Wednesday, October 01, 2014 596-SURF , 596-WAVE , 922-BONG , 638-RUSH , 572-SURF(MAUI) , 241-SURF (KAUAI) , 324-RUSH (BIG ISLAND)
   
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Winds Statewide in a glance ...go HERE (this link is also the 4th drop down under weather)

Sep 24, 2014 4:51 AM HST

Update
Winds have increased to advisory criteria at multiple sensors on Mauna Kea during the past hour. A wind advisory has been issued for the Big Island summits through 6 pm today. No other changes, previous discussion follows.

Synopsis
A moderate to locally fresh trade wind flow will focus shower activity over windward slopes today, although showers will reach some leeward areas as well, especially on Kauai. Showers will increase tonight through the end of the work week, as an area of enhanced moisture along a weak shear line moves over the island chain from the north. The wet trade flow will persist over portions of the state through the weekend.

Prev discussion
 scattered showers have developed across parts of the Hawaiian coastal waters during the night, although measurable rainfall at the gauges has thus far remained confined to Kauai. A 1024 mb surface high centered about 900 miles northwest of Kauai continues to drive moderate to locally fresh trade winds across the state. Satellite imagery reveals a diffuse shear line approaching the state from the north, extending far southwest and west from an intense low pressure center in the distant northeast Pacific. Aloft, a deep layered anticyclone is centered near the position of the surface high to our northwest, with dry and stable conditions prevailing aloft over the region. The 12Z soundings show that a well defined inversion is lacking, but stable conditions prevail in the mid levels, with pwats ranging from a below normal 1.21 inches at Hilo to an above normal 1.65 inches at Lihue. Mimic-tpw satellite imagery shows the leading edge of a wide east-west oriented band of higher moisture extending eastward from Kauai to about 80 miles north of Hilo. This moisture band is shifting to the southwest, and will spread across much of the state later today.

A wetter trade wind pattern is forecast to develop across the state during the next 24 hours. Continued moderate to locally fresh low level trade flow will generate increasing shower activity across windward and mauka areas, as the axis of enhanced moisture along the old shear line sinks southward and eventually stalls across the central part of the state. Showers should remain focused primarily across windward areas today, but expect a significant increase in rain chances across the smaller islands tonight through at least Thursday night. Windward areas will see a rather wet pattern during this time, with above normal rain chances across leeward areas as well. A more typical pattern seems likely on the Big Island, with scattered showers windward, and widely scattered afternoon seabreeze-driven showers on the Kona coast. Relatively stable conditions aloft along with quick motion of the showers in the continued trade wind flow should keep most rainfall amounts light to moderate. Uncertainty increases late Thursday night through Friday night, when the GFS shifts the enhanced moisture axis temporarily back to the north of the state, while the ECMWF keeps the moisture axis overhead. Have leaned toward the wetter GFS solution for now.

Uncertainty increases further during the weekend and into early next week, when many of the models continue to call for a mid to upper level trough to move near or over the island chain from the southeast. This increasing instability aloft could act on the stalled axis of enhanced moisture over the state to trigger periods of localized heavy rain, along with the potential for isolated thunderstorms. Trades may weaken somewhat during this time as well, allowing for slower shower motions and potentially a transition into a more widespread convective/seabreeze pattern. The 00Z ECMWF and GFS appear to have come into somewhat better agreement during this time period, but at this point confidence remains too low to fully buy into this potentially active weather scenario, and we continue to leave mention of heavy showers or thunder out of the official forecast at this time. The forecast for the weekend and early next week may require significant refinements as we move through the next few days.

Marine
The overnight ascat pass found areas of 25 knot winds in the Alenuihaha channel and south of the Big Island. The 12Z Lihue sounding also continues to show low level winds of greater than 20 knots, and the high resolution WRF models maintain advisory levels winds in the typically windy waters around Maui county and the Big Island through today and tonight. Have extended the existing small craft advisory through 6 am Thursday morning. The wind forecast for Thursday through Saturday remains uncertain, with models depicting a belt of enhanced trade winds dropping southward across the coastal waters along an old shear line. Some of the guidance indicates advisory level winds during parts of this time period as well, and the SCA may need to be extended further.

No major swells are in store through the weekend. A series of small, about 2 to 3 feet, swells from the NNW to NNE will pass through late this week and through the weekend. A small SSW swell will arrive Saturday, with a potentially more significant long period southerly swell forecast to arrive early next week. Otherwise, short period trade wind driven seas are expected to build a bit through the next couple of days, with surf along east facing shores remaining well below advisory levels.

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

Small craft advisory until 6 am HST Thursday for Maalaea Bay, Pailolo channel, Alenuihaha channel, Big Island leeward waters, Big Island southeast waters.

 

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