Hurricane Lowell aims at Kauai as Hawaii's third storm in three weeks

Hurricane Lowell was a Category 3 system with sustained winds near 120 mph as it made its northward turn toward the western Hawaiian islands on Monday. The National Hurricane Center's track put the centre about 100 miles west of Niihau, or 150 miles west of Kauai, from late Monday night through Tuesday morning. Kauai County sat under a hurricane warning. A tropical storm warning covered the same islands. A flood watch ran across the state through Tuesday.

This is the third major storm to point at Hawaii in a little over three weeks. Governor Josh Green declared a state of emergency on 3 September, before Lowell's final turn, because Karina was still in the basin and Lowell was already a major hurricane. The state activated 129 National Guard members and sent six high-water vehicles to Kauai. Two state parks closed to campers.

Rain is the first threat that will arrive even if the eye stays offshore. Forecasts called for 6 to 10 inches on Kauai and Niihau, with isolated totals above 14 inches. The Big Island was told to expect 4 to 8 inches, with a few spots up to a foot. Steep terrain on Kauai turns those numbers into flash floods and slides. Oahu was looking at tropical-storm gusts of 30 to 40 mph and heavy rain from Monday afternoon.

Surf is the second threat and may be the one that lasts longer. South and west-facing shores were forecast to see a south-to-southwest swell building to warning levels of 20 to 30 feet at Kauai and Niihau by Monday evening. Oahu and the west-facing Big Island, including the Kohala coast, were in the same swell train. The pulse coincides with the month's high tides, which raises the odds of road overwash and harbour closures. East-facing shores still had 7 to 10 foot surf from the remains of Tropical Storm Karina.

Lowell reached Category 5 on 2 September, then fluctuated as dry air and shear nicked the eyewall. An upper-level low north of the islands pulled the storm north after a ridge to its north weakened. That turn is why Kauai County's watch was upgraded to a warning on 7 September. The Papahanaumokuakea Marine National Monument, the chain of atolls northwest of the main islands, is also inside the warning area.

Labor Day traffic and the storm's closest approach fall on the same window. That is an operational problem for the airports and for anyone who planned to leave Kauai on Monday night. The National Weather Service in Honolulu said tropical-storm-force winds could reach Kauai and Niihau late Monday and run through the night, with gusts up to 74 mph in the county. Those gusts are enough to drop trees onto two-lane roads that already flood.

El Nino has loaded the central Pacific this season. Named storms have a longer runway of warm water and less shear than a neutral year. Hawaii rarely takes a direct hit from a hurricane. It does take glancing blows that dump a foot of rain on one island and leave another under a high-surf warning for days. Lowell is built as that second kind of storm if the centre stays west of Niihau. A small eastward wobble would change the wind field on Kauai in a hurry. That is why the warning is up even though the official track keeps the eye over water.

Green's emergency order and the Guard deployment are the state's way of moving assets before the roads close. Once the swell hits 20 feet, small boat harbours on the west sides become unusable. Once the rain crosses six inches in the mountains, the same valleys that flooded in recent storms will flood again. The question for Tuesday morning is not whether Lowell remains a major hurricane at the moment of closest approach. It is whether Kauai's drains and the coastal roads hold through a night of both rain and surf.

Residents on Niihau and the west side of Kauai have the shortest clock. The rest of the chain has a longer rain-and-surf clock that runs into Tuesday. The third storm in three weeks does not need a landfalling eye to earn its place in the season's record. It needs only to put 14 inches of rain on one island and 25-foot faces on another. That is the forecast Lowell carried into Monday.