Weather Analysis, A Step-By-Step Guide—Part 3, Learning About the Upper Level

500-mb level and wind shortly after model run time. The low south of New England is Hurricane Jose, and we can certainly see why it stayed stationary for so long.

In the last three chapters in this Online Book I detailed how we download weather information and analyze it to plan our cruises out as much as two weeks from forecast time.

And, as I said at the end of the last chapter, if you simply want to leave it at that you will still have way safer and more fun cruises and voyages than the many cruisers who just look at a small GRIB covering their immediate area.

But for those who want to put in the effort to take their weather-related cruise and voyage planning to the next level, there is one more step: Gaining an understanding of how the upper levels of the atmosphere affect surface weather...yes, I'm writing about the 500-mb reports and prognosis.

OK, I'm not going to kid you. Gaining a useful understanding of the 500-mb charts and GRIBs is one of the tougher intellectual challenges I have taken on.

But, on the bright side, I can also say that even a smattering of upper-level weather understanding can dramatically improve cruises. In short, it has been well worth the effort.

Not Just For The High Latitudes

By the way, some say that upper-level analysis is only applicable for those of us who sail in the upper-mid latitudes and high latitudes. That may, or may not, have been true once, but these days, particularly in winter, we are frequently seeing upper-level strong-wind-belt troughs that reach down as far as 25 degrees, or occasionally even lower, resulting in mid-latitude-type gales in the tradewind belts where winter passages are made by yachts.

In fact, some years ago, Phyllis and I took a nasty caning well south of Bermuda because of one of these south-digging upper-level troughs, and the same system reversed the tradewinds and caused havoc as far south as the Leeward Islands in the Caribbean, well below 20 N.

So my thinking is that if we are going to voyage north of 25 N or south of 25 S, it's worth the effort to gain understanding of the upper levels.

Why I Use The 500-mb Chart

Whenever I mention using the 500-mb charts, someone is bound to accuse me of an inflated ego—just imagine...me? How could they think that?—because clearly (they say) I think that I can out-analyze the models and the professional forecasts in determining what will happen at the surface by looking at the upper-level situation.

To which I say...in my most genteel way...bullshit. I may be deluded about some things, but I clearly understand that I'm just not that smart.

Rather, I have two reasons for looking at 500-mb information:

An Uncluttered View

First off, it's a great way for me to take a clear and uncluttered look at weather trends, particularly in the mid and high latitudes, without getting confused by the details—the reason I look at the 500 mb is exactly because I'm not smart enough to see the big picture when confronted with too much detail.

In fact, the 500-mb forecast is kind of like the view from 35,000 feet, as business people are wont to say these days...actually, it's the view from 18,503.94 feet—wow, I crack myself up¹.

So, for example, when planning our cruises as much as two weeks out, while I can and do look at the tracks of lows on a surface pressure GRIB (as detailed in the last chapter), that often gets messy and difficult to parse. But when I zoom out to the 500-mb level, it gets way easier to get my head "out of the crabgrass"² and focused on the big-picture trends.

Identification of Dangerous Stuff

500 mb 8 days after forecast time, showing more energy coming to Atlantic Canada in the form of a trough over the Great Lakes and the remains of Hurricane Maria threatening Nova Scotia. It's important not to fixate on the exact position of Maria, since it's unlikely to be correct this far out. But what is useful is to note that there is a possible set up for Maria and the trough to combine into a nasty extratropical storm. If returning to St John's, Newfoundland from Greenland, it might be smart to divert to Labrador.

My second reason for using the 500-mb charts is that they warn me of potentially dangerous weather in a way that looking at the surface charts does not.

OK, please lock in and read slowly and carefully, as this is the most important part of this chapter, maybe of the whole Online Book:

Weather forecasts, whether created by a computer in the form of a model, or by a meteorologist looking at a group of models, are only the most likely of several possible scenarios.

And, as we get further from forecast time, the probability of a completely different scenario happening goes up...a lot.

But the cool thing is that by looking at the 500-mb forecast, we can see situations developing that will sometimes create un-forecast dangerous weather.

And this in turn allows us to think ahead about what we will do if one of the alternative scenarios develops.

And better still, we can, using this information, make changes to our plans well ahead of time. For example, we might:

Sifting Sources of 500-mb Knowledge

So what do we need to learn so that we can use the 500-mb charts to plan safer and more fun cruises, and where should we look for that knowledge?

To answer that, I have just finished reading, or rereading, several dense weather texts, including a couple suggested by commenters as a possible source of upper-level understanding, which is why this post has been delayed a bit. Never let it be said that I don't go the extra mile for our members.

Based on that effort, I have bad news and good news:

Making it Easy...OK, Easier

Let's start with the good. I'm going to share a list of the knowledge we need to focus in on to get competent at planning with upper-level charts. Sort of a study guide that will save you wading through, or at least trying to fully understand and memorize, a huge amount of brain-busting stuff that, while interesting and sometimes useful, you don't need to know for our purposes here.

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Stein Varjord

Hi John.
As I’m a combination of nerd and annoying know-it-all, it’s embarrassing 🙂 to admit that I know practically nothing about this topic. Obviously, that situation can’t be allowed to continue too much, so I’ll have to dig into it. I probably can’t give interesting feedback anytime soon, but I will try.

Thanks for this valuable topic. I actually love to find new important topics to learn about.
PS
(Don’t tell anybody about this comment, since officially I already know everything.)
😀

Hans

Hi John,
thank you for this valuable online book. I’m still very grateful for your excellent advice back in 2011. Your recommendation of sailing to Newfoundland and laying Snowball up for the winter wasn’t only good with regard to the wind situation but it also gave me the opportunity of getting to know my – meanwhile – much cherished Newfoundland and its people.
An easy passage it was not though, unless you call being hove to in 50 knts. easy. But it’s true, this was because I tried getting home against your advice and relented only after the storm.
Cheers
Hans

Chris

I immediately bought the Chen & Chesneau book and have been digesting it for the past three days. Holy Cow, these guys know their stuff. In a past life I was a young weather officer in the USAF, having received my met training at St Louis Univ. I want to write my profs and tell them they MUST add part 1 of this book to their curriculum. The book adds an incredible job of connecting the upper level charts to the conditions on the ground, creating an understanding of the forcing functions that make our weather. I have to apologize (30 years late) to those many Army aviators whom i supported without this book as my guide. Thank you, John, for the recommendation. I can’t wait to sit at my nav desk with the upper air package streaming to my computer. Batteries don’t let me down. Ha!
Chris
S/v Morning Winds
Minneapolis

Philippe

May be the 10 hours are a bit underestimated.
Since you have to read and re-read and do it all again. May be this is just me that is a slow learner.

Alex Fontes

Hi John, I carry the Damned Book onboard. Tried binge-reading it in the past but didn’t get much progess. I am currently awaiting a weather window to sail from Fiji to New Zealand, and just saw this article. The Damned Book is sitting on my lap again, I have your proposed steps at hand, and will give it a go. Perfect timing ! Thank You

Alex Fontes

Hi John, I think it is already working. One departure alternative we were considering would have us arriving to NZ under a “Difficult Zone” of an upper level trough (Damned Book, pg 76). And four models have been saying it was doable. Finally this morning the GFS is indicating the formation of rotating surface low at the spot, winds up to 40kts, right where the “Difficult Zone” is. Of course that means we will stay longer in Musket Cove 🙂

Max Shaw

Great timing – thanks John – as I delve back into trying to gain a better understanding of the 500mb charts as we head out of the tropics. I have just reread Steve Dashew’s weather book but also found succinct and up to date summary from Gulf Harbour Radio. David and Patricia provide a volunteer weather service for those of us in the South Pacific. They are both ex-cruisers and David was a professional meterologist. If you are interested, a link to his 500mb article is here: https://www.yit.nz/resources/weather

Much thanks again,

Max
SV Fluenta

Ernest

Just a note to fellow Europeans who would want to buy the “Damned Book” (Heavy Weather Avoidance and Route Design) via amazon.de or other european Amazon stores – the hardcover is priced quite hight (EUR 183), but when ordered from amazon.com in the US, the hardcover including shipping comes at EUR 52,- – so beware…

Stan Creighton

Hi John,
I bought the “Damn Book” and it is good. However, by 10 hours I was still totally confused. I simply could not visualize the concepts in 3D. I finally found a link that clarified a lot for me so thought I would share it with you and others on the site:
http://www.atmo.arizona.edu/students/courselinks/spring12/atmo170a1s1/coming_up/week_3/lect9_upper_level_charts.html
Stan Creighton
MV Buffalo Nickel

michael

Stan,
that linked lecture note you provided was great! very helpful to begin to understand difference between surface and upper air charts, as well as terms such as trough, ridge, etc.
you may like the following NOAA tutorials:
http://www.weather.gov/jetstream/upperair_intro
http://www.weather.gov/jetstream/constant_intro

Carl E.

Hi John,

In case you haven’t heard about this:

http://gcaptain.com/notice-to-mariners-changes-to-noaa-marine-products-could-make-heavy-weather-avoidance-more-difficult/

The author doesn’t sound happy.

George L

“damned book” just about sums it up. While I appreciate, how far ahead of its time it was, this book would benefit from a serious edit.

Matthew Clark

I was trying to prep for what the big book had in store for me with some googling and I found a nice little primer by Joe Sienkiewicz and Lee Chesneau – https://maritimesafetyinnovationlab.org/wp-content/uploads/2020/04/Mariners-Guide-to-the-500-MB-Chart.pdf