Daily Weather

May 19, 2015 4:00 AM HST

Synopsis
Another day or two of trade winds before they take a breather as a front approaches the main Hawaiian islands from the northwest. With the light and variable winds, mugginess will settle in for about two days before the cooling moderate trades return for the weekend. The front is expected to reach Kauai Friday evening, then dissipate as it moves down the island chain. Remnant moisture from this front is expected to get hung up over the windward coast of the Big Island over the weekend. Leeward Big Island will have some afternoon showers each afternoon.

Discussion
Relatively dry condition prevails across the main island Hawaiian islands for the exception of windward Big Island where passing showers persisted. Only within an hour, have these trade showers increased affecting the windward and mauka sections of the smaller islands. Models are showing an elongated east to west band of clouds aimed at the Big Island for the next couple of days.

So far the forecast is on track for diminishing trades and muggier conditions for the second half of the work week. The weakening trades is being brought upon by an approaching front from the northwest. This front is currently nearing Midway atoll which is located 1170 miles west northwest of Kauai. More info is available about this front in our high seas marine bulletin for the north Pacific. Once the light and variable winds settles in, daytime heating coupled with onshore sea breezes will result in locally cloudy skies over interior and leeward areas of the smaller islands. These clouds are slated to be heavier Friday afternoon for all islands with scattered showers, accentuated by a passing mid/upper level trough. The ECMWF is a bit more vigorous with this trough than the GFS, showing a cool pool of minus 9 c temp skirting the islands. The GFS has a much smaller area of cool air, staying well away from the islands.

A few hours later, early Friday evening, the front reaches Kauai, and then gradually falls apart thereafter as it works eastward. Upper level support for the front will have wane on Saturday as it out runs the front. Remnant moisture from it is expect to reach the Big Island Saturday evening. And there the moisture stays trapped until Monday as a shallow northeast wind flow keeps the moisture from going anywhere else. As the low level winds veers more easterly Monday, clearing skies will come to the windward coast of the Big Island.

The northeast winds following the front will usher in a slightly cooler air mass. Also, the strength of the northeast trades will be tempered with moderate speeds at best. This is due to a blocking, high amplitude high sitting off the west coast which will prevent the next high from approaching the main Hawaiian islands too fast.

The layer of high cirrus clouds will gradually thin out through this afternoon.

Aviation
Easterly trade flow will dominate through today, providing clouds and mainly light showers mostly the windward and mauka areas. The winds have diminished enough, allowing the cancellation of airmet tango. No airmets are in effect. Mvfr conditions may occasionally occur across windward sections, but VFR will dominate across the terminals today.

Marine
Small craft advisory /sca/ remains for the typically windy waters around the Big Island and Maui. The SCA is set to expire at 6 am Wednesday.

A series of south swell will be reaching the islands through this weekend. The first of two is set to arrive this Thursday. The buoy down at american samoa started picking up on this swell Sunday and has remained steady at 6 to 7 feet and 13 seconds since. The second swell from the south is due in on Sunday. Surf from both these swells will be below advisory level for the south facing shores. A series of small NW swell will continue across the north facing shores the rest of this week. A moderate wind-driven short period swell along east facing shores will continue through today, then decline Wednesday through Friday as trades weaken. Please refer to the collaborative nearshore swell and wind forecast /srdhfo/ for more details.

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


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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