The Observatory said the No 8 signal is unlikely unless Cempaka moves closer or strengthens.
Cempaka is expected to move in a circle over the next few days in a distance beyond 100 kilometers from Hong Kong - a threshold for triggering signal no 8.
The observatory in Macau, which is closer to the storm, said it is likely to hoist the higher signal.
It was centered 190 kilometers southwest and moving at 41 to 62 kilometers per hour at 7pm.
"Winds over the estuary are expected to strengthen [this morning]," Hong Kong Observatory's scientific officer Tong Yu-fai said.
Cempaka is forecast to move north slowly across western Guangdong midweek, despite its direction remaining uncertain.
This came as tropical cyclone In-fa moves east of Taiwan toward southeast China.
In-fa is centered around 1,787 kilometers east of here, and moving at 90kmh at 2 pm yesterday.
If In-fa and Cempaka move close to each other, a phenomenon called the "Fujiwhara effect" could happen.
But Tong said that is unlikely.
"They are almost 1,900 kilometers apart for now, and therefore their circulations do not meet each other'," he said.
"Cempaka's movement remains slow due to its weak steering flow, and there is great volatility in its future movement ... In-fa's movement is much bigger in comparison."
Early yesterday, Hong Kong saw the amber rainstorm warning signal being issued twice - at 12.30am and 2.35am, with more than 30 millimeters of rainfall recorded over parts of the territory.
A tree fell on a car in Kowloon Tong early yesterday.
Occasional showers with thunderstorms and squalls are expected today and tomorrow.
Hotter, sunnier and wetter. Britain's weather in the last 30 years was different to the preceding three decades.