Assuming a west-northwest course that it would maintain throughout its existence, generally favorable atmospheric conditions allowed for gradual strengthening, with the cyclone attaining hurricane status a day after formation.

Continued strengthening progressed for several days up until August 4, when Iselle reached peak intensity with maximum sustained winds of 140 mph (220 km/h) and a minimum barometric pressure of 947 mbar (hPa; 27.96 inHg), making it a Category 4 hurricane.

Thereafter, Iselle encountered hostile environmental conditions and quickly weakened before making landfall on the Big Island on August 8 as a moderate tropical storm.

Upon making landfall, Iselle brought torrential rainfall and caused strong winds which resulted in widespread power outages and downed trees.

[7] On August 1, Iselle began to develop an eye,[8] and this progressive pattern of organization prompted the NHC to upgrade the storm to hurricane status at 00:00 UTC the following day.

[4] Although favorable conditions allowed for the uninhibited growth of Iselle early in its development,[9] moderate wind shear briefly impacted the hurricane on August 2, preventing thunderstorms from intensifying and causing the storm to become slightly malformed vertically.

[4][10] However, these conditions abated shortly afterwards, allowing Iselle to resume strengthening and reach Category 2 hurricane status late that same day.

[4] Consequently, Iselle reached peak intensity at 18:00 UTC with maximum sustained winds of 140 mph (220 km/h) and a minimum barometric pressure of 947 mbar (hPa; 27.96 inHg).

[4] Although wind shear began to decrease thereafter, abundant dry air caused Iselle to rapidly weaken, dropping to Category 1 intensity on August 6.

[4] On August 7, the upper-level trough to the north caused an increase in wind shear, the storm's eye had faded and the CPHC estimated that Iselle had weakened to a minimal hurricane.

[29] In anticipation for the potential impacts of both Iselle and Hurricane Julio, Hawaii governor Neil Abercrombie signed an emergency proclamation, thereby allocating resources to future relief and mitigation operations.

[34] The United States Coast Guard urged boaters and mariners to exercise caution as waves were expected to reach 10–15 ft (3.0–4.6 m) in some offshore locations.

[33] Tropical storm-force winds affected much of the state as Iselle moved through, except Niihau, the rest of the Hawaiian Islands all reporting gusts over 39 mph (63 km/h).

[57] A geothermal plant in Puna released toxic hydrogen sulfide gases upon losing its power sources, prompting officials to urge an exodus of the immediate area,[58] although a number of nearby residents reported health impacts.

However, the storm damaged roads leading to two polling stations on the Big Island, prompting officials to decide the election by absentee ballot in those areas.

[71] After the storm, Hawaiian Electric Industries worked to restore power across the Big Island, aided by trucks removing trees and debris from roads.

Due to the damage, Congresswoman Tulsi Gabbard remarked, "I'm urging FEMA, Hawaii County, the governor and President Obama to expedite the process of declaring Iselle's devastation a natural disaster so that the people of Puna can receive immediate help.

"[76] On August 25, Governor Abercrombie formally requested for federal assistance, after the state depleted its disaster fund,[66] but that was rejected by FEMA due to insufficient damage.

[77] The FEMA decision was reversed on September 12, when the federal government approved aid for rebuilding public buildings, as well as future disaster mitigation.

[79] With estimated damage cost of $148 to 325 million (2014 USD), Iselle became the third-costliest tropical cyclone to ever hit the U.S. state of Hawaii, even after accounting for inflation.

[80][81] In addition, Iselle was the third-strongest tropical cyclone to ever make landfall on the main Hawaiian islands after hurricanes Dot and Iniki.