Does anyone understand weather forecasting?

E

Ex-SolentBoy

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Apart from the fact that it is very difficult of course.

Today's wind at Hurst Castle shows about 40 knots with gusts over 50. http://www.weather-file.com/hurst/graph.htm

However, every other website I look at shows lower wind speeds forecasted. Tis is not in anyway unusual. We nearly always see anemometer speeds higher than forecast.

Leaving the specifics of today's forecast aside, why is it that forecast winds are nearly always lower than actuals on the Isle of Wight? It's not sea breezes as its just the same in continual overcast.
 
weather forecasting is one of the only jobs in the world where if you make a town halls of it, you still keep your job. I remember years ago taking the yachmaster theory, i completely guessed at the weather questions, no matter how I attempted to understand cold fronts warm fronts etc it went right over my head! local weather is far easier to understand, still, I can't be good at everything.
 
Ex-SolentBoy said:
Leaving the specifics of today's forecast aside, why is it that forecast winds are nearly always lower than actuals on the Isle of Wight? It's not sea breezes as its just the same in continual overcast.
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From what little I understand of forecasting, and a few visits to the Met Office, its a mathematical model they use with a bit of human interpetation.

Actually I think they do a pretty good job and are getting better, if only they did not need to send me Yellow and Amber alerts of rain all the time, just wish they would look out of the window a little more often; I am currenlty sat a few miles from the.

I am sure Simon will be along later to fill in the details and his site/YouTube clips are really good.
 
Ex-SolentBoy said:
Apart from the fact that it is very difficult of course.

Today's wind at Hurst Castle shows about 40 knots with gusts over 50. http://www.weather-file.com/hurst/graph.htm

However, every other website I look at shows lower wind speeds forecasted. Tis is not in anyway unusual. We nearly always see anemometer speeds higher than forecast.

Leaving the specifics of today's forecast aside, why is it that forecast winds are nearly always lower than actuals on the Isle of Wight? It's not sea breezes as its just the same in continual overcast.
Click to expand...
But not on the Coastal forecast which has been forecasting and is now forecasting 8-9 which is about what it is doing.

One of the problems with other sites is they forecast land wind speeds which are about 2/3rd those at sea. Especially xcweather ....
 
It sometimes seems a bit like economic forecasting. I think it was JK Galbraith who said that the only function of economic forecasters was to make astrology seem respectable....
 
>Does anyone understand weather forecasting? Apart from the fact that it is very difficult of course.

Actually it's quite easy just look at the Jetstream, anywhere it is over will get wet and windy weather. Then look at the synoptic chart to get windspeed and direction plus whether highs (good weather) or lows (bad weather) are on the way. That's what we usually do.

If you want to test that these are the two links you need:

http://www.netweather.tv//index.cgi?action=jetstream;sess=
http://www.metoffice.gov.uk/weather/uk/surface_pressure.html
 
I trust the met office. Every single one of the other weather sites you might use will get their data from them.

I also accept that it's not a science and that they won't always be right.
 
I agree that forecasting is both complex and difficult to explain. I try at http://weather.mailasail.com/Franks-Weather/Background-To-Forecasts. These brief notes may (or may not help.)

Weather models are – or try to be – mathematical representations of the physical processes that drive the atmosphere. There are many drivers that act and interact with each other, hence the complexity. The core equation goes back to Newton. . His third law says that if you apply a FORCE to a MASS then you get an ACCELERATION/

There are always forces in the atmosphere –

• Pressure gradients
• Gravity
• The Coriolis effect which acts as a force.


The forces are changing continuously. Therefore the atmosphere is in a constant state of flux. The forecast is always trying to hit a moving target. That is why I despair when I see claims such as these that show understanding neither of meteorology nor of the realities of weather prediction.

....exceptionally accurate
....high-resolution
...precise high quality,
Incredible detail....
....down to 1km resolution

Computers speeds are measured in the peta-flop range but are puny compared to the atmosphere. Observing systems cannot measure the weather on a small enough scale. There is always chaos on all scales from dust whirls in the street to showers and thunderstorms to the disturbances that create hurricanes and Atlantic depressions.

My take – born
of my background as a meteorologist and a cruising sailor – is that, by using GRIB files for 8 days ahead, I know when I can make decisions on a 5 or 6 day timescale and when I cannot do so. For the first 24 hours, I am well aware that no forecast can give me the detail that I will experience. The forecasts – GMDSS combined with GRIBs will help me to be aware of what might happen. After that, it is a case of experience, nous and commonsense.
 
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In this case there might also be a case of geography, in certain winds the ananomter at hurst can read noticeable higher than the surrounding area as the wind can "funnel" there.

Compare yesterdays wind at Hurst with the wind at Lymington there is a signification difference.

As I type there is a 5 knot 25% difference. When using "live" weather from the net you do not always get the full picture about the local environment and its effect.
 
ChrisE said:
But not on the Coastal forecast which has been forecasting and is now forecasting 8-9 which is about what it is doing.

One of the problems with other sites is they forecast land wind speeds which are about 2/3rd those at sea. Especially xcweather ....
Click to expand...

Remember XCweather displays live (or more or less live) wind culled from (largely) airfield automatic weather stations, plus a few buoys. Almost all land stations.
It's not bad really and I find it as useful as the professional forecasting done for offshore operations. It's less use West of Shetland mind you :)

In my normal sailing area - West of Scotland / North Irish Sea, I find forecasting generally over- estimates conditions by 1 to 2 BF, So if the number 3 is used - the chances are you'll have nothing to sail with.
To get a good 15Kt sailing wind , you will often be listening to dire mutterings of 5-6 and "Strong wind" warnings.
I'm not advocating ignoring Met marine coastal forecasting, just realise it needs a pinch of salt and some independent thought as well. ;)

oh... and a good New year to all....:D
 
onesea said:
In this case there might also be a case of geography, in certain winds the ananomter at hurst can read noticeable higher than the surrounding area as the wind can "funnel" there.

Compare yesterdays wind at Hurst with the wind at Lymington there is a signification difference.

As I type there is a 5 knot 25% difference. When using "live" weather from the net you do not always get the full picture about the local environment and its effect.
Click to expand...

If you are near the coast then, yes, obviously, the geography can greatly affect the wind speed and direction. You have only got to sail to realise that. Even if the NWP models got the gradient winds spot on, they will not calculate local winds accurately. To model, for example the effect of Start Point would mean using models with grid spacings down to a few 100s of metres even then variations due to gusts would produce different effects.
 
Tomahawk said:
Frank

Just done a search on peta-flop.. on your web site..


Search Results
0 pages found out of 279 pages searched.:confused:
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Sorry. I had not noticed the title of your post. I thought you meant the ideas as a whole. Put it down to the morning after - or, in the case, the morning.

You would have to go to Google to find pet-flops. When I wrote those pages computers were not quite as fast as they are now. To save you looking, 1 peta-flop = 1000 million million. In 1960 the Met Office computer had a (then) stagering speed of around 3000 flops.
 
Tomahawk said:
Frank
Just done a search on peta-flop.. on your web site..
Search Results
0 pages found out of 279 pages searched.:confused:
Click to expand...

FLOPS (FLoating-point Operations Per Second) is a measure of computer performance.
Peta is a prefix denoting 10^15 or 1000000000000000.

franksingleton said:
Sorry. I had not noticed the title of your post.
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You are not alone, I find it annoying when people don't reference the title in the body of the post, makes quoting more difficult.

Must learn to type faster :)
 
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