By Gilles Simard
With the implementation of the aerodrome forecast in TAF code in June 1996, some
questions have been asked on how to decode or interpret the Probability (PROB)
group.
The following article will give you a rapid understanding. Gilles Simard, a
meteorologist at the Eastern Canada Aviation Weather Centre (ECAWC) for the
Meteorological of Canada (MSC), is currently the operational lead for the
forecast centre and has extensive experience in Aviation Weather Training for
DND and the MSC. Gilles can be reached By E-mail:
Gilles.Simard@ec.gc.ca
Probability Groups
An aerodrome forecast (TAF) is an aviation-specific prediction that describes
the most probable weather conditions at a given site at the most probable time
of their occurrence. What happens when the forecaster’s analysis indicates that
a given condition may occur, but may not?
In this case a probability group, represented by PROB30 or PROB40, is used to
describe the possible occurrence of a weather event. It does not mean that a
given weather event will occur 30% or 40% of the time.
In accordance with international standards, a probability of occurrence of less
than 30% is not considered sufficiently meaningful to appear in an aerodrome
forecast (TAF).
On the other hand, if the probability of occurrence is expected to be 50% or
more, then the forecaster uses a BECMG, TEMPO or FM group.
As a result, only the values of 30% and 40% are allowed in conjunction with a
probability group. If the weather condition described in the probability group
actually occurs, then an amended forecast is issued to reflect the change.
For example, the forecaster expects the development of thunderstorms in the
general Montréal area between 1900 UTC and 2400 UTC and wishes to convey this
information to the users, but he or she is uncertain thunderstorms will occur
specifically at the Dorval International Airport:
…P6SM SCT040 PROB40 1924 3SM TSRA BR BKN008 OVC040CB…
Once thunderstorms are observed on radar, satellite pictures and/or METAR/SPECI
and lightning is reported by the Canadian Lightning Detection Network (CLDN)
moving toward Dorval, the forecast is revised to indicate the expected arrival
of the thunderstorms:
…P6SM BKN040 TEMPO 2023 21/2SM TSRA BR OVC020CB…
The use of PROB is restricted to phenomena that could effect aircraft
operations, comprising:
• Thunderstorms
• Freezing precipitation
• Ice pellets;
• Snow grains;
• Low level wind shear below 1500 feet AGL; and
• Ceiling and visibility values such as alternate and lowest approach thresholds
as provided by NAV CANADA to the Meteorological Service of Canada.
In summary, the use of PROB allows forecasters to alert users to the development
of weather that could effect aircraft operations, when the event likelihood at a
specific aerodrome is less than 50 per cent.
The probability group can also be used when the forecaster’s confidence is low.
This is often the case with thunderstorm activity, where predicting individual
cells is next to impossible.
On the other hand, once the weather condition described by the probability group
develops sufficiently to be tracked by the forecaster, the affected aerodrome
forecast is revised to indicate higher levels of confidence and/or likelihood of
occurrence at the site.