Tropical weather analysis - February 25, 2013

By: KoritheMan , 8:34 AM GMT on February 25, 2013

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Tropical Cyclone Rusty

Tropical Cyclone Rusty is taking aim on the northwestern Australia coast. As of the most recent advisory from the Bureau of Meteorology, the following information was available on the cyclone:

Wind gusts: 140 kmh
Location: 17.7°S 119.0°E
Movement: Near stationary
Pressure: 970 mb
Category: 2 (Australian scale)

Rusty looks about the same as it did six hours ago. Convection is trying to wrap around a cloud-free area that is not yet deserving of being called an eye. Recent satellite and microwave fixes suggest that Rusty has moved very little over the last few hours, save perhaps a small nudge toward the east-southeast. CIMSS total precipitable water data, along with the actual satellite signature, suggest that Rusty may have entrained some dry continental air from northwestern Australia.



Figure 1. Latest infrared satellite image of Tropical Cyclone Rusty. Image credit: NOAA Satellite Services Division (SSD).

The intensity forecast is rather tricky, and depends almost entirely on the evolution of the cyclone circulation. Rusty is an unusually large tropical cyclone, and experience has shown that these cyclones tend to respond rather slowly to seemingly favorable atmospheric dynamics. Given this, and my positive bias for Hurricane Isaac last year, I will choose to err on the side of caution, forecasting a somewhat weaker cyclone than the JTWC in their most recent (6z) forecast for the system, where they are showing a Category 3 (SSHS). While this type of intensification is possible if Rusty can develop an inner core, I would like to see some more evidence of such before I go along with such a forecast. One positive for strengthening is that the most recent AMSUB image shows a somewhat better defined cyclone signature, with the cloud free spot alluded to showing somewhat better definition, perhaps the beginnings of an inner core. There is no obvious reason why Rusty should not strengthen up to landfall, as there is little vertical shear and there is plenty of high oceanic content in the region, even toward the coast.



Figure 2. Southeast Indian Ocean Tropical Cyclone Heat Potential (TCHP) as of February 23, 2013. Values of 80 or over are considered to be conducive for rapid intensification. Image credit: AOML NOAA.

Following the typical progression with landfalling tropical cyclones, Rusty should weaken rapidly after landfall, although the inland decay rate will probably be a little minimized with this storm relative to an average tropical cyclone given the enormity of the storm circulation.

5-day intensity forecast

INITIAL 02/25 0600Z 60 KT 70 MPH
12 hour 02/25 1800Z 65 KT 75 MPH
24 hour 02/26 0600Z 70 KT 80 MPH
36 hour 02/26 1800Z 85 KT 100 MPH
48 hour 02/27 0000Z 95 KT 110 MPH...AT THE COAST
72 hour 02/28 0000Z 70 KT 80 MPH...INLAND
96 hour 03/01 0000Z 25 KT 30 MPH...INLAND POST-TROPICAL/REMNANT LOW
120 hour 03/02 0000Z...DISSIPATED

Note that the forecast intensities given above are based on the Saffir-Simpson Hurricane Scale, which utilizes maximum 1-minute sustained wind speeds. The Australian scale is quite different, which bases the severity category on maximum wind gusts. Initial wind speed is based off JTWC.

Also, the times provided within that table are based on Eastern Standard Time.

Since Rusty is a slow mover, determining when and where the system will make landfall is a little muddled. In general, the most reliable guidance suggests that it will strike a little to the east of Port Hedland, but given the erratic nature of the track, this is uncertain. In fact, the cyclone warning area covers quite a large stretch of coastline, effectively detailing the uncertainty associated with this storm. Perhaps the greater challenge is predicting the timing of landfall. The GFS and ECMWF, which have historically been very reliable, show landfall just after 48 hours, and that's what I will go with.

CIMSS steering data and upper-level wind analyses show that a weakness trails between a developing tropical low to the west and Rusty. This sort of synoptic evolution tends to favor erratic motion, and Rusty has been behaving in precisely that fashion. The cyclone is still forecast to begin a more definitive southward movement later today. Little deviation from that general motion is expected up to landfall, with the possibility of a more southwestward movement by day three and four. Virtually all of the global models lose the circulation by 120 hours as Rusty moves well inland.

Interests along the coast within the warning area should begin their cyclone preparations, as Rusty's large circulation will begin to impart rainbands over coastal areas later today as the synoptic steering increases and drives the system southward. Interests within the cyclone watch area should carefully monitor the progress of the tropical cyclone in case any significant track deviation occurs. Unfortunately, I haven't been able to find a decent geographical map to draw an actual track forecast on, so if you guys know how to find one, feel free to elucidate.

Given its slow motion, Rusty poses a heavy rainfall/flash flood threat. An earlier TMI image showed rainfall rates up to 1 inch per hour to the north of the center over water. Rainfall could reach 10 to 20 inches, with localized areas of 25 to 30 inches. I am seeing a similar flood situation that occurred with Hurricane Isaac in southeast Louisiana last year.


TROPICAL CYCLONE ADVICE NUMBER 15
Issued at 2:43 pm WST on Monday 25 February 2013

A Cyclone WARNING is current for coastal areas from Cape Leveque to Mardie.
A Cyclone WATCH is current for coastal areas for adjacent inland areas of the
Pilbara including Marble Bar, Nullagine and Millstream

At 2:00 pm WST Tropical Cyclone Rusty, Category 2 was estimated to be
295 kilometres north of Port Hedland and
345 kilometres west of Broome and
was near stationary.

Tropical Cyclone Rusty is moving slowly towards the coast. It has recently
moved slowly to the east southeast but it is likely to resume a more southward
track later this afternoon or this evening.

Gales are expected to develop on the coast between Wallal and Whim Creek during
this afternoon, possibly extending north towards Broome this evening or
overnight. During Tuesday afternoon gales could extend west to Karratha and
begin to extend inland towards Marble Bar and Millstream.

Further intensification is likely as the cyclone approaches the coast. There is
a HIGH RISK THAT RUSTY WILL CROSS THE COAST AS A SEVERE TROPICAL CYCLONE.
However, the slow motion of the cyclone means that the crossing time and
location is uncertain.

Rusty is a large tropical cyclone and its slow movement is likely to result in
rainfall that is heavier than that associated with a typical tropical cyclone.
Very heavy rainfall is expected in near coastal parts of the eastern Pilbara
and western Kimberley over the next few days. During Tuesday and Wednesday
widespread very heavy rainfall is likely to lead to MAJOR FLOODING in the De
Grey catchment. Significant flooding is also likely in the Fortescue catchment
and in Pilbara coastal streams.

Rusty's intensity, size and slow movement is also likely to lead to a VERY
DANGEROUS STORM TIDE as the cyclone centre nears the coast. Tides are likely
to rise significantly above the normal high tide mark with DAMAGING WAVES and
VERY DANGEROUS COASTAL INNUNDATION.

DFES State Emergency Service (SES) advises of the following community alerts:
BLUE ALERT: People in communities between Cape Leveque and Mardie, including
Port Hedland and Karratha and extending to adjacent inland areas including
Marble Bar and Millstream, need to prepare for cyclonic weather and organise an
emergency kit including first aid kit, torch, portable radio, spare batteries,
food and water.

Details of Tropical Cyclone Rusty at 2:00 pm WST:
.Centre located near...... 17.7 degrees South 119.0 degrees East
.Location accuracy........ within 55 kilometres
.Recent movement.......... near stationary
.Wind gusts near centre... 140 kilometres per hour
.Severity category........ 2
.Central pressure......... 970 hectoPascals

The next advice will be issued by 6:00 pm WST Monday 25 February.

Cyclone advices and State Emergency Service Community Alerts are available by
dialling 1300 659 210

A map showing the track of the cyclone is available at:
http://www.bom.gov.au/cyclone

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About KoritheMan

I'm just a 23 year old with an ardent passion for weather. I first became aware of this interest after Tropical Storm Isidore struck my area in 2002.

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