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Weather forecasting.

Patagonia's weather patterns by Gregory Crouch
Informacion basica para prevision de clima en Patagonia

Forecasting models

There are a number of numerical models that are used to forecast future weather conditions. Using the current conditions, which are obtained from weather stations, satellites, balloons, buoys, etc, an algorithm is applied obtaining the weather conditions 10, 20 or 30 minutes ahead, to which the algorithm is applied once more, repeating the process until the desired length of forecast is obtained.

Unfortunately for southern Patagonia, the economic interests in the area are limited, so there is limited allocation of resources to observe current weather conditions and produce a reliable forecast. Weather stations are few and far between and unlike the ocean east of Florida, very little is known about ocean temperature and currents in the South Pacific and off the coast of Antarctica, and their influence on weather patterns in the southern end of the Americas.

There are only three models that can claim to be independent. All other models and weather forecasting services are interpretations based on their results.
- The US’s National Oceanic and Atmospheric Administration runs the Global Forecast System (GFS) which produces a worldwide forecast four times a day for a period of 16 days.
- NAVGEM is a model run by the Naval Research Laboratory in Monterey, CA to be able to keep an eye on GFS accuracy.
- IFS (Integrated Forecast System) is a model operated by the European Centre for Medium-Range Weather Forecasts (ECMWF), that delivers forecasts every twelve hours and for a ten day period.

As a result of US law, NOAA’s GFS model is the only global model for which all output data is available; hence it is the most relied upon model. NAVGEM also seems to disclose most of their data, enough to serve as a point of comparison with GFS. ECMWF makes available for free only a small amount of data, so it is unfortunately of little use to anyone pursuing a non-monetary profit oriented activity such as climbing.

Sites like Windguru, The Weather Channel, Accuweather, Weather Underground, Meteo Exploration, Free Meteo, Snow forecast, etc, have no models of their own and produce no “original” data. They rely on the GFS model to make interpretations of future conditions, so comparing them is fairly pointless. It is far more interesting to compare different models than different interpretations of the same model. Plus, since NOAA’s makes all the GFS data available you can make your own interpretation without needing someone else to digest it for you. As mentioned earlier, the only two models that you can easily compare are GFS and NAVGEM.

Meteograms versus maps

Meteograms and weather maps offer the same data but in different formats. Looking at them for a few hours you will get used to understanding both. Weather maps allow for a much deeper understanding of the current and future weather conditions and also allow you to assess the reliability of a forecast, so they are my preferred alternative. That said, the meteogram is quick to obtain, requiring a short time to load and read.

NOAA’s GFS is most easily viewed as a meteogram (a weather chart), while NAVGEM is most easily viewed as weather maps. In their site the US Navy makes available both GFS and NAVGEM maps, in a format that makes comparing them very easy. Please note that many internet cafes in Chalten have connections that for some unknown reason do not allow you to open the Naval Research Laboratory’s webpage. Try several until you find one that does, or use Wetterzentrale, a German site that has both GFS and NAVGEM maps available that can be opened easily from anywhere. Note that the resolution of the Wetterzentrale maps is lower than the US Navy maps.

Update times

GFS and NAVGEM maps, and GFS meteograms are updated four times a day. In El Chalten those approximate times are: 9AM (for the 06z forecast), 3PM (for the 12z forecast), 9PM (for the 18z forecast), 3AM (for the 00z forecast).

Zulu time

All forecasts are in “zulu” times, which is UTC time (Coordinated Universal Time, which is equivalent to GMT). Depending on the time of the year and on daylight saving time, UTC is three to four hours ahead of Argentina, so 12z is 8 or 9AM and 00z is 8 or 9PM (of the previous day).


When using coordinates to retrieve a meteorogram, make sure you use coordinates that are for the mountains, not for Chalten. Although the grid for this part of the world has a resolution of around 20km the information between them varies greatly. Coordinates in all meteograms are expressed in decimal degrees, for example Cerro Torre is: -49.3 -73.1. Don't forget to put the minus in front, otherwise you will end up with a forecast for Kazakhstan and, although the mountains are reportedly beautiful there, it might not be what you are looking for.


All weather forecasting pages offer a number of variables; the main ones are sea level pressure, precipitation, wind speed and direction, temperature, humidity, dew point and in some cases solar radiation and cloud cover. Without having to become a meteorologist you will be able to make fairly accurate assessments of the upcoming weather conditions.

Mean sea level pressure

As far as barometric pressure, more often than not good weather happens with mean sea level pressures higher than 1020 hectopascals (hPa). However, good weather can happen with low pressure if the isobars are far from each other. In those cases decent climbing weather can happen with pressure as low as 1000 hPa. One important note, 1 hPA equals 1 millibar (mb or mbar).


Temperature at times is shown not at a defined elevation, but at a certain “barometric pressure height”, expressed in hPa. In those cases refer to this approximately: 1013 hPa=sea level, 800 hPa=1800 meters, 700 hPa=3000 meters, 600 hPa=4000m, 300 hPa=9000 meters. Some meteograms show the temperature at 10m or at 850 hPa height, while others show the 0º isotherm at how it varies in elevation, either in meters or hPA height. The US Navy maps show how the 0º isotherm at 850 hPa moves north to south.


Precipitation is expressed in millimeters per x hour period in the meteograms, while it is expressed in millimeters for the previous 6 to 12 hour period in the maps.


When looking at wind keep in mind that good weather usually happens with wind from the South or SW, while most storms come from the W or NW. The wind speed is expressed in knots (1kn=1.852 km/h), in kilometers per hour (km/h) or in meters per second (10m/s 36km/h, 15m/s 54km/h, 20m/s 72km/h, 25m/s 90km/h, 30m/s 108km/h). Wind in some cases is expressed at different hPa heights and in others at a fix height. For the latter you will have to know the area to know what 11kn of wind at 10m really feel like in the area you are in, and what wind is climbable wind for your pain tolerance levels. For the “classic” NOAA meteogram that shows wind speed at “10m over sea level” I would suggest that winds over 10 knots are too high to climb if you are on Fitz or Torre. Note that also available from NOAA are windgrams that chart wind speed and direction at different heights for the duration of the forecast model. In the maps the wind is expressed in flags. The flags show the direction of the wind while the tails indicate the intensity, each tail being 10 knots. You can also guess wind from the maps by looking at the distance between the isobars. Wind is at times shown not at a defined elevation, but at a certain “barometric pressure height”, expressed in hPa. In those cases refer to this approximately: 1013 hPa=sea level, 800 hPa=1800 meters, 700 hPa=3000 meters, 600 hPa=4000m, 300 hPa=9000 meters.

Dew point

The dew point is the temperature at which the air is saturated with water vapor. The difference between the air temperature and the dew point indicates the relative humidity. In a good weather period more often than not you will see both very far apart. Relative humidity is expressed in weather maps in shadows and can be a good indicator for cloud cover.


For any forecast information keep in mind that only the information for the first three days is reliable and that often times anything after five days it is pure fantasy. That said, big weather systems, big high pressures can at times be forecast two weeks in advance. In 2012 we had such an event, when two weeks before we observed a 1042 hPA high pressure system move east past New Zealand to join forces with the stationary Pacific high pressure, resulting in a one week good weather period in El Chalten. This type of observation can only be made in weather maps. For the past few years many Argentine climbers stay at home, tracking the weather and only fly or drive to Calafate when they see “la ventana” coming. In the 2009-2010 season, Swiss climber Walter Hungerbuehler did the same from Switzerland, completing an ascent of Fitz Roy in 9 days Bern to Bern.

Weather maps

The NAVGEM or GFS models are available in map form (links to the right), in which the most interesting things to look at are:

- 850 hPa Temperature [C], winds [kts] and Rel. Hum. [%]

- Previous 6/12-hr Precipitation Rate [mm/12hr] and Sea Level Pressure [hPa]

Click “All” immediately right of the desired set of maps you want to see, or load a specific map by clicking in the desired time slot. The date and time is listed in the upper right hand corner of the main menu, or in the lower left hand corner of each map.

For those that are checking the weather from a server that does not allow you to open the US Navy pages go to Wetterzentrale where you will find a low resolution version of both NAVGEM and GFS maps.

The little IFS model data available from the European Centre for Medium-Range Weather Forecasts can be obtained here.

The easiest for last... you can find NOAA’s meteogram here.

From their main menu and after entering the desired coordinates, you can obtain meteograms or windgrams with every bit of info you might phantom necessary.

Meteogram instructions

For a “ready to use” basic meteogram follow these steps:

On the boxes enter the coordinates to your objective. Cerro Torre would be:
-49.3 for latitude (don’t forget the minus in front)
-73.1 for longitude (don’t forget the minus in front)

click “CONTINUE”

From the menu choose option “GFS 0-192h”

click “GO”

in the next page click “Next” without changing anything

in next page:
x Default with Winds
x Speed and Direction

Don’t select anything else from the other menus

Ignore output options

At the bottom of the page enter access code.


In this meteogram you will find:
wind speed and direction
850 mb temperature (approx 1500 meters)
2m temperature and 2m dew point
sea level pressure


NOAA GFS meteograms

GFS weather maps

NAVGEM weather maps

IFS weather maps (the little info available for free from ECMWF)

GFS weather maps (Polar)

NAVGEM maps (Polar)

Wetterzentrale GFS and NAVGEM weather maps

NOAA Seasonal climate forecast (predictions for the next six months)

Meteo Chile


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