UKC

Hi there,

I'm really interested in weather forecasts for the Winter and I get a kick out of sitting about on UKC or Winterhighland looking at snow forecast threads. However, folk are always posting these complex and colourful pressure charts called GFS runs, or 850pa charts or something. Here is an example of one of these charts for Christmas day: http://www.winterhighland.info/forum/file.php?2,file=9476

I know this looks very dramatic and Daily-Express-esque but I don;t have a clue what it means. Obviously I know about high and low pressure but is this just a pressure chart like the isobar charts you see on telly? Is there a website where I can go to learn how to read these kind of forecasts?

In reply to TheseKnivesMan: There's been a few "how to do it" threads on the weather forum on Snowheads - nazawonson(?) seems to be the guru.

In reply to andy:

Aha! Thanks man I've found a beginner's guide on that forum.

In reply to TheseKnivesMan:
GFS is a global weather forecasting system run by the US National Weather Service and it is the most popular weather forecasting system on the internet as the output is global, a reasonable resolution and free. The amount of computer power required to generate a meaningful weather forecast is significant, and well beyond the means of a two-bit website relying on popup adds to fund, which explains its popularity for syndication.

The ensemble forecast is a collection of forecasts (at a slightly lower resolution). The forecasting algorithm is run multiple times (I believe it's around 20) with subtly different starting conditions and the variation between the individual forecasts can be compared for numerical stability. If they all match up, then the forecast has a high confidence. If they all wildly diverge with different answers after 2 days then the forecast is probably pretty hopeless. This is quite a complex task when you consider how much data is involved.

The chart you showed looks like it shows one of the individual ensemble runs with the isobars (pressure) in white writing and the colours show the geopotential height at 500mb pressure. That's rather complicated but in short if the geopotential height is lower than normal then the weather is more likely to be stormy.
Your chart shows the height of the 500mb contour at 4900m over the North Sea, which is well below the normal height of about 5500m.

This particular run looks different to the main GFS run so perhaps that's why it was flagged up on the website.

In reply to richard_hopkins:

Thanks very much, I think I might have to do some prerequisite reading first to understand that a wee bit better. I failed maths and physics in school, so I'm not the best numbers person!