Influence of Topography on Precipitation in Hawaii


Hawaii is a wonderful place to live and work. Many people have a lifelong dream of visiting the Aloha State and the lucky ones who make it realize that it was a dream worth fulfilling. One of the number one reasons that Hawaii is such a destination for many is because of the weather. Sunshine and cool comforting trade winds out of the East Ė Northeast dominate the forecast more than half and up to 80% of the year. The regularity of the weather can make predicting it a little boring from time to time and it spurs jokes about weathermen in Hawaii having the easiest job in the world. But thatís not the case. Admittedly Hawaiiís setting in the Tropics means that the state doesnít deal with the same types of severe weather that plagues the Mainland United States. But forecasting in a barotropic environment can be a challenge. For one thing the model data isnít the best. Both the NAM and NGM models are available but they both suffer from boundary issues. Hawaii is literally on the edge of both of those models. The GFS, NAVY, ECMWF, and UKMET are global models but they donít always initialize well enough to come to some sort of forecast consensus. Iíve learned to trust the GFS most of the time but that doesnít always work. As a result precipitation forecasting can be tough. The climate of the region plays a big role in forecasting but there are days when the models arenít picking up on something and what was supposed to be a gorgeous day ends up cloudy and rainy. I bring this up because even if you had perfect model data it would be hard pressed to make a completely accurate forecast because of another thing that makes Hawaii paradise: the aina or land. (See final page for a list of Hawaiian words used in this paper).

The topography of the islands does such a huge job affecting the weather that synoptic scale models are sometimes worthless. In this paper I will discuss some of the ways the aina affects the rainfall in Hawaii. The examples I will use in this paper are taken from the island of Oahu. Itís the most populated of the eight major islands that comprise Hawaii and because it houses Honolulu thereís extensive data available on it. Weíll start off discussing windward and mauka showers. Anyone whoís followed Hawaiiís weather for even a small period of time will come into contact with that phrase. Like the name implies, windward and mauka showers hit the windward and mauka sections of the islands. Theyíre caused by the Pacific High which is the main influence on the weather for 50-80% of the year. It fuels the trade winds which evaporate moisture off of the ocean as they head towards Hawaii. Once they encounter the island they naturally hit the windward sides first. As the air drags over the land it piles up through frictional convergence and creates clouds. As the trades continue to push over the land they encounter the Koíoílau Mountains. These mountains are anywhere from about two to six miles from the Windward Coast. The windward side of the range is jagged and sharp rising thousands of feet into the air (average height about 2500 feet) over less than half a mile in some instances. As the air hits the backside of the range itís forced to rise even more so which creates our windward and mauka showers. A lot of the time showers stay confined to those areas. But exactly where the rain will fall is tricky. Knowing what spots have the best chance of seeing them requires one to know what the moisture characteristics of the air. But thereís only one official reporting site in Windward Oahu: Kaneohe Marine Corps Base Headquarters. Thatís one site for about 35 miles of coast.

Kailua Town is about a mile from the Windward Coast. Head mauka 3.7 miles and you come to Maunawili. Maunawili naturally gets more rainfall because itís closer to the Koíoílau Mountains. Itís also right next to the Olomana Ridge which has a peak of 1643 feet and enhances lift in the air. But sometimes Kailua Town is sunny and dry while Maunawili is getting hit with showers and other times Kailua Town is getting rained on while Maunawili is in the midst of a deluge. It all depends on how close the air is to saturation. Kaneohe is close to Kailua but subtle differences in dew point tend to go a long way.

About the only time we see windward and mauka showers becoming more scattered is when the high pressure in control is unusually strong and as a result our winds are as well. From time to time a 1036 to 1040 mb high will be the main weather maker. When this happens trade winds of 20-30 mph are common and gusts can often get into the upper 30s and in some places get to almost 50 mph (if not more). This is enough to push moisture that normally stays confined to windward and mauka spots farther leeward and our showers become a bit more scattered. Again pinpointing exactly where the heaviest rain will fall is difficult. Itís not hard if youíre making a forecast for the next few hours but once you start getting 12 to 24 hours out the accuracy diminishes. This also makes cloud cover forecasts difficult because when the winds are strong enough clouds and showers get pushed through pretty quickly. But if thereís ample moisture upstream then a steady flow of cloud cover will stay on tap and skies will be dominated by low clouds and weíll experience mostly cloudy skies even though our fair weather friend, high pressure, is in control.

There are certain weather situations that allow the islands to see showers becoming more widespread. One of those is the light wind pattern that allows for afternoon convection. The aina heats up faster than the surrounding ocean during the day. As a result thermal lows develop over the islands during the afternoon. This draws in the sea breeze and commonly leads to afternoon cloud cover over island interiors and in windward and mauka spots (donít worry, though, the sunshine isnít far away just head makai because itís beautiful at the beaches during this type of a pattern). Clouds are at their thickest in the late afternoon and thatís also when we have the threat for scattered showers. These showers can get quite heavy depending on how high the dew points are. Typically theyíll start out over central Oahu and then spread out. Sometimes they head towards Honolulu and sometimes they donít. Other times thereís a light rain in one area while in another rains are heavy enough to prompt a flood advisory. For the forecaster the best way around these problems is in the wording. Donít lay down any specific towns just warn about showers that could get heavy in windward, mauka and interior spots. Once the showers develop you can get an idea of where theyíre trending and provide a more accurate forecast.

The sea breeze set up is also one that allows the Leeward Coast (West Coast of Oahu) to pick up showers even though itís dry for most of the year. As onshore flow develops it encounters the Waianae Mountain Range. The hills here are on average anywhere from 2500-3000 feet with a peak of about 4000 feet. As air from the ocean hits the land it will form clouds and showers and give the normally dry Leeward Coast some much needed rainfall from time to time. But just because a sea breeze develops doesnít mean weíll see showers (and in some cases extensive cloud cover) over the Waianae Range. Again it all depends on the moisture content of the air coming off the ocean. In this region there are some unofficial sites but the closest NWS reporting site is in Kalaeloa which is nearby. However, because the topography can vary so much over short distances itís not always useful.

Whenever a front passes through the state or a Kona Low develops to the North the island of Oahu can expect widespread rainfall from the event. And even though the showers are scattered the topography still influences how much rain will fall in individual spots. Out ahead of the event we experience Kona winds and showers tend to develop to the South of the state and push over the islands. Even though rainfall tends to be widespread, spots with the steepest topography tend to get the heaviest rain. An example of this is in Kaaawa. About a mile inland thereís a mountain thatís almost 2100 feet high. This quick change in topography provides added convergence and leads to enhanced rainfall. There are other spots up the Windward Coast that have a similar set up and these areas typically see the most rainfall during these events. One spot near Kaaawa got a foot of rain in a 48 hour period in March 2006 (and thatís based on a nearby rain gauge, heavier rain could have fallen as you headed mauka). Topography and rainfall in Hawaii go hand in hand. The mountains across the state do a good job forcing orographic precipitation and enhancing frontal as well as convection based rainfall. I have read a few books on the weather in Hawaii and it tends to be very accurate on a broader scale. But specific mesoscale areas, like individual towns, donít tend to get a whole lot of specialized attention to them in literature (if itís there I havenít been able to find it). This is fine because after awhile the forecaster tends to get a feel for what areas are going to see rainfall; and of those which will see the heaviest rainfall. Thatís why the forecaster needs time to learn the ways of the land. Once he has a good handle on that aspect forecasting in paradise will not only be easier but when he does get the weather right, it will be that much more enjoyable.



2. Hawaii Atlas & Gazetter: Topo Maps of the Entire State; 1999

Hawaiian Words Used in this Paper

1. aina (eye-na): the land
2. mauka (mao-ka): towards the mountains or inland
3. Koíoílau (Ko-oh-low but low is pronounced like in the word allow) Mountains: Major mountain range on Oahu (although not a range in the technical sense because it was formed from a single mountain)
4. Makai: towards the sea
5. Waianae (Why-uh-nye) Mountains: Mountain range on the West side of Oahu again not a range in the technical sense)
6. Kalaeloa (Kah-lie-loa)
7. Kaaawa (Kah-ah-ava)
8. Kona Low: Low pressure system that forms in the area of the islands with at least one closed isobar at the surface supported by a cut-off low in the upper levels
9. Kona Winds: Winds with a Southerly component to them