Question for trough (at the GFA)

[Edmonton boy]

GFA, purple line is called trough
which is extended low pressure
but to be honestly, I don't know what does that mean for pilot
even my instructor didn't can answer what does affect for flight
trough is not the front, right? front is blue(cold) and red(warm) line and accopanied by rain
any idea in terms of like wind, chance of LLJ??, chance of rain??, chance to be front??
if trough exist in the GFA, what is the most important affecting for pilot?

[pelmet]

To me, it means a much higher risk of bad weather....of some sort.

[Shinyjetsyndrome]

A trough is an extension of a low pressure system and is therefore an area of convergent winds. When winds converge, the chance for nasty weather is higher (forced rising of air). Troughs often have similar conditions to that of the low itself i.e. extensive cloud, precip, low vis, and stronger winds.

[Edmonton boy]

thank you for reply, you said extensive cloud, precip, low vis, and stronger winds.
but above GFA graph, nothing to say that.
even saying SKC P6SM left of through

which one is correct then??
even thought GFA near the through didn't say nothing, do I have to think that the weather would be bad??
confusing.....

[smooth]

https://www.youtube.com/watch?v=Oanyt6QeZ1A
found this on Youtube, well explained

thank you for reply, you said extensive cloud, precip, low vis, and stronger winds.
but above GFA graph, nothing to say that.
even saying SKC P6SM left of through

which one is correct then??
even thought GFA near the through didn't say nothing, do I have to think that the weather would be bad??
confusing.....

[CpnCrunch]

Looks like it may be a lee trough caused by the 25kt winds over the rockies, combined with the low pressure system moving east. So probably the main thing to worry about would be the wind and turbulence. It's the kind of weather I wouldn't want to be flying down low in a small plane anywhere west of Calgary. Further east, it will probably be smooth above a few thousand feet, and you'll get a nice tailwind if you're flying east.

[Shinyjetsyndrome]

thank you for reply, you said extensive cloud, precip, low vis, and stronger winds.
but above GFA graph, nothing to say that.
even saying SKC P6SM left of through

which one is correct then??
even thought GFA near the through didn't say nothing, do I have to think that the weather would be bad??
confusing.....

If you read what I wrote again, you’ll see that I said there’s a higher chance for bad weather. It’s not an absolute and you’d need the right conditions i.e. Instability and humidity. Meteorology is hardly ever black and white. It sits in the grey area quite a bit.

On this occasion, my best guess is that the low sitting over central Alberta is pulling the air from the Rockies over to the prairies which causes the foehn wind (large scale subsidence on the Lee side of the mountains essentially). This is very dry wind which lowers the humidity a fair bit in the prairies. This is why it’s looking pretty nice cloud wise, but there’s a decent amount of wind on the FD’s. Normally the foehn effect/chinook would cause a region of higher pressure, but Alberta is infamous for the summertime localized low pressure caused by intense daytime heating. On days after we get rained on, we get pretty sizeable CB’s in the summer from the heat on the prairies.

[pelmet]

.

[pelmet]

thank you for reply, you said extensive cloud, precip, low vis, and stronger winds.
but above GFA graph, nothing to say that.
even saying SKC P6SM left of through

which one is correct then??
even thought GFA near the through didn't say nothing, do I have to think that the weather would be bad??
confusing.....

Try thinking that it is more likely that the weather COULD be bad.

I remember several times back in skydiving pilot days where there was what the forecasters called an Upper Trough. Weather started off in the early morning as sky clear. But with daytime heating, there was TCU by 10am and some CB’s by noon.

I guess the less stable or rising air led to the bad weather but it started out nice and clear.

Had a great time flinging my eager craft though footless halls of air among the TCU buildups(that High Flight poem reminds me of those flights).

Anyways......a trough was passing by but it had been good weather to start with.

[xyzzy]

thank you for reply, you said extensive cloud, precip, low vis, and stronger winds.
but above GFA graph, nothing to say that.
even saying SKC P6SM left of through

which one is correct then??
even thought GFA near the through didn't say nothing, do I have to think that the weather would be bad??
confusing.....

The trough itself is also in AB so you'd want to take a look at that GFA as well for the annotations related to it, as they will not show on the BC GFA.

[455tt]

GFA, purple line is called trough
which is extended low pressure
but to be honestly, I don't know what does that mean for pilot
even my instructor didn't can answer what does affect for flight
trough is not the front, right? front is blue(cold) and red(warm) line and accopanied by rain
any idea in terms of like wind, chance of LLJ??, chance of rain??, chance to be front??
if trough exist in the GFA, what is the most important affecting for pilot?

You are almost there.

So a Trough as you state is an area of extended low pressure.

A Ridge is an area of extended high pressure.

Low pressure means the air is rising: it expands and cools, and if there is sufficient moisture, it cools to its dewpoint and cloud forms. This means, generally, bad weather.

High pressure on the other hand is descending, it compresses and heats and moisture evaporates. This means, generally, good weather.

So as a pilot you can generally anticipate poor weather at a trough and this is useful to know.

Notice how on the particular Clouds and Weather GFA that you provided that there is no cloud (scalloped edges) and the trough parallels the mountains. This is what we see with mountain wave patterns: rising/descending/rising air in a wave formation. You should also check out the Icing, Turbulence and Freezing Level GFA for the same time to see if moderate/severe mountain wave turbulence has been forecast in the area of the trough. Mountain wave turbulence can be hazardous to flight operations.

[DadoBlade]

Both warm and cold fronts lie in troughs. Note that all the weather systems shown on the GFA provided are upper disturbances. On GFA's a dashed purple line denotes either an upper trough, or an upper ridge. The upper trough shown on the GFA provided is known as a lee trough which usually hangs around the foothills of the Rockies, and characteristically brings southwest winds to the southern Alberta prairies. Also note the TROWAL, (TRough Of Warm Air aLoft) originating from the upper wave extending west through southern Yukon.

[co-joe]

The key to what happens with a trough are the properties of the air mass it is in. If the airmass is moist and unstable expect convective wx, is the airmass is dry and stable likely nothing will happen. Remember the definition of stability has to do with lapse rate.

Think about the relationship between the environmental lapse rate, the dry adiabatic lapse rate, and the saturated adiabatic lapse rate with respect to stability. How quickly will the air in the trough rise, and will it lead to wx.

Another indicator of bad or even dangerous wx is when that trough carries warm air aloft, in the form of an occlusion. A trough of warm air aloft (trowal) is where you can get the risk of freezing rain.