Weather decisions in summer - ISOL TS, SHOWERS

I believe you. But in the context of this thread, I was more talking about wanting to cross by flying above the terrain, and above the rising cumulus.

Between 10:30 and 11:30, we barely managed to fly Salzburg – Mauterndorf – Villach – ILB at FL120 remaining on top, but we could still see the clouds rise by the minute. It was indeed quite a stable day and no CBs developed as far as I could see, but even those big CUs become an issue for those wanting to fly on top.

Monday (return flight) was the same. We took off from Portoroz at 9:40 local time, so by 11, we were past Salzburg, but we still needed 11500 feet and a few turns to stay on top.

Ah, I see…I just read the last couple of posts ;-)
Yep, for IFR operations or VFR “on top” an early crossing is always advisable in summer.

Thomas_R wrote:

while all GAFOR routes in the area were classified as “MIKE” (marginal) for the whole afternoon due to forecast showers and thunderstorms.

@Thomas_R I’m not quite sure whether a misunderstanding of the GAFOR forecast might be present here.

GAFOR does not take into account showers or thunderstorms. The GAFOR code is chosen simply based on visibility and terrain clearance of clouds (cloud base above terrain).

In your picture I can see some mountain tops that reach the clouds. They don’t seem to penetrate the clouds, although I cannot see the higher mountains in the back clear enough. So a prevision of at least “MIKE” for a mountain crossing seems totally appropriate to me.

See wikipedia or AUSTROcontrol for an explanation of GAFOR.

For alps crossing the GAFOR routes are indicated, so the terrain clearance is given for that route (and not, like in Germany, for an area). Because else we’d see mostly XRAY in Austria or Switzerland.

par wrote:

In summer, there is a common pattern of afternoon thunderstorms. Do you try to navigate between them?

After an experience of last weekend I’m quite interested in this topic, too.

I wanted to do a short training flight to shoot an instrument approach at a nearby airfield. Weather looked good at home and my iPhone told me “clouds but no rain”. Off I went to arrive at the airfield where only minutes ago a severe rainshower had set the field totally wet. I was quite surprised and checked actual weather to see three CBs around, developing and moving fast, impossible to tell what picture is there in 20 minutes.

I didn’t feel up to the task that day and wasn’t interested in flying VFR either (which would have been possible below clouds). But later I puzzled about how would I’ve done it. So what do you guys do? I concluded that best thing would have been to get on top ASAP (it was 7 octas to overcast, but you could see the sun shining through, that looked promising enough to be able to climb through rapidly) and then to request avoidance vectors and to not enter clouds again before descent. Is this feasible, say, in Germany? With the latest weather check on ground one would have had say ten minutes where nothing turns really bad.

UdoR wrote:

GAFOR does not take into account showers or thunderstorms. The GAFOR code is chosen simply based on visibility and terrain clearance of clouds (cloud base above terrain).

Well, you don’t think that showers or thunderstorms might affect visibility and cloud base height?

UdoR wrote:

In your picture I can see some mountain tops that reach the clouds. They don’t seem to penetrate the clouds, although I cannot see the higher mountains in the back clear enough. So a prevision of at least “MIKE” for a mountain crossing seems totally appropriate to me.
[…]
For alps crossing the GAFOR routes are indicated, so the terrain clearance is given for that route (and not, like in Germany, for an area). Because else we’d see mostly XRAY in Austria or Switzerland.

The relevant cloud base height is assessed in relation to a reference height which is determined for each GAFOR route individually. This reference height allows you to follow the GAFOR route and cross any mountain passes safely. It has absolutely nothing to do with mountain tops.

The relevant GAFOR route in the picture is right along the valley which I’m following, and it did not have any significantly higher terrain to cross. So no, the “MIKE” classification was absolute nonsense.

UdoR wrote:

GAFOR does not take into account showers or thunderstorms. The GAFOR code is chosen simply based on visibility and terrain clearance of clouds (cloud base above terrain).

Austria and Switzerland do indeed indicate TS or LC or similar information below the codes in order to give an idea of what is causing the codes used. So it is not quite correct that they don’t take them into account.

UdoR wrote:

For alps crossing the GAFOR routes are indicated, so the terrain clearance is given for that route (and not, like in Germany, for an area). Because else we’d see mostly XRAY in Austria or Switzerland.

It is correct that in Switzerland and Austria GAFOR is based on routes rather than areas like in Germany. For each route, a reference altitude is given and the codes are relevant to those reference altitudes. Those altitudes are set to the maximum height on that route needed to fly it, which also means that considerable parts of the route may be flyable below those heights, but the pass is not. This has to be taken into account when assessing the flyability of a route. Under normal circumstances and with average experience, O and D are considered flyable while M and X are not.

The classification refers to clouds and visibility over the reference altitude on that given route. So from the pic alone it can’t be determined if the forecast was accurate or not. However, in convective situations, pinpointing where convection pops up during the time is next to impossible, so one has to work on the base of probability. If for the route and the area there is a high probability that convection will exist and that the cloud level will be relevant vs the refrerence altitude, what is done is to give the classification for that probability. True, this can in that case often be a bit pessimistic, however, particularly in the alps, one single CB or TCU with associated showers can totally close a route if it drops below the ref altitude or shuts down the visibility. If there is a high probability of convective cloud, the associated classification has to be put. In the case here, it was perfectly visible that there was convection. So M was probably the correct code to forecast.

Mooney_Driver wrote:

If there is a high probability of convective cloud, the associated classification has to be put. In the case here, it was perfectly visible that there was convection. So M was probably the correct code to forecast.

With that classification you could close all GAFOR routes in the Alps every time the sun is shining and there is no significant high pressure area. So I think it’s a little more complex than that. Air mass properties, lability, humidity, and wind systems all go into the equation to estimate the extent of convection. If clouds do not develop into TCUs / CBs they can be convective all they want, but it won’t be a problem for aviation. On the contrary, glider pilots rely on that.

I do agree that it’s good to be a little bit on the pessimistic side regarding aviation forecasts. But if a route is perfectly flyable with several thousand feet of ground clearance and very good visibility, it’s just not “marginal”. Period. And by the way, as I wrote above, apart from an initial shower at the eastern end of the Alps, there was not a single shower on the rain radar for the whole area during the whole afternoon.

I guess the TBM owners’ club is doing a fly-in to Crete

Is it correct to assume that if there is no sferics(lightning) activity in a cloud, even if it’s a tall raining cloud it should be ok to go through, and I should expect maybe some light turbulence at worst?

No, because a CB can build up in a few minutes.

The concern is regarding “tall”. You can get a fair amount of rain from clouds which are not that thick; say 5000ft tall, and then you should not get lightning. But anything a lot bigger is risky.

Anything with “TS” or “CB” is a no-go unless you can avoid it all visually and with a good distance horizontally.

That is an occluded front which usually means TS is likely.

No topic is too basic

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