Cold outbreak, first week of September

Paul Mallinson · Unread post by **Paul Mallinson** » Wed 05/09/2007 07:35

This is a test. (Tried posting a couple of times and nothing turned up).

Paul

Paul Mallinson · Unread post by **Paul Mallinson** » Wed 05/09/2007 07:49

Yesterday was a good case of what we call over-running. This occurs where the low level southerly undercuts the west or northwest flow aloft. This results in an area of precipitation occurring behind the front where the low level flow is from the southeasterly quadrant. This effect occurs most notably in Canterbury but also in Otago. Then, when the winds are light in the cold air, moderate to heavy precipitation can cool the temperature profile. That is as snow falls into air with above freezing temps, it chills this air as melting occurs and so the temperature drops to the zero mark and the snow level lowers. The surface temperature follows suit as the melting layer comes down. We saw this yesterday when temperatures, before the moderate precip, were in the 5 to 7C range. With the moderate precip, the temps fell to about 1 to 3C. When the precipitation lightened up, the temperatures rose again.

The very strong echoes on the Canterbury radar yesterday (the red areas etc) were showing the melting going on at low levels.

This lowering melting layer situation was the reason why the snow occurred during the day i.e. before the really cold trough (and sleet/graupel showers) arrrived at night. These situations are tricky to forecast well and it is mostly these situations that deliver the big snowfalls in Canterbury.

Cheers

Paul

gllitz · Unread post by **gllitz** » Wed 05/09/2007 08:06

Paul Mallinson wrote:Yesterday was a good case of what we call over-running. This occurs where the low level southerly undercuts the west or northwest flow aloft. This results in an area of precipitation occurring behind the front where the low level flow is from the southeasterly quadrant. This effect occurs most notably in Canterbury but also in Otago. Then, when the winds are light in the cold air, moderate to heavy precipitation can cool the temperature profile. That is as snow falls into air with above freezing temps, it chills this air as melting occurs and so the temperature drops to the zero mark and the snow level lowers. The surface temperature follows suit as the melting layer comes down. We saw this yesterday when temperatures, before the moderate precip, were in the 5 to 7C range. With the moderate precip, the temps fell to about 1 to 3C. When the precipitation lightened up, the temperatures rose again.

The very strong echoes on the Canterbury radar yesterday (the red areas etc) were showing the melting going on at low levels.

This lowering melting layer situation was the reason why the snow occurred during the day i.e. before the really cold trough (and sleet/graupel showers) arrrived at night. These situations are tricky to forecast well and it is mostly these situations that deliver the big snowfalls in Canterbury.

Cheers

Paul

GREAT stuff, Paul!! Thanks for that!!

Willoughby · Unread post by **Willoughby** » Wed 05/09/2007 10:42

Just recently a sharp drop in temperature with the cold southeasterly change thru here now, 9C with light rain breaking up.

gopolks · Unread post by **gopolks** » Wed 05/09/2007 11:11

Not one flake on the port hills.

southernthrash · Unread post by **southernthrash** » Wed 05/09/2007 11:25

fair bit on dunedin hills this morning. The ski areas got plenty, and at the end of the day that's all that matters.

Gutted at Gary's absence, haha

Michael · Unread post by **Michael** » Wed 05/09/2007 11:54

This is still supposed to be Winter.The days are still shorter than the nights.

TokWW · Unread post by **TokWW** » Wed 05/09/2007 14:21

Ahem - in between we get Spring and Autumn - Spring has started with wind and rain up north here but mild temps and the lawns need mowing second time... willows are budding into leaf, the sweet daphne is nearing the end of its flowering cycle and daffodils are up... what more can I say... ?

Janos · Unread post by **Janos** » Wed 05/09/2007 14:31

southernthrash wrote:fair bit on dunedin hills this morning. The ski areas got plenty, and at the end of the day that's all that matters.

Gutted at Gary's absence, haha

There you go Inny binny might be some truth about the troll that cruises the weather forums waiting to pounce on those who really want snow and not deliver.

David · Unread post by **David** » Wed 05/09/2007 15:36

Wow! What a cold day here... 9.9C with todays maximum being 11.4C, wind turned southeast with light rain most of the day.
My largest day to day max temperature range, yesterday a max of 19.5C, 8C colder today.

Inny Binny · Unread post by **Inny Binny** » Wed 05/09/2007 16:10

There you go Inny binny might be some truth about the troll that cruises the weather forums waiting to pounce on those who really want snow and not deliver.

That may have been the last southerly change to bring snow to low levels for this winter/spring.

I think from now on I will join in with those who are very pessimistic to make the demons go away.

Storm Struck · Unread post by **Storm Struck** » Wed 05/09/2007 18:37

Nice icy shower went through this morning from about 10am-11am around the Harewood area with large sleet then snow briefly

.
Also reports of heavy snow showers in Hornby,Tai Tapu,Hallswell at around the same time.
Mt Herbet and the hills surrounding are completly coverd probly the most ive seen on there this year, and I would imagine places like Akaroa got some snow overnight and into this morning Tich?.
Remember Inny Binny as over the last 4-5 years has proven to us winter can often come and go with blasts like this right up until mid December, and lets hope so because this also increases the thunder chances

.
Cheers
Jason.

TokWW · Unread post by **TokWW** » Wed 05/09/2007 19:32

Photos, photos... c'mon guys - stop talking about it and show us!!

tich · Unread post by **tich** » Wed 05/09/2007 19:46

Mt Herbet and the hills surrounding are completly coverd probly the most ive seen on there this year, and I would imagine places like Akaroa got some snow overnight and into this morning Tich?.

I'm in central Chch and got some sleet overnight. Talking to my parents, they said there was snow down to about 400m around Akaroa this morning.

Must've been fresh snow on North Island high country today, even though AA site didn't say any Desert Road or any other highways were closed.

tich · Unread post by **tich** » Wed 05/09/2007 19:53

This lowering melting layer situation was the reason why the snow occurred during the day i.e. before the really cold trough (and sleet/graupel showers) arrrived at night. These situations are tricky to forecast well and it is mostly these situations that deliver the big snowfalls in Canterbury.

Not one flake on the port hills.

Those must be the situations that deliver snow to low levels on the plains, while often sparing the Port Hills - couldn't see any up there late yesterday once cloud levels rose. Hard to imagine that there wouldn't have been a dusting on them this morning, but by midday when clous had cleared, it must've all melted.

Fujita Phil · Unread post by **Fujita Phil** » Wed 05/09/2007 20:02

Inny Binny wrote:That may have been the last southerly change to bring snow to low levels for this winter/spring.

I think from now on I will join in with those who are very pessimistic to make the demons go away.

Nah there another one on its way next week

David · Unread post by **David** » Wed 05/09/2007 20:58

I see that temperatures are already 0C in parts of Canterbury.
Some parts could maybe go down to as low as -5C there tonight?

Inny Binny · Unread post by **Inny Binny** » Wed 05/09/2007 21:10

Yes, just hitting zero now.

Could be a good frost tonight.

mikestormchaser · Unread post by **mikestormchaser** » Wed 05/09/2007 21:43

Well i have to say i am not concentrating on snow for this year anymore, i think thats the last time ull get a S cold enough to bring snow to sea level, but if it puts me wrong then by all means put me wrong mother nature!

Storm season coming up now so all eyes and thoughts on it and hopefully a good one! big frost tonight, bloody cold out there
cheers

David · Unread post by **David** » Wed 05/09/2007 22:06

tornadoman18 wrote:Storm season coming up now so all eyes and thoughts on it and hopefully a good one

Hopefully we'll get a cyclone in the next cyclone season (Dec-May), haven't had a particularly memorable one for a while

NZstorm · Unread post by **NZstorm** » Thu 06/09/2007 07:53

Hopefully we'll get a cyclone in the next cyclone season (Dec-May), haven't had a particularly memorable one for a while

Last time we had a decent ex-tropical cyclone in this neck of the woods was way back in 1997. Been some near misses since.

Just to stay on topic, looks icy in the SI with CH down to -4C this morn.

Willoughby · Unread post by **Willoughby** » Thu 06/09/2007 11:14

STUNNER of a day here after a frost of 0.8C, currently on 10C and clear

Probably a bigger frost tonight...

Wildland · Unread post by **Wildland** » Thu 06/09/2007 11:22

tich wrote: Must've been fresh snow on North Island high country today, even though AA site didn't say any Desert Road or any other highways were closed.

National Radio reported slight delays on the Rimutaka Hill Road due to light snow in their 3 p.m. news yesterday, but there was no follow-up in the 4 o'clock bulletin, so the event must've been short-lived.

We had some of that "snizzle" in Tawa for about 10 minutes after 2 p.m. yesterday but otherwise it was just a dreary, cold southerly day.

tich · Unread post by **tich** » Thu 06/09/2007 14:20

National Radio reported slight delays on the Rimutaka Hill Road due to light snow in their 3 p.m. news yesterday, but there was no follow-up in the 4 o'clock bulletin, so the event must've been short-lived.

There wasn't any road snowfall for the Rimutaka Rd yesterday, as it was only forecast above 600m in the Wairarapa. However, the summit of the road is only just below that altitude.

I wouldn't be surprised if the frost in Chch this morning was a record or near record. Only up to 10C so far; pretty chilly for a sunny day in September.

Wildland · Unread post by **Wildland** » Thu 06/09/2007 15:28

tich wrote: There wasn't any road snowfall for the Rimutaka Rd yesterday, as it was only forecast above 600m in the Wairarapa. However, the summit of the road is only just below that altitude.

Are you implying - if it wasn't forecast, it couldn't happen? Or do you know that there wasn't any snow on the Rimutaka Hill Road yesterday afternoon?

What I suspect happened is that traffic slowed while people rubber-necked at snow flurries, but I couldn't be sure.
I simply quoted a National Radio news report.

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