Area Forecast Discussion for Hawaii

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Issued: Apr 20, 2014 3:34 PM HST

Synopsis
Strong high pressure to the north of the state will create locally windy trades tonight and Monday. The trades will ease off a bit as the high moves away by midweek. The strong trades will continue to blow passing windward and mauka showers over to leeward sides at times through about Monday. Drier air and fewer showers are expected for much of the work week.

Discussion
Surface high pressure analyzed about 1000 mi almost due N of PHNL at 18Z keeping locally breezy trade winds blowing. The high is expected to continue to move slowly E over the next couple of days. At the same time, upper level ridging will build over the surface high, helping to lower and strengthen the relatively weak subsidence inversion currently over the islands. This should help to increase wind speeds a bit in the boundary layer tonight and Mon. So far, only Kamuela has been flirting with advisory level winds, but a few more spots could see winds near or at advisory level later on, thus will leave the wind advisory in place for now. It is worth reiterating that this advisory is not in effect for all parts of all islands, but mainly for areas immediately downwind of the smaller mountain ranges and over headlands.

The trades will relax a bit Tue and Wed as the high to the NE moves away. Another high will move N of the islands Wed night, but this high will be a little weaker than the current one. It will stall out N of the islands, in almost summerlike fashion, keeping locally breezy trades blowing all the way into next weekend.

An area of somewhat deeper moisture is seen on the mimic precipitable water animation riding in across the islands. This will continue overnight into Mon, keeping some passing showers going for windward and mauka areas. The brisk trades will continue to be strong enough to push some of these showers over the mountains briefly to nearby leeward locations. The airmass should dry out again by about Tuesday, with fewer showers expected for the rest of the week.

Aviation
Moderate to strong trade winds from sfc to about 10k with subsidence inversions between 5k and 8k aloft expected through 24 hours and beyond. Will be keeping airmet tango for mt wave turb to the lee of all islands in effect indefinitely.

Clouds and showers will favor the typical windward and mountain areas tonight with a few of these showers drifting into the leeward portions of the smaller islands. Mvfr cigs in showers are presently isolated but current trend is for increased moisture in the lower levels. Will likely need an airmet for mt obsc overnight for windward slopes and coasts of the Big Island, and possibly the smaller islands as well.

Few reports of moderate upper level turbulence across the state today but conditions are expected to remain isolated. No airmet issued or anticipated, but will give a non-airmet mention in the WA0HI.

Marine
The university of Hawaii high resolution WRF model shows winds peaking just below gale force in the Alenuihaha channel tomorrow. There is a chance we could have to upgrade the small craft advisory /sca/ to a warning if the models are a bit underdone on the winds, but confidence is not high enough yet to justify that. We have also issued a high surf advisory /hsa/ as east facing shores are continuing to see surf heights increase, and many areas will exceed the 8 feet face advisory criteria.

Watches/Warnings/Advisories
Wind advisory until 6 pm HST Mon for Waianae coast, Oahu Koolau, olomana, Waianae mountains, Lanai, Kahoolawe, Maui windward west, Maui leeward west, Maui central valley, south Big Island, Big Island north and east, Kohala, Big Island interior.

Hsa until 6 pm HST Tue for Kauai windward, Oahu Koolau, olomana, Molokai windward, Maui windward west, windward Haleakala, south Big Island, Big Island north and east.

Sca until 6 pm HST Mon for all Hawaiian waters.


R BALLARD/ASB