Area Forecast Discussion for Hawaii

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Issued: Jul 30, 2015 10:00 AM HST

Synopsis
High pressure far northeast of the state and a series of weak tropical disturbances passing to the south will maintain locally breezy trade winds through the weekend. The trades will bring some showers to windward areas, mostly during nights and mornings. Remnant moisture from a former tropical depression may affect the islands over the weekend, bringing muggy weather and increased showers for windward areas, especially for the Big Island.

Discussion
Water vapor imagery shows an upper level high southeast of the state, with a ridge extending over the islands. At the surface, high pressure is located far northeast of the state. The remnant low that was tropical depression Eight-E is located far east of the Big Island, and tropical storm Guillermo is far to the east-southeast. 12Z sounding from Lihue showed an inversion near 4-5kft and a precipitable water of 1.4 inches. Early morning total precipitable water imagery shows similar to slightly higher values over and upstream of the state, with much higher values in excess of 2 inches near 140°W associated with the former tropical depression.

A typical trade wind pastern is expected for the next couple days. Between the high to the northeast and a low passing far south of the state, the gradient across the islands will support locally breezy trade winds. Showers have been focused across windward sections, with rainfall amounts generally a tenth of an inch or less over the past 12 hours. Forecast soundings/time sections show inversions and moisture depth remaining steady through the end of the week. A similar weather pattern is expected through Friday night, with showers focused mainly across windward sections, greatest at night and during the morning, with some showers across leeward Big Island and Maui during the afternoon.

Former tropical depression Eight-E is expected to move west-southwest and pass by south of the state over the weekend. The main impact expected for Hawaii will be an increase in moisture, as well as a continued enhancement to the trade winds. 12Z models show most of the moisture associated with the former depression impacting east and southeast sections of the Big Island. There is the potential for some of this deeper tropical moisture to spread to the smaller islands as well, but 12Z gfs/ecmwf have backed away from this and now focus the bulk of the showers from the Big Island southward. Moisture depth will likely extend up to summit level, though temperatures are expected to remain above freezing through the period.

The National Hurricane Center in Miami is issuing bulletins on tropical storm Guillermo, which is nearly 2000 miles east-southeast of Hilo. The current forecast is for Guillermo to move west-northwest and strengthen, potentially crossing 140°W into the Central Pacific Hurricane Center area of responsibility on Saturday. Impacts to Hawaii will be very dependent upon the track and speed of the system, which is highly uncertain this far out in time.

Aviation
Expect trade winds to increase to moderate to locally breezy today and maintain through the period. As of 20Z wind speeds are below the threshold for widespread low level turb, however some areas may experience localized light to mod turb in the lee of the terrain. We will continue to monitor and airmet tango for low level turb may be needed later.

Clouds and showers will focus along windward and mountain areas, with prevailing VFR cigs/vis at all island terminals.

An upper level jet a few hundred miles northwest of Kauai may produce isol light to mod turb between fl300 to fl450 northwest of the area.

Fire weather
With locally breezy trade winds, we may reach the red flag wind threshold of 18 kt again. However, dewpoints are slightly higher than yesterday, and we are not expected to reach the relative humidity threshold of 45 percent. Therefore a red flag warning in not anticipated, but fire danger threat remains elevated.

Marine
A small craft advisory continues for the typically windier areas near Maui and the Big Island. As the remnant low that was former depression Eight-E passes south of the state, it may strengthen the gradient enough to warrant expansion of the advisory to additional channels near Kauai and Oahu.

No major swells are expected for the next several days. Surf is expected to remain below the advisory threshold along all shores.

The National Hurricane Center in Miami is issuing bulletins on tropical storm Guillermo, which is nearly 2000 miles east-southeast of Hilo. The current forecast is for Guillermo to move west-northwest and strengthen, potentially crossing 140°W into the Central Pacific Hurricane Center area of responsibility on Saturday. Impacts to Hawaii will be very dependent upon the track and speed of the system, which is highly uncertain this far out in time.

Watches/Warnings/Advisories
Small craft advisory until 6 am HST Sunday for the Pailolo and Alenuihaha channels, Maalaea Bay, and waters south of the Big Island.


BRAVENDER/EATON