Citing EU-OPS:
CAT.OP.MPA.185 Planning minima for IFR flights — aeroplanes
(a) Planning minima for a take-off alternate aerodrome
The operator shall only select an aerodrome as a take-off alternate aerodrome when the appropriate weather reports and/or forecasts indicate that, during a period commencing one hour before and ending one hour after the estimated time of arrival at the aerodrome, the weather conditions will be at or above the applicable landing minima specified in accordance with CAT.OP.MPA.110. The ceiling shall be taken into account when the only approach operations available are non-precision approaches (NPA) and/or circling operations. Any limitation related to OEI operations shall be taken into account.
(b) Planning minima for a destination aerodrome other than an isolated destination aerodrome
The operator shall only select the destination aerodrome when:
(1) the appropriate weather reports and/or forecasts indicate that, during a period commencing one hour before and ending one hour after the estimated time of arrival at the aerodrome, the weather conditions will be at or above the applicable planning minima as follows:
(i) RVR/visibility (VIS) specified in accordance with CAT.OP.MPA.110; and
(ii) for an NPA or a circling operation, the ceiling at or above MDH;
or
(2) two destination alternate aerodromes are selected.
(c) Planning minima for a destination alternate aerodrome, isolated aerodrome, fuel en-route alternate (fuel ERA) aerodrome, en-route alternate (ERA) aerodrome
The operator shall only select an aerodrome for one of these purposes when the appropriate weather reports and/or forecasts indicate that, during a period commencing one hour before and ending one hour after the estimated time of arrival at the aerodrome, the weather conditions will be at or above the planning minima in Table 1.
[...]
On the other hand:
CAT.OP.MPA.245 Meteorological conditions — all aircraft
(a) On IFR flights the commander shall only:
(1) commence take-off; or
(2) continue beyond the point from which a revised ATS flight plan applies in the event of inflight replanning,
when information is available indicating that the expected weather conditions, at the time of arrival, at the destination and/or required alternate aerodrome(s) are at or above the planning minima.
(b) On IFR flights, the commander shall only continue towards the planned destination aerodrome when the latest information available indicates that, at the expected time of arrival, the weather conditions at the destination, or at least one destination alternate aerodrome, are at or above the applicable aerodrome operating minima.
(c) On VFR flights, the commander shall only commence take-off when the appropriate weather reports and/or forecasts indicate that the meteorological conditions along the part of the route to be flown under VFR will, at the appropriate time, be at or above the VFR limits
CAT.OP.MPA.246 Meteorological conditions — aeroplanes
In addition to CAT.OP.MPA.245, on IFR flights with aeroplanes, the commander shall only continue beyond:
(a) the decision point when using the reduced contingency fuel (RCF) procedure;
or
(b) the pre-determined point when using the pre-determined point (PDP) procedure,
when information is available indicating that the expected weather conditions, at the time of arrival, at the destination and/or required alternate aerodrome(s) are at or above the applicable aerodrome operating minima.
So, should a commander of an operator's flight opt to commence a flight when the forecast at the ETA is above the minima, or the forecast needs to be above the minima for the period from one hour before the ETA till one hour after the ETA?
