Fascinating Article on the Use of 500mb Charts

[Symphony]

Originally Posted by Emouchet

500mb is a valuable extra bit of information for planning offshore passage.

Even without getting into the details, it is possible to explain the value of locating the 5640 height contour position and its relation to Force 6 (summer) or Force 8 (winter) maximum wind speed on the surface.

I am always surprise that this simple concept is not at least mention in basic offshore route planning training session. It is so dam simple and brings so much confidence on route choice. This has been released public now for more than 20 years...

+1 on this technique/ weather feature
+1 on Dashew's Weather book

I try to remember not to blindly accept expert advice not because I am better than the experts - I certainly am not - but because weather is super localized. Each of us is in the position to use our senses, barometer, wind and seas activity, etc. to do the best job of forecasting significant weather affecting only our boat. The expert forecasters are not looking at your particular 50 square miles... they are looking at large scales trends, averages, extremes.

When people think the forecaster is wrong because they got a S wind when the forecaster said north, they must consider when and where they were.

On the same level, the 540 mb chart is also on a global scale, but knowing what's happening on top, surface, and local will give any reasonable sailor a good idea of what's going to happen in the next few hours or minutes.

there's also the NOAA Atlantic Briefing Package - http://www.opc.ncep.noaa.gov/shtml/A_brief.shtml

[Cpt Pat]

Originally Posted by IdoraKeeper

There is no nautical equivalent to TAF, or METAR.

Wouldn't it be nice if there were? As a pilot, I can get a METAR for tiny airports in Podunk, but there's no equivalent for us boaters. Sad.

I use these charts for my pond (Pacific). GRIBs don't give you the big picture.

http://www.opc.ncep.noaa.gov/shtml/P_brief.shtml

[estarzinger]

as has been mentioned, 500mb is in gfs and can be got via gribs (not just by wefax).

the accuracy is pretty good, just for fun here is a chart showing a 24hr forecast overlay-ed on the actual (NWS does a good job continuously monitoring accuracy and providing data on it). Can you do better accuracy? (there are numerous other accuracy/quality charts if you want to look at it in different ways, I just presented this one because it shows what you would see in the grib)



I will comment that the article cited is from 2008 (and is a redraft from 1995), the dashew book from 1998 , and lee's book from 2008 (also mostly from 1990's thinking/background). Quite a lot has developed/improved since then (48hr forecast accuracy is about 4x what it was when dashews books was released), and it is pretty much all about numeric models now (Lee left just before numeric models took all over).

as mentioned, the two key user inputs are (1) what your actual conditions are, and (2) assessment of confidence.

[philiosophy]

Paul, you have sumed it up in a very simular way to my thinking. When the fronts show up all bets on those gribs are off. That is a quote by two different Meterologists here in New Zealand. One a retired American and the other a Kiwi. It seems to take the grib models quite a while even days to get it right after a front shows up. Especially in areas like New Zealand to the Islands where there are a lot of quick forming fronts and fast changing wind direction. I came to realize I wanted something while a bit harder to understand(Weather Faxes) than some very lovely and easy to read graphics ($500.00 a year) that were just when I needed them not at all accurate. You can believe what you want but the gribs fail because they arebased on an algorythmicprogram and run on with not nearly frequent enough input. I don not pay for those lovely graphics any longer. Back to old school what we used on the tugs offshore and my partner and I will be reading these books mentiond to refresh and learn in more detail how to use the 500 millibars information and anything else. Much thanks for this very important forum.....Kim

[Paul L]

Originally Posted by philiosophy

Paul, you have sumed it up in a very simular way to my thinking. When the fronts show up all bets on those gribs are off. That is a quote by two different Meterologists here in New Zealand. One a retired American and the other a Kiwi. It seems to take the grib models quite a while even days to get it right after a front shows up. Especially in areas like New Zealand to the Islands where there are a lot of quick forming fronts and fast changing wind direction. I came to realize I wanted something while a bit harder to understand(Weather Faxes) than some very lovely and easy to read graphics ($500.00 a year) that were just when I needed them not at all accurate. You can believe what you want but the gribs fail because they arebased on an algorythmicprogram and run on with not nearly frequent enough input. I don not pay for those lovely graphics any longer. Back to old school what we used on the tugs offshore and my partner and I will be reading these books mentiond to refresh and learn in more detail how to use the 500 millibars information and anything else. Much thanks for this very important forum.....Kim

The GRIB forecasts out 48 to 96 hrs do pretty well between NZ and the islands. It is the longer forecasts that tend to have large unknowns. 'Old school' didn't even try to forecast out 7 days.

[ArmyDaveNY]

Thanks for posting this. It was a very interesting read and is quite useful.