More info on our warnings

General principles

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 : 

 

Code green : nothing special to report

The forecast weathertype doesn't justify issuing a warning. 

 

Code yellow : be careful

The yellow code can be issued maximum 48 h (and sometimes less depending on the incertainty) before the occurrance of the phenomenon. The chance of meeting the criteria is of more than 65%.

 

Code orange : be ready and follow the instructions

The orange code can be issued maximum 24 h (and sometimes less depending on the incertainty) before the occurrance of the phenomenon. The chance of meeting the criteria is of more than 65%. Be careful and follow the instructions of the responsible authority.

 

Code red : take the necessary measures and strictly follow the instructions

The red code can be issued maximum 12 h (and sometimes less depending on the incertainty) before the occurrance of the phenomenon. The chance of meeting the criteria is of more than 65%. Bring yourself, others and, if possible, your personal belongings to safety and follow the instructions of the responsible authority.

 

Interpretation of the warnings

We use the following definitions, which have an approximate value :

  • "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

The minimum chance of occurrance of the phenomenon must be of 65%. If the chance is less than 65% then a lower colour code is attributed the phenomenon (red to orange, orange to yellow and yellow to green). Practically, the RMI will often give a lower colour code 48-24h before the phenomenon than the one given to the same phenomenon 24-12h beforehand.

 

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.
 

About the warnings for heat and cold : they are not necessarilly linked to the spells of heat and cold waves. The latters refer to a longer period, thou it is alsopossible to give warnings for strong but shorter heat or cold spells. Moreover, the crucial criteria for a heat wave only take the maximum temperatures into account, while we also take the minimum temperatures in the calculation for heat warnings. It seems that high minimum temperatures also influence the mortality rate (mostly with older persons).

The RMI also gives warnings for risks of CO intoxication (without colour code), when the atmospheric conditions (severe temperature inversion by calm weather) disrupt the normal evacuation of flue gases from heating systems.

 

Some phenomenons are sometimes very local and almost impossible to correctly forecast. They won't then be considrered as crucial to determine the colour code of the warning.

 

How do we work ?

The first warning will often be placed for a wider region due to the incertainty factor. Then, as the risk is coming closer in time, it allows us to work on a more local scale if the confidance level indicates that the phenomenon will occur on a smaller area.

From code orange, and for slippery conditions due to black ice or snow, and as for thunderstorms (phenomenons that can rapidly evolve from a certain region to another), hourly and geographically more detailed warnings are elaborated, until the orange warning is over or when, during a long code orange period, the risk is temporarilly weaker.

The RMI can depart from the colour codes associated with the criteria. In such a case, it will be justified in the accompanying text. The timing of the phenomenon can be crucial in this : weekend, peak hours, incertainty of the forecast, extention of the concerned zone of the phenomenon...)

About the thunderstorms : it is often impossible to forecast the number of the lightning impacts and the damage they can cause, or the speed of the wind gusts that are often very local. Thereby, no precise thresholds have been determined for those thunderstorms parameters. The criteria for the wind are then adapted to the gusts that are not linked to thunderstorms, but to the gusts that occur during widespread storm, for instance.

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 forecast.

Please note that some weather conditions can live at the limit of 2 colour codes and that other factors can influence the final choice of code, such as the impact on the community (peak hours or already problematic situations : widespread floodings, road traffic already stuck...). Our weather office will then justify its choice in an accompanying text.

 

 

RMI : rather meteorological warnings than forecast of damage

Dangerous meteorological phenomenons and damage are often linked together, but the RMI stands for the weather forecast and not for the damage caused by them. Their impact can also be influenced by local factorsthat we ignore, taht are not within the scope of our expertise, or that are not in the competence of the federal level (local hydrological management pfreventing floodings, the state of some buildings influencing their resistance to severe weather phenomenons...)

The impact of these phenomenons can also be differently appreciated by certain groups of persons (festivals, outdoor events...), or different activity sectors or companies.

Please find here under a recap table with the thresholds and the non-exhaustive risks associated for the different colours codes and per warning types. Nevertheless, we insist on the importance of the main warning text written by our weather office.

 

Warning
Wind
Wind gusts between
80 and 100 km/h
(70 and 90 km/h with leafed trees)
Wind gusts between 101 and 130 km/h
(91 and 120 km/h with leafed trees)
Widespread wind gusts of more than 130 km/h
(>120 km/h with leafed trees)
Rain
10 to 30 l/m² in 1 hour* or

20 to 40 l/m² in 6 hour or

25 to 50 l/m² in 24 hour
31 to 50 l/m² in 1 hour* or

41 to 60 l/m² in 6 hour or

51 to 100 l/m² in 24 hour
Already flooding problems and heavy rain still forecast, or

> 50 l/m² in 1 hour* or
> 60 l/m² in 6 hour or
> 100 l/m² in 24 hour
Thunderstorm
10 to 30 l/m² in 1 hour* or

20 to 40 l/m² in 6 hour or

25 to 50 l/m² in 24 hour or

local strong wind gusts

or

hailstones of 1 to 2 cm
31 to 50 l/m² in 1 hour* or

41 to 60 l/m² in 6 hour or

51 to 100 l/m² in 24 hour or

widespread strong wind gusts

or

hailstones of 3 to 5 cm
Already flooding problems and heavy rain still forecast , or

> 50 l/m² in 1 hour* or
> 60 l/m² in 6 hour or
> 100 l/m² in 24 hour

or

widespread extreme wind gusts or
hailstones > 5 cm
Slippery conditions
1 to 5 cm fresh snow in 6 hour or

5 to 10 cm fresh snow in 24 hour or

(very) local black ice
or (very) local frost or ice patches
3 to 5 cm fresh snow in 1 hour* or

6 to 10 cm fresh snow in 6 hour or

11 to 25 cm fresh snow in 24 hour or

widespread black ice or ice patches
Everywhere slippery and fresh snow expected or

> 5 cm fresh snow in 1 hour* or

> 10 cm fresh snow in 6 hour or

> 25 cm fresh snow in 24 hour or

intense and almost generalized black ice
Fog
Widespread, visibility
< 500 m or locally visibility < 200 m
Widespread, thick fog, visibility < 200 m
Does not apply
Storm surge
5,60 to 6,20m TAW in Ostend
or
6,60 to 7,20m TAW in Antwerp
6,21 to 6,50 m TAW in Ostend
or
7,21 to 7,50 m TAW in Antwerp
> 6,50 m to in Ostend
or
>7,50 m to in Antwerp
 
* the hourly forecast of certain values are broadcast 1 hour beforehand