Prolonged wet spell for West Coast

[tich]

I was going to head over there for a few days, more especially to South Westland (I really want to see Jackson's Bay), but the long-range forecast for that area seems to be summed up in one word - rain. All the models seemd to support this forecast -except for a recent GASP prognosis. GASP were (I think they've changed it now) suggesting a high to move over the South Island on Tuesday.

I'm heading to Stewart Island instead. I know the forecast is for somewhat unsettled weather there as well, but not persistent rain. Anyway I think that the main settlement area on the island is somewhat sheltered by the Mt Anglem complex in westerly and northwesterly flows. But I can't imagine it being nearly as warm as Christchurch was today.

Ben
Christchurch

[Michael]

You should go to Jacksons Bay in a Southerly -It would be a lot better then but the crap coming around the roaring 40's doesn't look promising to do that for a while.
I like to plan my trips to the weather when I can&when I can afford -ie go to the east coast in a NWer,Go to Nelson or Tauranga in a SWer,go to Alexandra in a large High in Summer,or the southwest coast of the North Is in a NE.-Bet you get my drift

[Thunder]

In Alexandra / central Otago last summer there were quite a few thunderstorms from the heat lows that were forming within those big highs over the South Island, so if you go there during a big high in summer you could possibly get a Ts or two. That would be ideal Micheal <G>

Cheers
Aaron Wilkinson

[tich]

Well, the weather turned out not so bad for Stewart Island. It rained pretty solidly in Invercargill when I got there on Sunday night, and there were still some showers about on the island on Monday, but Tuesday and Wednesday were nice and sunny, though the cloud increased on Wednesday afternoon back in Invercargill. Obviously a ridge must have moved onto the far south early this week. The days were mild, but there was some light frost in sheltered parts early on Tuesday.

I've been told that island temperatures are generally milder than the mainland - snow for example only falling about once a year at Oban, and rarely settling.

[Michael]

We just got it up here instead

[NZstorm]

Hi

Re snow at Oban, stewart island.

Oban is very sheltered from the south and tends to miss most of wintry showers. Also, there is a bit of coastal warming effect which tends to result in sleet rather than snow. Quite often, Invercargill will only get sleet as well (being near the coast). Towns further inland such as Gore and even Winton can get the now due to radiative cooling.

[Weather Watcher]

Towns further inland such as Gore and even Winton can get the snow due to radiative cooling.

"Radiative cooling" may be responsible for frosts in Gore and Winton, but I doubt it is a big factor in bringing snow to these places.
More likely, the snow at Gore/Winton is a result of the atmosphere being cooled from above by the falling/melting snow during a cold or prolonged snow event. This precipitation-cooling results in a nearly isothermal 'freezing layer' (near 0C), which can extend down to the surface in inland areas. Near the coast, the relatively warmer sea surface temperatures (around 8-10C about Southland in Winter I think) warm the lowest layer of the atmosphere and prevent this cooling effect reaching near the surface. Hence the lack of snow at Invercargill (or Oban?).
Also, Gore and Winton are 50-100m amsl.

[NZstorm]

Hi Weather Watcher,

I agree that a layer of nimbostratus can drop the surface temperatures through precipitation cooling. And that can mean the difference between a rain or snow event.

But I think radiative cooling plays a roll in snow levels in Southland because of the showery nature of the weather there. There tends to be fine breaks in between showers which helps to cool the surface temps and makes it easier for snow to settle.

[Weather Watcher]

There tends to be fine breaks in between showers which helps to cool the surface temps and makes it easier for snow to settle.

I presume you mean at night - during the day, the sun shining through those fine-breaks will heat the surface, not cool it.
Also, for radiative cooling to be effective, you need light winds near the surface (not a windy southerly) otherwise the relatively warmer air from aloft will be mixed down.
With "radiative cooling", you are cooling the atmosphere from below. This means you still need to develop a near-isothermal layer (near 0C) right up to the freezing level, otherwise the snow will melt in that warmer layer between the ground and the freezing level.
The only way radiative cooling brings snow to lower levels is if it occurred the previous night in clear skies, and the snow shower has moved over top of it once this cold isothermal layer has been established. This allows the snow to fall to the surface without melting. If it does melt, then hits a near surface freezing layer, you generally end up with freezing-rain (not more snow).

[NZstorm]

When i was living in the United kingdom I observed ice pellets.
Melted snow that had frozen in a cold layer near the surface. An unusual weather as it sounded like hail but fell as a steady precip of small elongated ice pellets. An occluded front was over the area at the time with polar air mixing with milder air creating layers of warm and cold air in the atmosphere.

Re the radiative cooling. You would be suprised the impact that radiative cooling does have on wintry precipitation. But I agree, if there is a freash wind, then its affect would be reduced. But having grown up in Otago, I know from my own observations that sleety showers by day can turn to snow at night (due to radiative cooling).