Severe Thunderstorm Forecasting

[NZstorm]

I came across this URL by estofex in Europe. It is a good introduction guide into forecasting severe convection and worth a read for anyone interested in trying to forecast thunderstorms.

http://www.estofex.org/guide/

[NZstorm]

I notice they mention the Spanish plume in the thunderstorm forecasting discussion for Europe. A Spanish plume is warm dry air in the middle atmosphere which over runs humid surface air to produce instability. The warm dry air acts as a cap and allows daytime heating to raise the instability levels (loaded gun). We see this phenomena in parts of the world that are known for severe thunderstorms. There would be no tornado alley if it wasn't for the warm dry air from Mexico. And the supercells of NSW often have warm dry air in the mid levels. This is the missing ingredient for severe thunderstorms in NZ.

[NZstorm]

A sounding here from Oklahoma this year, The warm air mass between 850-500mb creates the unstable environment when over running moisture. The 850mb temp is about 20C. 500mb -5C.

[NZstorm]

A sounding here for a thunderstorm day over the North Island. The 850mb temp is about 10C which doesn't allow for much of a build up of CAPE. This is a common feature of NZ thunderstorm set ups, a lack of warmer air aloft to act as capping.

[jamie]

Ha! Ours is hardly unstable and the wind barbs look horribly messy.

[Tornado Tim]

A sounding here for a thunderstorm day over the North Island.

It would be good if they have soundings in Waikato rather than go by Auckland ones.
The general trend should be the same though.

Ha! Ours is hardly unstable and the wind barbs look horribly messy.

But, If I remember correctly the 27th was kind of a dud for Auckland (which the sounding is for), CNI (Around Mid to Northern Waikato) had CAPE values to 2500j/kg.
But that was a forecast of course and we dont have a balloon anywhere near where the storms went off so that day and the others around then will kind of be unknown.

Perhaps I should ask MS to make a Radio-sonde Balloon setup for Hamilton AWS? (that would effectively change that station to a METAR rather than SYNOP).
METAR data in NZ is really sparse.

[Thunder]

You could use the figures from the sounding at Whenuapai and then add in surface obs for the CNI, the eventual sounding might be slightly off but it should give a fairly good indication unless the upper air data is quite defined, use this program.

http://www.downunderchase.com/storminfo/boltware.html

There may well be other programs but I haven't looked.

In the case the upper air data is quite defined you could use the modeled soundings from ARL (NOAA READY, get the txt results), put in your specific lat and long figures then get a surface ob and put it into a program like above. The very latest modeled upper air data isn't too bad it's the surface figures that it doesn't deal with too well in NZ but perhaps it's getting better. All in all isn't ultimately as good as a real sounding from a certain location but it will give a good indication .From what I remember when I went to metservice to have a look around they were using the modeled soundings then inputting their forecasted afternoon temps / dewpoints to see what could happen. There was one really enthusiastic guy there but I can't remember his name, was cool to see.

Thanks for that link NZstorm, yes lack of cap here alot of the time...... sigh.

Cheers
Aaron

[NZstorm]

Models compute various forms of CAPE. MUCAPE which is the most unstable CAPE and is computed using the surface dp. I have never been a fan of this number because it doesn't allow for mixing. MLCAPE mean layer CAPE is a better number and this is what the gfs uses. It uses the lowest 100mb moisture I think.

[Thunder]

Yes agree there NZstorm, didn't know about those different forms of CAPE but understood about the effect of lower level mixing and taylor skew t's when I draw them up on my computer to represent that, more or less I'm just visually looking at the graph to see how unstable things are or could be etc.

Cool, didn't realise GFS did that.

[Tornado Tim]

Thought id add another beneficial index for forecasting severe thunderstorms:

This is Total Totals, which is assessment of Storm Strength.
Essentially it is a calculated by using the 850mb DP (Dew Point), 850mb Temperature and 500mb Temperature.

Say if 850mb temperature = 5°C and DP @ 850mb is 1°C, 500mb temperature = -20°C.
It would work like this:
(5°C - (-20°C) + (1°C - (-20°C))) = 46

So the Total Totals for this situation is 46.

A rule of thumb:
TT Values less than 45 are not ideal for Severe Convection.
45-50: Thundershowers more likely, Thunderstorms possible.
50-53: Thunderstorms Likely, Low-mod risk for an isolated severe storm.
53-55: Moderate Risk of Severe Thunderstorms
55+: Severe Thunderstorms Likely.

Do not take this Index on face value, use it in conjunction with other Index's such as a CAPE, LI and SHEAR profiles, as you can see this value completely ignores CAPE.
(actually its a good idea in general to never take an index by itself, use other profiles as well)
You also cant use this index in high elevations due to it taking 850mb temps and DP which may well be near the surface.

[NZstorm]

I have had a look back at recent large hail in NZ. I'm defining large hail as stones above 3cm. I can find 8 events since 2005 that look legit.

Dec 14 2005 Winton, Southland
Mar 8 2006 Waimate, Sth Canterbury
Nov 17 2008 Ashburton.
January 3 2009 Amberly, Nth Canterbury
January 14 2009, Kaimai Ranges, Bay of Plenty
May 11 2009, Papamoa, Bay of Plenty
Dec 14 2009, Methven, Canterbury.
July 9 2011, Waikanai, kapiti Coast.

The Insurance Council on there storm damage page have the Ashburton 17/11/08 hail claims at $11million!

[Tornado Tim]

The Insurance Council on there storm damage page have the Ashburton 17/11/08 hail claims at $11million!

So that didn't include flooding etc? flip thats heaps for just hail alone!

[NZstorm]

yes, its all hail damage to cars, buildings and cropping around Ashburton area.

[NZstorm]

I have put up the NZWP on the day of the Papamoa event that looks to represent the typical large hail environment for NZ.

I am calling this the Polar/low CAPE/high shear set up. I think most large hail occurs within this type of set up. The environment is highly sheared with mid level winds 35-50kts, and the storm structure is low top supercell.

[Thunder]

Some great info, thanks for sharing this

[tgsnoopy]

What about the Kaimai Hail on Wed 14th Jan 2009? Was it similar?

[Weather Watcher]

I have had a look back at recent large hail in NZ...

Dec 9 2009, Methven, Canterbury.

I think this one should be December 14 2009 - Large hail (3+ cm) mostly around Mid/South Canterbury, tornado near Methven.

[NZstorm]

Thanks for the correction weather watcher.

The less common set up for large hail in NZ is those that occur due to the advection of subtropical air onto NZ. I am calling this the warm/high cape/high shear set up. High dewpoint air is heated in the afternoon to yield instability under an upper trough. The Winton (Heddon Bush), Southland event was an example of this. CB tops are much higher, usually 12km.

What about the Kaimai Hail on Wed 14th Jan 2009? Was it similar?

I would put that in the same category as the Winton storm.


The other 6 events on the list were the polar type.

[NZstorm]

Of the severe weather that can occur around a thunderstorm, tornadoes seem to be the most reported severe event in this country. Typically there are 10 to 15 damaging tornado events reported in the media a year in NZ.

Large hail (20mm+) events are reported on between 1 to 5 days a year from the little bit of research I have done. I'm sure a lot of large hail would go unnoticed due to it being very localized.

[Manukau heads obs]

I guess you could add large hail accumulations too to that list...

[NZstorm]

A good test of the potential for supercells is the 0-6km shear vector. Its the vector difference between the surface wind and the upper level wind around 6km. Both vectors are plotted on a hodograph and the difference measured. But often you can calculate the vector by eyeballing the numbers as with the example below. Rule of thumb, you need around 40kts for a supercell, 50kts is ideal.

Example, the hail storm over southern Hawkes Bay Monday. Surface wind SE 10kts, 6km wind NW 35kts. I've taken those numbers off the gfs model 00Z on the day. The vector difference is 45kts. So from a shear aspect there was enough shear for a supercell.

[NZstorm]

Destructive flash floods from thunderstorms are quite rare in NZ. They are most possible when we have warm moist subtropical air over the country in summer.

Memorable flash floods occurred over the west of Northland and at Pukekohe, south Auckland on 21 January 1999. The insurance council had claims of $5million and two lives were lost. 200mm was recorded at Pukekohe (90mm in 2 hrs).

The NZWP morning sounding showed the 500mb temperature -8.9C. The surface dewpoint 22C.

[Manukau heads obs]

yup, I remember M/S called it super saturated air mass
there was a bit of sea breeze convergence in a line from Pukekohe to here that day
I recorded 50mm each hour for 1.5 hours (150mm total, but places further north of me recorded over 200mm)