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

Artane2002 wrote: »

the UKMO phases them but the bottom low at T96 looks suspiciously deep.. we really need the UKMO on board soon..

hatrickpatrick wrote: »

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.



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!

highdef wrote: »

Correction: The TWO blizzards in FI are very pretty though

AuntySnow wrote: »

West based NAO imminent
Thats going to dilute the effects of any polar vortex movement cold spells in Ireland and largely replace them with blandness
Certainly no prolonged cold anyway
That will be reserved for Greece
At best you'll be looking at fronts creating snow events in Central and Eastern England as they bump into glancing blows of continental cold there
As we say down these parts,it's good night Joe doyle

AuntySnow wrote: »

No I do not
I use the word theoretical in most if not all of my posts here when talking about what they're showing
Didn't get it into that one because someone rang me,thought it was there
Word of caution though
Those are anomaly charts
372 is not terribly FI in anomalies
My opinion is we won't know until at least Sunday,what way this is all heading
My expectation is a cold to very very cold outbreak episode at the end of next week to be honest
But like everyone, I just don't know

Gonzo wrote: »

right now it looks like this upcoming spell could be a battleground between the mild south-west Europe and cold north-east Europe with the battleground over Ireland and the UK. If you want cold and snow Scotland and north-east England look like safe bets and the fun starts there from Thursday. Still looks up in the air for us and we may not have an answer till later in the weekend or even early next week.

If we get really lucky and a proper cold spell gets going for us the road to this one has not been an easy or straight forward one!