Area Forecast Discussion for Hawaii

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Issued: Apr 23, 2014 4:00 PM HST

Synopsis
Breezy and gusty trade winds will continue into Saturday as a strong ridge holds several hundred miles north of the islands. Modest rainfall will remain focused across windward slopes, though some showers will be briefly carried to leeward areas on the trades. Look for an increase in shower activity as a band of moisture moves through Thursday night and early Friday. Trades will weaken Sunday and Monday.

Discussion
The islands will remain locked in a breezy and gusty trade wind flow into Saturday. Surface ridging centered about 700 miles north of the state between 30°N and 35°N will be driving the fresh to strong trades. In spite of the ridge being currently split by a dissipating front north of the islands, the pressure gradient over the islands remains a rather strong with little change since yesterday. The afternoon Lihue sounding shows the inversion up near 8000 ft due to a moisture band skirting just north of the garden isle. The Hilo sounding captured the inversion at a lower 5500 ft while recent aircraft out of hnl have detected the inversion down near 3500 ft. Given the low inversion away from Kauai, there will likely be a good amount of gusts in the 30's with localized gusts over 40 mph over and to the lee of island terrain tonight.

The gusty trades could be a notch stronger on Thursday and Friday. A 1029 mb surface high will pass along the ridge during this time. The GFS and ECMWF show 850 mb winds, roughly around 5000 ft, cranking up from the current 20 to 25 kt into the 30 to 35 kt range. Strong mid level ridging should maintain a low inversion height, but a complicating factor will be a shower band, currently north of Kauai, expected to move through Thursday night and Friday. If the inversion stays low and strong on either side of the band, we could see pockets of advisory level wind gusts of 50 mph over and downwind of terrain. Conditions appear to be border line at this time, but we will be closely monitoring new model data for signs of stronger winds. Gusty trades will be down only slightly on Saturday.

Modest rainfall will be focused across windward slopes with an uptick in shower activity expected Thursday night and Friday. A shallow band of moisture currently along 23°N will skirt just north of Kauai through the day on Thursday then pass over the islands Thursday night and early Friday. Aside from this feature, no other organized areas of moisture are currently forecast through at least the weekend.

Trade winds will finally back off on Sunday. An upper level trough will develop over the central north Pacific, eroding the surface ridge north of the state and sending a front southward. Trade winds will slowly decline on Sunday and bottom out at moderate strength by Monday. The front will dissipate before reaching the islands, but we could see some shallow remnant moisture ride in during the middle of the week as trade winds gradually build.

Aviation
Gusty trade winds continue, requiring airmet tango for mechanical turbulence to the lee of the mountains. Anticipate this airmet to remain posted through at least Thursday.

Vfr conditions are expected to prevail, however low clouds and showers being carried in on the trades will bring passing MVFR conditions, possibly IFR conditions, to mainly windward and mountain areas. Winds are strong enough that some of the showers may blow to leeward areas of the smaller islands.

Marine
Trade winds will remain in the fresh to strong range into Saturday, with the strongest winds expected Thursday and Friday. The small craft advisory /sca/ has been expanded to cover all waters in anticipation of the strengthening winds in addition to increasing combined seas. The SCA is in effect through Friday afternoon but will probably be scaled down to the typically windy waters around the Big Island and Maui on Saturday night or Sunday. Winds will be northeasterly and at their minimum late Sunday and Monday, then increase again during the middle of next week.

With the islands squarely placed in a belt of trade winds, windward buoys have been registering seas of 7 to 8 ft at 8 to 9 seconds today. Some increase in wind-driven seas is expected through Friday, and this will result in rough surf near the advisory level for east facing shores. The increasing seas also contribute to the need for the expanded SCA. Expect a decline late Saturday and Sunday.

A northwest swell will continue to build into tonight. The latest readings at buoy 51101 show that the swell is bigger than wavewatch guidance and will result in significant wave heights just below the north shore advisory criterion on the smaller islands late tonight and Thursday. The swell will drop Friday and Saturday with a small reinforcing northwest swell due late Sunday into Tuesday.

Not to be left out, south facing shores are seeing some swell energy from the southwest. This swell will fade on Friday with low odds for some small south-southeast swell this weekend.

Watches/Warnings/Advisories
Small craft advisory until 6 pm HST Friday for all waters.


DISCUSSION/MARINE...WROE

AVIATION...MBALLARD