Model Output Discussion

[nick sussex]

I'm beginning to not like this run now, we dont want that ridge in the atlantic meeting up with the euro high,not enough energy going into the southern arm of the jet here, euro high and some inversion cold looks odds on but after this still alot of questions. We now rely on the Arctic high getting a move on moving sw and a shortwave running off the main trough under the advancing ridge from the ne, its now getting rather complicated. If we could just get some energy in the southern arm of the jet running towards Biscay it would make things look much better.

Edited December 5, 2009 by nick sussex

[Iceberg]

Two charts from the 00Z and 06Z both just in the reliable timeframe.

I've scribbed on the 06Z one to show the extent of northern blocking which is greater on this run, meaning more chance of a potential NEerly.

The 00Z has the undercutting lows which allows the High to keep the orinitation for an easterly given the small cold pool that moves across.

[Cal]

Maybe Nick but that Artic High ridging down is a whopper with some very cold air in the mix.

http://91.121.94.83/modeles/gfs/run/gfsnh-0-180.png?6

[Iceberg]

Yep come 240 and come the mega cold from the NE....

[Snowmadsam]

interesting model runs recently... wonder what the 6Z will bring out?

fingers crossed that ECM will soon join in with GFS cold-spell wise

UPDATE: 6Z looking lovely

Edited December 5, 2009 by snowmadsam

[londonsnow]

6z having a fine old time in the T+200's. a giant basket of 510 air swaggering towards our eastern shores

[Nick L]

Although it only lasts for 2 days, the good thing is that the easterly is still there!

[cooling climate]

The 06z run although very cold turned out a lot messier and not as good as I imagined

the reason for this was a low development over the pole at t180 thus cutting off the heights

over the Arctic and everything sinking south.

When you consider that the 0z at this time had a 1040 high over the pole I suspect that the

GFS may be up to its old tricks of developing to many shortwaves only to drop them on the

next run.

[clark3r]

Some excellent charts on offer today.

Let me begin by saying that at +144 we have excellent agreement in the ensembles with the mean in Cambs at 1030mb.

http://charts.netwea...bridgeshire.png

Now the next hurdle is can we get the cold airmass to our NE to advect SW. In recent Decembers when we have had a HP sitting over the UK there has never been an opportunity of the HP retrogressing due to LP systems running to our N. However take a look below at the Iceland ensembles and rather than there being a deep Icelandic LP we have higher pressure than the norm.

http://charts.netwea...mslReyjavic.png

So in summary there are 3 possible outcomes.

1. HP over the UK but the cold airmass to our NE moves S and doesn't reach the UK.

2. HP over the UK retrogresses and the cold airmass moves SW and reaches the UK.

3. HP over the UK retrogresses but only E areas tap into the cold airmass.

Over the next few days the models are likely to show all 3 outcomes but at the moment I wouldn't want to say which one it will be.

will be intresting to see if the meto change their 15 day outlook today?

[bluearmy]

will be intresting to see if the meto change their 15 day outlook today?

http://www.knmi.nl/exp/pluim/vijftiendaagse/index.html

one suspects they may mention the likelihood of it being cooler than average in the south towards the end of the period.

we dont want to see that mean wind speed dropping too low. a block with an inversion will give us cold and grey but no snow. encouraging to see the control with three increases in wind speed, two of which correspond to some snowfall.

dissapointing parallel which fails to drive energy under the block at the same time as sending some over the top. half way to GEM 00z misery there. notes of caution.

http://meto.gov.uk/weather/uk/ee/ee_forecast_weather.html

yes to the continetal feed but the ecm ens must point to a sinker of sorts - given the mean in holland, would think the southeast would hang onto cold (likely 'faux')

Edited December 5, 2009 by bluearmy

[cooling climate]

The parallel run does the same breaking down the -AO with some of the vortex returning

over the pole and the heights sinking back into n/e Siberia.

Is this a new trend?

One to watch for sure.

[Iceberg]

The alternative GFS 06Z run shows how things can go wrong though with the High sitting limpet link over the UK, Still likely to be very chilly. It does still show several shots though from either the east or north east if we became a little luckier.

[snowmadchrisuk]

Overall it keeps us to far west on the 00z / 6z . ECM looks a little better IMO . After the runs of yesterday though a few milder outcomes were to be expected to show up. Once that high starts building next week anything can happen and we will have to go from there. Late FI on the GFS 6z is the worst case as it topples the high over us and takes us back to square one.

[Methuselah]

The Devil is in the detail...Although I'd hate all that dry, cold, fog-prone dross that the 06Z solution would provide, I doubt (hope not!) things will develop quite that way. That said, we are almost reaching the point where a marked cooldown is certain. Aren't we??? :unsure:

What shape and orientation will the HP have on the 12Z??

[Iceberg]

It shows how good some of the runs have been if we start to complain about a run that brings uppers of -14C in the south east !.

Not to mention the snow.

[snowmadchrisuk]

The Devil is in the detail...Although I'd hate all that dry, cold, fog-prone dross that the 06Z solution would provide, I doubt (hope not!) things will develop quite that way. That said, we are almost reaching the point where a marked cooldown is certain. Aren't we??? :unsure:

What shape and orientation will the HP have on the 12Z??

It is all to do with this stupid Atlantic trough interacting with the high, we need it to move back west or fill, or failing that it would be even ok if it just moved East delaying our cold for a few days but allowing pressure rises behind it.

[phil nw.]

The alternative GFS 06Z run shows how things can go wrong though with the High sitting limpet link over the UK, Still likely to be very chilly. It does still show several shots though from either the east or north east if we became a little luckier.

Yes along the lines of 00z GEM which keeps too much energy over the top later on.

Still the first step is looking odds on now with the Euro high ridging North around T120hrs.

After that then there is more than one option but i think a few days of settled cooler weather under the High close by looks favorite at present.

[Duane S.]

Remember folks, we are dancing with the devil with this HP cell. High Pressures rarely retrogress in the manner we are hoping this one will, although, this time it is plausable that it could. I think we will have to content ourselves, initially, with an inversion high similar to that of last Xmas before a shot at the real cold just before Xmas itself.

[Iceberg]

I don't like the look of the GFS ensembles, they arn't dire my along way, but there is a majority clustering now keeping things averageish 850 wise. A wobble or a trend. ?

[cooling climate]

Cartes des ensembles GEFS control take it any day

http://www.meteociel...2&code=0&mode=1

PTB 9 trying to nudge -18c 850's into the southeast corner.

[SMU]

Morning All-

OK so we have overcome a couple of hurdles overnight-

*) the models all show the vortex getting kicked west,

*) More importantly the shortwave trough responsible for the WAA into Iceland is very much on the cards-

Its now odds on that central Europe will be battered from an intense cold wave from the East- i would place the Western extent of the old ( in terms of almost guarenteed) is Berlin-

THe ensemble Consensus is high & the mean is begining to dip below -10c...

http://www.wetterzentrale.de/pics/MT8_Berlin_ens.png

I still have Concerns though on how the Advetion process manifests itself across the low countries- we desperatly need that second trigger low to ensure the path of the cold remains on a westerly track & is not deflected away to our favourite Winter haunt- Greece-

THe operational has one vvv

http://91.121.94.83/modeles/gfs/run/gfs-0-240.png?6

THe ECM looks a little flabby-

& Would certainly require undercutting to get the Easterly flow going...

I Suppose the biggest WIN for me this morning is the ECM Ensembles...

http://www.knmi.nl/exp/pluim/vijftiendaagse/Data/15DAAGSE_06260_NWT.png

Day 11-14 is excellent & the control is mouthwatering-

Remember the 12z yesterday was a bit progressive IMHO- I think we go- from unsettled- >> to Dry & cool SE flow- >> Chilly Easterly under the current block- then POSSIBLY finally the trigger low coming down from the North & the northern flank delivers the proper cold...

Were not home & dry yet, the jet could hold enough strength to lose the recurve back west & thus we miss the boat on the Easterly-

Steve

[SP1986]

With a high over the country, there would always be an inversion at the top of the boundary layer.

This is not entirely true, I've seen many episodes of high pressure in winter overhead which have not morphed into inversions.

There seems to be a specific set of things needed for an inversion.

- The high needs to be fairly slack (though not always)

- There needs to be quite warm air going over cold air, at atmospheric level

- There always needs to be pollutants in the air as this traps moisture below the layer helping the cold air stay below in the layer

Unfortunately this does not always occur (2003 is a good example of not occurring I think), especially if there is 'fresh' Atlantic air being pumped in/around the stagnant high by a nearby low.

[Frosty.]

Were not home & dry yet, the jet could hold enough strength to lose the recurve back west & thus we miss the boat on the Easterly-

Steve

It wouldn't be a surprise if we did miss the boat but the signs are still good for a slow cooldown after the milder weather which may then accelarate towards a significant or mega cold spell after mid month. It seems that by next weekend the unsettled weather will be gone with high pressure taking charge but then it depends how the hp orientates and whether we can tap into a colder continental flow and even if we can't, there should be some faux cold to enjoy.

[SP1986]

In my own humble experience, slow changes to cold rarely become exceptional cold. Usually exceptional cold occurs pretty much at the click of a finger with a defined pattern change. However slow pattern changes can give rise to prolonged albeit moderate cold, similar to what we saw last winter for example and to be fair I don't think the majority on the form would moan if something similar to last year occurred

[nick sussex]

I think we should have a much better idea of where the pattern could eventually go this evening, the first major switch in the ensembles occured yesterday on the gfs 12hrs, we've now had all 4 different runs from the gfs with varying solutions, in terms of verification stats at day 5 currently the gfs 12rs is ahead of the 18hrs followed just behind by the 00hrs and then finally the 06hrs.

Overall i think we can lock in the euro high to deliver some quieter and colder weather especially for the se. Then as Steve M mentions the problem then of getting the cold to back westwards, this in recent years has been very difficult.