Fronts And Troughs

[PeggySue]

Hi,

I am having difficulty finding out how to use the GFS charts. The netweather guides are helpful and explain what is shown on the charts but the guides don't explain how to use the data. For example how do you figure out where the fronts and troughs are and how are they moving?

Also other threads suggest the best way to learn is follow the model thread but could someone point me to this thread please?

Many thanks

[Paul]

Hi Peggy Sue, in it's simplest the sense the way to track fronts and troughs is to use the precip charts such at the one below:

These are viewable from the GFS viewer:

http://www.netweather.tv/index.cgi?action=nwdc;sess=

Comparing these to the fax charts will give you an idea of what is happening too:

http://www.netweather.tv/index.cgi?action=...;type=fax;sess=

The fronts are the lines with triangles (cold front), half-circles (warm front), or both (occluded front) on them. Troughs are marked as thick black lines with no symbols on them.

Hope that helps

Paul

[johnholmes]

welcome Sue,

I'll do you a favour if you will do one, my favourite request.

drop your nearest town into your avatar and for that a quick lesson from me?

so hoping you keep your part

As Paul suggests, follow the ppn but another and one used by forecasters is to see how the dewpoint changes, I'll come back with examples in a bit

watch the wind changes at sea level, as a front goes through they veer (simply means change in a clockwise direction), ahead of the front they do the opposite,

that will do to be going on with.

I'll drop the charts in shortly, bit occupied at the mo as many others are watching the 12z GFS prediction running out!

Edited November 16, 2008 by johnholmes

[johnholmes]

hi Sue

I hope this helps a little. Please feel free to ask or pm me, you may not have that ability as a new member for a while but I'm always happy to answer questions.

Showing how to pick a front out

we will do a trough another day!

The one we see today (Sunday 23 November) is not the best example but it will do to start with.

The first chart is the actual analysis by the UK Met Office showing fronts and isobars. Cold fronts are shown with the sharp points on (on TV they will be blue); warm fronts with a bobble rather than sharp point, and on TV are shown in red. When the cold front catches up the warm front we call it an occlusion, shown in purple on TV and a mix of the two types in black and white.

12z Actual surface chart showing low/high centres and fronts/troughs

chart for the British Isles for 12z

on your pc, click on the 100% button( probably in the middle of the top on next to top line) and change to, say, 200%, to get an enlarged view, then back to 100% for the next comment?

Ahead of the front note two things, although the dewpoint is show (see NW Guides for what this is, and many others also), is shown on the map I've coloured some values in to make them clearer. Note the differences, higher one side of the front (see the chart above) and lower the other side. We call this a cold front as the dewpoint, and to a lesser extent the dry bulb temperatures (again see the Guides) is lower north of the front.

If we look at how the NW Extra shows this, remember its now past 12z so we are looking at what its predicting for 00z; check the surface winds and the dewpoints on the charts, also, although it's a very weak front, look at the precipitation (ppn) chart and see if the ppn matches where the front is shown.

The colours on here are NOT at the surface but give an indication of temperatures at about 500mb, 18,000ft, so ignore them for now. Note where the high and low centres are and the direction of the ISOBARS, again see the Guides for a definition); the surface wind tends to blow along the isobars.

to be honest this does not really help us to find the front as its on a large scale.

So, no problem, lets look at a chart with isobars and dewpoints on to see if that helps

If we take the value of 9C, near enough to our original 10C, its easy enough to see where our front is. Notice that its orientation is beginning to change. Why? Its because the winds to the west and north of the high centre, the circle with the number 1036 (its 1036 millibars-mb for short) and its how we measure pressure, has winds blowing from the south to south west pushing that front north, this time as a warm front, quick peek at the 12z Met O Fax chart for 12z tomorrow to show this

and we can see its shown from se England to the low centre off nw Scotland.

Back to our original cold front, to see if there is any ppn left on it at 00z

and you can see there is just some fairly faint blue patches where the dewpoints change; most of the ppn is now associated with the northward moving warm front its now becoming.

I hope that helps a little. It probably seems very complicated, it is, which is why I've only given a very rough idea on things and I hope no one jumps in to supposedly 'correct' me. In meteorology there is almost always a complication or a way of making it more complex. Best to do it simply and chop a few corners until we learn a bit more. Then we can start to introduce the complications that are there.

When being taught how to find fronts on a basic forecasting course, one is usually given a succession of charts, wind, ppn, temperature, dewpoint, cloud types and heights, changes in pressure and a number of others. In total ignorance, for some, one then marks in the areas of significant change or similarity. Loh and behold at the end of the exercise they will correspond. Obviously a very good example is shown where all the things that are supposed to happen, do; in practice only some of them will fit the supposed pattern. After some experience it becomes relatively easy to find these, discontinuities, or fronts, along with the main areas of differing pressure the highs (anticyclones) and lows (depressions). A basic rule is that the wind blows clockwise (in the northern hemisphere) round a high and anti-clockwise round a low.

Hope that helps, please ask away any time, there is always someone on here that will be able to answer your questions. Enjoy the forum and the site.

[PeggySue]

John and Paul,

Thank you both for your comments. This is exactly what I was after.

I have read John's tutorial a couple of times and I think I get it (although I don't see anything in the thickness + SLP chart yet). I'll give it a go over the next few days and get back with any supplementary questions.

You clearly spent a lot of time on the tutorial and I think it would be worth putting it with the guides.

Many thanks for your efforts; they are appreciated.

[PeggySue]

Hi John,

Bad news: I fell at the first hurdle!

From your annotated chart I decided I was looking for veering winds and decreasing dew point as the front passed and deduced that the front you show was moving south. So I had a look at the charts for 12:00Z today.

First I took a look at the met office forecast:

Here I saw a low north of Scotland, a warm front approximately down the center of the UK and a cold front through Ireland.

I hunted for the Dewpoint + SLP chart you had used and couldn't find it. Thinking that it might be part of the full member suite rather than in the Lite package I tried to take up the free 7 day upgrade but couldn't find the link to upgrade.

I had more success with the Rainfall radar overlayed with pressure, dew point, and 1000hPa wind.

The precipitation appears to a beginner as a text book pattern but I note the dew point is increasing after the warm front and the winds look odd! Taking the warm front to be at the western trailing edge of the precipitation: The winds before it are NNE but they back N in the middle of the warm sector before veering NE before, during and after the cold front.

I wasn't sure if the 1000hPa winds were representative so I had a look at the 850 hPa winds:

This was 5 minutes later for the live rain data but I assume the winds are from the same forecast. Here the winds veer very slightly at the warm front but appear to back after the cold front.

So this leads to some questions:

1) Is it correct to assume that the front in your example is moving south?

2) Should we use the 1000hPa surface winds or the 850 hPa?

3) Assuming the answer is 1000 hPa is there an obvious anomaly to explain the wind backing?

4) When looking for the dew point changes are we looking for any change or is there some special significance with the 10 C of your example. From the GFS chart there is a noticeable gradient around (after?) the warm front but no such gradient around the cold front.

Thanks for taking the time with this and I hope others will find it helpful. Don't worry about using technical terms or maths; if I don't understand a term I know how to find out what it means!

At the moment I am trying to build my knowledge so I can make an assessment of the confidence with forecasts. Future weather is clearly statistical in nature but the only forecasts I have found with confidence intervals are the ensemble forecasts. The rest imply the same level of certainty in the outcome for all aspects of the forecast. Ensembles are good but very course (6 hours) with no indication if the forecast precipitation is for the last 6 hours, the next 6 hours or +/- 3 hours!

Ultimately I need to be able to judge if the precipitation and visibility forecasts for Dartmoor in Devon are dead certs or wishful thinking! The NMM model looks most promising. It clearly recognises Dartmoor as different from the rest of Devon. I have seen cloud and rain patterns that I recognise as typical of the region. Having the knowledge to know which forecasts to believe and which to take with a pinch of salt would be helpful though.

[johnholmes]

okay Sue, I'll come back with a detailed answer,

just as a quickie

behind a cold front the dewpoint lowers

behind a warm front it increases

behind either front it will usually veer?

more later when I get a chance to look in more detail at your reply

keep going you seem on the right track to start.

[johnholmes]

reply to Sue

1) 1)Is it correct to assume that the front in your example is moving south?

Correct:-in the example I gave the cold front was moving south.

2) Should we use the 1000hPa surface winds or the 850 hPa?

best to use the msl(1000mb) flow

3) Assuming the answer is 1000 hPa is there an obvious anomaly to explain the wind backing?

I suspect that you have looked in the very far north where things are complicated by the actual low itself, I’ll explain the winds in that case another time if I may!

If you restrict your view to stations that are over southern Scotland, away from the low centre, you will notice that ahead of the front they tended to BACK but once the front was through, dewpoints rise and the winds VEER

4) When looking for the dew point changes are we looking for any change or is there some special significance with the 10 C of your example. From the GFS chart there is a noticeable gradient around (after?) the warm front but no such gradient around the cold front.

10C special=no

every case will have a figure which seems to be about ‘right’ for that instance; effectively in winter the number will be lower than the number one chooses in summer.

Have a read of the Guide on Airmasses;

dewpoints are always higher in Tropical air to those in Polar air. So behind a cold front they will drop and behind a warm front they will rise?

hope this explains your questions, if not simply ask again.

Ye, I hope that by doing a public explanation, some who are perhaps shy of asking may well learn. For anyone in that bracket please do ask, no one will belittle you, least of all me, we all have to learn at some point. I continue to learn almost every day about meteorology. One aspect I know very little about is lrf work, so I am really looking forward to listening to 3 with far more understanding than me tomorrow evening!

John

re your comment about ensemble forecasting Sue

The Met O may be able to help you with these queries, in that they do use the system to allocate % probabilities when they issue forecasts to paying customers based on the output from their ensembles.

[PeggySue]

Hi John,

Many thanks for that. It is coming together but it always takes me a couple of goes to get it! I will persevere over the next few days.

Incidentally my signature is in honour of your time slip where you quoted next Sunday's chart (23rd) rather than last Sunday's chart in your original email!

[johnholmes]

aw, beyond me chuck, I'm a simple fellow!

it will soon all fall into place, enjoy the wonderful world of meteorology, it gave me a career and a lifelong interest in the weather.

I'm always happy to try and answer any question and happy to pass it to someone better able to answer than me where necessary.

[PeggySue]

Hi John,

Thanks for your help with this. I have spent a few days looking at fronts, dew points, wind directions, and precipitation and I can correlate fronts with sharp dew point gradients, veering winds and precipitation. I have noted that the correlation isn't always that good, especially around the center of lows and the fingers of the fronts (The ends away from the low center). This is actually quite helpful because I am trying to establish which parts of the forecast can be relied upon and which parts can't.

I have read the airmass and Frontal Depression guides but I am still not able to create a 3D picture of a frontal depression in my mind. I can visualise the plan view with warm and cold air initially running parallel to each other and a frontal wave disturbance (e.g. jetstream?) causing an unstable vortex which deepens and a frontal depression is born with winds circulated anti clockwise around it.

I can also visualise a section through a low where air currents flow up, out and down but I can't put the two together. I can't visualise how a packet of air might be travelling up and around at the same time. I assume the air in the warm sector stays in the warm sector but circulates in some way. If this is the case, a packet of air behind the cold front stays their but gains on the warm front, pushing the warm air upwards. Again, if this is the case, the wind generated by the pressure gradient and geostrophic force is actually a component of the whole depression moving and rotating not winds circulating around and around the low like a merrygoround. Perhaps I am confusing the timeing of the plan and section views. Do you know of a video, animation or explanation that might give a 3D representaion in time?

As I say I am trying to establish the confidence level of the forecasts and have a couple of questions but I will start a new thread on that to keep things tidy.

PeggySue

[johnholmes]

I'll come back to you and try to explain when I've a bit more time

[kumquat]

Thanks for the thread, both JohnHolmes and PeggySue. Very enlightening. I also struggle somewhat with 3D visualisations of what the airmasses are doing so any further help/pointers to links would be appreciated.

D.

[johnholmes]

not sure if anyone still wants any explanations, if so please ask and I'll try and help?