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Metcheck Forecast Room - Meteorological Discussions
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Sometimes, the front page articles just aren't big enough to cover all our thoughts and ramblings on the developing weather.
By using graphical analysis from the weather models, as well as full forecasting terminology, you can see how events are being discussed here at Metcheck. The discussions aim to provide unparalleled access to forecasters thoughts and you may even start talking like us too ;-)
Here are the latest discussions from the Metcheck forecasters.
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| Current Discussions (PREMIUM ONLY) |
| Ensembles - Their Use In Forecasting |
There are many tools available to a meteorologist for analysing the weather. Satellite and radar images for current conditions, to the various weather charts that predict pressure, rainfall and wind speed for the coming days. There is one piece of information that is highly important and they are ensembles...
Up to present the discussions have contained forecast charts but we will now be discussing and using ensembles in some of the discussions that are issued. So here is a run down on how they are used and why they are important.
Take a look at the below ensemble graph for Manchester;
It looks rather messy doesn't it, but don't worry!. I will now explain in full how these images are of great use and importance.
Firstly, let's start with the axis of the image/graph and what they mean;
1) The left hand side of the image has numbers going from +20 all the way down to -30. These values are temperature values at the 850mb level and are in degrees celsius (oC). (If you are unsure as to what 850mb temperatures are and how they are used, take a look at the discussion on 850mb temperatures and their use in forecasting - http://www.metcheck.com/V40/UK/FREE/discussion.asp?DiscussionID=17)
2) The right hand side of the image has numbers going from 40 down to 0. These values are rainfall measurements in millimetres.
3) Finally the values across the bottom are self explanatory and represent the days ahead from when the image was produce.
From the above information you will now be aware that there are two pieces of information that can be gathered from these charts. The first is the predicted 850mb temperatures and also the predicted rainfall. The 850mb temperature predictions are across the middle of the image and the rainfall predictions are at the bottom.
So what do all these coloured lines mean I hear you ask?...
Well, in weather forecasting we use a variety of information to help us discover what the weather is likely to be in the coming days. This would be very difficult indeed if we only had one or two pieces of information available. The more information available for analysis means we will have a better understanding of what the weather will likely be.
That is what ensembles give us. They give us a direct view of not just one but a variety of possible outcomes. These 'outcomes' are known as 'members' and these members combine to make an ensemble.
So in the image above, we can view numerous different members each giving a predicted 850mb temperature and rainfall total. Now for the very important part...
What meteorologists look for in the image above is a congregation of the lines (members). The more tightly packed together they are the GREATER the confidence is of that particular value being correct whether it be the 850mb temperature or rainfall. When the lines are more widespread this means there is uncertainty and much LOWER confidence of what the predicted 850mb temperatures and rainfall will be.
So, again take a look at the image once more;
You will notice that during the first few days of the graph the members (lines) are tightly packed and congregated together. This means that confidence is high that the 850mb temperatures will be around 8C on the 5th of September for example.
In contrast later on around the 9th to the 11th of September. The members become spread apart, so there is low confidence surrounding the predicted 850mb temperatures at this time.
Of significance as well is the lack of predicted rainfall throughout the entire ensemble, from the 2nd all the way to the 17th of September.
FORECASTER: HUGO
Discussion Added : 02 September 2007 |
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