Peter wrote:
I don’t know the full details (I know some people on here do so maybe someone will post them) but AOC ops can disregard a PROB30 unless it is a PROB30 of a persistent condition such as fog.
Not only AOC ops, but it applies (will apply) to you and me as well (part-NCO), and to Specialised Operations. Oddly enough, there is no mention of this in part-NCC! Also — which is a bit interesting — these rules are AMC for SPO and NCO but GM for CAT!
This is the text for NCO, there is the same text for SPO but a table for CAT. (The table for CAT also mentions wind conditions.)
AMC1 NCO.OP.160:
(e) In a period indicated by TEMPO (alone), TEMPO FM, TEMPO TL, TEMPO FM TL, PROB30/40 (alone):
(1) deteriorations associated with persistent conditions in connection with e.g. haze, mist, fog, dust/sandstorm, continuous precipitation should be applied;
(2) deteriorations associated with transient/showery conditions in connection with short- lived weather phenomena, e.g. thunderstorms, showers may be ignored; and
(3) improvements should in all cases be disregarded.
(f) In a period indicated by PROB30/40 TEMPO:
(1) deteriorations may be disregarded; and
(2) improvements should be disregarde
In my dispatcher days we had the following rule:
Prob alone was taken as valid
Tempo alone was taken as valid
Prob/Tempo was disregarded.
my feel of this is that Prob30 is the weakest of all those forecasts and mostly put in there in order to CYA by the forecaster. It’s something he thinks MAY happen if lots of things go wrong, so he wants to be sure he mentioned it. Prob40 would be something he’s quite sure it may happen but not sure enough to make it non-conditional.
The combination of Prob-Tempo of course only makes sense for phenomena which can actually fulfil the TEMPO conditions at least halfways. SHRA, TSRA, clouds right at the forecast limit, e.t.c. Prob Tempo of FG or other stratiform stuff is seen regularly (again most of the time as a CYA forecast) but does only make very limited sense.
While in AOC ops, the TAF is often regarded as the definite decision tool and used to justify certain actions by the crew, we as GA do not really fall under this. We have to justify our decisions based on TAF’s and other forecasts only to ourselfs, there won’t be a flight ops department threatening you with dismissal if you divert based on a Prob Tempo or the likes.
Personally, I take the TAF as one of my decision tools, but hardly ever the only one. If I see a prob forecast for TSRA or TSGR, the predominant question is where they are coming from. Are we talking heat convection or are we talking a front. If there are frontal TS, I’ll take them very seriously indeed, as they WILL turn up and the prob only indicates that the forecaster is not sure WHERE. If they are normal summer convection stuff, I’d like to have more information on the local probabilities before I’d take those as a real threat. Mostly, if they are put in a forecast with a low prob, then they can pop up everywhere and nowhere and therefore are put in there just because there is a possibility but nobody can say with any degree of precision what will happen.
I’ve seen the convective situation in the last few days here at ZRH. Friday (the 13th….) was particularly nasty to forecast and we had one supercell crossing the country only very close south of the airport but not over it. The day after, one of those missed us to the north. Both carried hail and massive damage.
I now do observation here for over 14 years but I’ll be dammed if I ever saw a CB like that from below!! It rised quite a few eyebrows in the local met community…..
Another view of one of those CB’s, not even fully developped yet. Still, the whole cell created hail of quite respectable size…
