Fascinating Article on the Use of 500mb Charts

[Dockhead]

Here: Mariners Weather Log Vol. 52, No. 3, December 2008

I know some sailors like to just passively accept model outputs as GRIBs, and use those without trying to understand the weather any deeper than that, but the more I study, the more I don't like that approach, at least not for my boat!

[GordMay]

I’ve long been a “fan” of Lee Chesneau.

[Wood]

The Dashews also have much information on upper atmospheric weather and forecasts.

Their book are now free electronically.

Safe trip

https://www.setsail.com/free-books/

[Dockhead]

Originally Posted by Wood

The Dashews also have much information on upper atmospheric weather and forecasts.

Their book are now free electronically.

Safe trip

https://www.setsail.com/free-books/

Indeed! I bought the paper version and I'm re-reading it. Probably the best (read: most comprehensible) single source I've encountered.

[cagney]

Originally Posted by Dockhead

Here: Mariners Weather Log Vol. 52, No. 3, December 2008

I know some sailors like to just passively accept model outputs as GRIBs, and use those without trying to understand the weather any deeper than that, but the more I study, the more I don't like that approach, at least not for my boat!

It looks to me as if you just must get a copy of Chen/Chesneaus book "Heavy weather Avoidance and Route Design" from 2008. This book expands on the original article, and though it is mainly directed towards commercial shipping, still contains plenty for those of us who wants a deeper understanding.

Thomas

[IdoraKeeper]

Originally Posted by Dockhead

Here: Mariners Weather Log Vol. 52, No. 3, December 2008

I know some sailors like to just passively accept model outputs as GRIBs, and use those without trying to understand the weather any deeper than that, but the more I study, the more I don't like that approach, at least not for my boat!

Your a bit cryptic here. Can you describe what you are trying to achieve? There is no nautical equivalent to TAF, or METAR.

[Dockhead]

Originally Posted by IdoraKeeper

Your a bit cryptic here. Can you describe what you are trying to achieve? There is no nautical equivalent to TAF, or METAR.

Not trying to be cryptic. I mean that many sailors put all their faith in GRIBs, and don't look at weatherfax or any other higher level weather information in order to try to understand the bigger picture. Some argue that it's not even necessary.

[mikereed100]

After reading that article and others, years ago, I have made every effort to stay to the equatorial side of the 564 decameter line on the 500 mb charts on passage and it has served me well. in addition, following trends in the 500mb charts you can usually get some idea of where the lows are headed days in advance. I use GRIBs all the time, but like to use them in conjunction with surface and 500mb charts whenever available.

[S/V Alchemy]

Originally Posted by GordMay

I’ve long been a “fan” of Lee Chesneau.

I've attended two seminars he's held and own the 500 mb book he co-authored. It's hard work learning how to read this chart, but it pays real dividends when predicting weather likelihoods off soundings.

[Paul Elliott]

Perhaps we should be more specific when we talk about GRIBs. I assume that you are referring to the surface pressure and wind GRIBs. You can also get 500 mb GRIBs that cover the current conditions as well as the upcoming forecasts.

An email to query@saildocs.com with this content will get you surface pressure, surface wind, and 500mb data:

send gfs:20N,60N,160W,120W|2,2|6,12..96|PRESS,WIND,HGT

You can also get surface air temperature, sea temperature, and wave height data by adding these parameters: AIRTMP,SEATMP,WAVES. And there are still more parameters available. And you are not restricted to the GFS forecast model data -- there are other models you can use.

Of course the human-annotated WFAX-style synoptic and forecasts charts are also very useful.


Here are the available GFS model parameters:

PRMSL (pressure at sea level)
WIND (10 meters above surface)
GUST (at 10 meters)
AIRTMP (temperature 2 meters above surface)
SFCTMP (temp at surface)
RH (Relative Humidity 2m above surface)
LFTX (LiFTed indeX)
CAPE (Clear Air Potential Energy)
RAIN (Precip rate, mm/hr)
APCP (Accumulated precip)
HGT500 (500mb height)
TMP500 (temperature at 500mb level)
WIND500 (Wind velocity at 500mb level)
ABSV (Absolute vorticity at 500mb)
CLOUDS (Total cloud cover)
Also WAVES can be added to include sign wave height from the WW3 model.

saildocs.com GRIB general info: Requesting grib files from Saildocs (2010-07-08)

saildocs.com GRIB model info: Available Saildocs Grib Data

I suggest that you try these options before heading off beyond easy internet range, as the specifics do change from time to time. Make sure it works while you can still figure it out.

[Sailormantx]

The Dashew's weather handbook is indeed an excellent reference, including their chapter about using the 500 mb forecast and tying it all together.
GRIB files are just one of many useful sources of weather information today but are not available offshore unless you have some means of communication. Weatherfax can be heard almost anywhere (except Greenland) with a shortwave receiver. It should be pointed out to people who are new to this that GRIB files are the raw output of computer forecast models. Weatherfaxes are actual forecast prepared by meteorologists from the model outputs as well as other sources. Big difference!

[conachair]

Another couple of must reads IMHO >

https://www.amazon.co.uk/Barometer-H...5276460&sr=1-3

https://www.amazon.co.uk/Modern-Mari...marine+weather

[Paul L]

Every weather forecast that I use has already taken into account the 500mb situation. This includes the long range surface wind GRIBs which we primarily use while offshore. It is pretty unlikely that I will ever get to the expertise level where I will interpret the 500mb data and combined with the other data better than these already do. If I could I might write my own world model and automate it.
I did attend one of Chesneau's weather seminars and have read the Dashews book - can't say that reading a 500mb chart has made any difference in any of my passage planning so far.

[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...

[Paul Elliott]

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...

I've not seen convincing evidence that the surface grib forecasts are any less reliable than what humans can do by looking at the 500mb data. The GFS and other models do make use of all the other factors, including those that generate the 500mb charts. What the gribs don't show is the degree of forecast uncertainty, and someone looking at both the 500mb charts and the surface charts may be able to tell when a forecast is problematic.

I'm not saying that the gribs are any *more* accurate than the human-generated forecasts. I know Stan Honey, and he certainly looks at the 500mb data. He wouldn't bother unless it provided some value. But I think it's more in the nature of knowing the amount of chaos in the forecast and routing to minimize the danger of getting caught by an unanticipated shift.

For that matter, that's what I do when I use the surface gribs for wx routing. I'm no Stan Honey, but certain surface patterns are inherently chaotic, and recognizing these lets me avoid sailing into a situation where the conditions might easily turn unfavorable. I also look at the WFAX charts, but that's mainly to see the fronts and developing low-pressure systems that the gribs don't show or forecast well.