Convective Period- 18th to 23rd Nov

Tornado Tim · Unread post by **Tornado Tim** » Mon 18/11/2013 09:41

Right, time for a new thread about the Convective Period this week.

Metservices T-storm outlook today:

Metservice wrote:Daytime heating and sea breeze wind convergences are expected to trigger afternoon and evening showers over inland parts of both Islands, especially from Gisborne to Wairarapa, including the Central Plateau and also from inland Nelson and Marlborough to North Otago.

For the ranges of Gisborne, Hawkes Bay, the eastern Bay of Plenty, eastern areas of Taupo and Taihape, and also the ranges of Nelson and Marlborough, there is a high risk of thunderstorms during the afternoon and evening with a moderate risk of isolated SEVERE THUNDERSTORMS developing, while for Wairarapa the risk of thunderstorms developing is considered moderate with a low risk that one or two may be SEVERE. SEVERE THUNDERSTORMS WATCHES have been issued.

These thunderstorms are expected to be slow producing localised heavy rain with intensities of 15 to 25mm/hr, as well as hail of 5 to 10mm in diameter. However with SEVERE THUNDERSTORMS localised downpours of 25-35mm/hr are possible.

Early in the evening, there is a low risk that thunderstorms drift from the ranges of the central North Island into the Bay of Plenty and most of Taupo.

Additionally, a lower risk of thunderstorms extends out from the main areas to take in the central North Island high country and inland areas of north Canterbury.

Finally, a front is forecast to move onto Northland late Monday, bringing increasing showers to the area. Some of these showers could be heavy, with a low risk of isolated thunderstorms from this afternoon.

Elsewhere, no thunderstorms expected.

Richard · Unread post by **Richard** » Mon 18/11/2013 10:59

Even though i'm in the low risk area the clouds are building nicely,so fingers crossed.

Tornado Tim · Unread post by **Tornado Tim** » Mon 18/11/2013 11:05

High risk tomorrow for us Central Waikato Folk, I am more inclined to say its a Mod Risk, but then again things can change.

Metservice wrote: Issued at: 10:34am Monday 18 Nov 2013

Daytime heating and sea breeze wind convergences are expected to trigger afternoon and evening showers over the central North Island during Tuesday. From central Hawkes Bay, across to Taranaki and then north to Waikato, Bay of Plenty and Gisborne, there is mostly a moderate risk of thunderstorms during the afternoon and evening, which could bring localised heavy rain of 10-25mm/hr and hail 5-10mm in diameter. However, in the ranges of Gisborne and the eastern Bay of Plenty, and also about western parts of Waikato and Waitomo, the risk of thunderstorms is considered high, with the potential that localised heavy rain rates could exceed 25mm/hr in these areas.

Additionally, in the west of Northland there is low risk of thunderstorms during the afternoon and evening.

Elsewhere, no thunderstorms expected.

Tornado Tim · Unread post by **Tornado Tim** » Mon 18/11/2013 11:43

Looks like an isolated storm has already developed on the Gisborne ranges..... rather early....

Richard · Unread post by **Richard** » Mon 18/11/2013 11:53

yes i see that,early alright,could be a sign that they could be nasty later.

jamie · Unread post by **jamie** » Mon 18/11/2013 12:57

Some mid level cap in place? Seems to be by what I'm seeing. Big fists breaking that cap though.

borris89 · Unread post by **borris89** » Mon 18/11/2013 13:32

Yea some nice looking build ups over the northern Hawkes Bay ranges at the moment, popped up very quickly

cbm · Unread post by **cbm** » Mon 18/11/2013 13:33

To my Southeast.

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Southwest.

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NZstorm · Unread post by **NZstorm** » Mon 18/11/2013 20:12

A problem with this set up is there is no low level cap, so showers develop freely and weaken afternoon instability.

Can add Southland to the regions of the country affected by afternoon showers/thunderstorms tomorrow (Tuesday).

jamie · Unread post by **jamie** » Mon 18/11/2013 21:22

yea great views to the South east from 1pm onwards today. Looked best at about 3pm i think.

Tomorrow could be good but again starting early.

Tornado Tim · Unread post by **Tornado Tim** » Mon 18/11/2013 23:01

NZstorm wrote:A problem with this set up is there is no low level cap, so showers develop freely and weaken afternoon instability.

Yea its a shame that when wee get setups like this, the cap is missing. A cap is very much beneficial when storms form as only the strongest convection gets through. Also caps can act to accelerate updraft speeds when breached.

Think of a breached cap like rocks in a river that protrude out hindering water flow, the gap's or opening's in the rocks has much more force behind it accelerating the flow that goes between the rocks.

Single and Multicell storms on the cards tomorrow due to sea breeze and wind convergences. Winds aloft are forecasted to be slow so storm motions will be slow, however I think that storms will only turn "Severe" briefly because they will most likely be in the CB collapse stage(ie: Microburst stage where the core gets dumped). We dont have abundant CAPE, nor do we have the heat of Jan/February.

Id expect a line to develop from North Waikato down to central mid Waikato, where the winds meet. The stuff preceding that will be unorganised and should collapse quickly. ie: dont chase that stuff.......

Thunder · Unread post by **Thunder** » Tue 19/11/2013 00:11

Well thunderstorm corner it shall not be called during this convective period. The usual spots about the upper South Island, and inland parts of the North Island are the spots to be. And as NZstorm notes, Southland tomorrow.

There were some bubbly clouds about Mt Somers today, a few showers developed in the foothills late afternoon / evening. Saw a weak Cb when I drove further inland too.

aw1_20131118.jpg

aw2_20131118.jpg

jamie · Unread post by **jamie** » Tue 19/11/2013 01:13

Tornado Tim wrote: Id expect a line to develop from North Waikato down to central mid Waikato, where the winds meet. The stuff preceding that will be unorganised and should collapse quickly. ie: dont chase that stuff.......

Yea what do u think about all the early unorganized crap putting a lid on the heating an killing any chance of a convergence. It happened in similar circumstances just 2 weeks ago. Or when ever that last convective period was.

NZstorm · Unread post by **NZstorm** » Tue 19/11/2013 06:51

Hoping the western Waikato sea breeze is late coming in today as that will hold off convection in that area. Also, west Auckland will get thunder too if we have good heating. High cloud cover looks to be a concern today though. Hopefully that messy high cloud in the north Tasman holds back.

GFS precip for today. Fairly widespread convection today. Modelling won't get the amounts right but it is good at showing regions affected.

Manukau heads obs · Tue 19/11/2013 07:06

NE wind here now...a good sign for a sea breeze convergence today

lots of middle layer cloud too...to slow things down a bit

jamie · Unread post by **jamie** » Tue 19/11/2013 08:32

Overcast at the moment here but it will burn off before long I think.

spwill · Unread post by **spwill** » Tue 19/11/2013 08:39

The more sunshine today the better, I will keep a watch on west Waikato this afternoon for a possible chase

Beautiful photography there Aaron

jamie · Unread post by **jamie** » Tue 19/11/2013 09:02

High risk of severe today. Wahoo

Tornado Tim · Unread post by **Tornado Tim** » Tue 19/11/2013 09:16

jamie wrote:High risk of severe today. Wahoo

Yes not often you have a high risk of severe storms coming from Metservice.

Metservice wrote: Daytime heating and sea breeze wind convergences are forecast to trigger showers over inland parts of both Islands from late morning. The most unstable conditions are expected about central parts of the North Island under a cold upper level trough, where there is a high risk of scattered thunderstorms this afternoon and early evening. For inland areas from Waikato to Taranaki and across to the main ranges of Gisborne and Hawkes Bay, these storms will be slow moving and as a result have the potential to be SEVERE producing localised downpours of 25 to 40mm/hr, along with hail. Rainfall of this intensity can cause surface and/or flash flooding as well as slips, and may make driving hazardous. Heavy showers and thunderstorms should diminish into the evening hours. Outside the central high risk area, there is a lower risk of isolated thunderstorms from western parts of Northland to northern Manawatu and Central Hawkes Bay as indicated on the chart.

Over the South Island, isolated afternoon and early evening showers are most likely over the ranges from Nelson and Buller down to the Mackenzie, as well as inland parts of Southland, Clutha and Central Otago. There is a low risk of one or two showers turning thundery in these areas.

Elsewhere, no thunderstorms expected.

I will probably be out chasing today, not entirely sure where though yet....... West I suspect later on in the day.

Tornado Tim · Unread post by **Tornado Tim** » Tue 19/11/2013 10:00

Looks like we have 2 opposing flows today (according to GFS and Altocumulus visible atm), which will make this set-up quite interesting. Dominant SW to Westerly flow up to 600hpa and a Easterly to NE flow above that. A SFC Westerly should favour a westerly Seabreeze and the Easterly seabreeze I think will be held back to the BOP coast.
The Flow inbetween the 2 opposing flows is quite weak, but once a storm develops It will be interesting to see how it looks.

GFS Indicating CAPE up to 1100j/kg and LI down to -4.

jamie · Unread post by **jamie** » Tue 19/11/2013 11:04

Towers to my south already

jamie · Unread post by **jamie** » Tue 19/11/2013 11:33

A few strikes being detected down to the south already.

Clear sky here now and towering cumulus popping up. Feels quite hot today with more humid conditions. currently 21.4/17.2

Manukau heads obs · Tue 19/11/2013 11:43

more blue skies over north waikato than there is here
that might be a good spot
CU developing there now

its 25C in Waiuku with a northerly wind there

Tornado Tim · Unread post by **Tornado Tim** » Tue 19/11/2013 11:58

Oh man I hope it holds off more, however SFC conditions are soo good its bad for the outcome.......
23 and rising with a 18 DP = early initiation of showers......

Everyone cross your fingers for stuff to hold off a bit longer please..............

jamie · Unread post by **jamie** » Tue 19/11/2013 12:16

not far from starting now!

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