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Christmas Eve Model Summary 12z


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Good afternoon All--

Well in the Absence of Paul B I have voulenteered to pick up the 12Z summary today, I will do the same boxing day- but tomorrow will only be an 00z Summary-

Images courtesy of Net weather extra-

Starting point Summary-



The festive period begins on a settled note, an all to familiar pattern for Xmas eve- High pressure slap bang over the UK, settled conditions but quite dull- temps a little above the season average-

As we move into Christmas day & boxing day we see amplification of the jet stream heading NE towards Eastern Greenland, this will encourage the main atlantic low pressure belt to take an anomalous tilt towards this region allowing high pressure to reside underneath the jet-

The flow though is classed as highly meridional & when this happens we find that the flow will bend back west or Southwest against the natural coriolis force-

As seen on the link above this flow curving back west has enough lift to produce a shortwave to move south / South West through Scandi & start to advect Colder air westwards towards the UK-

This synoptic evolution is not uncommon, & has been observed in recent years- However what precludes how cold it gets at the surface for the UK is how fast the advection is along the northern flank, is there an upper air cold pool on offer & is just as important is it migrating west or stationary-

In terms of Christmas & boxing day out to t48- using the theta E & 850 temps-


Nothing substantial is reaching the SE of the UK-


With Temps approaching the -1c levels across the Uk, we are left with low level surface cold coming rom the East-

If your unsure how this works as opposed to a Northerly then this was explained perfectly by RETRON this morning over on


The reason is that once the easterly sets in down here, we're going to get very cold temperatures compared to the 850s. With 850s of +1 to +3C we'll be seeing surface temperatures also of +1 to +3C. Cold Continental air comes across as a thin cold layer, sort of like a door stop (think of it as a thin wedge).

On the other hand, the northerlies will mix up the air and will have travelled over the sea, warming the bottom layers. So with the same 850s we'd be looking at temperatures hovering around +7C , much milder! However, the models show that some deep cold air will get caught up if a notherly manages to persist for more than a couple of days and with 850s approaching -10C then we'd be looking at daytime highs just above zero down here.


This rough-n-ready diagram should explain it. On the left is an east-to-west flow, with a cold tongue of air from the Continent (as we'll see Boxing Day onwards down here). On the right, a typical northerly with deep cold air aloft but much milder air for our part of the world. In time, the surface cold air will reach us (as shown on the Scotland end of the diagram) but it'll take a few days.

So the consensus out to boxing day & indeed to t72 ( day 3 ) Is that the UK has a dry, Settled very cold 'surface' feed from the South east, which will feel very festive-

In that period the chances of any precipitation, including snow is below 20%, just an outside chance in the extreme SE & NE coastal fringes-

this is because as well as an upper air cold pool missing we lack any real unstable air- highlighted here-


Generally heights above 552 DAM are considered stable & heights below this level become more condusive to convective potential-

there is little chance of this changing out to T72-

All 3 models agree- GFS UKMET

There is at this point again a small chance that a small shortwave or pterbation could develop in the Westerly flow that lowers the hights & uppers enough to bring some wintryness to the east & SE- the chances are highlighted to be low- & any disruption from this is even lower- probably below 5%

At this point we have total unity between the models, however things start to change upstream when we get past the T120 Mark & it concerns the SOUTHERN arm of the jet stream NOT the northern arm- All this talk of retrogression & the northern arm- when in fact we are being hindered by the southern arm of the split flow-

Lets talk about that- A split flow is basically where the jet stream is split into 2 strands, whatever amplitude & profile they have determines the height & SHAPE of the 500Mb high, & thats crucial here, we will get high pressure from 144 to 168, but the profile & energy is beginning to look unstable so hopefully I can illustrate whats going on-

lets take the Northern half first-

THe modelling 'seems' to have a grip on a deep vortex moving SE close to Scandi in the 144 timeframe-

On a LOW amplitude wave ( flatter greenland profile) the advection of the Bitter air will move NW>SE along the profile of the jet-



on a higher amplitude wave 'arc' of the jet will pull the advection at a sharper gradient South- If you notice the ECM 144 it is slightly sharper-


so have we got the Northern arm licked at 144- Yes NEARLY, whats yet to be finalised is the inflection point of the jet- IE where it changes direction from going NE to moving SE - we know its roughly going to be just east of Greenland-...

in terms of cold for the Uk the sharper the inflection point the better, so the cold does start out closer over the NE-

So with that in mind we should have fairly good accuracy & model agreement at 144 then ??? No...-

Everyone has ignored the southern Arm & this is more critical to US than the Northern arm because its the strand thats going to get low pressure to the south & south east of the -

The HUGE divergence tonight is around the model interpretation of the southern stream around 144 in terms of lattitude of flow & profile- this is one stream we want to stay FLAT angled directly EAST-

We seem to have spluttered out of gas in terms of retrogression from the High- Remember it will only move westwards IF there is no jet to impede it- that comes from either the Northern Stream OR the southern-

Crucially we have seen a marginal change in the outputs in the last 24 hours which changes the whole synoptic outlook around t144 & is in relation to the projected path & jet profile of the Shortwave trough ((Azores low if you like) in the South Western atlantic-

here is the operational GFS at t144-

Look where ive annotated the circle & the jet energy-


The interaction with the high over the Uk & the fact that the Southern arm is now recurving North shows the low never getting under the high-

The pathway is a recurve up the western flank setting up a sustained southerly flow & of course no retorgression-

Here it is at 168 in its full glory-

note the continued northerly Jet streak in the Southern Stream-

Encouraging heights to rise to the SE of the UK- see the Shortwave travelling North then NW around the High-

Now we look at the GFS ensemble data-


look at the ones with jet streaks in the Southern stream at 168 that are moving north or slightly NW-

These are generalyy the bottom 4 & 10-

Now look at the associated SLP at the same time to see where the high & shortwave is-


Note where the Sinkers Are... but just as importantly look at the best one number 4- why?

1) The sharperst inflection point & gradient over greenland-

2) All the Southern energy moving East...

Finally here is the 180 charts-



Note all the best have that southern jet profile away from arcing up towards the UK- & the sharp greenland gradient-

Interesting is that there is clear divergence tonight in the ensembles V the operational, we have probably all looked at the big 3's ensembles ( Lon/Man/Aber) & they illustrate the Operational to be an outlier-

heres why-

this is the Ensemble mean at 150 for the jet-


note the flatter profile to the southern arm-

When this situation occurs as it does sometimes does its difficult to pin you hat on what set of data is right, we all know the weather forum posters will use any data possible to present a case for cold & it does often pan out 50/50 where the operational has the right trend of the ens data- I can pick clear incidences of when both outliers moved to the other member-

But that is where the divergence is anyway.......

Look at the UKMO & ECM 144 charts-

The shortwave is NOT getting under the high & as a result impedes the blocking moving NW-

compare the 144 00z Chart-

Then critically the 192 Charts 12z V 00z- 12z

More shortwave energy on this 12z run than the 00z run-

the UKMO 144 12z is a halfway house between the ECM 12 & 00z runs.........

Worth note where the JMA gets it different-


A sinuous Southerly jet-

In Summary then- the 'POST' even larger teapot synoptics are beginning to look on a sticky wicket, but by no means is it cut & dried- especially as we remember that last week the GFS operation KEPT being the outlier against its ensemble mean - However the ops was right with the locale of the shortwave the ENS members werent- this time we actually want the op to be wrong around the shortwave to give us a fighting chance of those screaming easterlies-

If they dont watch the METO & JB update their forecasts back to average & close but no cigar for the UK once again....


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  • Location: Eastbourne and Larnaca,Cyprus .
  • Location: Eastbourne and Larnaca,Cyprus .

    An excellent read Steve many thanks, its so typical that something pops up to throw a spanner in the works! Anyway all is not lost and lets hope this is just a blip. B)

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  • Location: Castle Black, the Wall, the North
  • Weather Preferences: Spanish Plumes, Blizzards, Severe Frosts :-)
  • Location: Castle Black, the Wall, the North

    Enjoyed reading that Steve B)

    T+144 is the crucial timeframe then.

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  • Location: Peterborough N.Cambridgeshire
  • Location: Peterborough N.Cambridgeshire

    Excellent summary Steve.

    When you have a min I would appreciate your thoughts about the ECM ensembles and my summary of these on the model discussion thread.

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  • Location: Home near Sellindge, 80m/250feet, 5miles from Coast
  • Weather Preferences: Severe Storms and Snow
  • Location: Home near Sellindge, 80m/250feet, 5miles from Coast

    Excellent write up! B)

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  • Location: Leigh On Sea - Essex & Tornado Alley
  • Location: Leigh On Sea - Essex & Tornado Alley

    As has been said by the Others an Excellent Read and Very well presented. Clear and precise and to the point.

    Many Thanks

    Paul S

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  • Location: Sunny Scunny. 52m (170ft) A.S.L.
  • Location: Sunny Scunny. 52m (170ft) A.S.L.

    Superb explanation of how it all is interacting there Steve, many thanks :clap: . Fingers & Toes crossed eh :whistling:

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  • Location: South Shields Tyne & Wear half mile from the coast.
  • Location: South Shields Tyne & Wear half mile from the coast.

    Nice to take the time and effort to present a great summary in a total unbias fashion, telling it just as the 12z charts are showing, educational read too.

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  • Location: North Yorks, prev West Essex
  • Location: North Yorks, prev West Essex

    Merry Christmas. [santa is late tonight, as the kids are still awake]

    Although Joe B over at accuweather is generally thought to be a twit, he has forecast that we will have to wait a while before we get a decent cold spell.

    It would seem he is correct, if the models are anything to go by.

    FI is still FI, in my book, so can't really take any notice until next week. Things should slot into place by then.


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  • Location: Rossland BC Canada
  • Location: Rossland BC Canada

    The 00z GFS is not too horrid, let's say, it only has a bit of that milder SE'ly stuff around New Years Eve and then rather quickly reverts back to the gradual retrogression, swings the wind around more northeasterly and at one or two points looks cold enough for snow.

    It wouldn't transfix anyone, but we've seen worse, even yesterday.

    Merry Christmas to you all, and that big red bum in your fireplace needs a shove, just like the jet stream. :whistling:

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