Weather

When the criteria are met, the RMI broadcasts general warnings several times a day in case of dangerous phenomenons linked to precipitations, wind, thunderstorms, tides or slippery conditions. Those warnings are given by Belgian provinces and for a determined validity period. They answer to the following principles, defined on European level and using colour codes representing the intensity of the phenomena : 

The forecast weathertype doesn't justify issuing a warning. 

The yellow code can be issued maximum 48 h (and sometimes less depending on the incertainty) before the occurrance of the phenomenon.

The orange code can be issued maximum 24 h (and sometimes less depending on the incertainty) before the occurrance of the phenomenon.

The red code can be issued maximum 12 h (and sometimes less depending on the uncertainty) before the occurrance of the phenomenon.

We use the following definitions to describe the region and the phenomena:

• "Very local" : a region representing less than the quarter of the province
• "Local" : a region representing between a quater to the half of the province
• "Widespread" : a region representing at least the half of the province

We use the following definitions for the "slippery conditions" :

• "Black ice/freezing rain" : freezing rain or freezing wet snow
• "Fog deposits" : Hoar frost or moisture that will freeze on the road surface without that the road became wet due to rain or wet snow (which has previously fallen onto a not yet frozen road surface).
• "Frozen surfaces" : wet road sections or snow rests that later freeze due to cooling. First there may be a shower of rain or wet snow on a road that has not yet been frozen, which then freezes due to cooling.

Since weather forecasts are always probability forecasts and some weather phenomena can only be predicted over a larger area, this is taken into account when a colour code is issued.

A provincial colour code means the following :

• The thresholds of that colour are exceeded with at least 65% probability over a territory that is at least a quarter of the area of the province.
  or
• The thresholds of the next higher colour code (indicating a more intense character of the weather event) are exceeded with at least 65% probability in a territory that is less than a quarter of the province's area.
  or
• The thresholds of the next higher colour code are exceeded with less than 65% probability - but at least 15% - in a territory covering at least a quarter of the province's area.

With regard to heat and cold warnings, it should be noted that in addition to the cold wave and heat wave, which cover a longer period of at least 5 days, warnings can also be issued for an intense but short-lived cold or heat period, where the criteria are tested on a day-by-day basis.

The RMI also issues a warning for CO intoxication (without colour code) when weather conditions (a large temperature inversion in calm weather) impede normal removal of combustion gases from heating plants. This obviously depends, among other things, on the combustion appliance itself and the room in which it is located.

Often, an initial warning will apply to a wider area because of uncertainty. As the risk approaches, information on the local and regional level will become more precise if there is greater certainty that the event will occur in a limited area.

The RMI may, especially in fringe cases, deviate from the colour code associated with the criteria.The timing of the phenomenon may be decisive for this purpose (weekend, heavy traffic, uncertainty in the forecast, the size of the area where risks are expected,...). The weather room will then justify the choice of colour code in the accompanying text.

About the snow : we deliver information about the forecast accumulated snow that would contain enough water. "Dry" snow, containing more air, can give rise to (local) more important snow deposit, but it is often very difficult to predict.

The speed of heavy gusts, the size of hailstones, the amount of precipitation during a heavy shower, the number of lightning strikes, visibility, sometimes vary over distances of hundreds of metres or several km and are usually not predictable by location.

Although there is often a link between severe weather and damage or nuisance, we note that the RMI forecasts the weather, not the nature and extent of the nuisance or damage, which is sometimes also the result of local factors we do not know about or which do not fall within our domain of knowledge or federal competence (e.g. the local water management in case of heavy rainfall, which determines whether or not flooding occurs, the condition of buildings with regard to their behaviour in case of strong winds or lightning strikes, whether or not certain roads, bicycle paths or footpaths have been gritted or not, etc.).

The impact can often also be perceived differently per group or person (festivals, open-air events,...), per sector or per company.

We explain below the threshold values (in a clear table) and the non-exhaustive risks for the various colour codes, for each type of warning, but we emphasise that the main text, prepared by the weather office, is always the most important.