In Depth Model Discussion, Analysis and Summaries

[Glacier Point]

Thanks for that GP, everything has fallen perfectly into place for your assessments thus far. Some days ago you were saying that the Scandi High was something of an outside bet but presumably you now see that as very likely ?

Thanks Ian.

The fall in angular momentum was more than anticipated, which will allow more inertia to escape northwards helping to force the blocking ridge further east towards Scandinavia, but I think we are seeing just how keen the modelling is to shift the block back to Iceland in the medium to longer term. This makes sense that Scandinavian ridges, at least in more recent climatology, are products of eddies in the northern arm of the jet when this is more dominant during Nina (low angular momentum) years.

I think what we are looking at is a strengthening of the jetstream over southern Europe which will cut off the flow forcing the block back allowing it to migrate to where it wants to go.

GFS ensemble mean guidance days 6-10:

http://raleighwx.easternuswx.com/models/gfsensemble/00zENS6-10day500mbHeightAnomalyNH.gif

an east / NE'ly flow programmed and 850 values seven degrees below normal:

http://raleighwx.easternuswx.com/models/gfsensemble/00zENS6-10day850mbTempAnomalyEurope.gif

Days 11-15 develop the block in a highly favourable location for snow in the south of the UK:

http://raleighwx.easternuswx.com/models/gfsensemble/00zENS11-15day500mbHeightAnomalyNH.gif

Still going for a warm feb GP ????

As ever, under review. Stratospheric events look 'interesting' at the moment

[BLAST FROM THE PAST]

Thanks Ian.

The fall in angular momentum was more than anticipated, which will allow more inertia to escape northwards helping to force the blocking ridge further east towards Scandinavia, but I think we are seeing just how keen the modelling is to shift the block back to Iceland in the medium to longer term. This makes sense that Scandinavian ridges, at least in more recent climatology, are products of eddies in the northern arm of the jet when this is more dominant during Nina (low angular momentum) years.

I think what we are looking at is a strengthening of the jetstream over southern Europe which will cut off the flow forcing the block back allowing it to migrate to where it wants to go.

GFS ensemble mean guidance days 6-10:

http://raleighwx.easternuswx.com/models/gfsensemble/00zENS6-10day500mbHeightAnomalyNH.gif

an east / NE'ly flow programmed and 850 values seven degrees below normal:

http://raleighwx.easternuswx.com/models/gfsensemble/00zENS6-10day850mbTempAnomalyEurope.gif

Days 11-15 develop the block in a highly favourable location for snow in the south of the UK:

http://raleighwx.easternuswx.com/models/gfsensemble/00zENS11-15day500mbHeightAnomalyNH.gif

As ever, under review. Stratospheric events look 'interesting' at the moment

Quite excellent thus far Stew. Stew, I notice your last comment, I have signal for further SW events and thus I am now 50 50 re Feb. I'm picking up on a signal that Feb could have re-invigoured northern blocking and again positioning very favourable for cold for the UK. I'll get back on that but suspect I'll have my answer by midmonth. I do agree wholeheartedly with the snow chances 15-20 for the south....big period in my method. Great thread this.

BFTP

[kold weather]

Models are picking up on a stronger jet stream firstly suggested by the ECM last night but also hinted at several times in past GFS FI runs. As GP said the upper profiles look very interesting for a possible attack from the SW. Both the GFS and the ECM op don't go for that solution and instead just drop the high over the UK BUT there are quite a few ensemble members who go along with the idea of a possible attack.

Also worth noting that once again the models are probably under-estimating the strength of the upper block and therefore I suspect what we happen is a big low in the western Atlantic but is forced to shear off underneath the block, as other shave pointed out this is the best shot at one of those big snow events however its a fine line between the high being too strong and shunting it too far southwards or the upper high being too weak/east and the low just moving in without anything to stop it, just like pre-xmas.

In conclusion I think the models are somewhat over-estimating the Scandi high's upper strength and I expect the models over the enxt two days to slowly shift the heights anomaly more towards Iceland again...

[Neilsouth]

In conclusion I think the models are somewhat over-estimating the Scandi high's upper strength and I expect the models over the enxt two days to slowly shift the heights anomaly more towards Iceland again...

Kold n others , what are you thoughts on this end of weeks north east , easterly?. Which would you favour being the more possible outcome gfs/ukmo or the ecm/gem

[Shiny_Bottom_1]

Quick question for those in a bit more know.

From experience over the last 7-10 days, where we have seen rain when 850's and H500 have been good enough for snow, how important is the 0oC Isotherm?

Personally, I would have though if 850's were <-5oC and 500's <528, the 0oC would also be in correlation with this (down to sea level). Is it in fact unusual to have an Isotherm level higher than than 0m under the conditions of the 850's and 500's we currently have?

Thanks.

[BrickFielder]

850hPa temperatures below -5 degrees C suggest snow is possible but it does depend on the conditions in the lowest layer of the atmosphere. 850hPa temperatures below -10 degrees C increase the chances of snow, but quite often are associated with drier air in the UK meaning there is less precipitation.

The zero degree isotherm shows the height at which temperatures are freezing, so if that height is generally below 200 meters then this is usually not enough for the falling snow to melt( the time it takes for snow to fall 200 meters). Other good indicators are dewpoints below zero and wet bulb temperatures below zero. Remember though without precipitation you get nothing.

[johnholmes]

Quick question for those in a bit more know.

From experience over the last 7-10 days, where we have seen rain when 850's and H500 have been good enough for snow, how important is the 0oC Isotherm?

Personally, I would have though if 850's were <-5oC and 500's <528, the 0oC would also be in correlation with this (down to sea level). Is it in fact unusual to have an Isotherm level higher than than 0m under the conditions of the 850's and 500's we currently have?

Thanks.

best you get yourself a cuppa-then go to the Guides and for heaven knows how many times this advert read the 'will it snow'

It has, I think, every variable you need to find out if you will get snow.

Don't forget the MOST important??

does the model show any precip-if not forget it.

hope that helps

[izi]

A little help understanding something from the more experianced please.

The cold front moving south tonight which is currently over north scotland seems to break up in southern scotland and then reform in yorkshire area on some forecasts.

Can someone explain what may cause a front to 'split up' and reform later? Always interested me this, its happened a few times and never understood it.

[johnholmes]

I have no actual proof but I would suggest topographical in origin, perhaps a relatively weak part of the front at the time?

[izi]

I have no actual proof but I would suggest topographical in origin, perhaps a relatively weak part of the front at the time?

the cheviots are around the borders which could explain part of it. Could you explain in laymans how a front would weaken at said point in this current synopsis?

Edited January 4, 2010 by izi

[Lettucing Gutted]

This makes sense that Scandinavian ridges, at least in more recent climatology, are products of eddies in the northern arm of the jet when this is more dominant during Nina (low angular momentum) years.

What are the other ways of forming Scandinavian Ridges?

Edited January 4, 2010 by Craig Evans

[Wayneywoo]

Mods, please delete this only after GP has posted.

Please give us an update Stewart

[Glacier Point]

Mods, please delete this only after GP has posted.

Please give us an update Stewart

[rubs lamp]... your wish is my command.

OLR plots identify somewhat disorganised convection with a weakish MJO-related signal entering the Maritime Continent consistent with phase 4/5 evolution. There is reasonable tropical supression in the western Indian Ocean which indicates a likley continued forward (east) motion in the tropwaves, albeit at low amplitude. Note also the continued tropical convection east of the Dateline consistent with a -NAO signal. You can see the clear storm track (-ve OLR) across the Pacific into the Atlantic towards the Azores indicative of a southward displaced jetstream.

http://cawcr.gov.au/...m/OLR/m.3d.html

The key thing I think here is that the MJO is continuing in its eastward migration, which interestingly the GEFS goes for but the operational has gone against. This takes us into the arena where (1) the tropical signal starts to get more favourable for westerly wind propagation into the tropics and (2) the continued progression towards phases 7/8 takes us towards end of January giving us a high latitude blocking signal as we progress through the second half of the month.

A strong mean zonal westerly wind anomaly still persists across 25N-40N. This is being outweighed however by easterly mean zonal wind anomalies across the high latitude in the northern hemisphere and mirror images of easterly winds across the tropics.

http://www.esrl.noaa...m.sig.90day.gif

With the MJO progressing, the easterly wind anomalies over the tropics should begin to ease off some. That should allow relative angular momentum to rise although it should be noted that total and relative angular momentum is below average at nearly -1SD, perhaps indicative that the atmosphere found it relatively easy to scrub El-Nino induced westerlies from the system, and also a possible indicator for the spring and summer which may not pan out quite to El Nino = good summer. I am unsure just how much relative angular momentum will rise, probably not as high as the hig water mark achieved last month.

Mountain and frictional torques are working to shift anomalies back and forward so the most probable evolution of the GWO is for a weakish signal in phases 0 and 4 tilted slightly towards phase 4. The composites for phase 4 are very ambiguous, but broadly towards a Scandinavian ridge type solution. Phase 0 is more affirmative towards blocking across the North Atlantic.

http://www.esrl.noaa...gcm/gwo_40d.gif

Our upwelling wave is continuing over the high latitudes likely to continue to favour blocking although its should be noted that this will begin to wane from mid January onwards.

Put these together and the reasoned assessment would be for a positive H5 anomaly (blocking ridge) centred between Iceland, Svalbard and northern Scandinavia, oscillating between Scandinavia and Iceland in the short to medium term and more towards southern Greenland in the longer term.

This pattern is at odds with some operational global model guidance which are keener to reduce heights to the north with more forward intertia in the northern arm putting pressure on the blocking structures. However, the ensemble mean guidance has been conistent in locating the centre of the positive height anomalies directly to the north of the UK around 70N.

Tonight's GFS, GEM ensemble means continue to suggest this. The CPC 8-14 day mean height anomaly represents a good synthesis of these model guidance tools:

http://www.cpc.ncep....y/814day.03.gif

With cold entrenched, that pattern is going to be very hard to shift with a continued east / south-easterly surface flow. Attempted breakdowns from the south-west (as flagged up previously) are starting to look quite possible opening up the potential for a very snowy phase of weather across southern parts of the UK.

So the extended forecast period looks to continue colder than average, possibly 850 hPa values becoming below (as opposed to well below at present) average at mid month. The key features look to be the exent of any rebound (increase) in relative angular momentum and the associated placement of the ridge 60-70N dictacting the weather pattern in our quadrant.

As posted before, there are firm indications of blocking coming to an end during February which will expose us to a more typical Nino winter, particularly as the wavelengths alter once more and set the trough in the Atlantic further west allowing a ridge to build over Europe.

Edited January 6, 2010 by Glacier Point

[bluearmy]

http://www.meteociel.fr/modeles/naefs_cartes.php?ech=192&code=1&mode=0&map=1

loving the updated NAEFS mean SLP - the way the blocking to our nw looks like a heart!!

http://www.meteociel.fr/modeles/naefs_cartes.php?ech=228&code=0&mode=0&map=1

the related anomolies at T228 are better for the 12z run than the 00z with the core of the positive returning towards greenland and the trough to our south more defined.

[BLAST FROM THE PAST]

Stew

Is Feb looking more uncertain for you currently? RJS and I went for Feb to turn quite mild but I'm seeing potential for this to be delayed somewhat.

BFTP

[Glacier Point]

http://www.meteociel...=1&mode=0&map=1

loving the updated NAEFS mean SLP - the way the blocking to our nw looks like a heart!!

http://www.meteociel...=0&mode=0&map=1

the related anomolies at T228 are better for the 12z run than the 00z with the core of the positive returning towards greenland and the trough to our south more defined.

Yes, those ensembles do suggest that the operationals, the GFS in particular, are being much too progressive.

Stew

Is Feb looking more uncertain for you currently? RJS and I went for Feb to turn quite mild but I'm seeing potential for this to be delayed somewhat.

BFTP

A lot will depend on the amount of westerly winds added over the next 2-3 weeks Fred. The amount of tropical convection in the Indian Ocean this time round was low, and it doesn't look like the MJO over Indonesia will do much either so I'm thinking another significant notch down in angular momentum during Feb which if anything will take us towards a more Nina like pattern.

[azores92]

Yes, those ensembles do suggest that the operationals, the GFS in particular, are being much too progressive.

A lot will depend on the amount of westerly winds added over the next 2-3 weeks Fred. The amount of tropical convection in the Indian Ocean this time round was low, and it doesn't look like the MJO over Indonesia will do much either so I'm thinking another significant notch down in angular momentum during Feb which if anything will take us towards a more Nina like pattern.

so perhaps a return to zonality for feb then GP?

[Yeti]

Yes, those ensembles do suggest that the operationals, the GFS in particular, are being much too progressive.

A lot will depend on the amount of westerly winds added over the next 2-3 weeks Fred. The amount of tropical convection in the Indian Ocean this time round was low, and it doesn't look like the MJO over Indonesia will do much either so I'm thinking another significant notch down in angular momentum during Feb which if anything will take us towards a more Nina like pattern.

Thanks for these fantastic assessments - I have a question though!

You mention upper temps returning to milder (though still below average) values - this presumably being down to the cold air simply running out of steam underneath the Scandinavian ridge. If we were to see blocking retrogress to GL/Iceland, would this potentially allow another stream of cold air to leak out of the Arctic down the eastern side of the block once again and (perhaps) drive this towards the UK, hence creating another cold reload later on in January?

Thanks

[Glacier Point]

Models reaching a cross-roads as tropical convective wave associated with the MJO heads into the far western Pacific, the evolution of which will dictate on the pattern into the next 3 weeks or so.

Current MJO-related tools identify it in phase 4:

http://cawcr.gov.au/...Last40days.html

http://cimss.ssec.wi...s/wavetrkP.html

http://cawcr.gov.au/...m/OLR/m.3d.html

Note in the middle link we can clearly identify our band of westerlies across 25-40N. If anything, these are intensifying as a result of relative angular momentum flux convergence:

http://www.esrl.noaa...m.sig.90day.gif

http://www.esrl.noaa...p.sig.90day.gif

The GWO (both tropical and extratropical wind signals) is now closely coupled with the MJO, both measures of flow patterns in phase 4:

http://www.esrl.noaa...gcm/gwo_40d.gif

Forecasts for the MJO illustrate where some NWP is having difficulty:

http://www.cpc.ncep...._CFSOP_GEFS.gif

GFS operational beating a retreat, GEFS heading towards phase 5.

Frictional and mountain torques are both trending upwards:

http://www.esrl.noaa...ltauf.90day.gif

http://www.esrl.noaa...ltaum.90day.gif

This should send tendency in relative angular momentum upwards. Already we are seeing some attempt for the models to resolve this with circumglobal troughs being toyed with in the longer range modelling - indicative of some movement of the GWO towards a high angular momentu base state (MJO phases seven / eight). With relative angular momentum also increasing (remember the more eastward the MJO the more relative angular momentum), the GWO looks to be nudging towards a phase 4/5 transition in the next 10-15 days. This signals Scandinavian ridge retrogressing.

Crucially, this pattern coincides with all models indicating a fragment of the PV dropping into western Russia t144-168. If we get a retrogressing pattern, there is a fair probability that the cold Arctic air mass will be advected westwards towards Europe.

So the analysis of the last two days is consistent with an Atlantic attack from the south-west with the cold block remaining in place. The snow threat increases considerably next week but these are likely to be trigger mechanism for height rises to the north to pull west. There may be some erosion at the margins but I would favour the block to largely remain and cold weather to remain entrenched the for at least the north and east of the UK, possibly all parts. Thereafter, the next reload to arrive from the NE ?

Edited January 7, 2010 by Glacier Point

[SteveB]

Hi GP.

How are AO & NAO fairing? plus is there any sign of warming in the stratosphere.

[Glacier Point]

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao_index_ensm.shtml

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.sprd2.gif

Both locked negative.

Stratsophere is cold and likely to remain so.

http://strat-www.met.fu-berlin.de/cgi-bin/times?plot=temps&alert=1

so somewhat suprising to see a lobe of the PV being projected to break off. Signs that our tropospheric upwelling is still in evidence.

[SteveB]

Thanks GP, maybe we don't need a warming event to keep the cold.

[chionomaniac]

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao_index_ensm.shtml

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.sprd2.gif

Both locked negative.

Stratsophere is cold and likely to remain so.

http://strat-www.met.fu-berlin.de/cgi-bin/times?plot=temps&alert=1

so somewhat suprising to see a lobe of the PV being projected to break off. Signs that our tropospheric upwelling is still in evidence.

That's a coincidence GP because I was just in the process of gathering charts to highlight this point.

The first chart I was going to post is of the cross sectional zonal winds through the stratosphere and troposphere for 2009.

When we compare last January just before the MMW we see that strong positive (westerly) zonal winds extend into the tropopause layer to about the 150 hPa level and resulted in a strong positive AO. These did not allow any weakening of the tropospheric vortex until after the MMW. When we compare these to last December, we see that the same strength positive winds are held far further up the stratosphere. This has weakened the tropospheric vortex to such an extent that repeated splits have occurred and hence the pattern of blocking with the lowest layer of tropospheric winds being consistently negative (easterly). This pattern has persisted into January as we can see below and hence the strongest blocking for years continues.

There are signs looking at the above chart that the stratospheric westerly zonal winds are dropping lower towards the troposphere but they are no way nearing the level where a strong positive AO will be the dominant player just yet. In fact it may be the end of the month before they do! So I would stick with GP and continued blocking wobbling around an axis north of the UK. With the stronger stratospheric winds still some way away from the troposphere the polar vortex can continue to fragment without the upper preventing influence as the latest forecasts suggest.

Regarding the possibility of a MMW, the latest forecasts have suggested that any upper warming will not disrupt the stratospheric vortex too much. However these forecasts have jumped around the last couple of days which they have done before when they are unsure. So something may just be round the corner.

c

[cooling climate]

Would the downward propagation of westerly winds in the upper stratosphere be held aloft

by the greater ozone level in the stratosphere at the 30mb level.

[chionomaniac]

Would the downward propagation of westerly winds in the upper stratosphere be held aloft

by the greater ozone level in the stratosphere at the 30mb level.

One would expect the increased ozone to have moderated the cooling seen at the 30hPa level which has now dipped well below average. What is surprising though is that the polar vortex hasn't increased dramatically in strength as a result at that level and remains slightly below average. This could be as a result of the modifying effect of the easterly QBO combined with the very cold tropical stratosphere which decreases the temperature differential.