'slow intensification' is a correct phrase and can be used in written English.
It is used to describe a gradual and steady increase or strengthening of something. Example: The slow intensification of the storm's winds caused widespread damage to the coast. In this sentence, 'slow intensification' is used to describe the gradual increase in the strength of the storm's winds.
The depression initially tracked west to west-southwestward with slow intensification, due to vertical wind shear.
Zelda continued its slow intensification rate, and as it turned more westward, it became a typhoon on the 1st.
It began a slow intensification trend, and peaked as a high-end tropical storm on 0600 UTC September 18.
Slow intensification continued, with Nadine peaking with winds of 90 mph (150 km/h) and a minimum barometric pressure of 978 mbar (28.9 inHg) on September 30.
Here, through the slow intensification of performance, and then also through a very subtle use of the camera: use of lenses, use of lighting...not trying to avoid the claustrophobia, but trying to take advantage of it.
Slow intensification ensued as Danielle headed generally westward and eventually became the second hurricane of the season on August 23, with hurricane-force winds extending outward for a 10 mile radius.
It strengthened into Tropical Storm Kirk on the following day, but persistent wind shear slowed intensification.
After a brief period of slower intensification, Hondo rapidly intensified into an intense tropical cyclone and reached its peak intensity with winds of 215 km/h (130 mph ) on February 7.
After the system passed between the eastern Bahamas and Turks and Caicos Islands on September 18, slow re-intensification occurred.
After becoming an intense tropical cyclone, wind shear began to increase, causing the eye to become slightly disorganized, temporarily slowed the intensification.
Gabrielle then strengthened slowly over the warm sea surface temperatures in a low wind shear environment, although land interaction slowed the intensification somewhat.
