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Model Output Discussions 12z 01/09/2016


phil nw.

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Actually the ukmo 12z is a little less progressive than the Gem @T+144 hours, I think there is a lot of uncertainty looking through next week, certainly a cold start anyway!:)

 

GEMOPEU12_144_1.png

Rukm1441.gif

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13 minutes ago, feb1991blizzard said:

Isn't it going to still be too far East though even with expected corrections?

If you look at northwestsnows link above

you can see that low to the NW fanning out - ideally when the model updates 00z we would like to more of a split along that line to a trough dropping south &the other getting further north

model resolution for this isnt very good especially GFS

expect the changes between 120-144 euros & 108-132 on GFS - ie half a day behind maybe a bit more

 

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The coldest air over the UK on GEM is Sunday and Monday

gem-1-96.png?12gem-1-120.png?12

gem-0-96.png?12gem-0-120.png?12

On Wednesday milder air tries to move in from the west but it doesn't quite manage it on it's first go but it has another try by the Friday

gem-1-168.png?12gem-1-192.png?12gem-1-216.png?12

gem-0-168.png?12gem-0-192.png?12gem-0-216.png?12

Looking at the models the colder winds should be easing by Tuesday

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1 hour ago, Nouska said:

I'm not terribly au fait with day to day weather and forecasting but is the ECM 168 hour chart not an example of diffluent blocking?

ECM1-168_hml7.GIF

I said diffluent in my post and only going by this diagram.

block.gif

Think it can be called Rex blocking as well - but as I said, more a background science person so maybe writing waffle.

My apologies for misrepresenting your post Take more care when reading posts Knocker. I'm not familiar with difffluent blocks but my main point still remains that we are not dealing with a block, or blocks, per se but transient ridging in what is a very volatile evolution within the general long wave pattern. And this has been the case for a while and why the chopping and changing with every run outside the near future and between models.

Again, my apologies Nouska,

EDIT

My understanding of a Rex block is an enclosed high poleward of an enclosed low which is what your diagram is showing.

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A more familiar sort of set-up with low pressure to the north and higher pressure from the south edging closer

12_225_mslp500arc.png

Whether this would be a long-term change or shorted lived remains to be seen a lot of weather to get through before this yet

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2 hours ago, knocker said:

Purely for clarification purposes and so we all know what is being discussed, what is  the definition of slider in the context of how it's used in this thread? Thanks

Hi Knocks I have posted a few examples previously as well as described what I believe to be the mechanism which actually begins before any potential block/trough disruption scenario and we already have what I would describe as a slider modeled at 24h also Blue has described the potential second slider scenario.

The mechanism is basically the cold dense air programmed to be over the continent being hard to shift as the relatively warm Atlantic air attempts to break through causing the troughs disrupt against the cold dense air mass (acting much like a blocking high similar to how we see the frigid landmass of Greenland repel and disrupt Atlantic lows) and cause them to "slide" down the Eastern flank of the Atlantic ridge which in turn would help reinforce heights over Scandinavia and potentially form a Scandinavian high.

It is a very typical battleground scenario we often see in the Winter months but fairly rare beginning of Novemeber I would imagine as there is rarely the depth of entrenched cold required across the continent this time of year.

In typical years the Atlantic usually wins out but occasionally we get enough energy disrupting and going SE instead of NE allowing for cold blocking patterns to form.

Hope that helps but I am open to different interperetations or corrections on terminology.

 

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10 minutes ago, Mucka said:

Hi Knocks I have posted a few examples previously as well as described what I believe to be the mechanism which actually begins before any potential block/trough disruption scenario and we already have what I would describe as a slider modeled at 24h also Blue has described the potential second salider scenario.

The mechanism is basically the cold dense air programmed to be over the continent being hard to shift as the relatively warm Atlantic air attempts to break through causing the troughs disrupt against the cold dense air mass (acting much like a blocking high similar to how we see the frigid landmass of Greenland repel Atlantic and disrupt Atlantic lows) and cause them to "slide" down the Eastern flank of the Atlantic ridge which in turn would help reinforce heights over Scandinavia and potentially form a Sacandinavian high.

It is a very typical battleground scenario we often see in the Winter months but fairly rare beginning of Novemeber I would imagine as there is rarely the depth of entrenched cold required across the vcontinent this time of year.

In typical years the atlantic usually wins out but occasionally we get enough energy disrupting and going SE instead of NE allowing for cold blocking patterns to form.

Hope that helps but I am open to different interperetations or corrections on terminology.

 

Mucka

I'm sure we could discuss the meteorological aspects of this for quite some time but it's really suffice to know what people mean in the context of the thread. :) So in this context the upper low on Friday is a slider and phases in with the cold air of the main upper trough. By the way have you noticed this evening's GFS is being quite wicked and shifting warmer air eastwards from the middle of next week.

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22 minutes ago, pdiddy said:

Looking at some of the GEFS, I suspect there will be a fair few different colder options, ranging from the Scandi high to another Greenland effort

 

Agreed, there are some GEFS 12z perturbations which show the scandi ridge still influencing the uk next midweek (P4 for example) and holding the atlantic at bay with cold air still in situ, it's an uncertain period and there is no guarantee that milder air will make significant inroads at all next week, at least the first half looks chilly, it could continue on the cold side away from the far west / northwest.

4_168_850tmp.png

5_168_850tmp.png

6_168_850tmp.png

15_168_850tmp.png

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Looking at the GFS ens the Op is on the cold side when compared to the mean from the 8th to 12th beyond this some days it's warmer than the mean and others its colder

gefsens850London0.png

A recovery in temps during next week is certainly possible but how high who knows maybe peaking around 12c in the south by late next week

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Just a quick mention about tonight,  The Gfs 12z shows a slight air frost and there is even a risk of a few freezing fog patches forming across parts of the Midlands and south which takes us into a crisp bright morning, especially further south.

12_18_uk2mtmpmin.png

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Looking at the Reyjavic pressure ens they peak at 1030mb this weekend followed by quite a sharp drop by mid-week by mid-month pressure steadies out around 1010mb ideally we want high-pressure showing for them like it will be over the coming days giving us some cooler / colder air

prmslReyjavic.png

*Note* The above is still the 06z ens

As this chart for the 9th shows with high pressure slowly building over the UK whilst for Reyjavic pressure gets down to around 990mb

h500slp.png

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The Ecm 12z is looking increasingly cold from the north this weekend as we get our first taste of Arctic air and some quite low thicknesses by sunday and the cold weather continues through the first half of next week, especially across England and Wales so there would be widespread frosts and chilly days with a peppering of coastal showers, some wintry across the n & e before a large ridge of high pressure to the w / nw  then slowly topples SE across the uk, still with frosty nights..certainly different to the 00z..colder and more blocked away from the far NW.

72_mslp850uk.png

96_mslp850uk.png

120_mslp850uk.png

96_thickuk.png

192_mslp500.png

216_mslp500.png

216_mslp850uk.png

216_mslp850.png

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h850t850eu.png

the high pressure laying down to our south west at 168hrs, is this not considered a bartlett high? i struggle to see this moving any time fast especially with the jet riding over the top and a weak polar vortex. Would i be right in saying that we can either hope for an easterly high to push it west/a north easterly to slide against it/it to link up with a long draw scandi high? 

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2 minutes ago, Snow and storms said:

h850t850eu.png

the high pressure laying down to our south west at 168hrs, is this not considered a bartlett high? i struggle to see this moving any time fast especially with the jet riding over the top and a weak polar vortex. Would i be right in saying that we can either hope for an easterly high to push it west/a north easterly to slide against it/it to link up with a long draw scandi high? 

Just your normal, run of the mill, Azores high. The image below is a Bartlett. 

bartlett.gif

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