Flying in the snow/rain boundary

[AlleyCat67]

I'm looking at a forecast a few days from now and wondering about the weather implications. It's going to be about 40 degrees at the surface, with rain forecast, and ceilings at around 4000'. Sounding forecasts don't suggest any kind of inversion - just a normal lapse rate to a freezing level around 3000'. So my assumption is that the precip would form as snow and then melt to rain by the time it hits the ground.

Are there any particular hazards associated with flying in a mixed rain/snow environment (assuming VFR below the ceiling)? For example, if the wings are slightly colder than the ambient air temperature could I expect refreezing? My inclination right now (if I even go) would be to fly low, keeping to the pure rain / >32F region closer to the groun.

 

[GeorgeC]

Depends where you're going, but remember to leave some margin between forecasts and reality. I was flying north of Allentown a few years ago, OAT near freezing, maybe 3000' ceilings. Despite the modest terrain, I felt like I didn't have many outs to lower ground, and the pucker factor was strong with just a mist of (unforecast) liquid precip.

 

[mcmanigle]

The rain/snow boundary is about the worst place to think about flying. You have the possibility of super-cooled or peri-freezing water, which will freeze on the control surfaces quickly, you will have fully saturated cool air going into the carb (admittedly holding less total water than warmer saturated air, but still), and assuming you're flying any distance, you will have small changes in air temperature by location and altitude that will be hard to predict exactly.

One possible scenario: if the rain/clouds are being caused by a frontal system, then possibly the precip is actually falling as rain from a slightly warmer cloud, then getting cooled in the 4k-3k range, and then is supercooled falling in the 3k-1k range where it is just wishing it could freeze on a nice piece of metal.

If it works with your trip, you would probably be safe with one of the following if you are very sure the weather will support it, or have an "out" if it changes:
- Climb to solidly "snow" layer outside of the precip (if you're sure there is one), fly through the "snow" layer, and then descend to destination outside of precip. (Here, I would be looking for at least -5C in the precip.) Snow can't freeze more.
- Stay well below the freezing level, e.g. +5C outside air temperature, and frequently check that nothing is building up on your leading surfaces to convince yourself that whatever you are flying through is not in fact super-cooled.
- Get above the weather entirely, climbing clear of clouds without penetrating visible moisture.

I would definitely not plan on lingering for any distance in the -5C to +5C range with visible moisture present, especially when you have an actual destination in mind, because that will lead to changes along the route. And hanging out just below the freezing level puts a ceiling as well as a floor on your flight, which is a lot of boundaries to put on your "outs."

I'm not going to pretend that there's no safe way to do what you want to do, but if you're asking how to do it on the internet, I wouldn't try it, at least without being very happy to land somewhere else for the night. Hell, I'm answering the question on the internet and I wouldn't do it.

 

[AlleyCat67]

Depends where you're going, but remember to leave some margin between forecasts and reality. I was flying north of Allentown a few years ago, OAT near freezing, maybe 3000' ceilings. Despite the modest terrain, I felt like I didn't have many outs to lower ground, and the pucker factor was strong with just a mist of (unforecast) liquid precip.

Yea, great point George. I know whenever I do a cross country below 2000' I get very sensitive to any change in engine noise! This is probably a case where the 3 1/2 hour car trip makes more sense.

 

[AlleyCat67]

The rain/snow boundary is about the worst place to think about flying. You have the possibility of super-cooled or peri-freezing water, which will freeze on the control surfaces quickly, you will have fully saturated cool air going into the carb (admittedly holding less total water than warmer saturated air, but still), and assuming you're flying any distance, you will have small changes in air temperature by location and altitude that will be hard to predict exactly.

One possible scenario: if the rain/clouds are being caused by a frontal system, then possibly the precip is actually falling as rain from a slightly warmer cloud, then getting cooled in the 4k-3k range, and then is supercooled falling in the 3k-1k range where it is just wishing it could freeze on a nice piece of metal.

If it works with your trip, you would probably be safe with one of the following if you are very sure the weather will support it, or have an "out" if it changes:
- Climb to solidly "snow" layer outside of the precip (if you're sure there is one), fly through the "snow" layer, and then descend to destination outside of precip. (Here, I would be looking for at least -5C in the precip.) Snow can't freeze more.
- Stay well below the freezing level, e.g. +5C outside air temperature, and frequently check that nothing is building up on your leading surfaces to convince yourself that whatever you are flying through is not in fact super-cooled.
- Get above the weather entirely, climbing clear of clouds without penetrating visible moisture.

I would definitely not plan on lingering for any distance in the -5C to +5C range with visible moisture present, especially when you have an actual destination in mind, because that will lead to changes along the route. And hanging out just below the freezing level puts a ceiling as well as a floor on your flight, which is a lot of boundaries to put on your "outs."

I'm not going to pretend that there's no safe way to do what you want to do, but if you're asking how to do it on the internet, I wouldn't try it, at least without being very happy to land somewhere else for the night. Hell, I'm answering the question on the internet and I wouldn't do it.

Great, detailed answer. It's possible that the precip will be widely scattered, in which case I'd feel confident about being able to dodge it, but yes if it's widespread probably better to give it a pass. And yet another case where the IFR rating isn't as helpful as one might think in the northeast...

 

[Jeff767]

The fact you have to post the question on here means you probably already know the answer.

 

[AlleyCat67]

The fact you have to post the question on here means you probably already know the answer.

I post here to learn something, not just be told not to go. Yeah, I already know that this is a no-go, but I realized that the meteorology at that interface isn't something I know much about.

 

[Sinistar]

Sound like a carb ice session for our 182.

 

[PaulS]

Sounds like an SLD situation. You have to remember MVAs when flying IFR. Depending on the tops, forecast and actual situation I might consider it with FIKI, but would need a definite fast out.

For me it's too far out now to consider scrubbing, I would wait until the night before to form an opinion and probably make the go/ no-go the day of the flight.

Once again, Scott D has a great site to help with these decisions, on top of that he'll do a brief with you, for a fee and help with the decision. I should get a commission.

 

[Rgbeard]

I’ve faced similar situations and in one instance chose to depart and climb quickly to 5,000, as it was clear above 4,000.

got there in three minutes and a trace of ice on the leading edges.

the rest of the flight was unremarkable

 

[Rushie]

I post here to learn something, not just be told not to go. Yeah, I already know that this is a no-go, but I realized that the meteorology at that interface isn't something I know much about.

Nothing wrong with making a no-go decision and still analyzing the weather as if you were going to go. Helps define or confirm your parameters.

 

[AlleyCat67]

Sounds like an SLD situation. You have to remember MVAs when flying IFR. Depending on the tops, forecast and actual situation I might consider it with FIKI, but would need a definite fast out.

For me it's too far out now to consider scrubbing, I would wait until the night before to form an opinion and probably make the go/ no-go the day of the flight.

Once again, Scott D has a great site to help with these decisions, on top of that he'll do a brief with you, for a fee and help with the decision. I should get a commission.

I had been thinking that a temperature inversion was required for SLD, but a bit of reading on the NASA website shows this isn't the case. Good to know. As far as carb ice, that's one good thing about the Grummans - you have to work really hard to get carb ice.

 

[AlleyCat67]

Nothing wrong with making a no-go decision and still analyzing the weather as if you were going to go. Helps define or confirm your parameters.

Right... the good news is that I can wait a day and substitute 20G30kt winds for airframe icing! Gotta love winter.

 

[David Megginson]

I had been thinking that a temperature inversion was required for SLD, but a bit of reading on the NASA website shows this isn't the case. Good to know. As far as carb ice, that's one good thing about the Grummans - you have to work really hard to get carb ice.

For SLD, what you need is some kind of lift — frontal, convective, terrain, etc — to keep the droplets bouncing around up in the colder air. Most common is ahead of a warm front. There is also the special case of very cold air blowing over warm, open water (Lake effect, freezing fog, etc)

I can tolerate a bit of rime icing, but I won't fly ahead of a winter/late fall/early spring warm front, period, in the Great Lakes area.