Chapter 8a: Basic Weather Forecasting

A weather forecast is an estimate of the future weather conditions at a specific location. Two things are needed to make a weather forecast:

Accurate data about the current state of the atmosphere
A solid understanding of the meteorological and atmospheric processes at play

Factors which affect the accuracy of a weather forecast include:

Quantity of current observations
Accuracy of current observations
Understanding of atmospheric processes
Familiarity with local topography
Computer model resolution and computing power
The chaotic nature of the atmosphere

Short-range forecasts (0-72 hours) are generally accurate. In medium-range forecasts (3-7 days), trends and patterns are handled well, but the details (precipitation) may be off the mark. Beyond 7 days weather forecasts become highly probabilistic and incorporate climate data.

Forecasting from observations

Forecasting from observations are primarily in the short-term (0-12hrs). In addition to local weather proverbs, the following general rules of thumb may forecast weather changes:

Steadily falling barometer potentially indicates an approaching storm.
Steadily rising barometer potentially indicates clearing.
There is little chance of precipitation continuing when the barometer is 1020mb or higher.
When temperatures during a storm drop below -15^oC, snowfall may diminish.
On clear and calm nights, valley temperatures can be colder than at ridgetops.
Valley fog clearing before noon can indicate fair weather.
Cirrus cloud can precede a storm by 24 hours or more
Thickening and lowering clouds (usually from the West) can indicate an approaching storm
Thickening lenticulars indicate increasing moisture aloft, and a possible approaching storm.
Snow plums on ridges indicate strong winds at higher elevations.
Thinning and lifting clouds indicate clearing weather.
The passage of a cold front is indicated by the lowest point in a baro-graph curve, a wind shift, and sudden appearance of heavily rimed snow crystals or graupel.

Avalanche Weather Forecasting

The mountain snowpack evolves over time under the influence of weather. The accuracy of an avalanche forecast is largely related to the accuracy of its weather forecast. The most important weather parameters affecting snow stability include:

Precipitation (type and rate, using SWE)
Temperature
Wind speed and direction (particularly at the ridge top)
Cloud cover and solar radiation
Relative humidity

Be familiar with how local topography affects weather. The orientation, elevation, and proximity to water sources are key factors. It is possible to see significant variations within the same mountain drainage. Familiarisation also helps identify patterns leading to significant weather events. Detailed and consistent weather observations help correct the nearest weather forecast to your area.

Professional Weather Forecast Products Specific to Mountain Meteorology

Avalanche Canada Mountain Weather Forecast (ACMWF) – Synoptic Based.

Produced daily throughout winter, focusing on avalanche conditions.
Includes graphical synoptic charts, a synopsis, an analysis, and a forecast up to 4 days.
Written for the mountain top.

Avalanche Canada Public Avalanche Forecast – Local Based.

Text-based weather forecast that supplements the ACMWF by tailoring the forecast to a specific region.
Includes Precipitation, temperature, and wind values up to four days. Wind speeds are typically at ridgetop, and temperature and precipitation typically at treeline.
AvCan forecast’s main disadvantage is they do not pick out nuisances or discontinuities.

Private Meteorological Consulting Services.

Environment Canada provide one-on-one at $2.99/min 5am-9PM on 1-900-565-5555.
WeatherBell Analytics gives access to graphical weather models at $200/year.
SimuAWPS – Modelled off the AWIPS, designed for meteorologists at $8.99/month.
Wunderground: Basic forecasting, user friendly.
SpotWx: Search-based engine for weather stations via Google Maps.