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NeilW · 14-10-2014, 15:25

I am struggling to use the 500mb charts for prediction of upcoming weather/wind.

I have read several texts on the subject and have a basic understanding of what they are, and what troughs and ridges are, and also that these are the steering levels of the atmosphere, which in many ways control the development of the surface features.

While many texts describe the 500mb charts as "essential" and "most meaningful", with the availability of 24, 48, 72 and 96 hr surface predictions, done by experts, what extra can I gain from studying the 500mb charts?

Thanks
Neil

barnakiel · 14-10-2014, 16:11

I do not use 500 as this data is already incorporated in forward surface forecasts.

I may be wrong in this so will be glad if anyone else chimes in with some for arguments.

b.

mikereed100 · 14-10-2014, 16:23

Neil,
While, as you point out, the information from the 500 mb charts is incorporated into the surface charts I find them useful to get an idea of what the weather will be doing over the next few days. In particular, I pay close attention to where the 5640 meter line is and do my best to stay below it, as well as try to steer clear of any cutoff lows.

belizesailor · 14-10-2014, 17:03

Even though the pros have considered them in their forecasts, I like to review them to develop an understanding of how/why patterns might shift.

My favorite view is to overlay them with surface isobar/forecast charts which can help make some of the influences more obvious.

In addition to 500mb charts you should take a look at upper level "stream lines" charts too, which show the wind flow and speed rather than just isobars.

http://www.weatheronline.co.uk/cgi-b...0&PERIOD=&WMO=

To understand why the pros made certain decisions in their forecast, and how confident they are in the forecast, its good to read the discussion notes such as published by NOAA. Also good to compare multiple models/forecasts.

I don't go to all this trouble every day, but when evaluating severe/tropic weather (tropical storm, hurricane...), I like all the information I can get.

Lodesman · 14-10-2014, 18:11

The closer spaced isobars on the 500mB chart give you an indication of the path of the jets at that level, which aligns with the jet stream (at about the 250mB level). The jet stream marks the boundary between two air masses, ie. the frontal boundary, so you assume the path of any associated lows along that boundary.

jackdale · 14-10-2014, 21:37

Like others I use the 500 mb chart to get a sense of the jet stream. The is essential in the PNW. Right now the jet stream is down through northern California, and there are a series of lows tracking through the Pacific heading for Vancouver Island.

Northeast Pacific WX Briefing Package

NeilW · 15-10-2014, 07:22

Thanks all for your replies.

I will continue to try and improve my understanding of these charts a little more.

I sense there is additional information I am missing that does not show up in the longer range surface forecasts, even if all it becomes is a confidence level in what the surface predictions are forecasting and the position of the Jet stream (although I thought that was more from the 250/300mb charts).

It is good to understand at least why the surface predictions are what they are.

BTW, I liked the weatheronline site. A lot of information nicely presented for those with internet access.

Neil

Paul L · 15-10-2014, 08:21

I'm sure the 500mb charts are useful to some cruisers. For me, I just don't see that I am very likely to ever be better at interpreting them than the pros who are creating the surface charts -- that I can use directly. Since most of my cruising is in areas that I am new too, I can't really take any personal history of weather situations and relate them to current 500mb charts. I sat through Lee Chesneau's weather class that was very focused on 500mb charts and can't say I got a lot out of it.
Daily GRIBS and an occasional surface chart works so far for me.

Lodesman · 15-10-2014, 08:35

500mB equates to about 18,000 ft whereas the jet stream occurs at the tropopause, roughly 35,000 ft or in the 200-300mB range. For actually forecasting the jet stream for aviation purposes the 200-300mB charts are more useful; but for the purpose of surface analysis, the 500mB chart is a little easier to use. You might get something from this: Mariners Weather Log Vol. 52, No. 3, December 2008

jackdale · 15-10-2014, 19:27

Quote:

Originally Posted by Lodesman

500mB equates to about 18,000 ft whereas the jet stream occurs at the tropopause, roughly 35,000 ft or in the 200-300mB range. For actually forecasting the jet stream for aviation purposes the 200-300mB charts are more useful; but for the purpose of surface analysis, the 500mB chart is a little easier to use. You might get something from this: Mariners Weather Log Vol. 52, No. 3, December 2008

True.