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PeggySue

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  • Gender
    Male
  • Location
    Newton Abbot, Devon
  • Interests
    Painting, Walking, Open Source Software.

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  1. Hi Terminal Moraine Many thanks for that. I will start two lists; one of situations to trust and one of situations to be wary of. Its a pity I haven't been paying more attention in the past!
  2. Hi, My aim in life is to be able to predict 2 or 3 days in advance if I am going to get wet while walking on Dartmoor! We all know that weather forecasts are uncertain but, apart from the ensembles, forecasts don't come with a measure of confidence interval. The NMM model is pretty good at predicting the general regions that will receive precipitation but the extent of the rain and the timing can be wrong in either direction. I'm new to this game but I get the sense that in some cases you can be certain 2 days ahead that a particular weather pattern will hit a given place within a given small window in time but in other cases you can't be sure of the timing or even of the event happening at all. Does anyone have any tips on how to divine the confidence in precipitation forecasts please? E.g. Look at these charts and if you see these patterns you can be sure that it will rain within two hours of the given time. Or look at these charts and if you see these patterns you can be sure that the event or its timing are anyone's guess! Many thanks.
  3. Hi, I note that Hgt500 + SLP is often used for the model summaries. I understand from the guides what the measures are but what is the chart used for? What does it tell us? Also I don't understand the subtleties of the wind stream chart. At first glance I thought the streams were wind isopleths but comparing the 06Z charts for today I see that, SW of Ireland, the streams are very close together but the barbs show a 10 Knot wind. I think I am missing something. Many thanks.
  4. 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
  5. Hi, I am trying to use the GFS Custom Charts but when I select a file and click on "file" to find out what the file is I get the error message: Software error: Undefined subroutine &main::printable called at subs.pl line 162. Is there a glossary anywhere that will help decode the file name structure please? I am running Ubuntu Linux and Firefox 3. Problem also appears in Opera 9.62. Thanks.
  6. 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!
  7. 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.
  8. 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.
  9. 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
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