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 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 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.

John was born and brought up in Bermuda and started sailing as a child, racing locally and offshore before turning to cruising. He has sailed over 100,000 miles, most of it on his McCurdy & Rhodes 56, Morgan's Cloud, including eight ocean races to Bermuda, culminating in winning his class twice in the Newport Bermuda Race. He has skippered a series of voyages in the North Atlantic, the majority of which have been to the high latitudes. John has been helping others go voyaging by sharing his experience for 25 years, first in yachting magazines and, for the last 20 years, as co-editor/publisher of AAC.

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