'Thunder trades' explained: A deeper look into the weekend storms

Updated: Apr. 16, 2018 at 8:22 PM HST
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HAWAII (HawaiiNewsNow) - The torrential rain this weekend was due to a wave of thunderstorms within Hawaii's trade wind flow — and not from one individual big storm.

Thunderstorms are a different animal when you take a deeper look at their convective nature. They are on a much smaller scale and they pop up individually when elements are aligned.

Weather models have a hard time projecting the intensity of individual thunderstorms compared to hurricanes, a cold front, or a Pacific storm. These storms are much smaller — they're usually several-miles wide compared to several hundred.

Ingredients have to come together along with other factors for these storms to produce record rain like we saw over the weekend.

From Friday to Sunday, there was cold air aloft from an upper level disturbance to the northwest, deep tropical moisture to the southeast, and instability within the trade wind flow.

It is hard to know the track or intensity of these individual cells especially since Hawaii is surrounded by a vast ocean. Many times, these ingredients can be present, but the development may stay over the ocean and not impact the islands.

Over the weekend, it all came together directly over the islands with the most intense rain impacting windward neighborhoods due to the wind flow. The trade winds acted like a conveyor belt — bringing a steady source of moisture Hawaii's way.

Then on top of that, the island terrain played a role in slowing these individual thunderstorms down, almost acting like a speed bump. There were about three back-to-back individual thunderstorms that developed over the north shore of Kauai.

These were associated with strong updrafts that were anchored to the terrain and the torrential rain was stationary for hours.

Hawaii streams and drainage systems cannot handle rainfall rates of three to five feet per hour.

Sadly, this led to the record rainfall seen on both Kauai and Oahu.

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