Daily Weather

April 3, 2015 5:00 AM HST

Synopsis
Trade wind speeds will ease slightly today, especially across the smaller islands, as a surface trough moves to a position north of Kauai. An increase in showers is expected through Saturday, especially over the windward Big Island, due to an area of enhanced tropical moisture spreading up from the southeast over the eastern end of the main Hawaiian island chain. Breezy trade wind weather is forecast to return later this weekend as a surface high strengthens to the northeast of the islands. Expect a somewhat typical springtime trade wind weather pattern early next week.

Discussion
The pressure gradient southwest of a nearly stationary 1033 mb surface high centered near 37°N 142°W, or more than 1400 miles northeast of Honolulu, is maintaining locally breezy trade winds across the eastern portion of the main Hawaiian island chain early this morning. However, a surface front located about 500 miles northwest of Lihue is moving slowly toward the northeast. The lower pressure associated with this feature is causing the winds over most of the western and central sections of the island chain to gradually weaken and shift to an east southeast direction.

Aloft, a mid-tropospheric trough is evident northwest of Kauai. This feature has caused a slight destabilization of the atmosphere near Kauai. However, moisture profiles from the early morning Lihue sounding and the latest satellite derived estimates of precipitable water /pw/ showed values of only about one inch in the vicinity of the smaller islands. There were scattered to locally broken low clouds and isolated showers mainly occuring over some windward and mauka sections of the smaller islands, while broken to overcast low clouds and scattered showers were near the windward Big Island.

The Hilo sounding showed slightly higher moisture values, with a PW of about 1.2 inches. However, an area of enhanced tropical moisture is currently evident about just south of the Big Island. The forecast models indicate this area will continue to spread up from the southeast over the eastern end of the main Hawaiian island chain today and tonight. Some of this enhanced moisture may also spread up over the smaller islands from later today and tonight into the weekend.

Output from the forecast models indicate the front northwest of Kauai will weaken to a surface trough later today or tonight when it moves to a position north of Kauai. Also, the mid-level trough will continue to shift eastward. The presence of these features will continue to slacken the pressure gradient across the area, leading to weaker east to east southeast flow over most of the smaller islands today. Parts of the Big Island may continue to see locally breezy trades today, with a slight decrease in wind speeds tonight.

The increased moisture combined with a slight destabilization of the atmosphere due to the mid-level trough will likely lead to increased showers especially over the windward Big Island from today into this weekend. The models also indicate there may be a modest increase in showers over portions of the smaller Hawaiian islands at the same time. There is also the potential for snow showers over the Big Island summits tonight and early Saturday morning, due to deeper moisture and the colder temperatures aloft associated with the mid-level trough.

During this weekend, the surface trough, as well as the associated mid-level trough are expected to lift north northeastward away from the islands. This should allow the pressure gradient to become reestablished over the area due to the surface high far northeast of the state becoming the dominate synoptic feature once again. This will lead to an increase of trade winds over the islands from this weekend into early next week. The global forecast models also continue to indicate a slightly drier air mass may spread across the area by early next week. Therefore, expect a somewhat typical springtime trade wind weather pattern across most of the island chain early next week.

Advisory and Warning Criteria
Location Advisory Warning
North-Facing Shores 15 Feet (8') 25 Feet
West-Facing Shores - Big Island 8 Feet (4'+) 12 Feet
West-Facing Shores - Remaining Islands 12 Feet (7') 20 Feet
South-Facing Shores 8 Feet (4'+) 15 Feet
East-Facing Shores 8 Feet (4'+) 15 Feet

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