Discussion

    Posted

    9 minutes ago, Steve Murr said:

    last one before later-

    Each run of the GFS is edging to the UKMO- minimal mention of this run however again if you compare 00z GFS mean V 06z GFS mean @72 its gradually sharpening up the curvature of the CAA to more of a straight easterly -

    The UKMO @midnight 00z Monday 

    3EB0E1EE-A993-4F33-B638-DF1A01F6609C.thumb.png.1bb205a1316e849f7d99c1f2c83ee568.png

    Has moderate snow showers pushing in from the continent - 

    The -12c Isotherm is over the midlands-

    169CD403-A06E-4505-A7C7-E5CE47F1BCBD.thumb.png.9fdbbb090dd4994d1ab4f834a923e8aa.png

    So rough approximations would say the based on the UKMO the -10c line would arrive across EA by 2pm-

    The arrival of snow flurries ~ 6pm.

    Any notion that the cold spell is being pushed back is incorrect-

    What the models are doing now ahead of the wave that I keep referring to - are developing a very shallow feature that would have a small milder sector of around -8 to -10c @850 - however importantly this is coming into 950 MB air / surface air & dewpoints that are already mixed to a PC airmass - meaning all the indicators for snow are perfect - The TheteE chart sums this up well-

    FC97FF79-7787-4567-8C41-4320AFF7F46C.thumb.png.2ea05e008f68108ae514460bbcc1a48c.pngC43F61A2-02BC-4E4E-A207-DF9CC5FFA368.thumb.png.7f0bc25341f56d72a9c50b5e59dbe996.png

    So this event would be all snow ushering in more snow-

    The wave behind it introduces the Vmax instability you can get -

    ~ 

    * -16 c air 

    * North sea Track over ~ 6-7c

    * Thermal Gradien 23c

    * Very Low heights / steep lapse rates

    T192

    1A2B257C-222F-4963-92AE-AB889EB66F48.thumb.png.bdcf6b8b460cc9c04ef7f6b7defad054.png07B5B73C-8E90-45DF-A14F-0DDBB065BFAB.thumb.png.7c358b3544f1ab5ceba18fc2c5437ce3.png

     

    Look at these 2 charts - If anyone draws a conclusion that it would be snowing in MOST places then thats a big misjudgement-

    ** Notice the -20c just in view at the top right **

    Have a great day

    S

     

     

    Last para Steve did you mean to say "if anyone draws a conclusion that it would NOT be snowing" ?

    • Like 2

    Model output discussion - here comes the beast!

    in Forecast Model Discussion

    Posted

    10 minutes ago, bluearmy said:

    the 06z drifts a little towards the ec theme but in a way that is still good for uk cold/snow. Hopefully this is where the middle ground will be.  What we don’t want is a gfs step by step movement in a single direction, run by run. 

    Sorry Blue, for us novices, can you briefly explain re "step by step movement in a single direction, run by run". Thanks.  

    • Like 1

    Model output discussion - here comes the beast!

    in Forecast Model Discussion

    Posted

    2 minutes ago, danm said:

    I'm not an expert by any means, but have a decent enough general understanding.

    High/steep lapse rates - high rate of change (decrease) in temperature as you travel upwards through the atmosphere. So with relatively warm sst's over the North Sea (e.g 6c) and 850 temps of, lets say, -12c. That gradient between relatively warm water and very cold temperatures at the 850 layer of the atmosphere creates instability which will spark off shower activity. If the 850 temps were only -4c for example, then the lapse rates would not be as steep which would result in less instability and less shower activity. 

    In terms of thicknesses, someone more experienced can explain this to you, however thickness is a measure of how cold/warm a layer of the atmosphere is. The lower thicknesses are better for snow prospects.

    Many thanks Dan. If I am understanding correctly then, with regards precipitation, in the coming days we shouldn't be looking so much at what there is on the near continent as any precipitation will be determined by the high/steep lapse rates & sst's over the north sea ie in this instance, the low countries might see very little snowfall whereas we "could" see more and therefore the references to lake effect snow.        

    • Like 2

    Model output discussion - here comes the beast!

    in Forecast Model Discussion

    Posted

    4 minutes ago, danm said:

    As John Hammond (ex-BBC) stated on his online video yesterday, models are notoriously poor at picking up convective precipitation (showers). He explained that they tend to vastly underplay the amount of precipitation that will fall further inland in this type of scenario.

    The bottom line is this - we will never really be confident of the exact amount and location of snowfall until we are within 24/48hrs of the event. Even then the models can be wrong and very much becomes a case of nowcasting and radar watching. 

    With regards to the cold spell next week - the excellent model runs over the last few days have excited many of us due to all the right ingredients being in place for a convective easterly, as opposed to a dry, cold easterly. What are those ingredients? Very low 850 temps (sub -10), low thicknesses, high lapse rates, relatively warm sst’s over the North Sea, late February sun being stronger than midwinter (which can fuel shower activity inland) etc etc. 

    Ofcourse, if high pressure builds over us and we lose that feed of very cold 850 temps and pressure builds, then shower activity will be cut off or reduced. 

    So don’t pay too much attention to precipitation charts beyond a few days, or even less. Many of the best convective snow events have popped up at short notice. If I remember correctly, the Feb 2009 Thames streamer was picked up within 24/36 hrs.

    Thanks for the explanation Dan. To help us complete novices out, can you briefly explain please in layman's terms what you mean by thicknesses and high lapse rates.  

    Model output discussion - Post SSW - Will it turn cold?

    in Forecast Model Discussion

    Posted

    15 minutes ago, Catacol said:

    Just to raise heads a bit from the short/medium term operational charts - SSW mean reversal now set to end around 27th Feb, a couple of days early than initially forecast

    u_65N_10hpa_gefs.png

    with not much scramble evident... so likely to be fairly accurate now. This will bring an end to 16 days of winds in reverse, with a downwelling response to follow. By 1st March ECM has a vortex showing a recovery to weak westerlies, but still a net easterly flow at 45/65 N

    [ECMWF 240 hour forecast from February 19 2018 12 UTC to March 1 2018 12 UTC: zonal mean zonal wind]

    Longer term this vortex looks unlikely to me to recover sufficiently for a return to an atlantic westerly pattern of any strength. GLAAM is set to fall... but how low? Nino tendency beginning to look more and more of a factor, though increase in trades easterlies in the net 2 weeks may help pump up sub tropical high pressure belts and push the storm track a little further north once again. However next cycle of torques looks set to begin shortly, lag impacts for 10 - 15 March and this would assist in maintaining a meridional pattern at our latitude, especially when laid on top of a weak vortex and potential for neutral GLAAM.

    End product? No automatic end of this cold pattern in sight. If the trades get a foothold then, as per MetO forecast I could see milder air at times nudging in to the SW... but otherwise this really could be a long and sustained spell to end winter. ECM monthly images in posts above not a surprise.

    :-)

    Based on the above, would you say that GFS model predictions indicating the cold spell could end by around 8th March is likely and therefore a 2 week cold spell is unlikely..   

    Model output discussion - Post SSW - Will it turn cold?

    in Forecast Model Discussion

    Posted

    45 minutes ago, Man With Beard said:

    Slightly unexpected cluster on the ECM ensembles this morning:

    ec-ens_nat_z500scenarios_2018022000_192.

    Cluster 2 looks a touch more northerly than easterly. Won't stop it being cold but might change the longevity of the cold pool over the UK and the volume of snow that could be achieved.

    Hmmm so we have this and signs from GFS that the Atlantic could get through by 8th March. If I have been reading correctly this seems at odds to what was being predicted before ie no end in sight to the cold reload after reload. All a long way off and there will be changes but the first signs of a wobble perhaps. If the ensembles are correct of course.  

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