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Weather => Forecasting => Topic started by: Lost Soul on August 19, 2012, 02:04:07 PM

Title: Unpredictability of Forecasts
Post by: Lost Soul on August 19, 2012, 02:04:07 PM
Just a timely reminder to not put all your faith in weather forecasts no matter where you got them from.  The bottom line advice of course is never go walking without your waterproofs.  You might just get caught out when you are least expecting to.

This morning's weather forecast for the south-east of England told us we will have a nice sunny day.  The highly localised and specific forecasts for Gatwick and Shoreham airports indicated likewise.  Yet at midday in the Gatwick airport vicinity we were treated to lots of thunder and lightning.

‘Oh bother wonder where that came from’ I can imagine hearing somebody say.  Needless to say the Gatwick and Shoreham forecasts were quickly changed to show thunderstorms and showers respectively for this afternoon.
Title: Re: Unpredictability of Forecasts
Post by: adi on August 19, 2012, 05:43:08 PM
LOL all the main storm forecast organisations had issued warnings for storms across the south east today.

http://www.estofex.org/ (http://www.estofex.org/)


Met Monkey

Net Weather

Those being the main ones but the other smaller companies and the crack pots have been rubbing there nuts too.

The charts have shown that the potential was there for a couple of days now and although the atmosphere was capped it was only a small cap and the weak upper level trough was enough to force the atmosphere.

But yes you are right always leave home dressed for the conditions but carrying for the worst.
Title: Re: Unpredictability of Forecasts
Post by: Lost Soul on August 19, 2012, 07:13:22 PM

As you are someone who knows about these things.  Of the web sites you have listed above, if you had to make a choice and just reccomend one of them as being the most useful for the UK which one would it be?
Title: Re: Unpredictability of Forecasts
Post by: adi on August 19, 2012, 07:32:41 PM
Ok I am going to be a bias here and say Net Weather, I used to be on their forecast team. Micheal Fish is on their Forecast team along with John Holmes who used to be the head forecaster for the RAF and Met Office. John taught me a lot when I first started on the forecasting team.

I would not bother getting caught up in the forum, it can become quite cut throat at times.

But for around £25 a year you can get access to 5 minute rain radar and ATD lightning info.

And for around £80 a year you can have access to all the weather data they have.

If you don't want to pay money then Estofex publish European storm risk forecast but they are only interested in severe weather forecasting along with most of the others.
Estofex also offer access to storm relative GFS data sets    http://www.estofex.org/modelmaps/browse_gfs.php?time=0&type=trop (http://www.estofex.org/modelmaps/browse_gfs.php?time=0&type=trop)

I hope that helps, any questions feel free to ask.

Title: Re: Unpredictability of Forecasts
Post by: Lost Soul on August 19, 2012, 07:55:46 PM
Thanks, that narrows the field I will check them out.
Title: Re: Unpredictability of Forecasts
Post by: adi on August 19, 2012, 08:01:38 PM
Lost soul what info are you after exactly? There might be other options.
Title: Re: Unpredictability of Forecasts
Post by: Lost Soul on August 20, 2012, 07:32:04 PM

Thanks for the offer.  I guess the best way for you to be able to give me a useful answer is for me to explain how I go about getting my weather information and what I feel is missing.

Now I do most of my walking and getting lost on the South Downs and the Weald.  Occasionally I venture out to the Peak District or even Snowdonia. 

I start by looking at the Met Office's online regional forecasts and synoptic charts for say 6 and 12 o’clock to get an overview.  Thereafter I start to hone down the information in an attempt to make it much more location specific.  If I’m in an area where a mountain forecast is issued I will look at those.  However, in my local area we obviously don’t have them but we do have the national park forecast for the South Downs.  Which somewhat narrows down the south-east of England to a near coastal strip of 100 miles long by 20 wide.

Next port of call is to look at the aviation forecasts for Gatwick, Southampton, Shoreham etc which provide very specific localised forecasts.  Nevertheless, by extrapolating and combining them an idea can be formed on what is going on where across the intervening landscape.  I will also look at the rain today website http://www.raintoday.co.uk/ and check out the rainfall radar to see what is out there and if it coming in my direction or not within the next 3 or 4 hours. 

Finally, I will have a look at the actual met reports on this website http://www.xcweather.co.uk/  (a website origionally aimed at the gliding fraternity).  It provides half hourly reports from all of the UK reporting stations including of course Gatwick, Southampton etc.  This way I can get a quick assessment of how accurate the forecast is for those places.  And an overview of what might or might not be on its way; in a not too dissimilar manner to the rain today site.  Before anybody says that an awful lot of work.  It’s not really, it takes about three or four minutes to review and provide the assessment.

Yesterday, those sources indicated a nice day was in store for us.  But oh dear come mid-day, where I live, we got hit by thunder and lightning.  As it was happening I checked back on the forecasts I had previously checked earlier in the day to see if I’d missed anything or, if something had been updated.  No, they were all still saying nice day.  Except rain to-day, this is historic information and showed some stuff trotting in from the channel.  It was at that point I was prompted to make my post about not believing all you hear about the weather and to take your raincoat with you.

Your response to my post was to say that a number of organisations that predict convective weather were in fact predicting thunderstorms.  So going back to answering your original question how can I help you.  Well the answer is I just need somewhere to go where I can get quick and easy information on convective weather that compliments the other weather sources I use and so enhances the picture for me.

Interestingly I was reading in the paper today that forecasters actually knew thunderstorms were on their way for the South of England.  But for whatever reasons chose not to mention it.  The BBC was particularly embarrassed and forced to give an on-screen apology.

So Adi if you have a recommendation I would be grateful.  Many thanks.
Title: Re: Unpredictability of Forecasts
Post by: adi on August 20, 2012, 08:43:59 PM
Hey don't beat yourself up. From your posts it is clear you have an understanding of the weather. The BBC gets its weather from the Meto. The Meto are damned if they do and damned if they don't for a number of years now. They have been accused of giving too much info and not enough. It is important to understand they are a national weather service (in fact they are and international weather service) they are charged with providing weather info to everyone because of this they only have to be 80% right, on average they are around 86% right. They have the power to tell you what is happening in your garden but if they did that for everyone people would get upset if the meto did get it wrong for their garden. So they forecast using wide open sweeping arms and hand movements so as not to mislead and inform as many people as possible. 

Regional forecasts are more accurate than national forecasts. The Meto can forecast storms and even tornadic weather but they have to balance cost and level of information they give out. It is very easy for them to give too much info and cause panic in the public, they need to avoid this by all costs. 

What you do is well beyond what most people do. You are probably correct more often than not by doing what you already do.

Rain today is a great site, that gives free 15 min rain radar which is a great resource. Only Net weather offers a better service that in 5 minute rain radar but that requires a subscription and only you know it you want to pay for that sort of detail. I am not sure what advantage 5 minutes offers over 15 minutes unless you’re a storm chaser. It is important to remember the limitations of rain radar though. It can rain somewhere without showing on radar and visa versa.

XC Weather is a great observation site that uses METAR data issued by stations located around the country (world) often found at airfields. Used to be manual stations where someone manually recorded the data on the hour every hour, nowadays most stations are automatic and need no human interaction. It is important to realise that the updates are always an hour behind. So a 0000 GMT reading is published at 0100 GMT so during BST the data is released 2 hours behind. They do not adjust for BST.

This is going to take some time to answer this fully so I will add to it over the coming days. I will tell you what I do for a quick forecast. Using the free charts available on Net weather.   
Title: Re: Unpredictability of Forecasts
Post by: Egg on August 20, 2012, 10:57:26 PM
Some excellent info that I hadn't come across before in your post Adi.
On a slightly more frivolous note, this was sent to me recently.
http://www.theweatherstone.co.uk (http://www.theweatherstone.co.uk)
Seems to be 100% reliable?
Title: Re: Unpredictability of Forecasts
Post by: adi on August 20, 2012, 11:30:00 PM
and likely to kill you in a tornado... lol
Title: Re: Unpredictability of Forecasts
Post by: adi on August 22, 2012, 08:09:45 PM
Sorry I have not finished answering Lost Souls Question I have been busy.

I will try to answer it fully tomorrow.
Title: Re: Unpredictability of Forecasts
Post by: adi on August 23, 2012, 01:19:08 PM
Ok when I want to check the weather for the next couple of hours or even a couple of days what I do is. Visit http://www.netweather.tv/index.cgi?action=nwdc;sess= (http://www.netweather.tv/index.cgi?action=nwdc;sess=) the data found here is free to view and the data defaults to GFS which is perfect for our purpose.

On the top row you will see ‘Fax’ (1) these are the UK fax charts so I always take a look at these first. This gives me a good idea of where the pressure systems are, what air masses will affect me and an idea of where the wind is coming from.

I then always click on ‘Compare’ (2) and compare the SL pressure charts. No model is truly accurate so by comparing the 4 different models you can get an idea of the complexity of the atmosphere at that time. If the charts look very similar then the models will be relatively accurate but if they don’t look similar you know each model is churning out different results because the atmosphere is very complex and models are struggling to predict what the weather will do. Normally when thinks are complex the weather will do something in between all the models.

Ok next I go back to the ‘GFS’ model and underneath the model selection there is loads of numbers from 0 to to 384 (3). These numbers are 0 + hours from the model time so if it is Thursdays 23/08 06 run it is hours from 6 am. GFS produces 4 runs a day at 00, 06, 12, 18 hours.

Under the numbers you will see 2 drop down menus called ‘select chart type’ (4) Chose the chart you want from the first drop down menu, The second is there so you can see two charts at the same time I very rarely use this.
Select you chart then you can advance the chart through the hours ahead. Although the charts go out to 384 hours after 180 hours are known as FI (Fantasy Island) because accuracy drops off pretty fast although they can be used to give you an idea what might happen. 

Title: Re: Unpredictability of Forecasts
Post by: adi on August 23, 2012, 02:15:29 PM
So I have looked at the Fax charts and the Compare charts so I know what the air masses are doing and I have a good understanding whether I am under high pressure or low pressure and what systems are starting to encroach into my area and whether there are frontal systems associated with it. And most importantly it gives me a clue to wind direction I can expect, if I know the wind direction I know what sort of weather is associated with wind from that direction.

So now I go down to the drop down menu and select the UK wind speed/direction chart to take a closer look at the winds and advance the chart through the hours I am interested in. Wind info gives me a lot of information. If winds are moving away from in different directions I know that air is sinking and if it is sinking it is not going to rain and the sky is going to be clear. If winds are converging then the air is rising, convection is happening, I will see cumulus cloud and other convective cloud, I can expect convective showers or thundery showers and if the conditions are right thunder storms or even worse. In winter this convergence may lead to snow showers.

Next I look at temperature, This gives me some idea of what kit I will need, if I am on a multi-day trip I am normally more interested in temperature during the night time hours because I want to select kit that best suits the conditions, I am mainly a hammock user these days and temperature is the main concern when swinging.

Next I might look at precipitation as most people think this is important. Rain for me is an inconvenience. However during winter there are some additional precipitation charts, snow risk and precipitation type which try to predict chance of snow and if it precipitation is rain, sleet or snow. These are useful as a guide only snow forecasting is difficult at best.

One chart that is very useful is the ‘Height of the 0° isotherm’. As a guide snow starts to melt once it has fallen through 600’/182 m of air above freezing so if the 0° isotherm is within 182 meters of ground level then the chance that precipitation is going to snow is high.

Finally during the warmer months (all year round for me) I check the UK CAPE & Lift Index chart to see how strong rising air is rising, As mentioned above rising air is convective and convective air can lead to thundery showers, thunder storms and worse.

If you are interested in cloud cover I suggest you look at satellite images which are freely available on the net.

As I said before forecasting at this level is not an art it is a case of interpretation of the data available. We can take it back further, right back to raw data if you are truly interested but that takes a lot of time to understand and more time to forecast, we are only interested in what weather we can expect on our trip.

Please remember we do not have crystal balls, the models are exactly that, models they do not predict the future so the charts should not be taken as 100% accurate.

If you want to make a quick check of what is going to happen over the next couple of hours whilst you are out it is worth just checking a rain radar  http://www.raintoday.co.uk/ (http://www.raintoday.co.uk/) this will show you what is heading your way.   
Title: Re: Unpredictability of Forecasts
Post by: Callum on August 23, 2012, 03:35:56 PM
I had never heard of Netweather.tv before and have tried this Adi, using your very easy to follow instructions, and it is actually very straight forward to use, one for the Favourites tab. The weather in the Lake District is usually predictable in that it is unpredictable ;)

Which site do you recommend for the clearest and most up to date cloud coverage images?
Title: Re: Unpredictability of Forecasts
Post by: adi on August 23, 2012, 04:50:11 PM
There are hundreds Callum

But the site I find most useful for now casting is http://meteocentre.com (http://meteocentre.com)

Their satellite imaging is found here http://meteocentre.com/sat/get_sat.php?lang=en&area=eur&map=_vis (http://meteocentre.com/sat/get_sat.php?lang=en&area=eur&map=_vis)
Title: Re: Unpredictability of Forecasts
Post by: Lost Soul on August 23, 2012, 06:47:04 PM
Thanks for that Adi.  Will have a look at it shortly and inwardly digest.
Title: Re: Unpredictability of Forecasts
Post by: Callum on August 24, 2012, 09:02:56 AM
This is not good Adi, I now know just how bad the weather is going to be this weekend in  the Lakes :(
Title: Re: Unpredictability of Forecasts
Post by: Pete McK on August 24, 2012, 01:16:30 PM
Still trying to get my head around this one, will post questions if I remain stuck ::)
Title: Re: Unpredictability of Forecasts
Post by: adi on August 24, 2012, 01:30:59 PM
I posted a simple method of forecasting for a trip.

I don't fanny trying to explain how I would go about forecasting for a storm out break with a tornadic potential to the amount of accuracy that I can get myself withing 1000 meters of the tornado if it forms.

Although I can do it I don't think I could ever explain how it is done.