The Wettest Place in the U.S.

Last week I traveled to an island where more precipitation falls each year than any other location in the U.S.  In fact, some people believe it is the wettest place on earth.

The location?   The Hawaiian island of Kauai.  Upon Kauai lies Mt. Waiʻaleʻale, which reaches 5148 ft and where the annual precipitation is roughly 460 inches a year.  An amazing total that eclipses our Olympic Peninsula, the wettest location in the lower 48 states where a measly 160-180 inches a year falls on the windward slopes.

Mount Waialeale is usually engulfed in clouds!

Kauai is a roughly circular island at 22N that is usually located in the northeast trade winds (these winds are from the northeast).  Roughly 30 by 30 miles, it has high terrain at its center that descends abruptly to the northwest (Na Pali) coast, and more gradually to the east and southeast (see map).

As moist subtropical/tropical air ascends the eastern/northeastern slopes it is cooled to saturation, resulting in clouds and persistent precipitation on the windward slopes and over the crest.   In contrast, precipitation declines rapidly on the lee side away from the incoming wind (see precipitation map).  So on eastern side from Lihue (southeast side, the biggest town) to Princeville (on the northern side) 40-100 inches a year is observed.   In contrast, the southern side from Poipu to Barking Sands Navy base is much drier, with some locations getting less than 25 inches.  Barking Sands is the venue for top secret military activities.

If you want a sunny vacation, then Poipu area is a good bet, but if you enjoy a moist tropical environment the Lihue to Princeville stretch is for you.   The profound difference between the windward and leeward sides of Kauai (and most Hawaiian islands) is illustrated by some satellite imagery.  Here is an example from a google satellite image--clearly the winds were from the east.

When the air is less stable, the lifting causes stronger cumulus and cumulonimbus clouds to form and the windward side and mountain crests become engulfed in clouds.  Here is an example of such conditions on March 22nd, when I was trying to enjoy some sun.  As a meteorologist, I retreated to the sunny southwest side of the island to a Kauai coffee plantation.  You have to do what you have to do.

Now, trade winds from the east to northeast don't always blow, particularly during the winter and early spring when weather disturbances can move past the islands to the north.
In fact, on Saturday the remains of a front moved through Kauai and the winds switched to the north.   The result?  The clouds moved more to the northern side of the island...here is the associated satellite image (from today, April 2).  See the difference?

There is a National Weather Service radar on Kauai, but for some reason it is located on the southern, dry side of the island and thus unable to view approaching trade wind showers and the heavy precipitation on the northeast side of the island since the terrain blocks the radar.  Go figure.  Here is a sample image from this radar.

Last Wednesday, moist, unstable air approved Kauai from the northeast, producing 15-20 inches during one day over the NE side of the island.  The result was river flooding (the main road was closed in Hanalei) and ten hikers were trapped on the Na Pali slopes and had to be helicoptered out (see pictures).

Perhaps the meteorological highlight of my trip came during a visit to a mountain waterfall.   The sun broke through as showers fell around us and I looked down into a valley.   An amazing sight (picture courtesy of my son, Nathan).  Look carefully, you can see a double bow.  Truly paradise.