Area Forecast Discussion for Hawaii

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Issued: Mar 06, 2015 10:00 AM HST

Synopsis
Brisk and cool northeasterly winds will continue to produce rather wet conditions across windward slopes through tonight. Winds will decrease during the weekend as a surface ridge to the north is eroded and displaced toward the islands. Deeper moisture south of the Big Island may push far enough north to increase rain again over the Big Island on Sunday or Sunday night.

Discussion
The islands lie within a transition zone between a strong 1039 mb surface high 1500 miles to the northwest of Kauai and a nearly stationary front about 400 miles to the northeast of the Big Island. Moisture pooled near the front is getting caught up in the brisk low level northeasterly flow and is being transported over the state, especially across the eastern end of the island chain. The wetter windward gauges picked up around an inch on Kauai and Oahu overnight, while quite a few windward gauges on Maui county and the Big Island picked up more than 2 inches. Little rainfall occurred across leeward areas.

A breezy and cool northeasterly flow will continue to produce rather wet conditions over windward areas in the short term. Forecast models show only slight changes to this scenario through tonight, with the main difference being a gradual decrease in available low level moisture. As a result, expect continued wet conditions across windward slopes of all islands into tonight, especially on Molokai, Maui, and the Big Island, with some decrease in rainfall amounts compared to last night.

An increase in rainfall is possible near the Big Island beginning Sunday, while all islands will see a decrease in the cool northeasterly winds. The current longwave pattern features deep troughing just north-northeast of the state flanked by deep ridging. A shortwave aloft will rotate through the base of the trough late Sunday into early Monday. This will weaken the surface ridge currently to our north, leading to a decrease in the northeast winds. Deeper moisture will likely be drawn over the Big Island and possibly as far north as Maui, creating the potential for another round of wet weather and summit snowfall on the Big Island, though not as heavy or persistent as the last round of steady rain. In addition, expect increased high clouds state-wide.

This pesky band of deep moisture hangs around even into the middle of next week. The 00Z/06 ECMWF still eventually wants to bring heavier showers up over Maui and the Big Island, but the trend is later, and farther south than before and more in line with what the GFS has been persistently showing. The northern islands should remain under moderate and rather cool northeasterly low level flow within mainly windward showers.

Another prolonged episode of very strong winds for the Big Island summits appears likely as strong low pressure in the mid-levels of the atmosphere presses south toward the islands this weekend. In the meantime winds will likely remain near advisory levels for Mauna Loa and Mauna Kea through about Saturday, then ratchet up to warning levels for Sunday and beyond.

Aviation
Breezy northeast winds will continue today. Airmet tango for low level turb over and south through west of the islands is posted. This will likely continue into Saturday.

Passing low clouds and showers under the breezy northeast winds will bring periods MVFR ceilings and mtn obsc over windward slopes. Prolonged periods of MVFR cigs are occurring at PHMK, PHOG, and PHTO as the majority of clouds and showers have been focused over windward areas of Molokai, Maui, and the Big Island this morning. Airmet sierra for mtn obsc is posted. Otherwise, VFR conditions should prevail at sites today.

Airmet tango for tempo light to mod turb between fl250 and fl400 is posted as an upper level jet sit overhead.

Marine
A strong, large 1039 mb high about 1500 miles NW of Kauai is keeping trades more northerly than usual and in the fresh to locally strong range, with small craft advisories /sca/ needed for those areas where the NNE winds are accelerated around island terrain. Sca-level winds will persist through Fri night but should drop on Sat as a narrow ridge of high pressure pushes S toward the islands. Rough seas are also running close to SCA criteria as well.

A complex of low pressure areas milling around south of Alaska the last few days created a couple of fetches of NW and NNW winds south of the Aleutians. These winds were mainly below gale force but still enough to bring a couple of small NW swells to the islands. The current small NW swell will be winding down as we head into the weekend while the forerunners of another arrive on Friday.

In addition, the guidance shows another complex low developing about 1100 miles NNE of the islands on Saturday and lingering into next week. This one will have a fetch of northerlies that may reach gale force, which will send a larger swell that will likely reach the islands about Sunday. The swell will again be rather short fetch owing to the proximity of the low. It will likely be sub-advisory level for north facing shores, but surf heights along E facing shores exposed to such a swell may reach advisory criteria by Monday as well as a possibility for harbor surges for Kahului and Hilo harbors.

Watches/Warnings/Advisories
Wind advisory until 6 pm HST Saturday for Big Island summits.

Small craft advisory until 6 am HST Saturday for Kauai northwest waters, Kauai leeward waters, Kauai channel, Oahu leeward waters, Kaiwi Channel, Maui county leeward waters, Maalaea Bay, Pailolo channel, Alenuihaha channel, Big Island leeward waters, Big Island southeast waters.


DISCUSSION...WROE

MARINE...R BALLARD

AVIATION...EATON