The Coldest May since 1992

[J07]

Here's a turn up for the books, after Nov-April all coming in above average and with January being the warmest for 10 years and driest in history, we've finally seen a below average month here. These, as I mentioned on another thread, are far more common than in the UK.

http://www.niwascience.co.nz/ncc/cs/monthly/mclimsum_08_05

http://www.niwascience.co.nz/__data/assets...194/0805sum.pdf

(there are a few daft errors in the PDF file)

All the main centres in NZ came out cooler than normal, but also sunnier and drier than normal. Clearly, the cool temperatures of this month have been skewed by clear, cold nights. This matches with increased frost recorded.

In terms of the most interesting statistics:

Extreme Wind Gust

In a month of interesting weather statistics (if not single events), this is one I did not expect. You would not often find such a strong gust in autumn....most likely they would come August-October. It also is noteworthy that this gust was an easterly, as opposed to the westerlies that prevail in that region.

The gust was a rather impressive 114mph at Mokohinau, easily overtaking the 96mph recorded in Wellington during ex-cyclone Funa in January. I was quite flabbergasted on seeing this.

Cold Nights

Some places came out with minima 3C below normal. A few locations (mostly inland and in the south) had maxima a few degrees below normal. In terms of maxima, Wanaka (inland town in the mountains) had its coldest May on record with mean daily highs of only 10C.

Taupo (inland town near central plateau, about 800m ASL), in terms of mean daily temperatures had its coldest May ever, with a mean of 8C.

Frosts

Some inland locations had 24 days of ground frost.

Extremely Sunny in the West

Sunniest place was Hamilton, coming in with a rather stonking 180 hours of sunshine (this is a record for them). At this time of the year, that is really something, and it wouldn't surprise me if it came out sunnier than much of the UK this month; and given the daylength discrepancy that would be quite an achievement.

Extremely Dry in Places

Some of the statistics are hard to believe, for an autumn month. Several single digit rainfall totals. The most astonishing of all is 3mm at Motueka (top of the South Island) which accounts for only 3% of its average monthly rainfall!

This becomes even more interesting, when we track back 6 months, and what do we find? Motueka, again, recording only 3mm of rainfall.

With the dry summer, clearly they will have suffered a fair bit.

And Wet in others...

In terms of wetness, the east of the North Island, normally fairly dry, suffered a lot of rain with 200% of average recorded in places.

Auckland also saw 84mm of rain in one day, with 25mm falling in 75 minutes.

So, quite a contrast across the country, which is normally the case given the state of the topography.

Where I can, I will try to compare with the UK November 2007 statistics. Last autumn to mid spring is the time of year when there is most difference between the respective UK/NZ climates.

Mean temperatures:

UK: 7C (1C above average)

NZ: 10C (1C below average).

Extreme temperatures:

UK: Max temperature of 19C, Minimum temperature of -7C

NZ: Max temperature of 22C, Minimum temperature of -8C.

There is more variation in temperature across NZ, which is normal.

Max snowfall in one event:

UK: 8cm

NZ: 48cm

UK's was recorded at Sennybridge, NZ's at Albert Burn. This is 1300m ASL, so maybe that constitutes an unfair advantage! But if you have a station there....you've gotta use it!

----

Conclusions:

Plenty of records broken across the board.

On the whole, a fairly quiet month for singular events, but consistently very sunny and dry for many. It would have been extremely autumnal in the Waikato and Central Otago, both being sunny and cold, with snow falling in Otago and long periods of calm weather in Waikato to enhance the autumnal colours. :lol:

Worth noting that there is a continuation of the trend that NZ is currently experiencing for increased sunshine hours. Every month comes in above average for almost everyone.

I see the reverse of this in the UK, which in November seem to be picking out sunshine hours of 50-70 hours BUT with *increasing* temperature. Compared with NZ which is getting sunshine hours of 100-180 across the main cities, BUT with *decreasing* temperature.

I believe this ties in well with the belief of many on this forum that British nights seem to be getting cloudier, and this is skewing mean temperatures upwards. Here, all the statistics for May point towards a month dominated by clear conditions allowing nights to cool and frosts to form, skewing mean temperatures downwards.

This is all well and good, but during summer we had both increasing sunshine hours and above average temperatures. It seems fair to say that during those months, cool nights contribute less towards mean temperatures than in winter- given the longer daylight hours it is easier for a cool night to turn into a hot day.

So it seems our upward trend in sunshine hours continues. I do wonder why this is. Synoptically, it is very hard, if not impossible, to get all of NZ under cloudy skies. Given the distinct mountainous boundaries and significant ranges lying all over the place, you cannot get gloomy low cloud across an entire island. But then it is hard to say whether our synoptic weather patterns are changing. Regardless of what happens, it seems like we always end up with one of the main cities (at least) coming out with record breaking, or top-5, sunshine hours.

Can this be pinned on El Nino/La Nina? Well, last year came out sunny for many. Wellington is basically on track to equal last years 2200 sunshine hours, despite us having a La Nina in 2008 and El Nino during half of 2007.

Elsewhere, someone mentioned the Antarctic Dipole and how that may have an effect. I have no idea what that is.

[Gavin P]

Thats weird, becaause the UK has just had it warmest May since, you've guessed it, 1992. :lol:

[Lettucing Gutted]

Could your cold but sunny May be down to frequent low level highs combined with upper level cool troughs to allow maximum radiative cooling?

[J07]

Thats weird, becaause the UK has just had it warmest May since, you've guessed it, 1992.

No way! Uncanny! :lol:

[J07]

Could your cold but sunny May be down to frequent low level highs combined with upper level cool troughs to allow maximum radiative cooling?

Synoptically, I think the most high-pressure dominant period was in the middle of the month, about 10 days of pretty quiet weather. During this time, there was a 500hPa trough in the Tasman Sea, a Queensland low stalled beneath it and that whole situation was stationary for a significant period of time (relative to this part of the world).

We do get those cold highs in two particular places, Central Otago in the South Island the central plateau on the North Island. They did crop up pretty often during the month of May. Also is worth noting that places like Hamilton and Auckland had more frequent SE winds than normal, leading to colder air with lower dewpoints passing over them, and cooler nights. These SE's were partly from the synoptic situation, but in Hamilton there would have been a katabatic contribution from the localised cold high over the central plateau (Hamilton's katabatic wind is SE).