Example:
1.) 6°C on the ground, overcast at 6,000 ft, tops at 10,000 ft.. Forecast predicts light ice with a probability between 25% and 50% . Would you climb through it and why?
2.) -11°C on the ground, overcast at 1,000 ft, tops at 2,0000 ft.. Forecast predicts no ice. Go / no go?
3.) -1°C on the ground, overcast at 1,000 ft, tops at 8,0000 ft.. Forecast predicts moderate ice, between 25% and 50% probability. Go / no go?
Assumptions I'm going to make because you didn't provide the information needed:
- Your "forecast" is the icing model chart. (If not, what forecast product are you specifically referring to that gives icing information? Airmets don't give percentages like you have.)
- The system causing all of the above is a widespread stratus system under high pressure with crappy vis and low ceilings, since you don't say. But you need to know what's causing the forecast. A fast moving cold front which is sweeping through with high winds and mixes of icing and precip is a totally different animal than a hundred mile wide area of constant precip and low ceilings. They're not caused by the same things. You need to know what kinds of weather boundaries you're crossing all along the route.
- Without a skew-t, no inferences can be made about inversion layers either. You need that information. All along the route.
- PIREPs are not mentioned. You need that information. All along the route.
But I'll play. In theory anyway...
1) Since my home airport is at 5883, that one is no-go. I'm not a fan of departing nearly zero-zero with no ability to return in a single. Heh. Grin.
If you meant AGL, 6000' would put the bases at 12,000 for me, so I think I can work with that for visibility but working backward that gives a dewpoint of -10, and I doubt you're going to see a stratus layer causing much precip at 12,000 with air that dry. That's 30% RH. But you could probably fly BELOW it if you were definitely flying toward warmer weather. Colder, you'd be risking freezing precip. Inversions aloft, same problem. You need to look along the route of flight and know where precip is and where it's likely to move to and if there's a way out of it that keeps you in above freezing temps. In this assumption laden version. LOL. Stay in warm air.
As you can see though, you forgot an important detail: What altitude is the airport?
2) -11C at the surface and no inversions aloft? That stuff is all frroooooooozen. Go. Doesn't matter if you're talking AGL or MSL. It's frozen. Watch out if it's getting warmer where you're going. And you still need to look for inversions or other mixing along the way. Take a good coat, preflight is going to suck. Plan a while to pre-heat, too. Haha.
3) 1000 MSL is about 5000 below the surface here so I guess we'll ignore that possibility. Grin.
-1C at surface and clouds at 1000 AGL, says that's a temp/dewpoint spread of 1 degree (-2) and 93% RH. No inversion aloft, it should all be frozen but that's flirting with other problems. How widespread is that tight temp dewpoint spread and are we flying toward warmer or colder temps? 1000 AGL gives us the ability to return to the departure airport at least, but that could close up real quick with a tight spread like that. This one is likely a no-go for me but not for icing reasons necessarily.
I think you need to look at the relationship between cloud height and RH a bit more and also feed more data into these scenarios. Winter weather flying is a game that requires looking over a massive amount of data but also seeing the big picture of what's creating the weather system.
Any one of those scenarios could be blasted straight to no-go by winds or PIREPs saying things aren't as forecast or even the "wrong" direction temperature swing as you cross weather systems enroute. They're not nearly enough information to make a decision on.
I've given some examples of what types of thoughts you might look at without the proper additional info, though. Dewpoint spreads, temp at cloud base, temps aloft and whether there's an inversion, what kind of weather system are you coming from and going to, what are your "outs" and which direction are they, what's the weather forecast at the "outs" and is it getting better or worse, how widespread are the weather patterns and how stable, do the PIREPs match the forecast...
