On January 25, 2003, a low-pressure area formed within a monsoon trough about 300 mi (485 km) northwest of Fiji and moved to the east-southeast.
That morning, the Joint Typhoon Warning Center (JTWC) began to issue warnings on the system, designating it as 12P. Shortly thereafter, Cilla turned southeast hours later in the general direction of Tonga  Early on January 26, RSMC Nadi designated the low as Tropical Depression 07F, after attaining 10-minute sustained winds of 35 mph (55 km/h).
At the time, the slow moving system had a poorly defined center of circulation that was hard to identify via radar and satellite imagery.
In addition, most of the deep thunderstorm activity was displaced to the north and southeast of the center.
Later that morning, the JTWC reported winds of 35 mph (55 km/h); however, the depression did not become any better organized throughout the day.
Early the next day, RSMC Nadi upgraded the tropical depression to a Category 1 tropical cyclone on the Australian intensity scale and named it Cilla.
By 0600 UTC January 27, the JTWC reported that Cilla had attained 1-minute sustained winds of 40 mph (65 km/h), which according to JTWC data, was its peak intensity.
Subsequently, Cilla turned east-southeast.

Throughout the day, wind shear conditions lessened further; however, shower activity separated from the center, prompting the JTWC to issue its last warning on Tropical Cyclone Cilla later that day.
At 2100 UTC January 27, RMSC Nadi remarked that the cyclone had become better organized.
Upon attaining peak intensity, the storm passed fairly closed to Tonga.
At 0600 UTC January 28, Cilla reached its peak intensity, with 10–minute sustained winds of 45 mph (70 km/h) per RMSC Nadi.
At this time, the tropical cyclone was located about 400 mi (645 km) south-southeast of Pago Pago.
Rapidly moving, Cilla showed baroclinic characteristics, hinting that the system was a hybrid low, sustaining characteristics of both tropical and nontropical cyclones.
According to RMSC Nadi, Cilla weakened slightly as the storm lost organization due to increased wind shear.

On January 29, thunderstorm activity once again increased in converge around the center, though at first, the convection was sheared at times.
On 0000 UTC, Dvorak satellite intensity estimates yielded a 3.0, suggesting a tropical cyclone with 10–minute winds of 45 mph (70 km/h), Cilla's secondary peak intensity.
Satellite images indicated a banding pattern associated with the cyclone.
The JTWC briefly watched this system for regeneration, noting it had a "fair" chance.
However, continued wind shear began to weaken Cilla, and by 1200 UTC on January 29, Cilla was reduced to a tropical depression just before the system turned south-southeast.
With the center exposed from the deep convection, Tropical Depression Cilla transitioned into an extratropical cyclone at 1100 UTC the next day, on January 30.
The extratropical cyclone completely dissipated two days later.
On January 25, 2003, a low-pressure area formed within a monsoon trough about 300 mi (485 km) northwest of Fiji and moved to the east-southeast. That morning, the Joint Typhoon Warning Center (JTWC) began to issue warnings on the system, designating it as 12P. Shortly thereafter, Cilla turned southeast hours later in the general direction of Tonga  Early on January 26, RSMC Nadi designated the low as Tropical Depression 07F, after attaining 10-minute sustained winds of 35 mph (55 km/h). At the time, the slow moving system had a poorly defined center of circulation that was hard to identify via radar and satellite imagery. In addition, most of the deep thunderstorm activity was displaced to the north and southeast of the center. Later that morning, the JTWC reported winds of 35 mph (55 km/h); however, the depression did not become any better organized throughout the day. Early the next day, RSMC Nadi upgraded the tropical depression to a Category 1 tropical cyclone on the Australian intensity scale and named it Cilla. By 0600 UTC January 27, the JTWC reported that Cilla had attained 1-minute sustained winds of 40 mph (65 km/h), which according to JTWC data, was its peak intensity. Subsequently, Cilla turned east-southeast.
Throughout the day, wind shear conditions lessened further; however, shower activity separated from the center, prompting the JTWC to issue its last warning on Tropical Cyclone Cilla later that day. At 2100 UTC January 27, RMSC Nadi remarked that the cyclone had become better organized. Upon attaining peak intensity, the storm passed fairly closed to Tonga. At 0600 UTC January 28, Cilla reached its peak intensity, with 10–minute sustained winds of 45 mph (70 km/h) per RMSC Nadi. At this time, the tropical cyclone was located about 400 mi (645 km) south-southeast of Pago Pago. Rapidly moving, Cilla showed baroclinic characteristics, hinting that the system was a hybrid low, sustaining characteristics of both tropical and nontropical cyclones. According to RMSC Nadi, Cilla weakened slightly as the storm lost organization due to increased wind shear.
On January 29, thunderstorm activity once again increased in converge around the center, though at first, the convection was sheared at times. On 0000 UTC, Dvorak satellite intensity estimates yielded a 3.0, suggesting a tropical cyclone with 10–minute winds of 45 mph (70 km/h), Cilla's secondary peak intensity. Satellite images indicated a banding pattern associated with the cyclone. The JTWC briefly watched this system for regeneration, noting it had a "fair" chance. However, continued wind shear began to weaken Cilla, and by 1200 UTC on January 29, Cilla was reduced to a tropical depression just before the system turned south-southeast. With the center exposed from the deep convection, Tropical Depression Cilla transitioned into an extratropical cyclone at 1100 UTC the next day, on January 30. The extratropical cyclone completely dissipated two days later.
Cyclone Cilla