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Tropical Storm Iselle and Big Island Rainfall Reports

By: Christopher C. Burt, 7:58 PM GMT on August 08, 2014

Tropical Storm Iselle and Big Island Rainfall Reports

Tropical Storm Iselle made landfall along the southeastern coast of Hawaii’s Big Island early Friday morning bringing some fairly strong winds and big waves. However, it was the potential of torrential rainfall and possible flash flooding that was of greatest concern. So far, this has not materialized in any significant way, at least for Hawai'i's Big Island.

Although the final reports are not yet available it would appear that the rainfall may not have been as dramatic as expected given the topography of the island and the slug of moisture that Iselle tossed upon it. So far (as of 8 a.m. HST on Friday August 8th), the top amount reported has been 14.51” at Kulani (elevation 5,050’). Below is map of some other rainfall reports from across the island (not final totals):



A sketchy map of some rainfall reports on the Big Island as of early Friday morning. Note the lack of precipitation along the western shores where the rain shadow of Mauna Kea and Mauna Loa holds sway. This map, of course, will be updated later today or Saturday. Map from The Weather Channel (as of about 4 a.m. HST).



Rainfall reports for the Big Island as of 8 a.m. HST. By this time the rainfall had, for the most part, ended along the eastern shores including Hilo where a total of 3.34” was reported: not really a substantial amount for this normally very wet location and where 27.24” once fell in 24 hours (on November 1-2, 2000). Table from NWS-Honolulu. You can follow this link for updated rainfall reports for all locations in the state of Hawaii.

The heaviest rains have occurred on the windward slopes of the volcanoes and, at sea level locations, along the southeastern coastline. This is, climatologically, the norm year around as the map below illustrates.



A rough map of the Big Island’s annual average precipitation. There are few gauges in the region of heaviest rainfall along the steep slopes of the mountains just west of Hilo. Map from Hawai’i-Guide.com.

According to the rainfall atlas produced by the Geography Department of the University of Hawaii-Manoa the wettest location on the Big Island is a site known as Makakanaloa 2 located at 2,675’ elevation (19.810°N, 155.191° W) 13 miles northwest of Hilo and where an average of 7613 mm (299.72”) of rainfall accumulates each year. The gauge here was operated under the supervision of the State Division of Forestry from 1934-1953. The driest location, according to the rain atlas, is the Mauna Kea Observatory (13,631’) with an annual average precipitation (which includes snowfall) of just 207 mm (8.15”). Amazingly, the summit site is only about 18 miles west of the wettest site mentioned above.

Christopher C. Burt
Weather Historian

Extreme Weather Precipitation Records Tropical Storms Mini Blog

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

The rainfall distribution is a bit like on Runion island with a wet side and a dryer side with totals between 400 mm on the northwest coastand 10000 mm in the southwest on the slopes of the Fournaise volcano.
Compared to a city like Hilo there are less days with rainfall : Gros Piton Sainte Rose has 3732 mm of rain per year and 231 rainy days.
What I don't understand on Hawaii is why the area designated as Captain Cook still gets a lot of rain though it's not on the windward side.
Chateau--The Kona coast gets onshore seabreezes that develop showers/thunderstorms during the summer months. It is the only area of Hawai'i that has a warm-season maximum. This happens because the Big Island is large enough for the mountainous interior to interfere with the trades during the afternoons.
Thanks, Chris. It's nice to know things could have been worse, but weren't...

On my inaugural visit to the Big Island, the first thing I wanted to see--ahead of Kilauea or Hilo or the observatory or anything else--was the demarcation line between the wet and the dry sides of the island, since I'd long been fascinated by the idea of that sudden a switch between climates. The easiest and most approachable way to do that was take a little drive to Waimea, then head northwest on 250 along the spine of Kohala. There's a long stretch of that highway that pretty much rides the demarcation line, with Nevada-like dryness to the southwest and Panama-like wetness to the northeast. You can literally walk from one extreme to the next in less than a minute in places, such is the starkness of the divide. Here's a screenshot from Google maps that'll give you some idea:

strange the number of places that reported 0. Still mind boggling how localized it can be. Guess the tall mountains there made a big difference?
I usually get 100-120 inches of rain a year in my part of Trinidad,but our airport gets 60-80 and im only 8 km away,nut on the hills :)
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