On July 24, a Fujiwhara interaction between Noru and Kulap started.
As a result, Kulap became directly located to the north of Noru early on July 25, and this situation allowed cold dry air to intrude along the western and southern periphery of the typhoon, making Noru weaken slightly with the significantly shallower convective core.
The primary steering influence for Noru was still a near equatorial ridge to the south, but another ridge was also building in to the east, turning Noru northeastward and then northward.
Although vertical wind shear relaxed somewhat with the robust eastward outflow channel on July 26, the reduction of mid-level relative humidity and slightly depressed SSTs at 26 °C to 27 °C tempered intensification and even eroded the convective structure.
Having finished a cyclonic loop and tracking west-northwestward along the southern periphery of a deep-layered ridge to the northeast, Noru showed some signs of transitioning to a hybrid system, with an upper-level low nearby and a thermal structure more indicative of a subtropical storm.
However, Noru reformed a 35 km (20 mi) diameter eye late on the same day, and while dry air was still entrenched across the southern portion of the typhoon, the extent of the dry air had shrunk and the moisture envelope was expanding, forming a hospitable cocoon for the system to reside in.

On July 27, Noru began to track westward along the southern periphery of a deep-layered ridge to the northeast.
The upper-level environment was extremely complex, with a tropical upper tropospheric trough (TUTT) cell having developed just to the southwest, another upper-level low to the southeast and an induced anticyclone just to the south and extending over Noru.
The aforementioned TUTT cell to the southwest was also providing a weak source of outflow.
Moreover, the combination of the complex upper-level outflow and low vertical wind shear was helping Noru to maintain intensity.
At around 00:00 UTC on July 28, Noru slightly weakened into a severe tropical storm when approaching the Bonin Islands.
Dry air completely surrounded the system with weak convection wrapping around a ragged eye; moreover, vertical wind shear was high, but the shear vector was in general alignment with the storm motion, with resultant relative shear values in the low to moderate range.
At the same time, the steering flow transitioned to the subtropical ridge centered over the East China Sea, causing Noru to track southwestward along the eastern boundary of this STR.
Despite warmer SSTs at 28 °C to 29 °C which supported intensification for Noru in the area, the complex upper-level environment with a TUTT cell to the southeast and a ridge far to the north combining limit outflow, while dry air continued enveloping the typhoon and isolating it from any significant moisture.
On July 24, a Fujiwhara interaction between Noru and Kulap started. As a result, Kulap became directly located to the north of Noru early on July 25, and this situation allowed cold dry air to intrude along the western and southern periphery of the typhoon, making Noru weaken slightly with the significantly shallower convective core. The primary steering influence for Noru was still a near equatorial ridge to the south, but another ridge was also building in to the east, turning Noru northeastward and then northward. Although vertical wind shear relaxed somewhat with the robust eastward outflow channel on July 26, the reduction of mid-level relative humidity and slightly depressed SSTs at 26 °C to 27 °C tempered intensification and even eroded the convective structure. Having finished a cyclonic loop and tracking west-northwestward along the southern periphery of a deep-layered ridge to the northeast, Noru showed some signs of transitioning to a hybrid system, with an upper-level low nearby and a thermal structure more indicative of a subtropical storm. However, Noru reformed a 35 km (20 mi) diameter eye late on the same day, and while dry air was still entrenched across the southern portion of the typhoon, the extent of the dry air had shrunk and the moisture envelope was expanding, forming a hospitable cocoon for the system to reside in.
On July 27, Noru began to track westward along the southern periphery of a deep-layered ridge to the northeast. The upper-level environment was extremely complex, with a tropical upper tropospheric trough (TUTT) cell having developed just to the southwest, another upper-level low to the southeast and an induced anticyclone just to the south and extending over Noru. The aforementioned TUTT cell to the southwest was also providing a weak source of outflow. Moreover, the combination of the complex upper-level outflow and low vertical wind shear was helping Noru to maintain intensity. At around 00:00 UTC on July 28, Noru slightly weakened into a severe tropical storm when approaching the Bonin Islands. Dry air completely surrounded the system with weak convection wrapping around a ragged eye; moreover, vertical wind shear was high, but the shear vector was in general alignment with the storm motion, with resultant relative shear values in the low to moderate range. At the same time, the steering flow transitioned to the subtropical ridge centered over the East China Sea, causing Noru to track southwestward along the eastern boundary of this STR. Despite warmer SSTs at 28 °C to 29 °C which supported intensification for Noru in the area, the complex upper-level environment with a TUTT cell to the southeast and a ridge far to the north combining limit outflow, while dry air continued enveloping the typhoon and isolating it from any significant moisture.
Typhoon Noru (2017)