Over the past few days, first a tropical depression and later a cyclone have formed west of the Timor Sea. Ilsa – is the name of the tropical storm - will continue to strengthen over the next two days and then make landfall on the north coast of Western Australia.
Emergence and course so far
As early as the beginning of April, global weather models indicated a possible development of a tropical storm between Australia and Indonesia for mid-month. Understandably, forecasts over such a long time horizon are subject to large uncertainties. However, over the past few days this trend has manifested itself and finally a tropical depression formed on Easter Monday. This is the precursor of a tropical storm, which in that region is also called a cyclone. (Not to be confused with a cyclone, which in technical jargon describes an ordinary low. More information and differences between tropical and extratropical lows can be found in this blog).
As soon as such a system has reached the status of a tropical storm, it is designated by the respective responsible services. The current cyclone has been named Ilsa. Ilsa currently has average wind speeds of 130 to 150 km/h, making it a category 2 (out of 5) cyclone on the Saffir-Simpson scale (on the Australian scale for tropical storms, these winds already correspond to category 3 out of 5, although there are also a few differences in the general definition that are not explained further). It is currently located just under 500 kilometers northeast of Port Hedland and is moving southwest at about ten kilometers per hour.
Fig. 1: Further train path of Ilsa with the wind strengths to be expected in time; Source: Joint Typhoon Warning Center
Further course and development
The models agree on the further track of the storm. According to them, after the current southwest drift, it will first move south and finally southeast, where it will probably make landfall between Port Hedland and Broome on Friday (local time). That Ilsa will continue to strengthen over water along this path is certain, but the models do not yet agree on the intensity of the strengthening. This is mainly due to a slightly different track, which gives the storm more or less time to intensify. If one believes the latest model data of the American global model (GFS), then the cyclone should have a core pressure of 930 to 940 hPa at landfall. The European model (ECWMF) models a minimum pressure of about 960 hPa, but also an earlier landfall of several hours. An even more hasty but rather unlikely variant is shown by the German ICON model. According to this model, the cyclone would make landfall as early as Thursday evening with a center pressure of about 970 hPa. Depending on the scenario, average winds of between 160 and just under 200 km/h must therefore be expected at the coastal section in question.
Fig. 2: Wind speeds at 10 meters early Friday morning (local time) according to GFS.; Source: weathermodels
Reasons for the rapid strengthening
In order for a tropical storm to develop well and quickly, on the one hand the energy supply from the warm sea surface must be given, and on the other hand the winds at different altitudes should not vary too much from each other (wind direction and speed; wind shear). Ideally, the top layer of water should have a temperature of at least 26-27 degrees. As the following map impressively shows, values of partly more than 30 degrees are currently measured in the region. Likewise, the wind shear in the immediate vicinity of the storm center shows extremely low values (lower map).
Fig. 3: Surface temperatures of the water (reddish tones > 25 degrees, violet-pink > 30 degrees).; Source: Earth Nullschool (dargestellte Daten von UK Met Office)
Fig. 4: Wind shear in the troposphere according to ECMWF (the lower the values, the better the cyclone can develop).; Source: weathermodels
Dangers from Ilsa
In addition to the strong winds, tropical systems must also expect large amounts of precipitation. In a relatively narrow range, partly more than 200 mm of rain are expected in the coming days. This may not sound like much at first glance, but if you classify this according to the region, this corresponds to more than half of the typical annual precipitation in some areas. For example, in climatological terms, Port Hedland receives about 315 mm of precipitation per year (for comparison; in the Swiss Mittelland, the annual average is about 1000 mm, and in Switzerland between 1200 and 1300 mm). In this dry climate, such heavy precipitation cannot usually be absorbed well by the soils, as a result of which a large part of the water runs off above ground and can thus lead to flash floods.
Fig. 5: Expected rainfall totals in the next 60 hours according to GFS; Source: weathermodels
Another factor in the coastal region that should not be neglected is the high swell caused by the storm. As shown in the figure below, waves of 10 to 15 meters can be expected in the affected coastal section. In addition to coastal erosion, such rough seas also pose challenges for one of the world's largest export ports for bulk cargo in Port Hedland.
Fig. 6: Wave height shortly before landfall of Cyclone Ilsa according to GFS; Source: Tropical Tidbits
Climatology of tropical storms in the Australian Basin
The Australian Basin encompasses parts of the South Indian and South Pacific Oceans. Typically, this refers to the area between the 90th and 160th degrees of longitude east. The tropical storm season officially starts on November 1 and lasts until the end of April, although off-season storms can also form (then mostly south of Sumatra). Data have been recorded in detail since the 1969/70 season. During this period, an average of eleven tropical storms formed annually in Australian waters, with four making landfall each year. Since the turn of the millennium, a somewhat decreasing trend has been observed, with an average of only nine tropical storms per season. Ilsa is the sixth named cyclone this season. In February, Cyclone Freddy already made headlines, which among other things set a new global record as the most energetic tropical storm.
Fig. 7: Cyclone Freddy over the West Indian Ocean on February 12; Source: Wikipedia
