Area Forecast Discussion for Hawaii

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Issued: Nov 25, 2014 9:30 AM HST

Synopsis
A cooler and somewhat drier air mass has settled over the islands and will linger for another couple of days. A modestly strong high pressure system will settle north of the islands Thursday, resulting in breezier trades for the rest of the week and the weekend. A stable air mass will keep the frequency of trade showers down and confined to the windward and mauka areas.

Discussion
Post frontal clouds persist over the islands this morning with a shallow layer of stable low clouds mainly between 25 hundred and 7k feet. We expect this stratocumulus /sc/ deck to be thinning out gradually over the smaller islands through out today and into this evening. Windward Big Island will be slower in clearing in the reason to be explained shortly. But we are hopeful for a bit more Sun tomorrow. Thus far light showers or light rain have been focused across windward Big Island overnight and this morning will likely continue through tonight.

The situation is we are dealing with a low level trough at around 6 to 7k feet with the NE to SW oriented axis over Molokai. The trough is nearly stationary but weakening, and it is forecast to gradually fill in through Wednesday. Thus we expect the sc deck to decrease but not totally clear out the smaller islands today. Beneath the trough is a N to NE E winds, mostly modestest in speed. Above and east of the trough is a light to moderate SW wind flow. The GFS solution has this trough totally filled in over the Big Island by Wednesday night, so more Sun and less showers for this part of the Big Island tomorrow.

The dissipation of this trough is being brought upon by higher pressure building in from the N and W where a modestly strong surface high is expected to settle in 11 hundred miles north of Kauai Thursday night. This high will linger north of the islands over the weekend but weakening from a Max central pressure of 1031 mb to 1026 mb Monday morning. The result for the islands is strengthening trades to locally strong and breezy from as early as Wednesday afternoon continuing into the weekend. We may encounter a slight uptick in trade showers with the increasing trades, and perhaps slightly warmer temps. As per GFS solution however, the air mass is forecast to remain stable with a strong inversion at 5 to 7k feet during this period of breezy trades. This inversion will tend to keep the frequency of the showers down and confine the shower to the windward and mountain areas.

Aviation
Overcast low clouds with tempo MVFR cigs and mtn obsc are impacting windward slopes of Oahu through Big Island this morning. These clouds are likely to hang around for much of the day. Have increased mtn obsc to cover windward sides of the mentioned areas.

Airmass is rather dry and stable so precipitation, if any, will be light. Moderate north to northeast across the islands due to a high pressure ridge to the north. Believe winds may not be strong enough to bring low level turb affecting imt S thru W areas of the mtns at this time.

Marine
The NW swell is still running slightly above guidance at 9 am HST, at 9 feet at 11 secs, resulting in surf of Max 15 ft along Oahu's north shoreline. The high surf advisory ends at 6 pm tonight but and wave watch guidance shows some linger above normal heights at least early in the evening. So more than likely the advisory will be extended to 6 am HST Wednesday. No significant swells are expected for the rest of this week.

The small craft advisory /sca/ for selected waters is due primarily to the above normal swell and not so much for winds. It is expect however, the trades start picking up strength Wednesday that will keep the SCA in place through at least 6 pm Wednesday for the typically sensitive areas exposed to the trades.

Watches/Warnings/Advisories
High surf advisory until 6 pm HST this evening for north and west facing shores of Niihau and Kauai and for north facing shores of Oahu Molokai and Maui.

Small craft advisory until 6 pm HST Wednesday for Kaiwi Channel, Maalaea Bay, Pailolo channel, Alenuihaha channel, Big Island leeward waters, Big Island southeast waters.

Small craft advisory until 6 pm HST this evening for Kauai northwest waters, Kauai windward waters, Kauai leeward waters, Kauai channel, Oahu windward waters, Maui county windward waters, Big Island windward waters.


DISCUSSION/MARINE...LAU

AVIATION...FOSTER