Topic: Tropical Cyclones 2007/2008 Southern Hemisphere Season

[Michael Bath]

First TC of the 2007/2008 developed this morning per JTWC within the Australian region at 91E. The low has been in the advisories for 2 days but finally reached the threshold.

https://metocph.nmci.navy.mil/jtwc/warnings/sh0108.gif

BoM Perth put out a tropical low warming later in the day.

[Mike]

Satpics don't show much yet - is it too far out!  There's definite movement afoot with this one.  One to watch considering they have got warnings out. Thanks, Mike for posting the thread - was not even monitoring considering the fine weather!



Here's the latest.....


At 0600UTC a tropical low was located within 20 nautical miles of
 Latitude eleven decimal eight degrees South [11.8 S]
 Longitude ninety one decimal seven degrees East [91.7 E]
Recent movement southeast at 8 knots.
Maximum winds 30 knots.
Central pressure 998 hPa.

AREA AFFECTED
Within 90 nautical miles of the centre extending to 200 nautical miles
in southern quadrants.

FORECAST
Sustained clockwise winds 30/40 knots in southern quadrants possibly extending
to northern quadrants should the low develop into a tropical cyclone in the next
12 to 24 hours. Rough to very rough seas and moderate swell.
                                                                               
                                                                 
At 1800 UTC 30 July: Within 60 nautical miles of 12.9 South 92.6 East
                    Central pressure 998 hPa.
                    Winds to 30 knots near centre.
At 0600 UTC 31 July: Within 100 nautical miles of 13.9 South 93.3 East
                    Central pressure 998 hPa.
                    Winds to 30 knots near centre.

Next warning issued by 1300 UTC 30 July 2007.

Mike

[Michael Bath]

Lasted less than 24 hours and just two warmings issued by JTWC. It looked better on satpics before it was classified TC strength.

Previous July TC was 20 - 27 July 1998

[Jimmy Deguara]

Hi Michael,

Previous July TC was 20 - 27 July 1998

I almost entered a post yesterday on this basis. Well the possible implications for us. The tropical cyclone threat may have excited some but being July I took no real interest. I was going to indicate my impressions of what the positioning and timing could mean with regards to the existence of a tropical cyclone in this region. But that would have complicated the discussion. Now that the tropical cyclone threat has eased, probably more of an appropriate time.

Well the north west cloud bands and them streaming down this way - what effect could this have in terms of an impact on eastern Australia. Well 1998 saw some serious east coast lows develop with a relatievly wet August experienced. Michael do you know if other years such as 1986 or 1990 for instance may also have had tropical cyclones/tropical activity in those regions during July? Or perhaps a good Indian/Bangladesh monsoon?

Regards,

Jimmy Deguara

[Mike]

Would this early TC and NWesterly inflow of moist air be as a result of a Rossby wave?  I'm not that familiar with Rossby waves, but do know that it is either oceanic or atmospheric.  That these waves originate from the northern hemisphere and at times a portion of this air/oceanic mass 'pulls downward' toward the equator at times?

As we have seen in the past with moist inflow streaming across central WA and NT into NSW and VIC that they have interacted with ECLs and created on large low pressure system.

Or is it just the MJO that is progressively getting closer causing all this?

Mike

[Michael Bath]

Jimmy - the earliest in those years was September. There have been a few occasions reported of weak TCs in July - at least since the satellite era. And they have all formed around 90E.

[Mike]

From the BoM issued yesterday for their outlook for the northwestern area:

Computer guidance suggests a low may develop in the vicinity of 12S 115E-122E on
Tuesday or Wednesday with an increased risk of cyclone formation later Thursday
or Friday. At this stage the likelihood of a tropical cyclone developing in the
next three days is:
Wednesday: Low
Thursday: Moderate
Friday:   High

MSLP maps show a progressive deepening low of 1000 to 996hpa off the coast of northwest WA by Friday.

This for Darwin  by JTWC:

TROPICAL DISTURBANCE SUMMARY:
(1) AN AREA OF CONVECTION HAS PERSISTED NEAR 11.9S 129.5E,
APPROXIMATELY 65 NM NORTHWEST OF DARWIN, AUSTRALIA. ANIMATED
INFRARED SATELLITE IMAGERY INDICATES A WEAK LOW-LEVEL CIRCULATION
CENTER (LLCC) WITH POORLY ORGANIZED DEEP CONVECTION FLARING NEAR
THE CENTER. THE DARWIN RADAR CLEARLY SHOWS CONVECTION ROTATING
AROUND THIS DEFINED CIRCULATION JUST WEST OF BATHURST ISLAND. SURFACE
OBSERVATIONS NEAR THE LLCC INDICATE SLP NEAR 1005-1006 MB. THE
251058Z SSMIS AND RECENT AMSU IMAGES DEPICT CONSOLIDATING BUT WEAK
DEEP CONVECTION. UPPER-LEVEL ANALYSIS INDICATES A DEVELOPING
ANTICYCLONE OVER THE LLCC WITH FAIR OUTFLOW ALOFT ENHANCED BY
TROUGHING TO THE SOUTH. MAXIMUM SUSTAINED SURFACE WINDS ARE
ESTIMATED AT 15 TO 20 KNOTS. MINIMUM SEA LEVEL PRESSURE IS ESTIMATED
TO BE NEAR 1005 MB. BASED ON THE LACK OF SIGNIFICANT CONVECTIVE
BANDING AS WELL AS THE POSITION NEAR LAND, THE POTENTIAL FOR THE
DEVELOPMENT OF A SIGNIFICANT TROPICAL CYCLONE WITHIN THE NEXT 24
HOURS IS POOR.

I'd like to see it move further north to gain some advantage of the warmer ocean temps and away from land.  Viewing the 7 day animation of the MLSP BoM maps might give an indication of how the monsoon trough is moving and the tropical charts don't even have it drawn on yet.  Checked out the wind profiles for new years day and there was some circulation shown on the GFS maps on WZ - but i'd like to see more stronger circulation!
Mike

[Carlos E]

In addition to the Tropical Low mentioned above, the Coral Sea, and the southcentral Indian Ocean have potential Cyclones by the weekend as well.

Sounds like an interesting way to end the year, or start the new one.

[Peter J]

I have noticed that the tropical low off WA coast has now been officially recognised by the BoM. and also has been highlighted on their global severe weather (cyclone/hurricane) page.

If we do get two cyclones (one in west, one in east), expect Melbourne to be very hot and dry in the forcast.

PJ

[Richary]

Just as a matter of curiosity - what is the definition on a cyclone as opposed to a low? When does it actually change status?

Is it wind speeds, or pressure? Or something else?

[Carlos E]

Hi Richary,

In our area, it's windspeed. Once a Tropical Low has sustained 10-minute winds of 35 knots (which equates to Gusts of 90km/h), it is assigned a name and called a Tropical Cyclone. Pressure does play a part, but with this system (which is 990 as I'm typing), it's pretty intense for a Tropical Low, and would be very close to forming to a Cyclone anyway. Once they do attain Cyclone strength, they can generally intensify faster due to the rotation.

EDIT - It might actually be 10-min winds of 40 knots, and not 35; I'm not 100% sure.

[Mike]

Added to that ....once the pressure falls below 1000hpa.  First it's a tropical low, then a cyclone.  So long as the BoM see circulation on a regular daily basis and organisation, increase of thunderstorms surrounding it then all that is taken into account when forecasting it on a three daily basis.  So long as an eye develops then it shows strong, deep circulation and maturity.  Best thing to do is keep an eye on the satellite animation loops and see how it all falls into place.  NASA, GOES loops are the pick.  you can go on the BoM site for all the information on cyclones at http://www.bom.gov.au/weather/cyclone/about/about-tropical-cyclones.shtml

 BoM NT are tipping for a probably low to form by Sunday or middle next week for the Gulf of Carpentaria.  Very early days and really depends on how much of the fuelling winds the cyclone on the west coast pinches from us.  If both cyclones form on either side of the country the NT will get a 'break' in the monsoon as all of its energy is being directed towards other events -

From what I've seen of the models the monsoon trough is going to go deep into the NT an afar southward whilst holding onto the low just below the GOC.  The trough will have to rise again back to open water for anything to spark a that low up.  It's happened before a few years back when the monsoon and a low got dragged to Katherine for over a week, flooded the town, moved back up with the trough and then formed a cyclone once over water again.

Mike

Mike

[Richary]

Thanks for the definitions. I knew there had to be a tipover point somewhere of when it changed from a low to a cyclone. Given the lack of activity forecast in Sydney for the next week I guess we will have to vicariously look on from a distance.

[Peter J]

Has anyone been tracking Cyclone Funa, as it has been a Cat4, and seems to have gone unnoticed?

Big Pete

[Richary]

I see the Bureau has issued a cyclone warning for Norfolk Island.

A CYCLONE WARNING IS IN FORCE FOR NORFOLK ISLAND
TROPICAL CYCLONE FUNA [950HPA] WAS CENTRED NEAR 29.7 SOUTH 172.2 EAST
AT 200200 UTC, OR ABOUT 450KM EAST OF NORFOLK ISLAND, AND MOVING
SOUTHWEST AT ABOUT 20 KNOTS.
ON THIS TRACK THE CYCLONE SHOULD PASS ABOUT 200KM SOUTHEAST OF
NORFOLK ISLAND TONIGHT.
GALE FORCE WINDS ARE EXPECTED FOR NORFOLK ISLAND FROM THIS EVENING
THROUGH TO MONDAY MORNING ALTHOUGH THE STRONGEST STORM FORECE WINDS SHOULD REMAIN SOUTHEAST OF THE ISLAND


Bit of a messy structure on the satpics though, with not much to the north and huge mass of rain from it swirling down towards the North Island of NZ.

Looking at the NZ Met service previously it was at 930 HPA so it has weakened a lot as it has moved south.