Topic: Extreme CAPE in NW WA

[Michael Thomas]

Yesterday, around Broome there were a few thunderstorms. Not sure about the severity or structure of these thunderstorms but one standout feature was the amount of CAPE. In the unmodified morning sounding, CAPE was ~4000 j/kg based on a temp/dew point of 30.3/25.4. The maximum temp/dew point yesterday in Broome was 33.6/26.5 which give a surface-based CAPE of ~6000 j/kg. It was worth mentioning however that the low-level moisture was quite shallow (approx. lowest 50 mbar in morning sounding).

http://soundings.bsch.au.com/skew-t.html?source=wyoming&lat=-17.9492&lon=122.2336&gribdate=&month=11&day=28&year=2010&hour=00

I have long believed that NW WA is the home of extreme CAPE in Australia. A strong capping inversion is usually present and it is not uncommon to have very rich dew points in the lowest 50 mbar. It might be interesting to keep an eye on tomorrow as well.

[Michael Thomas]

Some strong storms have formed 200 km south of Broome at the moment. Surface-based CAPE is around 3500 j/kg when replotting the Broome sounding with the current obs (33.7/23.2). Interestingly, upper-level winds are actually quite strong with 70 knot W'lies at 300 mbar. 500 mbar winds are on the weak side at 25 knots and surface winds are strong (19 knots at Broome) from the NW.

I would think that the conditions may be supportive of supercells. 500 mbar temps are about -9 C which is cool for the tropics. This may even mean that large hail is possible in stronger storms.

Michael

[Michael Bath]

I glance at the LI and CAPE charts each day before zooming into the eastern states for forecasting. It hadn't really sunk in the impressive values that happen from time to time in NW WA.  Thanks for pointing out what's going on.

The cumulonimbus structures would be an amazing sight for us storm-starved SEQ/NE-NSW residents !

[Michael Thomas]

Hi Michael,

I feel that this storm event in NW WA was pretty much ignored by most people. Not hard to understand since few people live out there. The setup really was quite decent, rich dew points (23C in Broome), a strong cap to keep storms isolated and strong upper-level winds to 'vent' thunderstorms. Winds were pretty weak from about 500 mbar down but still, I think us in NE NSW and SE Qld would be happy with 25 knots at 500 mbar, it's still enough for some degree of storm organisation.

This loop shows the event quite nicely-

http://www.theweatherchaser.com/radars/australia/national-satellite

You can see the storms form along a boundary which was moving up the N WA coast line. A shame the Broome radar wasn't updating for a couple of hours.

Edit: I imagine storms would be very photogenic out there. Explosive updraft set against the red earth, I think it would be pretty amazing to see.

[Jimmy Deguara]

Hi Michael,

Paul Graham, David Croan and I have talked about the storms in NW WA around Broome up to a few years ago in response to the high CAPE and also the boundary layer being shallow in terms of moisture. There is a photograph of an explosive updraft on the Bureau Storm Spotters Guide a few years back from this region if I am not mistaken.

Question is - do you leave the storms you have here in November / December and chase up there or Darwin for instance. Not a chance.

By the way Michael, I would not call 25 knots at 500hPa weak personally. If a storm traverses some sort of boundary with such high CAPE, ti still can remain organised for a sufficient period of time. A similar scenario in 2005 in SW Oklahoma yielded a tornado even with a sharp right moving storm along a triple point in such marginal conditions - hmm well the CAPE values were 6000J/kg which makes a difference.

Regards,

Jimmy Deguara

[Jimmy Deguara]

Hi Michael,

It was worth mentioning however that the low-level moisture was quite shallow (approx. lowest 50 mbar in morning sounding).

I wonder if the heating further inland may increase the dry bulb - dew point deficit keeping bases slightly higher limiting the extent that a storm could feed on such moisture / realised CAPE?

I replotted the sounding below for a temperature of 35C instead and the LCL was a little higher say at about 870hPa. Still, impressive CAPEs and updrafts nevetheless at 5,500 CAPE assumably.

Regards,

Jimmy Deguara

[Michael Thomas]

Radar loop for 20/11/10-

http://www.theweatherchaser.com/radar-loop/IDR172-broome/2010-11-30-00/2010-11-30-10

This radar loop shows the explosive development 200 km south of Broome. Note the radar was offline for 1.5 hrs.

[Michael Thomas]

Hi Jimmy,

Yes, I do think that the dew point depression would be greater away from the coast as a result of higher temperatures and/or lower dew points. When replotting the Broome soundings, 33C is not sufficient to remove CIN.

Regarding the wind speed at 500 mbar, words are really quite subjective. What I meant was that 25 knots at 500 mbar is not strong, at least for mid-latitudes. I would say that 25 knots at 500 mbar is on the weak side of what is needed for organised storms.

Michael