Rapidly Deepening Low and Possible Snow Event July 20-22nd

[Talbotm]

Very interesting topic this. Im not sure anybody really knows how they work. We think we do but given the different opinions it shows people looking at it differently. When both professional forecasters ie Weather Watch and Met Service are both picking 300m or thereabouts yet some of us not all are picking 500m or above. Just proves how hard it is to predict these things.

[melja]

M/S seem to be thinking the low will form a little further north and effect Cant/Marlb, im seeing 100+mm for the event here in Rangiora.
I see CWU calling it "one of the most significant storms in a number of years" Again, how many most significant storms can we have.

[Bradley]

M/S seem to be thinking the low will form a little further north and effect Cant/Marlb, im seeing 100+mm for the event here in Rangiora.
I see CWU calling it "one of the most significant storms in a number of years" Again, how many most significant storms can we have.

Yes and with the low forming further north it will allow more of a cold air intrusion hence their forecast of snow to 300m, could well end up being like the June 2015 event which changed from a south Canterbury snow event to mid Canterbury to the low forming further north less then 24 hours away from the event - forecasters nightmare!

[Chris W]

M/S seem to be thinking the low will form a little further north and effect Cant/Marlb, im seeing 100+mm for the event here in Rangiora.
I see CWU calling it "one of the most significant storms in a number of years" Again, how many most significant storms can we have.

It certainly looks pretty significant for many parts of the country, notably Otago and Canterbury. Some west coast-like colours appearing on this side of the mountains in the 3-day forecast.

Much colder air approaching on Tuesday? Goes beyond the range of the BOM viewer and I haven't seen GFS yet.

[NZstorm]

Very interesting topic this. Im not sure anybody really knows how they work. We think we do but given the different opinions it shows people looking at it differently. When both professional forecasters ie Weather Watch and Met Service are both picking 300m or thereabouts yet some of us not all are picking 500m or above. Just proves how hard it is to predict these things.

Climatology is a big part of weather forecasting, what typically happens in this type synoptic scenario.

[NZ Thunderstorm Soc]

I hope it snows in the upper echoes because we don't want the ruddy stuff here.
In the Herald today, the farmers were quite pleased as they cleared ewes from the recent snow dump.

[Bradley]

On the OneNews weather segment tonight they showed the low forming at the top of the South Island instead of the bottom of the South Island hence the Metservice going for snow to 300m instead of 500-600m which would be the mark if GFS and EC are to be believed...

[Razor]

This promises to be a really disruptive system for those travelling home for the end of the school holidays. Watching with interest from the comfort of our home fire place

[Richard]

Having the low forming near the top of the South Island instead of the bottom of the South Island is great news for hopefully getting that soaking rain needed here. If only this time.

[spwill]

The snow event will be inland, for the hill country and inland basins, Otago and Canterbury. Temperatures cooling through Friday into Saturday and there will be the cooling under the heavy precipitation for inland areas.

[TonyT]

Having the low forming near the top of the South Island instead of the bottom of the South Island is great news for hopefully getting that soaking rain needed here. If only this time.

But, be careful what you wish for. 50mm in 6 hours would do more harm than good! The latest ECMWF has backed off the rain a bit for North Canterbury and still wants to keep it a Mid and South Canty event.

[tich]

If the flow's more southeasterly than southerly, Chch should be spared the worst. Still should be wet and cold, but (hopefully) not a damaging storm.

[Richard]

If the flow's more southeasterly than southerly, Chch should be spared the worst. Still should be wet and cold, but (hopefully) not a damaging storm.

If it is more SE and not a S we shouldn't be sheltered by Banks Peninsula here like the last system

[TonyT]

If the flow's more southeasterly than southerly, Chch should be spared the worst. Still should be wet and cold, but (hopefully) not a damaging storm.

If it is more SE and not a S we shouldn't be sheltered by Banks Peninsula here like the last system

Last time you were sheltered from low topped shower clouds in the very cold air. This time around the air won't be nearly as cold and the dynamics will be supporting very much deeper and broader clouds, so sheltering won't be a big issue. Much more important is the question of where the rain band will be and how much uplift the air will get as it hits the coast, the Plains, the foothills, and of course, the main ranges.

[tich]

Wishful thinking, but don't really want a combination of heavy rain and severe gales on Friday night.

[Storm Struck]

I think the biggest potential issue with this system depending where it does cross, is not only the heavy rain or snow for higher elevations but also the winds and sea swells.
A low to deepen in a short amount of time from 990hpa in the Tasman then cross the Alps and deepen to 974hpa off the coast of Canterbury, depending on the positioning this could be simular to the 2000 southerly storm which brought 100kmh+ winds to the city and damage to the Lyttelton Marina.
If anyone has a map image from that day feel free to post

[Razor]

I think the biggest potential issue with this system depending where it does cross, is not only the heavy rain or snow for higher elevations but also the winds and sea swells.
A low to deepen in a short amount of time from 990hpa in the Tasman then cross the Alps and deepen to 974hpa off the coast of Canterbury, depending on the positioning this could be simular to the 2000 southerly storm which brought 100kmh+ winds to the city and damage to the Lyttelton Marina.
If anyone has a map image from that day feel free to post

I worked in Lyttelton in those days. A brutal storm, no doubt and quite unexpected. But having grown up in Wellington it was nowhere near the worst I've seen. Local conditions and poor engineering of the breakwater stick in my memory more

[NZstorm]

I can't see any significantly cold air likely to be present.

Cooling can come from aloft through snow melt. The heavier the precip, the more the latent heat absorption and so more cooling.
The process can allow snow to fall 600m or more below the recognised freezing level in.

[Richard]

I remember the first major snowfall in 1991 i got snowed in on a farm out the back of Waiau, that snow fall was what i called a drizzle snow, all very small flakes but lots of them, 100m was as far as you could see. When we finally got out i remember driving through the Culverden area and noticing the snowline was like someone had used a ruler, one line, same level right around, thick as right to that line too. When you get snowfalls with larger flakes the snowlines are less defined because the larger flakes reach a lower level than the small flakes.

[Richard]

Cooling can come from aloft through snow melt.

Same principle as say a micro burst from a thunderstorm ?

[Bradley]

If the Metservice is still saying 300m for the plains after the update this morning I'm going to tend to believe that, their is obviously something else going on if they keep to that 300m mark...

[NZstorm]

Cooling can come from aloft through snow melt.

Same principle as say a micro burst from a thunderstorm ?

That is evaporative cooling which is yet another method in which snow levels can drop. There needs to be some dry air aloft for that to occur.

[wembley]

The MeTService website has not updated it's forecasts for Central Otago for today

[Andy]

The MeTService website has not updated it's forecasts for Central Otago for today

Interesting to see how much rain/snow comes inland. Certainly won't take much to cause issues as the ground is cold and wet!

[melja]

Story on stuff with MS saying 100+ mm for chch city now

Sent from my Redmi Note 4 using Tapatalk