FI Charts ( T120+ onwards) Winter 2020 **READ MOD NOTE IN FIRST POST**

hatrickpatrick

The 0z GFS is a thing of beauty in terms of synoptics and temperature, but strangely seems to show no precipitation over Ireland or the UK during the heart of the cold spell. I'll assume this can be safely written off however, as I've always found the GFS precipitation charts to be almost entirely useless. Seems like it's game on according to this run!

The Parallel is unfortunately a total mess. I'm buoyed however by the various assertions that this particular 'upgrade' to the GFS seems to have an appalling track record for verification compared with the current operational. One wonders if this continues, will the rollout of the new model as the operational be delayed for re-working?

bazlers

Yes more encouraging signs. ECM on track too. Another good day of model watching hopfully albeit twists and turns im sure.

Villain

The GFS control run has gone crazy

bazlers

If this cold spell comes to pass it will be a lengthy one.

Villain

Villain wrote: »

The GFS control run has gone crazy

+276 hrs though so a bit like a le care novel
Exciting but fiction
Theoretically in an East southeast wind you'd have an almost universal snow shower fetch along the east coast
No obstacles or shadow
No marginality
Sea ice
Not much precipitation on that chart though but shower precipitation maps aren't at all reliable that far out or anything in fact

sicknotexi

GFS was a cracking run and ECM is decent but Icon, UKMO and GFS para took a step backwards. Let's hope we can see cross the board consistency in the next few days.

NMB

Personally I find GFS great for the trend. I usually look for a confirmation from UKMO. Bet the GFS takes a step backwards and the UKMO screams Beast and then they all agree the next day. What a polarcoaster ride whoop whoop. Loving this model watching!

Reversal

sicknotexi wrote: »

GFS was a cracking run and ECM is decent but Icon, UKMO and GFS para took a step backwards. Let's hope we can see cross the board consistency in the next few days.

At 144hrs, the 00z UKMO looks very similar to the 00z GFS to me.

NoCoDu

Very novice question here: I understand these models are FI. However at what point do they become more accurate/a possibility? I see a model above is 23rd January - does this mean that the possibility has moved out to this timeframe? I had thought we would now see these trends appearing closer to say 20th January/day 8 if they were starting to become a possibility? Genuine question.

Nqp15hhu

As always this cold spell will probably arrive sometime early February.

Nabber

AuntySnow wrote: »

Not much precipitation on that chart though but shower precipitation maps aren't at all reliable that far out or anything in fact

Beyond ~60hours I find Precip charts are just fun additions.

Cold air meeting warmer air with some tight gradients, some parts could see some decent snow.

TheMilkyPirate

Good over overnight runs, A historic heavy frontal snow event on the GFS for next Wednesday :pac: 0Z An amazing run overall



Broadly similar evolutions from the 3 main models at +144. UKM and GFS very similar.







Overall the pattern of amplification towards Greenland on around the 18th continues. Plenty more twists and turns before then

igCorcaigh

Noob question if I could...

What do the numbers (496 - 616) under the colour legend mean?

Reckless Abandonment

igCorcaigh wrote: »

Noob question if I could...

What do the numbers (496 - 616) under the colour legend mean?

Taken from UK met . Hope it helps.

Pressure decreases with altitude, and thickness measures the difference in height between two standard pressure levels in the atmosphere. It is proportional to the mean temperature of this layer of air, so is a useful way of describing the temperature of an airmass.

Weather charts commonly show contour lines of 1,000-500 hPa thickness, which represent the depth (in decametres, where 1 dam = 10 m) of the layer between the 1,000 hPa and 500 hPa pressure levels. Cold, polar air has low thickness, and values of 528 dam or less frequently bring snow to the UK. Conversely, warm, tropical air has high thickness, and values in excess of 564 dam across the UK often indicate a heatwave.

igCorcaigh

Reckless Abandonment wrote: »

Taken from UK met . Hope it helps.

Pressure decreases with altitude, and thickness measures the difference in height between two standard pressure levels in the atmosphere. It is proportional to the mean temperature of this layer of air, so is a useful way of describing the temperature of an airmass.

Weather charts commonly show contour lines of 1,000-500 hPa thickness, which represent the depth (in decametres, where 1 dam = 10 m) of the layer between the 1,000 hPa and 500 hPa pressure levels. Cold, polar air has low thickness, and values of 528 dam or less frequently bring snow to the UK. Conversely, warm, tropical air has high thickness, and values in excess of 564 dam across the UK often indicate a heatwave.

Ah, it's thickness, thanks!

Gaoth Laidir

Reckless Abandonment wrote: »

Taken from UK met . Hope it helps.

Pressure decreases with altitude, and thickness measures the difference in height between two standard pressure levels in the atmosphere. It is proportional to the mean temperature of this layer of air, so is a useful way of describing the temperature of an airmass.

Weather charts commonly show contour lines of 1,000-500 hPa thickness, which represent the depth (in decametres, where 1 dam = 10 m) of the layer between the 1,000 hPa and 500 hPa pressure levels. Cold, polar air has low thickness, and values of 528 dam or less frequently bring snow to the UK. Conversely, warm, tropical air has high thickness, and values in excess of 564 dam across the UK often indicate a heatwave.

Those are not thickness lines, they're lines of 500 hPa geopotential (the altitude at which air pressure is 500 hPa). Thickness is not shown on those charts.

Reckless Abandonment

Gaoth Laidir wrote: »

Those are not thickness lines, they're lines of 500 hPa geopotential (the altitude at which air pressure is 500 hPa). Thickness is not shown on those charts.

Is it not pretty much that same thing ?

Reversal

Reckless Abandonment wrote: »

Is it not pretty much that same thing ?

In a simplistic form, height of a singular pressure level, such as the 500mb level, is proportional to pressure. The height between two pressure levels is proportional to temperature. They show fundamentally different things.

lucernarian

Reckless Abandonment wrote: »

Is it not pretty much that same thing ?

No, in a very important respect. With cold air, whether high or low pressure, the distance between the 1000hPa altitude and 500 hPa altitude will be smaller than for warm air, as colder airmasses are denser.

In the charts above, they are just plotting the height of the 500 hPa level above sea level. In a cyclonic airmass (low pressure) there wouldn't be a 1000 hPa level to begin with as sea level air pressure might be e.g. 970 hPA. Those colour contours are showing where air aloft is rising (lower surface pressure) or descending (higher surface pressure).

nacho libre

The GFS chart for the 23rd is one you don't see every often - the vortex over NA is destroyed due to a link up between a gl high and pacific ridge that extends into the arctic. If it were to come off there should be a sustained cold spell instead of a brief one. If we have a southerly tracking jet sending lows up against that cold block some of you might start to worry about too much white precipitaion:)

Nqp15hhu

igCorcaigh wrote: »

Noob question if I could...

What do the numbers (496 - 616) under the colour legend mean?

They are the thickness. The distance between the average altitude of the 1000mb and 500mb pressure levels.

Colder air is denser so these values will typically be at a lower altitude in colder air.

528dam does not guarantee snow. You are usually looking at 520dam.

Nqp15hhu

Reckless Abandonment wrote: »

Is it not pretty much that same thing ?

No. It’s the height of that pressure level.

sryanbruen

nacho libre wrote: »

The GFS chart for the 23rd is one you don't see every often - the vortex over NA is destroyed due to a link up between a gl high and pacific ridge that extends into the arctic. If it were to come off there should be a sustained cold spell instead of a brief one. If we have a southerly tracking jet sending lows up against that cold block some of you might start to worry about too much white precipitaion:)

Very weird run, starts off with developing a secondary low that no other run has shown that I'm aware of which delays the onslaught of the cold but the lack of phasing the shortwave with the Atlantic low means it still goes onto produce an absolute stonker.

Crazy blocking. Take this scenario with truck loads of salt - which we would need if it was to verify :P

Gaoth Laidir

Reckless Abandonment wrote: »

Is it not pretty much that same thing ?

Nqp15hhu wrote: »

They are the thickness. The distance between the average altitude of the 1000mb and 500mb pressure levels.

Colder air is denser so these values will typically be at a lower altitude in colder air.

528dam does not guarantee snow. You are usually looking at 520dam.

As already pointed out, they are definitely not the same thing at all. A common mistake with those charts. However, the colours don't necessarily show where air is rising or falling. A cold pool aloft will have lower heights, but that could be above an area of high pressure at the surface, and vice versa.

Only where the surface pressure is exactly 1000 hPa is the 500 hPa geopotential equal to the 1000-500 thickness. Everywhere else you need to calculate it (to the 500 hPa geopotential add (subtract) 8.3 m per surface hPa below (above) 1000 hPa). Or just look at an actual thickness chart!

In any case, the 1000-850 thickness is better for snow forecasting. Sub-129 dam pretty much guarantees snow.

DOCARCH

Mod Note: Keep on topic please.

Gonzo

great to see the GFS back into it after yesterdays fairly disappointing series of runs. Would be good to see GFS, ECM, GEM and UKMO all lining up with each other from tomorrow/Thursday for complete agreement. If they start singing from the same hymn sheet by Thursday this cold spell will advance from FI into the much more reliable time frame and then we can start to get properly excited and start working out the finer details.

Billcarson

Billcarson wrote: »

With the gfs is though sometimes it can be the first to see a cold spell. Then it drops the idea before coming back to it again. Hopefully that will be the case this time too.

This mornings gfs runs the gfs has gone back to seeing a cold spell........funny that. Lol

Gonzo

Billcarson wrote: »

This mornings gfs runs the gfs has gone back to seeing a cold spell........funny that. Lol

I wouldn't be surprised if it drops the idea again, particularly that pub run.

this mornings ensembles graph has the perfect slope indicating a constant cooling over the next 2 weeks.



We may have low level cold to begin with but the depth of cold becomes more intense as we progress through the spell.

in the very unreliable extended range there are hints that it will stay genearlly cold right into the middle of February with wintry potential.



As for the ECM extended range it too is showing cold signals right into February for Ireland and the UK.

lucernarian

Gaoth Laidir wrote: »

As already pointed out, they are definitely not the same thing at all. A common mistake with those charts. However, the colours don't necessarily show where air is rising or falling. A cold pool aloft will have lower heights, but that could be above an area of high pressure at the surface, and vice versa.

Only where the surface pressure is exactly 1000 hPa is the 500 hPa geopotential equal to the 1000-500 thickness. Everywhere else you need to calculate it (to the 500 hPa geopotential add (subtract) 8.3 m per surface hPa below (above) 1000 hPa). Or just look at an actual thickness chart!

In any case, the 1000-850 thickness is better for snow forecasting. Sub-129 dam pretty much guarantees snow.

As a rough guide for understanding what's going on with the north Atlantic, I think it's fine to associate the height rises that come from warmer air reaching the stratosphere when e.g. SSW events are discussed here. Nothing wrong with pointing someone in the right direction with a general and brief comparison, of course the full explanation is a lot more complicated and there are better sources for that.

hatrickpatrick

NoCoDu wrote: »

Very novice question here: I understand these models are FI. However at what point do they become more accurate/a possibility? I see a model above is 23rd January - does this mean that the possibility has moved out to this timeframe? I had thought we would now see these trends appearing closer to say 20th January/day 8 if they were starting to become a possibility? Genuine question.

The general definition is anything from 120 hours onwards is FI, some would stretch that to 144. However, as you've probably noticed, a lot of this also depends on what's going on more generally. For instance, the current SSW event means that FI is even more unreliable than usual. On the other hand, a large block, for example that which settled over Ireland during that epic summer of 2018, can result in the charts stagnating for a very long period, during which FI becomes somewhat more reliable than usual as an indicator of how long such a pattern might last.

igCorcaigh wrote: »

Noob question if I could...

What do the numbers (496 - 616) under the colour legend mean?

Others have given very detailed scientific explanations of this, but if I could add more of a simplistic outlook, if I'm ever showing these charts to a friend I usually tell them that light green or cooler (towards purple) shows a low pressure system, while light yellow or warmer (towards orange and red) shows a high pressure system. In the Winter, this skews a bit, as light green also qualifies as high pressure relative to everything around it (the entire thing about atmospheric pressure is that it's entirely dependent on its surroundings - a system is only high or low relative to its closest neighbouring air masses, such that while green would normally not qualify as high, when it's surrounded by deep lows (dark blues and purples) it can.

Again another simplification, but very very broadly speaking, high pressure brings dry conditions and clear skies, low pressure brings potentially wet conditions and cloudy skies.

I realise that this is very much not the whole story but for the purposes of interpreting these charts particularly from the point of view of the average Irish weather system, I'd suggest that it's more than enough for someone who's new to reading the charts. The key thing about this to remember is that the colours really don't have that big an impact on whether we're going to get snow because what really matters is where the air is coming from.

What's important to understand is that high pressure systems rotate in a clockwise direction, while low pressure systems rotate in an anticlockwise direction (although the opposite is true in the southern hemisphere). This, combined with which direction the system approached Ireland from (west, east, south or north) is the key to determining if it's likely to bring snow or not.

Take this chart for today, for example (Note to mods: Only showing this to compare with a subsequent FI chart, as it has a very simple and easy to explain setup! This is still an FI post as ye will see further down )



What you need to visualise when you look at a chart like this (and to be honest, I've always wondered why Meteociel doesn't include these markings as a feature, I know @Sryanbruen frequently draws them in as I've done here, to illustrate his forecasts) is the direction on the airflow, based on whether the pressure is high or low. As I've mentioned, you can glean that from looking at the colours, so in this particular case, you need to visualise this:



As you can see from this image, there are two main points regarding today's weather in Ireland:

1: We are sandwiched directly between a high pressure system to the southwest (the Azores High, or Subtropical Ridge) and a low pressure system to the Northeast. This is why today's weather is so unstable, oscillating between rain and dry and back again.

2: Because of our position between these two systems, the airflow over the country is a northwesterly airflow - the air is coming from the northwest.

If you look at the temperature charts, you can see why this is important:



Superimpose those two images over one another in your mind, and you'll see that the northwesterly airflow we are currently under brings relatively mild air from the Atlantic. This results in rain if the pressure over Ireland is low, or dry is it's high.

Looking to the North and East, you can see the much colder air which the coldies here are dying to tap into - in order to do that, we need the pressure systems to align such that they are pushing air into Ireland from either the East or the North. South, West, or Northwest won't cut it for us because the air over the Atlantic is naturally warm and, crucially, an airmass which passes over the Atlantic will tend to itself warm up as it crosses, such that a cold system which approaches us from the West or Northwest has almost certainly warmed up too much for snow by the time it reaches Ireland.

Now, on to the juicier FI stuff I've drawn in the directions of the air flow and marked high and low areas, just as before (again, I've always felt that this would all be a lot easier to explain to folks if Meteociel had charts which showed arrows within the isobars and marked H and L where appropriate, but I digress)



As you can see, this places Ireland under a northerly airflow, as we sit on the Westernmost edge of a large polar low pressure system.



Again, superimposing the two charts together, you can see why this results in a cold outbreak, and why therefore, such charts are very exciting to those of us here who enjoy cold

Hope this clears it up a little, and again to the far more knowledgeable posters here I do realise that geopotential and pressure are not the same thing, but nine times out of ten for the lay person one can use these charts synonymously without problems arising!