Grek weather

[charles_reed]

Hi, Charles, I really do not understand your comment. I have long regarded the NOAA GFS as one of, if not the most useful of all GRIB (ie objective) forecast services available to sailors. That is particularly so now that it computes on a grid of around 12-13 km although only providing data on a ¼ degree ie 25 km grid. It is easily available over narrow bandwidth systems using zyGrib or Ugrib or by email.

Given broadband Internet access, the Turkish Weather-wise - http://212.175.180.126/DTS/sea.php - should be as good better over the first few days. Large scale charts are available as well as meteograms for specific locations and a route planning forecast. This uses the ECMWF data analysis (currently 15 km but due to come down to 8 km) with the WRF forecast on a similar sized grid.

The GFS and Weatherwise have better initial data inputs than the Athens U , Skiron, OpenWRF or any of the various commercially available GRIB type forecasts. As far as I know these all start with the GFS 25 km grid and add no more actual weather data to initialise their models. As the models themselves are all very similar, the main difference is in the data input. That is very much an over-all statement. On any individual occasion any one forecast may do better than the others. There will also be some gains on some occasions due to use of finer scale topography. On other occasions this gain will be lost because the meteorological input is less good.

The Greek Met service numerical model output should be as good as Weather-wise and the GFS, may be a little better at the higher resolution forecasts.

In practice there will probably be little difference in general terms berween all the various services. Preferences are often a matter of presentation and ease of use. For me, zyGrib is the easiest to use. I like the viewer. When I get output by email I still use the zyGrib viewer although some may prefer the Ugrib viewer.

I am always sceptical of the real value of high resolution forecasts because of the short lifetimes of small weather detail. To me, it always seems better to use global model output, especially with their current grid lengths, remembering that all models tend to underestimate stronger winds. To which you should always add your experience and judgement.

That is the message that I give in Reeds Weather Handbook.

Many years ago, you took me to task for swearing by using the NOAA output (admittedly @ the largest scale on zygrib) without any meteoroligist input.
However, I find your recent comments above unexceptional - though I find on balance nearly all forecasts over-forecast wind speeds. (Excepting meltemi, mistral and bora, where they frequently under-estimate).
What none of the forecasts appear to be able to do is to accurately allow for island effects (which can make a F3 on one side a F6 on the other) or local thunderstorms.
I too, prefer the latest version Zygrib (current stable one is 7 for Win and 6.9 for Linux), though I suspect the European model is more accurate.
The trouble is, without such post-trails one can get stuck in a rut.
I do find Western Approaches weather far better forecast than in the Med (which is a large lake with lots of land effects all round).

 

[franksingleton]

Many years ago, you took me to task for swearing by using the NOAA output (admittedly @ the largest scale on zygrib) without any meteoroligist input.
However, I find your recent comments above unexceptional –

In the early days of GRIBs (c 2001) it was certainly sensible to know how the professionals were interpreting the output. Nowadays, advances in modelling are such that objective (GRIB etc) forecasts are the best guidance beyond 2 days and, often up to around 6 days. In the short term I have always said that you should know what the human forecaster is saying.

………… though I find on balance nearly all forecasts over-forecast wind speeds. (Excepting meltemi, mistral and bora, where they frequently under-estimate).

Do you mean GRIB or subjective forecasts? As a general rule, I usually add one Beaufort to strong winds given by GRIB products. A grid can only represent weather features on a scale of around 4 or 5 grid lengths. For the current GFS that is at least 60 km. Even for the 4 or 5 km grid used by Met Service modellers that is still 15-25 km. Wind strengths vary on smaller scales than models can represent – even were they perfect. And they are not.

Forecasts produced by humans often seem to over forecast but that is a consequence of the “rules” for warnings. For the UK and it is probably similar for French forecasts a gale warning MUST be issued if a gale MIGHT occur within 12 hours. But a gale warning can only be cancelled when the forecaster is certain that there are no gales existing or expected. If you think about it, that will often give the impression of over forecasting. Also, the forecaster has to ensure that there is warning of possible weather hazards. When sailing I am never surprised when we have spells of wind up to one force stronger or less than the GMDSS is saying.

What none of the forecasts appear to be able to do is to accurately allow for island effects (which can make a F3 on one side a F6 on the other) or local thunderstorms.
I too, prefer the latest version Zygrib (current stable one is 7 for Win and 6.9 for Linux), though I suspect the European model is more accurate.
The trouble is, without such post-trails one can get stuck in a rut.
I do find Western Approaches weather far better forecast than in the Med (which is a large lake with lots of land effects all round).

Predictability is a limitation not understood by many sailors. It is possible several days ahead to know that there will be showers over the Solent. Six hours ahead it is impossible to say whether or not there will be a shower over the Nab Tower at midday. It would be not at all al certain that there will be a shower over the Solent at that specific time.

When it comes to small scale topographic effects. Take a look at http://www.puertos.es/es-es/oceanografia/Paginas/portus.aspx#. Select Isla Baleares and you will see that wind shadow effects are shown but on a far coarser scale than you will experience.

Some major Mediterranean features are well predicted days ahead – Mistrals and Boras for example can be seen on GRIB charts. Topographic effects are, of course, of great significance but how they are created depends on the large scale pattern. That is one reason why I never recommend use of those (private) forecast services that do not use detailed data analyses but claim to provide detailed forecasts. GIGO applies.

 

[Tony Cross]

I think that what Charles is saying is that forecasts are just that; an estimate of what the weather might be like given open seas and no land.

Frank seems to be saying that forecasts have become much more reliable of late but that it's still vital to consider the effects of local topography.

It seems to me that little has changed despite the advances in technology, all the forecasts we have access to are still estimates and its essential that we apply some common sense and local observations before making an decisions on what the actual weather might be.

It is, in my opinion, a huge mistake to interpret any online forecast as a statement of fact. Forecasts are simply a guide, and even though the accuracy of forecasts has increased over the years, the decision on what the weather will actually be is still down to us. Computer forecasts are a great tool, but they are not a statement of fact.

 

[franksingleton]

I think that what Charles is saying is that forecasts are just that; an estimate of what the weather might be like given open seas and no land.

Frank seems to be saying that forecasts have become much more reliable of late but that it's still vital to consider the effects of local topography.

It seems to me that little has changed despite the advances in technology, all the forecasts we have access to are still estimates and its essential that we apply some common sense and local observations before making an decisions on what the actual weather might be.

It is, in my opinion, a huge mistake to interpret any online forecast as a statement of fact. Forecasts are simply a guide, and even though the accuracy of forecasts has increased over the years, the decision on what the weather will actually be is still down to us. Computer forecasts are a great tool, but they are not a statement of fact.

The greatest change/improvement has been that the guidance for the longer periods ie up to ~one week has improved significantly. We can often plan up to a week ahead. When cruising we can often see that we should be looking for a sheltered harbour 5 or 6 days ahead. Weather is not precise. Nor are or ever will be forecasts. Anyone who claims to give precise forecasts is living in never-never land.