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Unusually Strong Hawaii Storm This Weekend May Bring Damaging Winds, Unprecedented Coastal Flooding, NWS Says
Published: February 10, 2019
An unusually intense storm will hammer Hawaii on Sunday with widespread high winds, potentially damaging coastal flooding in unusual locations of the island chain and even some mountaintop snow in at least one unusual location.
Hawaii's typically tranquil weather is usually dominated by subtropical high pressure and northeasterly trade winds, providing plenty of sunshine downwind of the islands' volcanic peaks.
From roughly October through April, though, Hawaii can be impacted by frontal systems known locally as kona storms.
These storms approach the islands from the northwest or west, with moisture-laden south to southwesterly winds, the opposite of the typical trade winds. Kona storms typically soak most of the islands, including those areas to the west and southwest of the volcanic peaks that are usually dry when the normal trade winds blow.
This weekend's storm is a strange one – not quite a classic kona storm, although the effects will be the same.
Instead of approaching from the west or northwest, this storm will plunge from south of the Aleutian Islands to near the Hawaiian Island chain this weekend.
Resembling a cresting wave, strong high pressure building north of the storm will force it to track due south, setting up a blocking pattern known as a Rex block.
The surface low will quickly intensify into the 980s millibars, a central pressure on par with some typical intense northeastern U.S. coastal storms known as nor'easters.
The upper-level core of this storm may be among the coldest for this part of the Central Pacific in the last 40 years, according to an analysis from Tomer Burg, an atmospheric science graduate student at SUNY Albany.
Forecast Impacts Through Sunday Night
Given the intensity of this storm, high winds won't simply be pinned to the highest elevations.
According to the National Weather Service, strong, possibly damaging north to northwesterly winds are expected this weekend not simply over the higher volcanic peaks, but also downwind of the mountains, potentially including parts of the Honolulu metro area and any areas particularly exposed to north to northwesterly winds. One rough analog to this is the downslope windstorms that occur along the Front Range of the Rockies from Montana to Wyoming and Colorado.
Current Radar and Winds
These winds should be strongest Sunday into Sunday night and are capable of downing trees, knocking out power, and, if strong enough, could lead to some structural damage over much of the island chain.
The highest winds will be on the highest volcanic peaks on the Big Island, where winds could gust to 120 mph Sunday into Sunday night.
Parts of Oahu and Kauai have seen wind gusts of 40 to 50 mph since Saturday.
Coastal Flood, High Surf
The north shores of Hawaii are no stranger to high surf generated from distant North Pacific storms in the winter months.
However, swells generated by this intensifying storm this weekend will produce "giant, disorganized waves" and could lead to "unprecedented coastal flooding" through Sunday along exposed north- and west-facing shores," according to a high surf warning issued by the National Weather Service in Honolulu.
NWS-Honolulu also warned of "extreme harbor surges" along north- and west-facing shores of Kauai, Oahu and Molokai and north-facing shores of Maui this weekend, stressing the potential for "significant damage to coastal property and infrastructure," with "evacuations and road closures possible."
Offshore wave heights north of the islands could rise to 60 feet by Sunday, according to the NWS – a danger to ships.
Other Potential Strange Impacts
There are other potential aspects to this unusual storm.
Given the cold nature of the storm, snow could fall not only on the typical Big Island summits of Mauna Kea and Mauna Loa, but potentially atop Haleakala, a broad shield volcano on Maui.
Low temperatures in Honolulu may dip into the upper 50s Monday. That may not sound chilly to most of you, but Honolulu International Airport has only dipped into the 50s 22 times this century, last occurring almost exactly three years ago.
That deep cold air aloft could also fuel some bands of thunderstorms over the islands Sunday into Monday, if the atmosphere is moist enough.
Those thunderstorms could produce brief strong wind gusts, perhaps even hail.
A supercell thunderstorm on March 9, 2012, produced hail over 4 inches in diameter on Oahu, a state record.
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