Gramet is of quite limited value for VFR, especially because low level cloud is not predicted reliably. Also, I would not give anything on a forecast so far in the future. For the general trend yes (which indeed looks bad for next week), but not for a precip prediction for a specific place, date and time.
Not disagreeing Bosco, I was taking it only to get (confirmation of) a very general impression. Still I’d be glad to understand what I get shown.
As for the relativity of forecasts: today we had thunderstorms and low cloud announced, and lots of rain, yet I had some nice flying though I did have to remain below 1500. Have seen far worse days, though. The worst I got was some drizzle on the windshield.
What escapes me is the colour of the “sky” at the end of the flight: where the rest of the picture shows the sky blue, deeper with increasing altitude, in the lower right corner it is very light blue, beginning with an abrupt vertical line, then more or less following terrain.
The model shows a certain likelihood of clouds in the boundary layer. The weather model GFS uses the “boundary layer” for everything close to the surface. That part of GFS is not very accurate, hence it’s limited usefulness for VFR flying. For the general weather situation, you can use the GRAMET, MSLP, TEMSI etc. and then a day before I would get an overview of TAFs. Obviously the best way to do that is to get an autorouter briefing pack which has it all graphically arranged 
So what is indicated is a (likely) presence of low cloud, to add to all the other trouble. Thanks for explaining, Achim, and thanks for all the effort that went into the autorouter website.
On a GRAMET, what does the column of pale cloud, often reaching ground level, mean in practice? (Over Tours, in the example below). The otherwise clear GRANET guide does not give much help in interpreting cloud types.
That’s the so called “boundary layer”. GFS is organized in a fixed set of cloud levels and the boundary layer is the lowest. This is where GFS is rather weak. When there is a probability of low cloud (stratus or fog), we shade the area. It means that you should look at the TAFs of nearby airports to get a more exact prediction. It’s usually only a concern at departure and destination aerodrome.
Ah, interesting. Actual and forecast weather at Tours today shows no cloud below 3500 and broken above, and therefore entirely useable VFR (it’s my regular fuel/pee stop). So as you say, that would seem to be the bit of the model which is least helpful
METAR LFOT 261130Z AUTO 33007KT 280V030 9999 FEW040 SCT048 BKN058 17/07 Q1023
TAF LFOT 261100Z 2612/2712 35008KT 9999 BKN035
Most of the time, a lightly gray boundary layer just means the visibility is less than from pole to pole.
In your case, the model probably overestimated the aerosol content…
I have more than once wondered how or where I can find a definition of this boundary layer or " Grenzschicht".
How high is it or how is the upper end defined?
Wetter3.de also uses it in their Grenzschichtbewölking prognosis.
