Low to cross North Island Monday

[NZstorm]

A low of tropical origin will cross the upper North Island Monday. Although this system will move through quickly it should bring a spell of very heavy rain to northern regions as it passes.

[David]

Yes I have been watching the model forecasts for this event, as metservice's severe weather outlook page mentions the possibility of weather warnings being required for Northland, Coromandel, Bay of Plenty and eastern parts of Auckland

Although I hope for the heavy rain (because I like it) it is really the last thing Northland needs - which was drenched several times during this July and August.
Whangerei recorded a total of 456mm for July

[TokWW]

There will be some moderately strong Nor-easters with it as well showing 20kts + on MetVuw yesterday, and WxSim was predicting some good rainfalls as well - I don't like wind and rain when I am trying to cycle 10kms to work...

[Cameo1]

Oh no not MORE rain. I have just planted hundreds of native plants along the creek at home and if it bursts its banks it'll ruin them all.....AGAIN I only need a few weeks without a storm so they can get a few roots into the ground.

[David]

Metservice has upgraded their confidence of heavy rain to moderate over Auckland

Here's their full statement:
Issued by MetService at 1:36pm Friday 24th August 2007

On Saturday a ridge of high pressure should cover the North Island with a westerly flow over the South Island. During Sunday afternoon and evening an easterly flow is expected to strengthen over the north of the North Island as a subtropical low approaches. Heavy rain is likely to develop in Northland late Sunday and continue into Monday. There is increasing confidence that rainfall amounts will reach warning thresholds.

During Monday the low will move further south onto the North Island though there is great uncertainty about its path from here on. Rain is likely to spread over all the North Island on Monday with an increasing likelihood of heavy rain for Auckland, Coromandel Peninsula, Bay of Plenty and Gisborne. There is also a risk of strong easterlies and heavy rain for Gisborne and Hawkes Bay on Tuesday depending on the track of the low.

[NZ Thunderstorm Soc]

Not looking good for Canterbury, sadly
Mostly fine or uninteresting weather eventuating from it, although it could be windy
Hope the skies will be clear for the eclipse

JohnGaul
NZTS

[David]

I've completely lost hope in being able to see the upcoming eclipse up here

[tich]

Oh no not MORE rain. I have just planted hundreds of native plants along the creek at home and if it bursts its banks it'll ruin them all.....AGAIN I only need a few weeks without a storm so they can get a few roots into the ground.

Isn't Auckland somewhat sheltered (at least a little) from easterly rain by the Coromandel Peninsula?

[David]

This can be true, but usually the rain bands move from the north or northwest - in these cases Coromandel gets more rain due to the elevation (heavier orographic rainfall) and Northland takes the brunt as the fronts lose moisture and intensity as they travel southwards. Auckland still receives moderate falls but sometimes can miss out - such as the Waitangi Day heavy rains up north where only 2mm fell here.

Still occasionally Auckland can receive significant falls during these events - most likely in the north and more likely here in the east than in central, western and southern parts.

[David]

Still a moderate risk of rainfalls exceeding warning criteria in Northland, Auckland and the Coromandel on Monday - and a severe weather watch has been issued for these areas

[TokWW]

This can be true, but usually the rain bands move from the north or northwest - in these cases Coromandel gets more rain due to the elevation (heavier orographic rainfall) and Northland takes the brunt as the fronts lose moisture and intensity as they travel southwards.

If you look at the MetVuw maps it shows Easterly and ESE so the weather is not going to come from NW?? How?

When you strong Low's with high wind velocity, things become more unusual than normal. I can't see the rain coming from the NW can you?

[David]

That forecast shows the wind blowing as an easterly, but the rain is coming from the north.

http://www.weatherforum.org.nz/phpBB3/d ... &mode=view
This shows the radar for the heavy rain event in 24 Jan 2006 - the rainband has moved down from the northwest, and their were easterly gales gusting 100km/h here in E Auckland. Coromandel was affected worse because the orographic effect. Think about it, when you get strong easterlies from a low pressure system - does the rain move across from the Gisborne area? It usually comes from N or NW.

The depression's direction of movement is also the direction the associated fronts generally move in - the wind directions differ at different points in the low because of the curvature of the isobars surrounding the low.

[TokWW]

But Gisborne is south of this region, we get a lot of occluded rain in these events coming across the Mamaku ranges from the BOP, it arrives in bands from the NE usually and not from the NW . Can you explain the occluded fronts moving in from the east? I know the weather macro moves across in opposite direction to the movement of the earth and hence the normal movement of weather "objects" west to east, or with the rotation of a low. from the NW.

How do you explain the Cyclone Bola and the weather that came from the EAST in that semi isolated incident? From Gisborne and up to here... we are north of Gisborne

Hey - I don't want to argue - I do want to learn, and I guess proposing arguments that need explanation help with the learning process. Thanks

Graeme

[David]

I am too young to remember cyclone bola, but I do know that it moved over the N Island from the northeast
I don't know as much about the weather patterns in your area - I know most about patterns here in Auckland and New Zealand's general patterns, I am still learning as well. I don't know about rain moving from the northeast or east where you live - but I can't remember any frontal band moving onto here from the east in the last 2 years - and I regularly view the rain radar.

It may be something to do with the topography - which might affect the orientation of fronts?

Had another thought - when the fronts cross the country often the whole band does not move together. Often one side remains slower moving within the low and the other end moves at a faster rate - so the front curls. This could mean the section of a front affecting Auckland from the north could curl and affect your area from the northeast.

[Manukau heads obs]

yes, when the rain is falling from upper level cloud systems, like in a warm front, its that southward movement of that warm front that means the rain does in fact move south, even though the surface wind is an easterly (underneath that frontal cloud sheet (which is overriding the colder surface wind))

[David]

yes, when the rain is falling from upper level cloud systems, like in a warm front, its that southward movement of that warm front that means the rain does in fact move south, even though the surface wind is an easterly (underneath that frontal cloud sheet (which is overriding the colder surface wind))

Is this why when we have rainy weather the clouds we can see from ground level appear to be coming from a different direction than the weather is coming from?

[TokWW]

Thanks for the explanation Brian - that explains what I have been seeing esp with the NE, because the warm front air is moving southwards from over the sea, laden with higher DP/humidity than that which is over the land. We often have cool air here and clear skies partly because of elevation.

Some of us (mainly myself ) need some video lessons on Climatology and weather systems

[Manukau heads obs]

i see heavy rain warning now issued for this system coming

[NZstorm]

Looking like quite an active little system to go through but rapid movement should prevent any major flooding problems. I notice the dewpoints are 19C at Norfolk Island now so some very moist air comin in as expected. The high dewpoints mean the system will be convectively enhanced. Could be thunder about eastern Northland through to BOP.

[David]

metservice has done poorly for this event - seeing the rain radars, looks like we are not going to get much more than the 1mm overnight
Whangerei only has had about 7mm and Coromandel hasn't had much either.
They were also wrong about the winds - strong easterlies are not here, but light southerly.

[Manukau heads obs]

yes, what has happened is what often happens with the lows from north...a hole in the rain bands....i.e rain to the SW and SE of the low center, but not much to the south of the low center

[David]

a hole in the rain bands

Many times have I been disappointed when this has happened with squall lines or bands of heavy rain. What is most disappointing is becoming excited about an approaching downpour or thunderstorm and then it swerves away or dissipates

[TokWW]

The path and strength have both changed and even MetVuw maps were quite out this time... fine mild day here in Tokoroa! Very slight 2mm rain this morning before 9am - that's all...

[David]

Strange... SE breeze picking up but temperature suddenly gone up to 16.3C here, clouds cleared away to sunshine

[David]

Michaels favourite weather tomorrow!:
As issued by MetService:

Northland and Auckland
Showers, some heavy today, with isolated thunderstorms. Gale southwesterlies tomorrow.