Guessing at the weather

Guessing at the weather
Viewfax displaying a GRIB file. Note the several 50+ knots arrows.

We are still sitting out a prolonged period of windy conditions that look set to last for some days to come. I thought that I would write a little about the sources of information that we use to ‘guess’ what weather we ‘might’ get. Here when we talk of weather we generally mean ‘wind’; provided that the wind is favourable what the rest of the “weather” is doing is almost irrelevant.

Here in the Chilean channels weather forecasting is a black art. The best forecasts are those provided by the Chilean Navy, we can listen to these on the SSB radio morning and evening on 4.146MHz and by using the Saildocs web-page retrieval system can have the text version emailed to us. The Chilean Navy provide a very good text based webpage for mariners at this URL if anybody wants to check it out ( ) and the forecast that we have been using in the southern channels is at ( ). We are now moving into the Puerto Mont area of responsibility and their forecast is not as comprehensive as the Punta Arenas one; hopefully that is because weather is less of a concern to them.

Our second and long range (further than tomorrow) source of weather forecasts are GRIB files that we download via email. Again I usually use Saildocs for this service because the GRIB request system built into Airmail (See the blog entry describing how we stay in touch) is very good and the integrated Viewfax GRIB file viewer very good – it doesn’t have many bells and whistles which I think helps it present the data clearly without distraction.

The GRIB system is normally very reliable but of late the forecasts sourced from NASA have been late and often are missing forecast periods (we can view the forecast in the GRIB viewer in time steps of 3,6,12,15 hours etc.). According to a message from Saildocs, who download their files from NASA, following an upgrade in mid-January the NASA servers have not been able to keep up with demand. I don’t know if it was the servers that were upgraded or the GRIB system resulting in larger files that the servers can’t handle, either way it’s not working well at the moment.

Luckily I use MaxSea as my electronic charting software and can request GRIB files from within it that come from the European meteorological system and servers which do not seem to be affected by the gremlins at NASA.

ViewFax screen capture with wind forecast

ViewFax screen capture with wind forecast

Why all this interest in weather and GRIB files? Well if you look at the image accompanying this blog entry you will see what I see when I look at the current forecast for 72 hours from now. The grey map shows the area of Chile where we are now, the little green boat shows our position, the arrows show wind direction (mostly northerly in this example) and the little feathers on the arrows show wind strength; a full feather denotes 10 knots, a half feather denotes 5 knots and a triangle 50 knots. As you will see there are a lot of 50+ knots arrows on the forecast – hence my present keen interest!

Now the thing about these forecasts is that they forecast the average wind-speed and are quite reliable in the open ocean but one should always, in these latitudes and in squally frontal conditions, expect gusts to around 30% greater than the average speed – with 55 knots ‘on the GRIB’ that means that we can expect gusts of around 70 knots. Inland we have to take local factors into account and the GRIBs are not as good inland as they are over the open ocean (in this part of the world at least). This is important because the GRIB files are generated by a computer model and do not operate on a small enough grid to take into account features the size of the channels here; which is why we tend to take more notice of the Chilean Navy forecast for the following day which is produced by Human meteorologists with local knowledge.

The channels tend to funnel the wind along their long axis and usually increase the strength, conversely some areas can be perfectly sheltered from winds from a particular direction. Features of any area where mountains feature are katabatic winds or shorter and often more violent williwaws (woollies in the Falklands) and here the woollies can be very vicious indeed. The obvious trick when choosing an anchorage is to pick a location protected from the direction where you expect the wind to blow strongest and free of woollies. We take other factors into consideration too, such as: ease of navigation/access, the view, hiking opportunities, a sunny aspect (for the solar panels and to dry any laundry), historical significance, wildlife, etc. but the overriding factor is always protection from the wind.

We always use our own observations to give us our very local and short range (the next few hours) forecast; wind direction and strength, temperature, cloud type, barometric pressure, etc.. “Tapping the glass” is still as important now as it was when Robert Fitzroy introduced the concept of forecasting based on observations of the barometer.

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