Christmas Low 22 – 26 December

Cyclone Tracy · Unread post by **Cyclone Tracy** » Sat 22/12/2018 09:51

I thought this deserves its own thread. A subtropical low has now formed NW of the NI in the Tasman that will deliver multiple fronts to the NI and later to parts of the upper SI during the Christmas period. The first warm front will move across Northland today and towards Auckland tonight. Tomorrow will see an increase in moisture from the tropics with the warm front stalling around the Upper NI. More details on the synoptics to follow as the system matures.

Weather models have a common agreement on the scale of the depression but vary on rainfall totals per region. UKMet currently going for around 250mm to 300mm in the Auckland and Coromandel areas, EC, GFS, Access, CMC more evenly spread with the higher falls of 100 to 200mm around Northland. It’s guaranteed there will be many upgrades and downgrades to come over the next few days

David · Unread post by **David** » Sat 22/12/2018 19:27

About 70mm modelled by GFS for here. This would take my station's yearly rain total to a whopping 1700mm!

Cyclone Tracy · Unread post by **Cyclone Tracy** » Sat 22/12/2018 20:31

As at 8pm low has now deepened to around 999 hPa in the Tasman with a small 2nd circulation nearer to the NI which is being consumed by the main low. Warm front dipping down into the upper NI containing initial moisture. Extensive storms near the boundary of the main trough back into the coast of NSW and Qld.

MS heavy rain watch remains in place from tomorrow for Northland, Auckland and Coromandel. Models holding solid on a big rain event but wiggling the mesoscale levels as expected.

NZstorm · Unread post by **NZstorm** » Sat 22/12/2018 21:56

I put a comment in the general thread earlier about this system as I didn't notice this thread had started.
Thunderstorms could be a significant player enhancing rainfalls.

spwill · Unread post by **spwill** » Sun 23/12/2018 07:56

For Xmas Day, GFS pointing to brighter spells arriving in the morning for Auckland/ Northland but heating/convergence leading to thunderstorms pm.

David · Unread post by **David** » Sun 23/12/2018 08:14

I'd expect the weather watches to be upgraded to warnings this morning. Looks like widespread totals of 100mm may be possible

Cyclone Tracy · Unread post by **Cyclone Tracy** » Sun 23/12/2018 09:41

Remnants from EX TC Owen now connecting down the atmospheric river towards NZ at 8am. Total Precip water around 50 to 70mm into the sub tropics. This has quite significant rainfall potential for the NI

spwill · Unread post by **spwill** » Sun 23/12/2018 11:23

Lightning active just NW of Northland this morning.

GraemeWi · Unread post by **GraemeWi** » Sun 23/12/2018 12:33

Thunder just now in Swanson

David · Unread post by **David** » Sun 23/12/2018 14:41

Latest GFS now showing approx 120mm in 24 hrs for Akl tomorrow

Cyclone Tracy · Unread post by **Cyclone Tracy** » Sun 23/12/2018 15:50

Water vapour increasing in the mid levels over Northland at the moment. Subtropical jet now running around 220 to 250 km/h over the NI. Isolated CG's west of Whangarei. As mentioned above, some models are starting to go big. Based on current moisture observations across the layers, its not surprising. MS might have upgrade watches to warnings soon, especially for those pitching tents in low lying areas.

David · Unread post by **David** » Sun 23/12/2018 19:13

Already some heavy falls around this evening. Had 10mm here in the last hour

David · Unread post by **David** » Sun 23/12/2018 19:28

17mm in the last hour. Where are the weather warnings from MS!

Cyclone Tracy · Unread post by **Cyclone Tracy** » Sun 23/12/2018 19:49

I think they maybe have taken a Christmas break

14mm so far here in that band. Circulation likeness echo appearing on the radar WNW of Auckland with an intense convergence on the east side. Interesting to see if that maintains its intensity on approach to land

David · Unread post by **David** » Sun 23/12/2018 20:48

Cyclone Tracy wrote: ↑Sun 23/12/2018 19:49 I think they maybe have taken a Christmas break

14mm so far here in that band. Circulation likeness echo appearing on the radar WNW of Auckland with an intense convergence on the east side. Interesting to see if that maintains its intensity on approach to land

Noticed it had some white on the radar, so very intense. Image attached for future reference.

Vertigo · Unread post by **Vertigo** » Sun 23/12/2018 21:05

Thats definitely a hook echo, and with a very intense bow wave in front of it! Heading to South Auckland/North Waikato.

MS needs to issue a severe thunderstorm warning on this one.

Edit: Its weakening.

jamie · Unread post by **jamie** » Sun 23/12/2018 21:36

I notice MetService have issued another watch and not a warning and pulled back on the duration of the event.

Warning for northland and coromandel tho.

Getting decent rain here. Already over 200mm for the month. Another 100mm will make it a super wet month.

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Cyclone Tracy · Unread post by **Cyclone Tracy** » Sun 23/12/2018 21:55

Based on current wind obs, the surface convergence is through the middle of Auckland ATM with a NW to SE steer above. Water vapour near -60c in the Tasman. Looking quite wet for parts of Auckland soon if that holds.

NIWA tweeting hourly rainfall record on the North Shore between 5 and 6pm. An indirect shot at MS for no warning maybe

David · Unread post by **David** » Sun 23/12/2018 22:29

Looks like SW Auckland could catch a soaking next few hours, and then down into the Waikato judging by the recent radar scans.

jamie · Unread post by **jamie** » Mon 24/12/2018 03:03

David wrote:Looks like SW Auckland could catch a soaking next few hours, and then down into the Waikato judging by the recent radar scans.

Yea I’ve already had 32mm. Rain has just woken me up.

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melja · Unread post by **melja** » Mon 24/12/2018 04:49

I thought you need 100mm in 24h or 50mm in 3-6h for a warning? I'm guessing that's why no warning

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Cyclone Tracy · Unread post by **Cyclone Tracy** » Mon 24/12/2018 05:50

melja wrote: ↑Mon 24/12/2018 04:49 I thought you need 100mm in 24h or 50mm in 3-6h for a warning? I'm guessing that's why no warning

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Spot on, which means MS currently think that threshold won't be exceeded in Auckland. Time will tell I suppose but some gauges have had a solid start.

David · Unread post by **David** » Mon 24/12/2018 05:59

Not much here since yesterday evening's downpour and the rain appears split either side of land in the current radar, so looking like they could well be right to issue no warning.

Also, GFS has reduced its forecast to 40-50mm more for today which is nothing remarkable. Time will tell...

Cyclone Tracy · Unread post by **Cyclone Tracy** » Mon 24/12/2018 07:09

Here is my 2 cents worth overlaying the 6.30am water vapour image. She is a little complex. Rich moisture in the mid level to the west of Auckland at 6.30am. One to watch is that surface trough north of Northland that will move south and squeeze against the front later today. MS warnings are tipping that around the Coromandel is the big squeeze zone but the eastern side of Auckland could go big if it wiggles west with the surface low movement.

Brassnz · Unread post by **Brassnz** » Mon 24/12/2018 07:40

35mm rain since midnight here, to go with the 25mm from yesterday. Things are pretty soggy at this point.

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