Model Output Discussion 4th Dec.2013-12z onwards.

[chionomaniac]

HiOn the model charts how can you tell if the energy is going over the top or underneath ?Sorry to sound stupid and it's probably really obvious .Thanks

It's best to look at the jet stream charts, TF. http://www.netweather.tv/index.cgi?action=jetstream;sess=6bae1b18f657f66dba7b0af76fd3a14f

 

There are not really any good examples of a split flow or southern positioned jet stream in the current run to illustrate what it is best to look for in order to show energy going underneath a block. Currently the flow across the Atlantic is strong and flat. When the flow meanders, then there will be better examples to show split flows with the energy going over or under.

[East_England_Stormchaser91]

Now that on Boxing Day, st Jude would have diddly squat on that. Britain would be torn apart.

[Jonan92]

 

I quite like this chart. We can see a chunk of the vortex moving towards Siberia, whil heights build in the pole and there is some space for the heights to build towards Greenland, but for that we would need the help of east-american low

[Frosty.]

The GEFS 12z mean shows a slow decline in temperatures although it's not as simple as that, through the first half of the run the temps are up and down but eventually they begin to drop continually and this is especially noticeable from T+240 onwards and suggests an increasing risk of wintry precipitation, and not just for northern high ground, some wintryness to lower levels at times further north and also across southern uk I would not rule out the risk of sleet or wet snow to bring a generally more wintry mixture, very windy at times with severe gales. Up to T+168 hours, the southeast has some weak ridging so the driest and brightest spells should be predominantly across the southeast but beyond day 7, the PFJ is forced even further south, almost as far south as central / southern france with the lower heights to the north west digging further south, so it could be rather wintry during the christmas period with the jet edging further south with time.

Edited December 14, 2013 by Frosty.

[Newberryone]

Disgusting run. I'd rather Uncle Barty settled down with a bottle of house sherry for the entire holidays than face that. Thankfully it's in FI

I fully respect your point of view but hey, where's your sense of adventure. Seriously though, I for one am ecstatic with what the models are generally showing for the foreseeable, especially the gfs 12z. Some of these charts are so disturbed it's like we're about to venture into some unknown territory. 

Interestingly, a work colleague informed me earlier today that this very mobile Dec pattern is to be followed by brutally cold weather for Jan and Feb.

We live in hope 

[Vorticity0123]

 To my mind that is part of the Vortex very close to the UK

Actually, the vortex is forecast to be positioned over Greenland as well as Northern Siberia (broad split) at T+240 (00Z run, GFS, 24 december, see map below). 

 

The main PV center over Greenland is of most importance for the current pattern.

As the general geostrophic flow to the south of the PV is from the west to east, a large amount of 'energy' from the polar vortex is being sent from Greenland to the Iceland-Scotland area. Because the main center of the PV is forecast to sit near the area of Greenland for the rest of the forecast period, there is a continuous pulse of intense low pressures moving toward the UK. The 'energy' is mainly caused by a very sharp temperature gradient (big temperature differences at a very short distance), creating a rather intense jet stream. To illustrate this, check the chart below:

 

The chart shows 850 hPa temperatures (T+240, GFS 12Z). Note the very big temperature difference to the east of Newfoundland (20*C max in less than 1000 km!), continuing till midway in the Atlantic. The big temperature difference creates perfect ingredents for the birth of intense low pressure areas. The low pressure areas are then being transported westward to reach the UK in a weakening state as the main source of energy weakens (a less steep temperature gradient).

 

Concluding, though it appears that the PV is located very near the UK, it is in reality located over Greenland. The intense low pressure areas are a result of the big temperature difference at great heights caused by the PV, being transported westward by the zonal flow, but they aren't the PV itself.

 

EDIT: The geostrophic flow is from west to east to the south of the PV, not from the east to the west. Sorry for the inconvenience.

 

Sources:

http://www.cpc.ncep.noaa.gov/products/stratosphere/strat_a_f/

http://www.wetterzentrale.de/topkarten/fsavneur.html

Edited December 14, 2013 by Vorticity0123

[Summer Sun]

Looking at the London ensemble the 850's never get any lower than -3c over the next few weeks

 

 

Highest snow risk for London to the 30th is 10%

Edited December 14, 2013 by Summer Sun

[Jonan92]

Looking at the London ensemble the 850's never get any lower than -3c over the next few weeks

 

 

 

Well, I wouldn't say "few" as only two weeks are forecasted in each GFS run

[Purga]

A very stable looking vortex in place over Greenland & N Canada which will be self perpetuating- typical chart below

 

 

The extreme cold is well established in that area and will lead to a potent supply of intensely cold air exiting N America and supercharging the jet stream. Canada isn't going to warm up in a hurry once this type of pattern has got entrenched with minimal solar insolation and a deep snowcover in place the frigid airmass is very stable.

 

Normally this type of pattern would last for at least 6 weeks if not longer as there are so many natural forcings all working together at this time of year.

 

Solid looking mean anomaly charts really underline the deeply established zonal pattern

 

 

It'll take some shifting - Oh well, maybe we'll get a cold March again?

Edited December 14, 2013 by Purga

[Swave Snow]

Actually, the vortex is forecast to be positioned over Greenland as well as Northern Siberia (broad split) at T+240 (00Z run, GFS, 24 december, see map below). 

 

The main PV center over Greenland is of most importance for the current pattern.

As the general geostrophic flow is from the east to the west, a large amount of 'energy' from the polar vortex is being sent from Greenland to the Iceland-Scotland area. Because the main center of the PV is forecast to sit near the area of Greenland for the rest of the forecast period, there is a continuous pulse of intense low pressures moving toward the UK. The 'energy' is mainly caused by a very sharp temperature gradient (big temperature differences at a very short distance), creating a rather intense jet stream. To illustrate this, check the chart below:

 

The chart shows 850 hPa temperatures (T+240, GFS 12Z). Note the very big temperature difference to the east of Newfoundland (20*C max in less than 1000 km!), continuing till midway in the Atlantic. The big temperature difference creates perfect ingredents for the birth of intense low pressure areas. The low pressure areas are then being transported westward to reach the UK in a weakening state as the main source of energy weakens (a less steep temperature gradient).

 

Concluding, though it appears that the PV is located very near the UK, it is in reality located over Greenland. The intense low pressure areas are a result of the big temperature difference at great heights caused by the PV, being transported westward by the zonal flow, but they aren't the PV itself.

 

Sources:

http://www.cpc.ncep.noaa.gov/products/stratosphere/strat_a_f/

http://www.wetterzentrale.de/topkarten/fsavneur.html

 

 

Thanks for the VERY informative Post

[shotski]

Actually, the vortex is forecast to be positioned over Greenland as well as Northern Siberia (broad split) at T+240 (00Z run, GFS, 24 december, see map below). 

 

The main PV center over Greenland is of most importance for the current pattern.

As the general geostrophic flow is from the east to the west, a large amount of 'energy' from the polar vortex is being sent from Greenland to the Iceland-Scotland area. Because the main center of the PV is forecast to sit near the area of Greenland for the rest of the forecast period, there is a continuous pulse of intense low pressures moving toward the UK. The 'energy' is mainly caused by a very sharp temperature gradient (big temperature differences at a very short distance), creating a rather intense jet stream. To illustrate this, check the chart below:

 

The chart shows 850 hPa temperatures (T+240, GFS 12Z). Note the very big temperature difference to the east of Newfoundland (20*C max in less than 1000 km!), continuing till midway in the Atlantic. The big temperature difference creates perfect ingredents for the birth of intense low pressure areas. The low pressure areas are then being transported westward to reach the UK in a weakening state as the main source of energy weakens (a less steep temperature gradient).

 

Concluding, though it appears that the PV is located very near the UK, it is in reality located over Greenland. The intense low pressure areas are a result of the big temperature difference at great heights caused by the PV, being transported westward by the zonal flow, but they aren't the PV itself.

 

Sources:

http://www.cpc.ncep.noaa.gov/products/stratosphere/strat_a_f/

http://www.wetterzentrale.de/topkarten/fsavneur.html

 

What a great post, sorry to post it again mods, clear and understandable and easy for the likes of me to learn from.

If only more posts were like this, there would not be so much confusion in here.

 

Shotski

[JK1]

It remains very unsettled on Christmas eve with some severe gales for some

 

 

A closer look at that low

 

would that low have any affect on the southern areas?

[Downpour]

I fully respect your point of view but hey, where's your sense of adventure. Seriously though, I for one am ecstatic with what the models are generally showing for the foreseeable, especially the gfs 12z. Some of these charts are so disturbed it's like we're about to venture into some unknown territory. 

Interestingly, a work colleague informed me earlier today that this very mobile Dec pattern is to be followed by brutally cold weather for Jan and Feb.

We live in hope 

I accept that mate but fancy a bit of mountain biking over the holidays!

[Ali1977]

What day time temps are we looking at in Xmas day then ?

[Frosty.]

There are a few stonking Gefs 12z perturbations tonight, P14 and especially P18.....which is stunning, it would be the first proper wintry blast with widespread snow and sharp frosts with ice days for some, it goes on and on and shows what could happen if the jet is forced far enough south, these 2 would bring a white christmas and there are plenty of others showing a stormy pattern during the festive period with cold zonality and wintry showers but these are the STANDOUTS tonight, mega P18  .

[Yozzer]

What day time temps are we looking at in Xmas day then ?

This far out difficult to say, but my guess would be somewhere between -14C and +15C. Could be wrong though.

[Gibby]

Good evening. Here is my slant on the latest report on the midday outputs from GFS, UKMO, GEM, NAVGEM and ECM for today Saturday December 14th 2013.

 

All models show a very mixed spell of weather over the next 4-5 days. In essence there will be periods of rain for all in mostly mild conditions offset by an often strong SW wind, reaching gale or severe gale force at times towards the NW. On Monday and Tuesday a respite in conditions look likely across Southern Britain as High pressure moves quickly East close to the South of the UK with something of a ridge from it giving less windy and colder weather for a day or so. By midweek a new storm system to the NW will push troughs across the UK with strong winds bringing heavy rain to all with gales and severe gales over all areas once more.

 

GFS then shows a colder and showery period with some wintry showers over the higher hills before further spells of stormy and wet weather storms in from the West on occasion. It looks like Christmas will be a potentially stormy period with heavy rain or severe gales more likely to be newsworthy rather than snowfall, though Northern hills will see some of this at times as temperatures fall back slowly to average and maybe a little below at times.

 

UKMO tonight shows the end of next week with deep Low pressure out to the North and NW with a broad and strong SW flow across the UK. It would be becoming milder again after a colder showery few days before the end of the run. Fronts crossing NE would again bring heavy rain across all areas but most prolific in the North and West.

 

GEM tonight shows fast changing weather patterns with rain and strong winds at times with temperatures never far from normal offset by the strength of the wind at times.

 

NAVGEM shows a wet and very windy period to end next week and the weekend with strong to gale and locally severe gale winds at times with bouts of heavy rain and showers for all areas at times, wintry over Northern hills.

 

ECM tonight shows deeply unsettled and occasionally stormy conditions on the run up to Christmas with copious rainfall for all areas at times with the most volatile weather likely towards Christmas. Colder conditions could accompany this period too when some wintry showers with snow in places might occur in amongst the spells of rain in some places over the Christmas period despite winds from a Westerly source.

 

The GFS Ensembles tonight endorse universal support for a very volatile spell of winter weather to come. There is spells of heavy rain and severe gales with very low pressure readings at times later across the UK. Temperatures will become less mild with time and there will probably be some snow on hills though this will be far from the main talking point of the weather over the period overshadowed by the potential of damaging winds and heavy rain by most members.

 

The Jet Stream remains extremely powerful throughout the coming few weeks with it's position gradually moving further South over the UK and then to the South in week 2.

 

In Summary it's probably the most mobile and boisterous Atlantic pattern of weather we have seen for some years at this time of year with the prospect of a very windy and wet Christmas looking more and more likely day by day. While some colder air will be entrained in the strong airflow at times with some snowfall at times on Northern hills the main talking point at the end of the period should tonight's charts verify will be the amounts of rain and potentially damaging strong winds that could buffet the UK at times over the period. Interestingly there could also be some very low air pressure readings at times across the UK with some near record low level readings occurring over the UK at times.

[bluearmy]

purga, to help the less informed on here, can you link to the occasions over the past few winters where that pattern has prevailed for six weeks on the back of a v cold canada?

 

the mean trough approaches as we reach xmas.  now here's the only question that really matters - will it get into w europe for more than a day or two ? if it does, then we, sitting above the PFJ will get cold enough to see some snowfall between xmas and new year, especially in the north of the uk.  the extended ens output has, until now, indicated that the next ridge will soon be along to reset the general sw/ne flow.

[Chris K]

It'll take some shifting - Oh well, maybe we'll get a cold March again?

 

Just like the Euro high that people said would last for weeks only several days ago?

 

Whilst this pattern may take some shifting....I don't think I will be relying solely on March quite yet! 

 

Uncertainty remains for the week ahead regarding the systems heading in from the Atlantic - never mind 2 months in the future.

Edited December 14, 2013 by Chris K

[stratty]

 

In Summary it's probably the most mobile and boisterous Atlantic pattern of weather we have seen for some years at this time of year with the prospect of a very windy and wet Christmas looking more and more likely day by day. While some colder air will be entrained in the strong airflow at times with some snowfall at times on Northern hills the main talking point at the end of the period should tonight's charts verify will be the amounts of rain and potentially damaging strong winds that could buffet the UK at times over the period. Interestingly there could also be some very low air pressure readings at times across the UK with some near record low level readings occurring over the UK at times.

 

I love a good Christmas buffet, so no better time for it I suppose! 

Seriously though, some of those charts look frightening for the strength of the sustained gales that could be produced. Nice to see some colder charts after Christmas springing up though Frosty!

Keep up the good work chaps!

[nick sussex]

Not much change tonight overall in the outputs, it's very much a stuck pattern but with some differences compared to many limpet PV zonal set ups I've seen in the past.

 

To use an analogy we're looking at the models trying to develop a horseshoe type set up, main PV over Canada and a couple of chunks edging south and one in Siberia, this is not a XXX rated 88/89 mild horror show type set up because they still do show some weakish height rises towards the pole and the pattern in the east Pacific may well deliver a bit of extra forcing to the PV if we can get more ridging north towards the Arctic.

 

 

The key is really to see how much forcing can be placed on the PV to disrupt, even if the main chunk stays in Canada this doesn't always automatically mean no cold, you can have a powerful jet but if it heads further south you can still end up north of PFJ.

 

The interest from the ECM at T240hrs is really where the output would go after that, beforehand on the face of it nothing stands out in terms of wintry potential, maybe some PM air and a little snow for the far north, more especially towards elevation, a lot of rain unfortunately and theres always the danger you get one of these fast developing lows that run east on the southern flank of the main trough, that is a concern especially with so many people on the move around Xmas.

 

Whilst the models keep hinting at some influence from the Pacific side and those weakfish height rises near the Arctic then that does add a little uncertainty going forward.

 

Overall my cold rating tonight edges up a notch to 3.5/10

 

Snow chances falling upto T240hrs 4/10 Sledge rating 1/10

 

White Xmas just using the bookies version 4/10 the north, 3/10 further south, its one of those that you might end up getting a surprise especially in any heavier showers, equally it might be that the UK is in the warm sector that day, its very hit and miss!

[Vorticity0123]

What a great post, sorry to post it again mods, clear and understandable and easy for the likes of me to learn from.

If only more posts were like this, there would not be so much confusion in here.

 

Shotski

 

Thanks for the VERY informative Post

 

Thanks for the positive reactions   I'm glad I could be of some help!

I have to admit that I'm still a beginner at meteorology, but I learn a lot every time I read through the forum!

 

A nice movie illustrating the low pressure formation form the temperature gradient is given below:

The explanation above is viewed more from a pressure gradient view. In the case of the Newfoundland gradient, the polar front is very active because of the very strong temperature gradient (explained in previous post), which is in turn very favorable for intense low pressure formation.

 

EDIT: In my previous post, I made a mistake. I stated that the geostrophic flow south of the PV is from the east to the west However, the it is at that point from the west to the east, not from the east to the west. Sorry for the inconvenience.

 

Source:

http://www4.uwsp.edu/geo/faculty/ritter/animation/atmosphere/cyclogenesis.gif

Edited December 14, 2013 by Vorticity0123

[bluearmy]

the extended ecm ens show that a euro trough is not the result of the 12z suite.  the jet gets a bit further south than previous runs (clear of n scotland) but begins to climb slowly north again in two weeks.  some subtle differences around the pole but nothing worth noting at this time.

[Downpour]

Just like the Euro high that people said would last for weeks only several days ago?

Whilst this pattern may take some shifting....I don't think I will be relying solely on March quite yet!

Uncertainty remains for the week ahead regarding the systems heading in from the Atlantic - never mind 2 months in the future.

Well exactly. Until a few days ago Barty's mutant relative was apparently here until late March according to some members. People have short memories. Last Christmas was an ugly wet washout - only to be followed by the coldest, snowiest, longest late-winter in London for what seemed like a generation. Edited December 14, 2013 by Downpour

[swfc]

well at 174hrs the 18z zips a cheeky little low threw the uk which wasnt on the 12z! barring that not much change so the wet ,widy weather looks to stay which i have very little interest in