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New Wettest Location for the U.S.A. Discovered?

By: Christopher C. Burt, 10:32 PM GMT on May 15, 2012

New Wettest Location for the U.S.A. Discovered?

A team of scientists from the University of Hawaii-Manoa Geography Department has recently published a rainfall atlas to the state and may have discovered a new ‘wettest spot’ in the islands and thus for the entire United States. In 1992 they set up a rain gauge at a location known as the ‘Big Bog’ on the edge of Haleakala National Park on Maui Island. They had originally estimated the rainfall at this site to be on the order of 180” per annum, but the second year of data (1994) saw an amazing 560” of precipitation fall. The 30-year (1978-2007) average annual rainfall was 404" at Big Bog (estimated from 1992-2007 data) vs. 393" at Mt. Waialeale.

Mt. Waialeale

A photograph of the eastern wall of Mt. Waialeale, a cliff known as the ‘Big Hole’ and the location where amazing orographics lead to the extraordinary rainfall totals measured near the mountain's summit. Photo by Paul Chesley.

For years the acknowledged wettest location in the U.S. (and one of the wettest in the world) has been a site located at 5,150’ near the summit of Mt. Waialeale on Hawaii’s Kauai Island. Precipitation records began here in 1912 when a ‘super’ 900-inch capacity rain gauge was installed.

A 900-inch rain gauge is prepared for transport to Mt. Waialeale’s summit in this 1928 photograph. U.S. Geological Survey collection.

A schematic of the rain gauge the USGS currently maintains on Mt. Waialeale. Graphic from USGS.

The rainfall records at the site have been more or less continuously monitored since 1912 with the exception of 1958-1963 (with annual inspections made until 1931 when monthly inspections began). Access to the site used to involve a grueling mule-back journey to the summit until helicopters came in to use in the 1950s.

Near the summit of Mt. Waialeale, where the rain gauge is located, there is a small pond but no trees. The persistent cloud cover and rainfall inhibits their growth. Photo by Steve Perlman for the Honolulu Star-Bulletin.

The reason for the amazing precipitation totals at both Mt. Waialeale and Big Bog are the persistent easterly trade winds that bring moisture from the Pacific up against the steep mountainsides of the Hawaiian Islands creating orographic lift below an inversion layer that normally exists at around 7,000’. This is why the higher peaks of Hawaii, like Mauna Loa (13,679’) and Mauna Kea (13,796’) on the Big Island are among the driest locations (just 8.4” precipitation per annum) in the state.

This graphic illustrates the climatic scenario common to the Hawaiian Islands where trade winds are orographically lifted producing persistent rainfall in mountainous zones between 3,000-6,000’. An inversion layer around 7,000’ caps the cloud tops. Graphic from ‘The Rainfall Atlas of Hawaii’, 2011 edition, University of Hawaii-Manoa Geography Department (see link at end of blog).

The actual annual average amount of precipitation that accumulates at the Mt. Waialeale site is open to various interpretations. For the entire POR (1912-2011—excluding 1958-1963) it is 423.13”. For the most recent 30-year POR (1981-2010) it is just 384.35”. The previous 30-year POR (1971-2000) saw an average of 399.85”. Many sources, including my book, Extreme Weather: A Guide and Record Book, and the Guinness Book of World Records, use a figure of 450-460”/year as the average. This figure was based on an earlier, wetter, period between 1938-1957. The peak-wet period would be 1921-1950 when the average was 471.67”. The single wettest year was 1982 at Mt. Waialeale (thanks in part to Hurricane Iwa) with 682.94” and the driest in 1926 with 218.00”.

Table of annual rainfall at Mt. Waialeale 1912-2011 with cumulative average annual rainfall rates for various periods of record overlaid. USGS graphic.

In any case, there are in reality only two figures which should be noted so far as the average annual precipitation at Mt. Waialeale is concerned:

1) the latest official 30-year POR of 1981-2010 with 384.35”

2) the entire POR from 1912-2011 with 423.13”

Other citations, such as the commonly used 460”/annum figure, are ‘cherry picking’ the data.

Rainfall map of Kauai illustrating the tremendous range of annual average precipitation. This map was produced for my book ‘Extreme Weather: A Guide and Record Book’ and erroneously uses the 460”/annum figure for Mt. Waialeale. Map by Mark Stroud.

Change in Precipitation Patterns in Hawaii

There has been a marked decrease in precipitation at the location since 1990 (the 2nd driest year on record was 1993 with 244.36”). The reason for this is complex. Here is what the authors of the Rainfall Atlas of Hawaii say on the subject:

“The maps comprising the 2011 Rainfall Atlas of Hawai’i depict average rainfall for the 30-year period ending in 2007. This gives an up-to-date picture of normal rainfall amounts and patterns. But, we must be aware that rainfall varies over time. For example, we have strong evidence that rainfall in Hawai’i is affected on a year-to-year time scale by the occurrence of El Niños and La Niñas. These events are part of a large-scale interaction between the ocean and atmosphere centered in the equatorial Pacific, known as El Niño-Southern Oscillation (ENSO). In particular, El Niño is consistently associated with lower than normal rainfall during winter months in Hawaiʻi. Because Hawaiʻi is relatively close to the center of action of ENSO, its effects are strong here. El Niños and La Niñas recur on average about every 3 to 7 years. This gives rise to large year-to-year variability in rainfall in Hawaiʻi.

Another, perhaps less familiar ocean-atmosphere interaction, known as the Pacific Decadal Oscillation (PDO), also exerts a strong influence on Hawaiʻi rainfall. The PDO is somewhat similar to ENSO, but varies much more slowly, with each phase lasting up to 30 years. During most of the base period for the 2011 Hawaiʻi Rainfall Atlas of Hawaiʻi (1978-2007), the PDO was in its positive phase, which is generally associated with lower rainfall in Hawaiʻi.

In addition to natural variations in rainfall, we are now aware of long-term trends that might be caused by global warming. Over the past 90-100 years, while the effects of ENSO and PDO caused large ups and downs, rainfall in Hawaiʻi has slowly declined overall. This decline has been especially apparent during recent decades, in part, because it coincides with the low rainfall phase of the PDO. However, the rainfall record and other evidence point to a downward trend in mean rainfall that may persist at least through the end of this century. One possible explanation for the decline has to do with the weather disturbances that regularly disrupt the trade wind inversion and produce widespread rainfall over the islands. These disturbances, often associated with incursions of mid-latitude weather systems into the Hawaiʻi region during winter, have declined in frequency, as storm tracks have apparently migrated northward. This shift in storm tracks is thought to be a result of global warming and is predicted to continue. If these predictions are borne out, we will continue to see ups and downs in rainfall in the future related to ENSO and PDO, but mean rainfall will decline, and drought will become more frequent.”

Big Bog

Big Bog is located at 5,440’ on the border of Haleakala National Park and the Hana Forest Reserve on the eastern side of Maui Island overlooking the town of Hana.

Map of Maui with inset showing location of Big Bog on the border of Haleakala National Park. National Park Service.

A team from the Geography Department of the University of Hawaii-Manoa led by Tom Giambelluca set up a rain gauge here in 1992 as part of a network of instruments (the HaleNet network) used for data the recent edition of Rainfall Atlas of Hawaii (see link at end of blog). In 1993, the first full year of operation, the gauge at Big Bog measured 338.90” of rainfall. This is particularly astonishing since 1993 was one of the drier years for the Hawaiian islands, in fact it was the 2nd driest year on record for Mt. Waialeale which picked up only 244.36”. The next year, 1994 saw 560.3” at Big Bog compared to 489.44” at Mt. Waialeale, the wettest year for Mt. Waialeale since 1990 and 2nd wettest since the big year of 1982 (and to 2011 still such).

Mean annual rainfall map of Maui and surrounding islands. Big Bog is located near the center of the maximum precipitation region on the eastern portion of the island. From ‘The Rainfall Atlas of Hawaii’, Dept. of Geography, Univ. of Hawai’I at Manoa.

The scientists have now estimated that the 30-year mean precipitation at Big Bog for the POR of 1978-2007 is 404.4”. The actual measured mean for the same period at Mt. Waialeale was 393.3”. Although there have been some months of missing data since 1992, the team has made careful estimates to fill the gaps based upon the group of nearby gauges in the HaleNet network and other statistical methods. The details of how this was done may be read in their atlas (link at end of blog).

A map of the HaleNet (Haleakala Climate Network) rain gauge network in eastern Maui. Big Bog has an SKN (state identifier number) of 276 and a HaleNet number of 164 which can be identified on this map. Image produced by Max Chen for Univ. of Hawai’i-Manoa Geography Department.

Afternoon sun shines over Haleakala Crater on Maui. The Trade Wind Inversion caps clouds at a lower elevation. The photo was taken at an elevation of 2470m (about 8,100’) from HaleNet site #161 (see above map), the Pohaku Palaha Station. Photo by rainfall atlas team leader Tom Giambelluca.

Comparing Big Bog to Mt. Waialeale

For the specific 30-year POR (1978-2007) it would appear that Big Bog is wetter than Mt. Waialeale. However, 15 of the 30 years of precipitation measurements at Big Bog are estimated so it will not be until 2023 that we will have an actually observed 30-year POR. Nevertheless, at least for the past 18 years that we do have records for, Big Bog does seem to be more or less consistently wetter than Mt. Waialeale. Thus one can conclude with a degree of certainty that this is indeed the new wettest location in the United States.

Of course, there could be even wetter spots somewhere in the Hawaiian Islands and there are, for certain, wetter spots at other locations around the world. That will be the subject of my next blog.

KUDOS: Special thanks to the authors of the Rain Atlas of Hawaii at the Univ. of Hawaii-Manoa: “Giambelluca TW, Chen Q, Frazier AG, Price JP, Chen Y-L, Chu P-S, Eischeid J., and Delparte, D. 2011. The Rainfall Atlas of Hawai‘i.”

Also thanks to Jan Null, Certified Consulting Meteorologist at Golden Gate Weather Services, for bringing the Big Bog story to my attention.

Christopher C. Burt
Weather Historian

Precipitation Records

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