Reading around on this forum, you'll have seen the term '500mb heights' used here and there, sometimes abbreviated to just 'heights'. Those who've read through the more detailed posts commonplace in the winter season will most likely have gathered that 500mb heights are represented on most charts as coloured contours.

Case in point, here's a link to a chart from today's ECM 12z operational run, for next Tuesday.

http://www.theweatheroutlook.com/twodata/chart.aspx?chart=/charts/ecm/168_mslp500.png?cb=446

As well as the 500mb heights in their colourful glory, we can also see white lines labelled with three or four digit numbers, which are lines of equal surface pressure (isobars).

Notice how higher values of surface pressure have a fairly good correlation with higher values of 500mb heights (the oranges and reds)... yet it's far from perfect. This comes down to the fact that 500mb heights are also related to the air temperature. The colour scheme used gives a clue as to how the two are linked; low air temperatures tie in with lower 500mb heights, which are the blues in the above chart, while high air temperatures are associated with the areas of higher 500mb heights represented by those fiery oranges and reds.

So to summarise:

Higher Surface Pressure

and/or                                          =  Higher 500mb Heights

Higher Air Temperature

Knowing this, it won't surprise you to learn that the above chart features a large 'plume' of hot air extending up through Spain toward the UK - you can see the tongue of higher 500mb heights associated with that.

Meanwhile, you can just about make out the lower heights associated with that area of low pressure affecting Scandinavia (to the NE of the UK). Notice how those heights aren't as low as those in the vicinity of Canada and Greenland (to the NW of the UK) despite the surface pressure being higher over there. This reveals the tendency for the air temperature to have a bigger say in what the 500mb heights get up to than the surface pressure - the only exceptions we tend to see are when you have unusually deep areas of low pressure at any time of the year, or very strong areas of high pressure in winter.

Here's some visual representations of the above points, and an explanation of the physics behind each one:

As you increase your height above the surface, you reduce the amount of atmosphere pressing down on you from above. This is why pressure always falls with increasing altitude. This means that if you start off at a low value at the surface, you can expect the pressure to reach the 500mb level at a lower elevation. In a region with no temperature change horizontally, the change if a very straightforward one. It's where you have changes in temperature that things get more complicated.

Here we have a typical situation within the atmosphere - an abrupt temperature change as a cooler airmass moves into a warmer one, marked by a cold front. 

Being denser, the cooler air has more mass per unit volume, and tends to have more of that mass closer to the Earth's surface. So when increasing elevation, the rate at which the mass pressing down on you falls is higher than in the warmer airmass. This means the 500mb level is reached sooner.

Notice that the above example has no horizontal change in surface pressure - this is unlikely to be witnessed in reality when there's a frontal boundary, as the surface pressure is usually a little higher under that cooler airmass.

I'll second that - well said. It would be great to have all that explained as I've never fully understood all this despite my interest in meteorology.

Its a complex and important bit of information and a detailed explanation will be excellent. It really will.

While you're waiting for the next part on 500mb heights (still in the works - lot's going on these days), here's an article I've put together about airmasses around low pressure systems:

https://docs.google.com/document/d/1K78A391AsCObep-bZ7hxuVPlV7q0GDn4LateTCEMdYk/pub

Let me know if you have any questions 

Very nice and informative article there on air masses. SC, if I may ask a question regarding Occluded fronts. Can thunderstorms or heavy convective episodes occur within a fully formed Occlusion? I have this idea that this, given the general dynamics of occluded set ups,  would be rare, if not near impossible but I may be wrong.

That's a great question which I'll have to have a good think about tomorrow, it's not something that's really crossed my mind before 

Saturday morning edit:

So I've had a read around to see if any exceptions to the rule can occur with occluded fronts, but it seems that the standard model does tend to hold true, with the cold front undercutting the warm front, the warmer air ending up as a 'wedge' that no longer reaches the ground.

Search for a visual representation and there's loads to be found, such as this one:

http://www.atmos.umd.edu/~meto200/4_1_03_lecture_files/slide0069_image206.jpg

Notice how there are actually cold-type and warm-type occlusions, depending on whether the undercutting air is colder or less cold than the air ahead of the warm sector.

In both cases, the lifting of the warmer air results in increasing temperature with height (an inversion), which tends to prevent air from rising in a convective fashion, as rising air parcels quickly end up colder than the surrounding air, lose their buoyancy, and sink (as colder air is denser).

Of course the incoming cold air can be forced upward by high terrain for example, but this tends to result in persistent stratoform rain - known as relief rainfall. This is why weak occluded fronts sometimes produce very wet days on the windward side of mountains.

If you had a high mountain sticking up into the warm sector and forcing air upward within that... then maybe some high-based convection could occur. This is an unusual case though.

So to conclude, your 'idea' looks very accurate to me Chunky Pea! 

Thanks Stormchaser. Great and detailed explanation that pretty much confirms my own 'suspicions' so to speak. Have always considered the Occlusion to be the most complex of all the frontal features - and the hardest to pin down.

It might be "the exception that proves the rule" but the most intense convective rainfall I have ever seen occurred under an occluded front (albeit only a partial one and not really a "typical" weather set-up I grant you)

I was living in Bromley in 1968 and experienced a series of severe thunderstorms over 36 hours caused by a stationary occluded  front as described in this met office write-up

http://www.metoffice.gov.uk/media/pdf/3/2/Southeast_England_Floods_-_15_September_1968.pdf

The weather can always surprise us

120 mm + in 24 hours That sounds like one serious event. Thanks Lanky for that interesting example. That reminds me of something similar that occurred here back in September 2010, but obviously not as intense as your example. Widespread totals of between 40 - 80 mm in 24 hours with some regions getting just over the 100 mm. Again, it was all down to an occlusion that became slow moving. There were thunderstorms reported (I seen lightning myself) as the front approached but this occurred near an actual triple point shortly before becoming a fully occluded feature later that night. Still, rainfall reached convective intensity for long periods throughout the event over the following 24 hours.

Curious cases there... perhaps the warm sector was forced upward rapidly, resulting in a rapid cooling and immense condensation of moisture?

I've never studied these sort of events, despite one of my modules in Reading being 'Hazardous Weather'...! I guess there's only so much you can cover in the space of a few months.

Now seems like a good time to do a little work on the use of ensemble mean and spread charts.

First example, the location of the low pressure expected Mon-Tue next week:

The ensemble mean chart on the left needs little explanation; it takes an average of the 50 ECM ensemble members and plots them with the usual manner of colour scheme for the 500mb heights and white lines for surface pressure.

Taken on it's own, you can get a good idea as to what the consensus is for that day and time, and how well defined the areas of high and low pressure are can give a sense as to how much variation there is when looking through all of the ensemble members.

The ensemble spreads are what takes this consideration of between-ensemble variation to the next level. What you see in the example on the right is a contour plot showing the range of 500mb heights across the ensembles. The brighter colours represent a greater variation, which can be due to differences in the intensity of a system or it's position.

Typically, it's areas of low pressure that are responsible for most of the variation, as they can be anything from 910mb to 1015mb whereas most areas of high pressure range from 1016mb to 1040mb. You also tend to find that they aren't as unpredictable in terms of location.

In the example shown, the location of the brighter region to the SW of the area of low pressure visible on the ensemble mean chart suggests that some ensemble members are tracking the system further to the southwest than is the current consensus. Some variation in system intensity will be factoring in as well.

To the west of the UK we see a broad band of greater variation, reflecting a fair bit of uncertainty regarding the location of that secondary low that the mean shows to the southwest of Iceland. This suggests that the mean chart represents an outcome that while most likely, is far from guaranteed.

Now we're going to look further ahead in time and consider the likely position of a ridge of high pressure expected to be influencing the UK, and the risk of low pressure to the southwest leading to unstable conditions with a chance of thunderstorms:

The ensemble mean is far from clear cut here, with a weak suggestion of that low to the southwest or south, and the ridge seeming to be just about in control but threatening to lose its grip across the south in particular.

Remarkably, the ensemble spread shows little variation from this mean outcome as far as the UK is concerned, with the main uncertainty appearing to be with regards to how far east that area of high pressure extends from the North Sea. Usually we see a lot of bright greens appearing at this sort of range, but in this case, the models are suggesting only shallow areas of low pressure near the UK, so much so that variation in position has little impact on the spreads.

So what we're seeing is a signal that high pressure is very likely to be situated close to the SE of the UK, but with some uncertainty over how far northwest it extends. While being confident that it will exist in some form. ECM really doesn't seem to like the idea of a low tot he southwest getting up to much, which is good news if you fancy some settled conditions and warm temperatures for the weekend.

Worth noting that the ensemble mean and spreads for today's GFS 06z show greater uncertainty as far as that high pressure is concerned, yet manages to have a mean chart that makes even less of the low pressure over Europe - which happens to be in stark contrast with the operational run and suggests that it was off on one.

Such is the power of ensemble forecasting tools 

Hi all 

Now that I'm back from holiday, and my workload is proving manageable for the time being, I'm looking to add more to this thread.

With respect to this, I'm wondering if there's anything in particular that people would like to learn more about?

Here's hoping July picks up again soon, it looks rather benign over the coming few days at least!

Turbulence Charts.. Are interesting....

Following various observations in the MOD thread regarding our lackluster June and July this year, I have been pondering the driving forces behind our misfortunes this past month - particularly to what extent the SSTs have played a role - and have decided to put forward a case for the SSTs playing at least a partial role in driving our summer weather.

While it's not as such a guide to chart interpretation, it does make use of a lot of charts so I figure this is the place to put it, to save starting up a separate dedicated thread (SST threads have turned up most years and always get lost in the doldrums before long).

The baseline idea behind my analysis is that where there's below normal SSTs to the north of above normal SSTs, there's an enhanced south-to-north rate of temperature fall (temp. gradient) which encourages the jet stream to locate in or close to that region (as it's an upper level wind that arises due to a temp. gradient in the atmosphere, which in summer tends to be very similar to that observed in the surface layer of the ocean, though rarely are they perfectly aligned).

Naturally, above normal SSTs to the north of below normal SSTs tends to bring about a weaker than normal temp. gradient, reducing the chances of seeing the jet stream venture across such regions for more than brief periods.

Okay - prepare your eyes, this is post is Long with a capital L 

First up, SST map for late June 2007 which reveals a very different setup to what we have now, as expected based on the BBC article that Nouska linked to earlier:

http://www.ospo.noaa.gov/data/sst/anomaly/2007/anomnight.6.28.2007.gif

Across the Atlantic it's nearly opposite to what we have had this year June-July.

Using the 2007 SST pattern alone (which grew gradually more pronounced through June and continued to do so through July), I can see an unusually low SST gradient from 40*N to 60*N across the eastern half of the N. Atlantic, which in theory promotes a weak, meandering jet stream. So the SSTs may have played at least a partial role in delivering us those slow moving areas of low pressure that summer... which happened to be laden with extra moisture as well.

Now check out 2012... look familiar?

http://www.ospo.noaa.gov/data/sst/anomaly/2012/anomnight.6.28.2012.gif

Yeah, that really was 2007 round two! Except that with a more pronounced band of below average SSTs near 40*N, there was support for the jet to tend to be stronger than seen in 2007, but at the same time tracking south of normal. As it happens, 2012 did see a lot of mobile weather (based on an unpublished team project I worked on early last year), with the wet months characterised more by a succession of vigorous storms running through the UK as opposed to slow moving systems parking themselves overhead.

If we now look at this year's anomalies:

http://www.ospo.noaa.gov/data/sst/anomaly/2015/anomnight.7.27.2015.gif

...we find that, despite much warmer SSTs just south of Greenland with the cold anomalies a lot less expansive, the boundary between the anomalously cold waters and anomalously warm waters to the south in 2012 is actually very close to the range of latitudes (around 40*N) across which it has been located through June and July this year. In fact the similarity is stronger than I was expecting!

Perhaps, then, this is a signature in the Atlantic SSTs that we should watch out for in the May-June period during summers to come, as an omen of a mobile summer lacking in decent pushes of warm air northward?

I can see how it could prove to be of some value... but I can't see it precluding such a season. The reason is that other factors such as the state of the Pacific (El Nino Southern Oscillation, Pacific Decadal Oscillation) and MJO events are widely accepted to be of equal or greater importance for defining our dominant weather patterns. Not only that but unusual situations over land can play a significant role - for example the anomalously dry, hot setup over NW Africa which has led to a lot of heat export to Europe this summer.

In fact, I strongly suspect that the heat over Europe has acted to steer the jet further north than would have been the case with normal temperatures in place. In this way it has effectively shifted the 2012-style washout up to Scotland while a drier, but still often cool regime has been frequent across the middle third, the southern third coming within a gnat's whisker of a warm June/July combination, instead receiving a lot of near average temps - until this past week, that is!

So to summarise, 2007 was a different beast in terms of the mechanisms by which it was so dire, but 2012 displays some notable similarities in terms of the overall N. Atlantic setup.

Now to attempt to wrap this up with some positive memories, here's a SST anomaly chart from 2006 - a great summer for many parts:

http://www.ospo.noaa.gov/data/sst/anomaly/2006/anomnight.7.15.2006.gif

The temp gradient looks weaker than normal around 40*N, perhaps a little stronger than normal near 60*N. Ideal for the jet venturing well north - nearer Greenland.

...and finally 2013's stunning July:

http://www.ospo.noaa.gov/data/sst/anomaly/2013/anomnight.7.15.2013.gif

A bit of a mess, but looks uninviting for the jet stream SW of the UK, the strongest temp gradients locating to the NW instead.

Alright. Time to have a nice lie down I think 

Terrific. Well researched and well thought out analysis Stormchaser.

It's been a while since I updated this but don't worry, I'm still at it - it's just that I have a blog now and that is where you can find my updates: 

http://www.weatherscientific.co.uk/2015/09/06/long-range-500mb/

Obviously my day to day model analysis and discussion still belongs here, and I hope to become more active again as and when the weather starts producing interesting situations again 

Excellent and very informative articles SC. Many thanks! 

Here's an explanation as to how the weather breaks down just in time for the weekend, summarising a few important aspects of thermodynamics along the way. The structure is designed to benefit those who have not studied meteorology but wish to get the ball rolling, or those looking to refresh their memory on the topic.

Let me know what you think - is this clear enough or could it be improved? 

http://www.weatherscientific.co.uk/2015/09/10/jet-stream-low-development-sep-15/

Gotta take my hat off to your expertise on meteorology. Although I do have a better grasp on it when compared to five years ago, there is still a lot more for me to learn.

yet another reason why TWO is the best

Are you ready for one of the more complex aspects to meteorology? This one's all about vorticity - it may leave your head in a spin but fear not, I went through the same process some 18 months ago.

http://www.weatherscientific.co.uk/2015/09/21/vorticity-relates-precipitation/

Let me know what you think and if you have any suggestions for improvements or even future topics!