Model output discussion - 20th Feb onwards

[Singularity]

Growing signs of a trough extension well south in the vicinity of the UK by the Easter weekend. About as typical as it gets!

I was hoping to see this trending west so that we could at least be on the warm side of things. As it is, GFS has us right on the boundary with a lot of rain possible, while ECM has us on the cold side with chilly winds at the surface.

This may be a consequence of the MJO looking more active than was anticipated into phase 5, which tends to encourage a stronger polar jet across the Atlantic at this time of year, though with the balance shifting more toward a pronounced Euro High as you move into April. Not sure exactly how the El Nino background modifies this signal but I imagine that in combination with the N. Atlantic 'cold pool' it could be adding further westerly momentum to the Atlantic jet stream(s).

[Singularity]

4 hours ago, knocker said:

I'm afraid I'm another thicko. Given that geopotential heights are essentially a function of temperature how can you have huge changes in temperature without significant changes of geopotential height?

Oh I don't know about that - your contributions have always seemed to be of an intelligent nature to me 

What I gather is that it is all relative; with a SSW that propagates down and splits the vortex strongly, you get a much bigger height rise for a given temperature change, as the lowering effect of the vortex is vacated. In this case, the vortex has barely relented and this has greatly moderated the height increase in relation to the large temperature rise.

So there has been a notable change, just not on the scale that could have happened. My use of the word 'significant' was a bit misleading if taken in the scientific sense - by that measure the changes are significant i.e. not by random chance. It seems that it takes a big transformation to be able to drive the tropospheric conditions without considerable modification by troposphere-led forcing e.g. tropical.

[IDO]

18 minutes ago, Nouska said:

What was the other year? - last technical SSW was 2013 and that sure did the business.

Yes, a lot to learn about the strat but it has never before been under such close scrutiny as a tool for bringing the much desired big freeze to a specific location. The science is advancing, expectations need to, as well.

I apologise if I am wrong but last year I read that an SSW occurred: http://www.wsi.com/blog/a-strong-stratospheric-warming-event-currently-underway-implications

I remember lots of expectations from members with the SSW forecast.

[TomDav]

28 minutes ago, IDO said:

I apologise if I am wrong but last year I read that an SSW occurred: http://www.wsi.com/blog/a-strong-stratospheric-warming-event-currently-underway-implications

I remember lots of expectations from members with the SSW forecast.

No Technical SSW last year.. That article was published late March and I believe was a final warming which occurs every year

Edited March 19, 2016 by TomDav

[knocker]

45 minutes ago, Singularity said:

 

What I gather is that it is all relative; with a SSW that propagates down and splits the vortex strongly, you get a much bigger height rise for a given temperature change, as the lowering effect of the vortex is vacated. In this case, the vortex has barely relented and this has greatly moderated the height increase in relation to the large temperature rise.

Sorry not following this. What is all relative? The fact that GPHs are a function of temperature is beyond dispute so what is the other mechanism that has such influence? Plus you are now saying you get a much bigger height rise for a given temperature change (again why?) which appears to contradict your earlier statement.

Quote

among the biggest discoveries being the fact that you can have some epic warming of the stratosphere without a large change in geopotential heights

What does "as the lowering effect of the vortex is vacated" actually mean?

Edited March 19, 2016 by knocker

[IDO]

25 minutes ago, TomDav said:

No Technical SSW last year.. That article was published late March and I believe was a final warming which occurs every year

It was maybe not a technical Major SSW but I though it was at least a Minor One? 

All semantics really as even the experts in the field cannot agree on a standard definition, as the following article testifies to:

Here

[Nouska]

2 minutes ago, IDO said:

It was maybe not a technical Major SSW but I though it was at least a Minor One? 

All semantics really as even the experts in the field cannot agree on a standard definition, as the following article testifies to:

Here

Two problems here ... Some of the expert bloggers are adding to the confusion in their recent postings and secondly, a minor warming will not cut the mustard as it does not create reversal of the zonal winds. The winds must reverse before propagation can take place but reversal does not always lead to propagation.

Great video by Adam Scaife  - explaining why reversal has to be first stage.

 

[IDO]

4 minutes ago, Nouska said:

Two problems here ... Some of the expert bloggers are adding to the confusion in their recent postings and secondly, a minor warming will not cut the mustard as it does not create reversal of the zonal winds. The winds must reverse before propagation can take place but reversal does not always lead to propagation.

Great video by Adam Scaife  - explaining why reversal has to be first stage.

 

Cheers, yes not straight forward.

If anyone is interested, still no conclusive evidence that GFS op, despite a few runs being consistent has nailed 10 day plus. The GEFS are a mixed bag and a extended period of westerlies is not a given yet, many still showing wedges of heights building mid to high latitude, no block per se. What looks almost certain is no deep cold or warm ups, just average fare, for London at least:

[Interitus]

6 hours ago, knocker said:

I'm afraid I'm another thicko. Given that geopotential heights are essentially a function of temperature how can you have huge changes in temperature without significant changes of geopotential height?

You are correct of course - the significant changes in geopotential height occurred in the stratosphere.

To quote Steenburgh & Holton - http://www.atmos.washington.edu/gcg/JR_site/papers/1993_3.pdf

Quote

Assuming hydrostatic balance, only two processes can alter the geopotential height of a given pressure surface:
1) a change in the mean virtual temperature of the column of the air below that pressure level, and
2) a change in the total mass of the air column due to net mass divergence or convergence.
Virtual temperature change above a given pressure surface cannot alone cause a change in the geopotential height of that surface.

In summary, an analysis of the complete geopotential height tendency equation shows that geopotential height may be directly influenced by temperature change in the underlying atmospheric layer. However, a temperature change in the overlying layer can only indirectly affect the geopotential height below that layer by inducing an ageostrophic circulation that results in surface pressure rises or falls.

So thinking of the atmosphere in terms of layers, warming of a particular layer increases the geopotential thickness of that layer which typically might increase the geopotential height at the top of the layer - but this depends on the sum of thicknesses of all the layers below as well. The height is proportional to the temperature of the whole atmosphere to a particular pressure level - warming aloft combined with cooling at the surface might mean there is no height change at the top of this depth of atmosphere.

It also implies that change in geopotential height or thickness makes no difference to sea level pressure. The only way to change surface pressure is increase or decrease total atmosphere mass above by convergence/divergence - increased heights is not the same as high pressure.

This can all be seen for example by looking at recent radiosonde data from the Polargmo Krenkelja station on Heiss Island, part of Franz Jozef Land in the Arctic, at 1/3/16 12z the height of the 20mb level was 25050m and -57.7°C and by 7/3/16 12z the height was 25990m with temperature -22.1°C. The majority of this 940 metre height rise came from an increase in the 50-20mb level of 720 metres.

Afterwards the 20mb height rose further to 26550 metres by 13/3/16 12z but the temperature was back down to -46.3°C and the 50-20mb thickness fell 250 metres. Instead the bulk of the increase came from changes to 100-50mb and 250-100mb layers which were now 410m and 310m more than the start of the month.

At the 500mb level, the next sounding on 14/3/16 0z saw a 500mb height of 5250m so also an increase from the initial 5140m, but with a 1000-500mb thickness of 5318m, a surface pressure of 988mb somewhat less than the 1012mb from the 1st. The temperature ranges of the 1000-500mb level increased from -18.9/-39.1°C to -0.7/-27.1°C with the arrival of low pressure into the region.

[wishingforsnow]

I recall one of Fergie's posts from a few weeks back stating that after a spell of anticyclonic weather things were likely to get more cyclonic again by the end of March though personally don't think it will be anything like what we experienced during Winter and hopefully not for as long either. Also I find extended forecasts are vague beyond 5-7 days and likewise models, and could change easily. I think what may happen is a week of particularly unsettled weather but it won't be as 'extended' as rain non stop for weeks on end like we saw in November, December and most of January. Met Office have also not ruled out settled spells inbetween the unsettled weather around Easter and early April. We are in a better position for longer or more frequent settled spells now that we have entered Spring although April looks like it will be slightly colder than average. A cold early Spring has been predicted on 3 month outlooks with encouragingly, warmer conditions towards the end into Summer 

Edited March 19, 2016 by wishingforsnow

[knocker]

C

5 hours ago, Interitus said:

You are correct of course - the significant changes in geopotential height occurred in the stratosphere.

To quote Steenburgh & Holton - http://www.atmos.washington.edu/gcg/JR_site/papers/1993_3.pdf

So thinking of the atmosphere in terms of layers, warming of a particular layer increases the geopotential thickness of that layer which typically might increase the geopotential height at the top of the layer - but this depends on the sum of thicknesses of all the layers below as well. The height is proportional to the temperature of the whole atmosphere to a particular pressure level - warming aloft combined with cooling at the surface might mean there is no height change at the top of this depth of atmosphere.

It also implies that change in geopotential height or thickness makes no difference to sea level pressure. The only way to change surface pressure is increase or decrease total atmosphere mass above by convergence/divergence - increased heights is not the same as high pressure.

This can all be seen for example by looking at recent radiosonde data from the Polargmo Krenkelja station on Heiss Island, part of Franz Jozef Land in the Arctic, at 1/3/16 12z the height of the 20mb level was 25050m and -57.7°C and by 7/3/16 12z the height was 25990m with temperature -22.1°C. The majority of this 940 metre height rise came from an increase in the 50-20mb level of 720 metres.

Afterwards the 20mb height rose further to 26550 metres by 13/3/16 12z but the temperature was back down to -46.3°C and the 50-20mb thickness fell 250 metres. Instead the bulk of the increase came from changes to 100-50mb and 250-100mb layers which were now 410m and 310m more than the start of the month.

At the 500mb level, the next sounding on 14/3/16 0z saw a 500mb height of 5250m so also an increase from the initial 5140m, but with a 1000-500mb thickness of 5318m, a surface pressure of 988mb somewhat less than the 1012mb from the 1st. The temperature ranges of the 1000-500mb level increased from -18.9/-39.1°C to -0.7/-27.1°C with the arrival of low pressure into the region.

Precisely. As a matter of interest I remember many years ago at Camborne we used to fly regular High altitude radiosonde balloons for Berlin and in those days we had to calculate the individual thicknesses by hand using a Vaisala thermodynamic diagram. They use to reach an altitude of around 40K. Times sure have changed.

Edited March 19, 2016 by knocker

[Singularity]

Thanks for the detailed explanation Interitus. I was trying to put things in a nutshell but didn't quite get the words right. It seems that the changes that a SSW can bring require a large increase in air column mass (i.e. density, hence pressure down below) via convergence. I believe it's along the lines of a reversal in zonal winds at the top leading to an accumulation of mass as it clashes with the still zonal winds below, this leading to a feedback process which brings the reversal down through the stratosphere. Don't take my word for it though as I've had to put climatology and theoretical stuff on one side for the time being while helping to get a business venture in motion - so I may be getting a little rusty in places for the time being. The situation will soon be recovered I'm sure 

Anyway, that propagation down has proved rather lackluster below something like 70 hPa this season.

 

Back to the usual model output discussion, and it's another dire Easter Weekend from the GFS 12z run, while UKMO looks like it could actually be stormier going forward from its day 6 chart. Having had such a lengthy run with high pressure in control in the run-up, I suppose the chances of a fine Easter Weekend were always going to be on the low side - rarely do we see much more than a fortnight of largely dry conditions across a large part of the country.

[Singularity]

  

Active weather fronts hitting the south Saturday daytime with some heavy rain and winds fairly strong for a time. The north cool and showery.

Generally cool, breezy and showery on Sunday (thinking high single digits maximums), persistent (frontal) rain clipping the SE perhaps.

Lighter winds with slow moving showers - heavy with downpours possible - on Monday, temps a little below average (just about double digits I reckon), while heavy rain grazes the far south, strong winds hitting France.

 

It's a bank holiday weekend, everyone!  

[Captain Shortwave]

14 minutes ago, Singularity said:

  

Active weather fronts hitting the south Saturday daytime with some heavy rain and winds fairly strong for a time. The north cool and showery.

Generally cool, breezy and showery on Sunday (thinking high single digits maximums), persistent (frontal) rain clipping the SE perhaps.

Lighter winds with slow moving showers - heavy with downpours possible - on Monday, temps a little below average (just about double digits I reckon), while heavy rain grazes the far south, strong winds hitting France.

 

It's a bank holiday weekend, everyone!  

Sometimes you get the feeling that using Murphy's law as a basis for weather forecast would prove to be more accurate. Bank holiday = wet 

One silver lining is the big potential for some big April showers with plenty of cold air aloft some a lot of instability coupled with an increasingly strong sun to drive home grown convection so hail and thunder look a distinct possibility. Of course as you have mentioned there is a risk of southern areas being stuck under the same weather front for a large part of the weekend which would be thoroughly miserable. At this point an unsettled weekend looks likely, but for me a convective Spring weekend isn't that bad and I would either consider interesting. A wet and grey weekend however is a big no.

[knocker]

Not exactly news but the GEFS and ECM anomalies are looking at the breakdown towards the end of next week. Both have deep upper troughs mid Atlantic moving east that portends some wet and windy weather for a few days.

NOAA is in agreement and keeps it unsettled until the end of the fourteen day period with the trough just to the west of the UK.

Edited March 19, 2016 by knocker

[wishingforsnow]

For what it's worth, personally I'd rather be experiencing periods of slightly cooler than average unsettled weather during early Spring as opposed to during the Summer if it has to happen. For instance, most of April last year was dry, sunny and warmer than average for weeks but what followed was a dull wet and cold May and a cool June. And the awful summer of 2012 followed a mostly dry winter and warm dry spring (bar April). 

Also remember that models often overdo stormy and unsettled conditions so who knows how stormy it will actually be or not.

Edited March 19, 2016 by wishingforsnow

[MP-R]

Wouldn't be surprised if we have a week similar to the final week of March 2006, where the emphasis was on rainfall rather than wind. Overall, March could turn out rather wet overall if the charts after next Wednesday transpire. 

[wishingforsnow]

13 minutes ago, MP-R said:

Wouldn't be surprised if we have a week similar to the final week of March 2006, where the emphasis was on rainfall rather than wind. Overall, March could turn out rather wet overall if the charts after next Wednesday transpire. 

In my opinion this Spring could be similar to 2006 as a whole actually.

[knocker]

According to this mornings GFS Friday not looking too bad but  a band of rain passing through on Thursday and then the next batch on Saturday. After that very changeable with on occasion some warmer air being advected from the south.

The ecm doesn't that much from the timing of the fronts.

Edited March 20, 2016 by knocker

[Frosty.]

The Gfs 00z op repeatedly pumps some lovely early spring warmth northwards across the UK in FI with temps close to 70f for favoured spots through early April.

[Gibby]

HERE IS MY LATEST ANALYSIS USING DATA SUPPLIED BY THE NWP OUTPUT COVERING 5 OF THE WORLDS MOST POWERFUL WEATHER COMPUTERS ISSUED AT 09:00 ON SUNDAY MAR 20TH 2016

THE CURRENT GENERAL SITUATION High pressure to the West of Scotland with a ridge of High pressure over Scotland will slip South over the next 24 hours with the ridge across more Southern areas tomorrow and Tuesday with more of a Westerly flow across the North of the UK from then.

http://www.wetterzentrale.de/pics/Rtavn001.gif

CURRENT SNOW FORECAST AND FREEZING LEVEL OVER THE UK The freezing level across the UK will remain in the range of 4000-6000ft across the UK in the coming couple of days with no risk of snow anywhere in the UK through that period.

http://www.snow-forecast.com/maps/dynamic/uk

http://www.natice.noaa.gov/pub/ims/ims_gif/DATA/cursnow.gif

MODELS-2 WEEK HEADLINE  Mostly dry and cloudy at first then becoming very unsettled across the UK over Easter and the end of the month. Temperatures returning to near normal.

THE GFS JET STREAM FORECAST The Jet Stream is quite weak currently with the Southern arm the strongest portion of the flow at the moment well down across the Med and North Africa. This will weaken through the week as the northern arm strengthens somewhat over the Atlantic through this week steered North across the UK for the Easter period and remains blowing North or NE across the UK in the post Easter period until the end of the month. 

http://www.netweather.tv/index.cgi?action=jetstream

GFS OPERATIONAL The GFS Operational Run today shows a much more unsettled pattern developing for the UK as we move through this coming week as the High pressure to the West weakens away South and SE through the first half of this week. This backs winds off to a SW point with cloud and rain spreading SE across the UK soon after midweek and setting up a more changeable Easter Holiday with Low pressure to the NW of the UK well in control. It won't be raining all the time however and it may well be that some drier spells are possible in the South and East with the heaviest rain in the North and West while temperatures return to nearer the seasonal average for many. This pattern then persists for the remainder of the run taking us into early April.

GFS CONTROL RUN The theme of the Control Run is very similar to the operational with the fine period coming to an end soon after midweek as a trough swings SE across the UK. This is then followed by a more mobile SW or South flow across the UK with rain and wind at times for all areas mixed with some shorter, drier and brighter spells with temperatures well up to the seasonal average for the remainder of the period.

http://www.wetterzentrale.de/pics/Rtavn1441.gif

http://www.wetterzentrale.de/pics/Rtavn3841.gif

http://www.meteociel.fr/modeles/gefs_cartes.php?code=0&ech=144&mode=0&carte=0

http://www.meteociel.fr/modeles/gefs_cartes.php?code=0&ech=384&mode=0&carte=0

GFS ENSEMBLE DAY 14 DATA  The GFS Clusters today show an 80% bias towards Low pressure across the UK or just to the West with a cyclonic flow across all areas of Britain with rain at times for all in average late March-early April temperatures. just 20% indicate something a little drier under slacker pressure..

http://www.weatheronline.co.uk/cgi-bin/expertcharts?LANG=en&MENU=0000000000&CONT=euro&MODELL=gefs&MODELLTYP=2&BASE=-&VAR=cpre&HH=372&ZOOM=0&ARCHIV=0&RES=0&WMO=&PERIOD=

UKMO The UKMO model this morning shows a change on the way as High pressure slips away South and SE through the week. Troughs of low pressure affecting the North from Tuesday extend South soon after midweek opening the door to more unsettled and windy weather for Easter with rain or showers at times in a strong SW flow as Low pressure anchors out to the NW of the UK.

http://www.wetterzentrale.de/pics/Rukm1441.gif

THE FAX CHARTS The Fax Charts show High pressure weakening over the coming days as it slips towards the South next week with fronts moving across the UK from the West heralding rain by soon after the middle of the week heralding a more mobile Atlantic weather pattern.

http://www.weathercharts.org/ukmomslp.htm#t120

GEM GEM this morning also shows a slow decline into more unsettled conditions under SW winds from midweek. However, it maybe that the worst of the conditions may hold off from the South and SE of the UK until later in the Easter weekend when all areas lie close to deep Low pressure with heavy rain and showers and blustery SW winds leading proceedings for all then out to the end of the month.

http://www.wetterzentrale.de/pics/Rgem2401.gif

NAVGEM NAVGEM also indicates a change to unsettled and windy weather as we move through the Easter period following an entrance into this more unsettled regime by a weakening cold front bringing the first rain for many soon after midweek. It maybe with low pressure held well to the NW that although windy the South and East may not see as much rain as points further North and West over Easter itself.

http://www.wetterzentrale.de/pics/Rnvg1681.gif

ECM ECM this morning once again shows the steady decline to Low pressure becoming influential to all of the UK. However, it takes a little time to do so and it will probably be Easter itself before the real unsettled weather takes hold across the far South with just patchy rain for a time soon after midweek opening the door to the West to more vigorous Low pressure and fronts later in the weekend. From then and into the post Easter period cyclonic conditions prevail with low pressure close to or over the UK delivering spells of rain and showers under temperatures no better than average for many.

http://www.wetterzentrale.de/pics/Recm2401.gif

ECM 10 DAY MEAN The 240hr mean Chart today shows Low pressure close to Scotland with the UK lying under a trough. the Jet stream is South of the UK with the likely weather across the UK being unsettled with rain at times for all under near to average temperatures.

http://www.wetterzentrale.de/pics/Reem2401.gif

NOTABLE TRENDS & CHANGES FROM PREVIOUS RUNS There remains universal support for the UK to enter a very unsettled spell for all areas over and beyond the Easter weekend

31 DAY HISTORICAL VERIFICATION STATS FOR GFS, UKMO & ECM The verification Statistics of GFS, UKMO and ECM show at 24 hours ECM ahead at 99.6 pts with UKMO at 99.5 pts and GFS at 99.4 pts.  At 3 days ECM is leading with 97.7 pts to UKMO's 97.3 pts while GFS lags behind at 96.5 pts. At 5 days ECM leads with 90.2 pts to UKMO at 89.2 pts and GFS at 85.5 pts. Then at 8 Days ECM has the leading spot from GFS with 65.0 pts to 59.7 pts. Finally at Day 10 GFS has the lead from ECM with a score of 44.7 pts to 44.6 pts.

http://www.emc.ncep.noaa.gov/gmb/STATS_vsdb/allmodel/daily/cor/cor_day1_PMSL_MSL_G2NHX.png

http://www.emc.ncep.noaa.gov/gmb/STATS_vsdb/allmodel/daily/cor/cor_day3_PMSL_MSL_G2NHX.png

http://www.emc.ncep.noaa.gov/gmb/STATS_vsdb/allmodel/daily/cor/cor_day5_PMSL_MSL_G2NHX.png

http://www.emc.ncep.noaa.gov/gmb/STATS_vsdb/allmodel/daily/cor/cor_day8_PMSL_MSL_G2NHX.png

http://www.emc.ncep.noaa.gov/gmb/STATS_vsdb/allmodel/daily/cor/cor_day10_PMSL_MSL_G2NHX.png

MY THOUGHTS  The models remain solid in support for a major change in weather type as we move towards the Easter weekend as for the first time for a couple of weeks Low pressure looks like regaining control over the UK under a SW flow. It maybe some time before the last influences of the ridge of High pressure finally gives ground sufficient to allow much in the way of rain to affect the SE of the UK but all areas should of seen a spell of rain at least by Good Friday. Following that SW winds look like strengthening markedly across the UK for all and this will allow temperatures to rise towards average at least though the effects of this will probably be lost on the strength of the wind and the fact that it will probably be raining for some of the time. Through Easter itself pressure will be falling across the UK and late in the holiday weekend and into the second week Low pressure looks likely either to be sitting over or close to the UK with bands of rain and showers with average Spring temperatures for many. Then if I was to take a stab at where we go weather-wise as we move into April I would suggest that on current evidence that further unsettled and Atlantic based weather looks likely to be the most likely weather type but with some pleasant drier periods under transient ridges allowing some Spring warmth to shine through at times. However, having said that it looks unlikely that any very warm or cold conditions notorious on occasions at this time of year looks likely given this morning's charts. 

Next Update Monday March 21st 2016 from 09:00

[knocker]

The ecm anomaly for the 30th has a familiar look about it upstream with the Aleutian LP, Ridge GOA and and trough down the eastern N. America. And downstream the trough in situ over the UK which will continue to give periods of unsettled weather and clearer interludes.

By the 4th April the EPS has the UK trough extending south to the western Mediterranean and heights building over Scandinavia. Thus a much more southerly quadrant flow and perhaps edging to one suggested scenario (METO 30 day) of low pressure to the south of the UK and HP to the NE inducing an E/SE airflow with more unsettled weather in the south and south west. Temps with this would perhaps be a little below average.

Edited March 20, 2016 by knocker

[Frosty.]

The Gfs 6z op shows a nice fine very mild end to march and beginning of April across most of the uk with temps into the low 60's F for some areas.

[marksiwnc]

Looks like a few surprises today weather wise

 Amazing sunny skies here and real warmth.  And you've guessed it met office shows cloudy all day. Shockingly crap computers 

[Costa Del Fal]

Usual story of symbol and even graphic forecasts being way way way too pessimistic with sun amounts. Good in a way that it makes us pleasantly surprised but not good for the tourist industry. How many people decide to change plans and stay in etc because the forecast is cloudy but the reality is often sunnier! 

Edited March 20, 2016 by Costa Del Fal