Front vs Trough

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Tornado Tim
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Front vs Trough

Unread post by Tornado Tim »

Hmm I checked BOM and M/S with the MSLP maps.
BOM plots a trough over the tasman yet M/S plots it as a cold front?
There is quite a diffrence as BOM state that is associated with the Low yet with M/S, the cold front is associated with the High (well it looks that way)..... Hmmmm?
Why the difference?
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Re: Front vs Trough

Unread post by Richard »

i cant see a lot of difference myself
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Re: Front vs Trough

Unread post by Manukau heads obs »

i guess the problem is that the front is not clear cut when you look at the satellite map
more a bunching up of shower lines in the tasman
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Re: Front vs Trough

Unread post by Michael »

It (front) separates across the main divide and lags on the eastern side.
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Re: Front vs Trough

Unread post by Myself »

When it comes to analyses, two people will always have two different results. That's just the nature of it, because the atmosphere works in its own way, and meteorologists have limited "notation" with which to denote "features". (Though actually the range of notation is more varied than what appears on charts available to the public). You also have features at different levels. Have a look at UK Met Office charts, and you will see them drawing upper level cold fronts and warm fronts (they looked like normal fronts but not coloured in). Sometimes these are not necessary, because if things fit into the "standard model" then you know where they are in relation to surface features. But, in unusual situations or if there is no surface feature it may be worth drawing.


The difference between the analyses that Tim attached can partly be explained by assuming that each analyst was using a different model. When it comes to analysing, you always start with model isobars, but adjust them for actual observations. The fact that one analysis has a low and the other does not....kind of shouts out loud that they are not starting with the same model.
It may be that no satellite imagery or surface observations were able to confirm/deny whether a surface low existed off the Canterbury coast (hence both analyses are OK), or someone got lazy and didn't check. In which case one is right and the other is wrong.

The BoM chart looks by far the "lazier". They have simplified the situation over NZ considerably. No adjustment of isobars for terrain (notice the MS chart has orographic troughing offshore Gisborne/Hawkes Bay whereas the BoM does not). No splitting of the feature across NZ. Most fronts will split in some way, and generally lag behind up the east coast as Michael points out above.
I think the BoM have just drawn to a GFS model field, and left it at that.
It appears they did not check observations over New Zealand, because if you follow their chart then the southerly change had gone through Kaikoura at 6PM yesterday. In reality, it had not. The MS chart has the change further back, correctly signifying that no change had surged up the east coast by that point.
If you followed the BoM chart, you would expect the southerly at Wellington within a few hours, but of course that did not happen (it came through just before dawn today).

As for trough vs front....given the significant drop in freezing levels, and temperatures behind this feature, it's kind of clear that it was a cold front. Very obvious thermal advection with it....so I can't really see a good reason why they called it a trough and not a cold front.

You would expect the MS to do a better analysis over NZ than the BoM! And in this case they definitely have. The BoM analysis is pretty lazy, it seems to have not looked at any NZ observations, or thought about the terrain. And fair enough, it is not their concern. But in that case, maybe they should adjust their chart domain and just not bother drawing over NZ area if they are going to be so sloppy about it.
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Re: Front vs Trough

Unread post by Tornado Tim »

Thanks for that Myself :)
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Re: Front vs Trough

Unread post by Manukau heads obs »

I agree with myself , he he
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Re: Front vs Trough

Unread post by 03Stormchaser »

As a side note why dont metservice print times on there mslp maps??
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Re: Front vs Trough

Unread post by NZstorm »

As a side note why dont metservice print times on there mslp maps??
Maybe they are not allowed to. The MSL prognosis charts come from other sources such as the Ukmet model. Not sure about the MSL analysis chart though.
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